This presentation will discuss the workflow and methods to characterize and manage fluid circulation in geothermal reservoirs from a geomechanical point of view. We will in particular focus on the following key-points: i) in-situ stress field characterisation, ii) hydraulic stimulation design and monitoring, and iii) multi-scale reservoir geo-mechanical response. Incorporation of uncertainties in engineering decision will be highlighted

Short advertizing presentations of selected PhD students to attract the audience on their poster room

Productions and injections of fluids in rocks can induce significant earthquakes. Induced seismicity is observed by hydraulic fracturing of shale and stimulation of enhanced geothermal systems.

Earthquakes can be caused by long-term developments of oil and gas fields. Understanding and monitoring of fluid-induced seismicity is necessary for controlling its seismic risk. The talk provides an overview of physical and geomechanical fundamentals of the seismic hazard, of approaches to its control and of various relevant case studies.

Summary: The episodic recurrence of seismic events suggests that fault zone mechanical and transport properties may vary cyclically. During the interseismic period, elevated temperatures in hydrothermal systems are known to affect the permeability of fault zones with time, and the dynamic evolution of structural permeability in the seismogenic zone may induce fault zone weakening through the generation of elevated pore pressures. Understanding how competing fracturing/sealing processes (gouge formation, hydrothermally induced healing, pressure solution) can lead to the development of overpressured patches along fault zones is critical, as they can act as nucleation sites and trigger earthquakes. Seismogenic fault fracturing can create considerable fracture permeability in and around the rupture zone which can initiate large fluxes of fluid through a variety of processes, and can result in hydrothermal deposition of economically important minerals such as mesothermal gold-quartz deposits. For example, earthquake slip transfer in dilatational jogs along strike slip fault zones creates space via extensional fracturing , lowering the mean stress and pore fluid pressure within the jog leading to a suction pump effect. Large coseismic fluid fluxes have also been shown to initiated by the ‘fault valving’ process first suggested by Sibson. Earthquake rupturing of low permeability seals can release previously trapped high-pressure fluids, that propagate through fault fracture network created by the mainshock as pressure pulses that have been directly correlated to aftershock hypocentres over periods of weeks. Experimental studies of permeability variations in fresh fractures at seismogenic temperatures have shown permeability can significantly recover over similar timescales of days to weeks and microfractures on the order of hours to days. However, at larger scales, a growing body of geophysical evidence exists for damage healing processes over such longer timescales in active fault zones. For example, the Landers fault zone in California was shown to exhibit an abrupt decrease in seismic velocity following the 1992 earthquake, interpreted as rupture induced fracture damage. This was followed by a time-dependent increase in seismic velocity lasting over ten years which was inferred to be fault zone healing processes. It is clear therefore that competing fracturing and sealing processes at different scales will have a significant control on fault zone fluid flow and a variety of geological processes. It is clear therefore, that a combination of field, laboratory and geophysical techniques is required to address such problems. Laboratory studies can quantify the temporal variations of permeability due to competing fracturing and hydrothermal sealing processes, and field and geophysical investigations are required to place the experimental results into a framework of constrained natural fault zone architecture. In this presentation, I will present a general background to these themes in the context of geothermal systems, an overview of my own research, and summarize areas that still need to be addressed.

Short advertizing presentations of selected PhD students to attract the audience on their poster room

Short advertizing presentations of selected PhD students to attract the audience on their poster room

In recent years, deep geothermal projects are on the rise in Germany, especially by the huge development in the Bavarian Molasse close to Munich. In spite of the moderate temperature, there are now 24 projects with numerous others in the planning or extension phase. In contrast, the Rhine Graben area has a much higher resource potential due to the higher temperature levels but only 5 deep projects are in operation so far.

The presentation will highlight the different conditions of this geothermal development, detail the challenges associated to environmental hazard and present new concepts for future geothermal utilization. Especially after the events close to Strasbourg, seismic hazard and the associated public acceptance have an overwhelming impact on the geothermal developments. New approaches by the scientific community are required to create the acceptance and broader support of this energy source, that is indispensable for a successful realization at least for a transition of heat from fossil fuels.

In addition to the typical production of heat from subsurface layers, there exists an increasing need for heat storage. These new concepts will support the sector coupling by the supply of renewables from base-load (e.g. geothermal) or fluctuant energy sources (e.g. wind or solar). In this context, also the new DeepStor project at KIT can be considered as a pathway into new geothermal utilization scenarios and to future applications.

Reykjavik is almost entirely heated by geothermal energy. Yet, recent growth of the city significantly increased the heat demand. Past experiences in Iceland’s capital region showed that hydraulic stimulation of existing geothermal wells is suited to improve hydraulic performance and energy supply. However, fluid injection may also trigger felt or even damaging earthquakes, which are of concern in populated areas and pose a significant risk to stimulation operations. Consequently, soft stimulation concepts have been developed to increase geothermal well performance while minimizing environmental effects such as induced seismicity.

In a demonstration project of hydraulic soft stimulation in October 2019, more than 20.000 m³ of water were injected into well RV-43 on Geldinganes in multiple stages and with different injection schemes. An a priori seismic risk assessment was conducted and for the first time the risk was continuously updated by an adaptive traffic light system supported by a sophisticated realtime microseismic monitoring. This work was performed within the H2020 project DESTRESS. We will discuss the entire workflow from planning to deployment and highlight the most important results and critical challenges.

The United Downs Deep Geothermal Power project is the first geothermal power project in the United Kingdom. The project consists of two deviated wells; a production well to a depth of 5275m MD and an injection well to a depth of 2,393m MD. Both wells target a sub-vertical, inactive fault structure that is thought provides enhanced permeability relative to the surrounding granitic rock. This talk places the project in the context of previous geothermal research carried out in Cornwall, summarises the concept and describes the site selection work carried out. It also outlines the preliminary geological findings.

This course presents the geothermal power plant of Soultz-sous-Forêts and the heat geothermal plant of Rittershoffen. Localization and geological context, history, major drilling and testing results, as well as plant design, will be briefly presented for both sites. The student will also have a virtual 3D visit of the Rittershoffen and Soultz-sous-Forêts geothermal plants.

Warming up of the event by the EGPD Organization Committee

Introduction to the program of the 2 days

Some virtual rules for fluidity in the online exchanges

Visit of surface installations and explanations

Farewell words from the Organization Committee.

Announcement of prices for the best posters.

Reading of application letters from candidate universities willing to host EGPD 2022.
If no application, call for candidates to the PhD participants who could encourage their universities.

Warming up of the President of CY Cergy Paris Université and MEET organization team.

Introduction of the 3 days.

Some virtual rules for fluidity of online exchanges.

Nowadays, no researcher, whatever in public or private organisations, can ignore or even underestimate the mechanisms and power of public funding, especially at European Union level. European Commission, through the Framework Programme for Research, Development and Innovation, supports the EU research through various so-called “funding instruments” according to its social, economic and competiveness needs.
Fabienne Brutin will provide to MEET winter school’s participants detailed information about overall funding rationale and instruments panorama, selection mechanisms and other tips, so they will be able to maximise their choices with the objective to boost their careers !

Summary:
Geothermal energy is becoming increasingly seen as a vital part of the 'energy transition' and an essential part of the sustainable development toolkit. A critical investment and implementation barriers to a global expansion of geothermal energy, however, is demonstrating its 'social licence to operate'. Gaining the broad societal acceptance of geothermal energy is hampered by low public awareness about the subsurface exploitation of geo-energy and poor community participation in energy projects development. Therefore, the key to gaining societal acceptance is effective communication of highly technical science to non-technical audiences, from local residents to political decision-makers.

In this talk, Pr. Iain Stewart will build on a 15-year partnership working with BBC television making popular earth science documentaries - intergrated with direct experience of public communications and communtiy engagement with a deep geothermal drilling project in SW England, to examine how effective public engagement can drive forward future geothermal energy projects.

N.B: This public talk will be disseminated at different levels in the European community in order to popularize geothermal energy (secondary teachers, general science associations, geology & geothermal associations, local-national-european authorities, decision-makers, environmental organisations, NGOs, research, industry, etc.)

Engineered Geothermal Energy Systems (EGS) follow a concept that involves an artificial improvement in the hydraulic performance of a reservoir with the aim of using it for the economical supply of heat or electrical energy. The improvement challenge is based on several unconventional methods of researching, developing and using geothermal resources that are not economical with conventional methods. In the introductory session of the MEET Geothermal Winter School 2021 I will present the EGS development based on the holistic requirements:
We need to be aware of the potential location-specific market for the provision of geothermal energy (heat or electricity). There are European destinations with easily usable hydrothermal sources for electricity and much more for heat supply. Other targets require different exploration strategies because they need to go deeper or meet safety requirements, as development is based on stimulation strategies that need to rule out harm to the public. We have to characterize the potential reservoirs, i.e. relevant rock and fluid properties, stress, permeability and temperature, and basic equations have to be considered. According to the EGS concept, we should have access to methods of safe reservoir technology. So-called soft stimulation approaches and tests are recommended, which require a deep understanding of all processes involved. After we have achieved the desired hydraulic performance, we need to set up sustainable geothermal fluid loops and use the heat on the surface of the earth that contributes to the energy needs to be met on site.

• Introduction with some motivation for geothermal energy (heat or electricity) provision
• European targets, with some potential maps and rough exploration strategies
• Reservoir characterisation, i.e. relevant rock and fluid properties, stress, permeability, and temperature and fundamental equations
• Reservoir engineering, i.e. soft stimulation approaches and testing
• Geothermal fluid loops and utilisation and
• Conclusions

The Upper Rhine Graben (URG) was historically the root for the development of a new type of deep geothermal energy aiming to exploit the heat extracted from crystalline rocks hidden by a thick sedimentary cover.

Based on deep drillings penetrating the crystalline basement of the URG at Soultz-sous-Forêts (Alsace, France), various geothermal concepts like Hot Dry Rock, Hot Fractured Rocks, Hot Wet Rock, and a more recent approach were defined by the geothermal community as EGS, Enhanced Geothermal Systems or Engineered Geothermal Systems.

EGS is more a technology than a concrete geothermal concept aiming to enhancing a low permeable well for reaching an economic threshold by applying thermo-hydro-chemical treatments.

Based on operating and under development geothermal sites operating in the Upper Rhine Graben, this presentation will illustrate how EGS technology is still on the learning curve and could produce electricity, heat or some strategic metals from deep fractured basements.

Every deep geothermal project starts with the exploration for the geothermal reservoir. The exploration aims to completely characterize the reservoir including the determination of its structure, extent, thickness, thermal field and petrophysical, hydraulic and mechanical properties.

This lecture will introduce standard exploration workflows for deep geothermal projects in various conditions (low-enthalpy sedimentary basins, mid-enthalpy basement reservoirs for enhanced geothermal systems as well as high-enthalpy volcanic geothermal systems). Different exploration methods from low-cost fieldwork-based approaches to high-cost geophysical surveys will be introduced and the main targets and outcomes of each method will be explained. Finally, the integration of the exploration data into one comprehensive model of the geothermal reservoir will form the basis to select drilling targets and define the probability of success which is the basis for investor decision and prospective risk insurance.

Especially EGS projects are dedicated to the exploitation of heat from reservoirs with limited permeability. For these kinds of reservoirs stimulation is part of the plan to promote economic viability of the project. Other projects not envisaged as EGS may need stimulation to further enhance or restore the connectivity of the well with the reservoir. Challenges derive from drilling the well, well design and completion, reservoir characteristics, hydrochemistry and in-situ stresses among other potential reasons.

The lecture aims to provide an overview of different techniques to enhance open geothermal systems. A focus will be on chemical treatment, its prerequisites, strategy and technical steps, as well as major risks and their mitigation measures.

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Abstract:
In recent years, geothermal energy has been increasingly considered as a major contributor in the future’s green energy mix. Reservoir stimulation techniques enable heat and power generation from conduction-dominated play types, found away from tectonic plate boundaries. In the UK, conduction-dominated plays of the hot dry rock and hot wet rock types can be expected around radiothermal granites and in sedimentary basins. A promising target of the latter type is the Lower Carboniferous Dinantian Carbonate play; a known hot spring aquifer with an assumed subsurface extent that spans areas of increased heat demand in the country. The confirmation of this stratigraphic unit as a regional geothermal play will be contingent on the successful identification of facies types with favourable porosity/permeability characteristics; or that are suitable for EGS development. Induced seismicity as one of the main risk factors associated with EGS should be an important focus during the development of the Dinantian play; considering it is underlain by crystalline basement and its proximity to densely populated areas. Currently, seismic risk mitigation in the UK is based on a traffic light system with prescribed threshold magnitude values at which a reduction of injection rate, or stopping of injection is prompted. A shortcoming of this system is highlighted by examples from different injection sites that experienced increasing seismicity after such mitigative measures. Injection protocols should be designed and adjusted with a focus on keeping the maximum induced magnitudes below a certain level. Here, a project is proposed that aims to help the development of best practice procedures to minimize seismic risks associated with the future EGS development of the Dinantian play.

Keywords : induced seismicity, enhanced geothermal systems, reservoir stimulation

In order to better know the pathways followed by the fluids in geothermal reservoirs, one has to look for clues. Among them, discontinuities, porosity and hydrothermal alteration have proven to be very efficient.

Discontinuities in general (fractures, faults, joints between debris flows or lava flows in volcanic settings, unconformities, grain joints at a smaller scale) as well as connected porosity (created by dissolution of primary minerals for example) are prone to fluid flow because of the voids they constitute.

When a natural hot fluid circulates in those voids, it interacts with the rock and modifies it creating the so-called hydrothermal alteration. This alteration is responsible for the modification of the structure (increase of porosity, decrease of density), of the chemistry of the rock (loss or gain of chemical elements) and of the precipitation of newly formed phases in porosity (dissolution of primary minerals followed by precipitation of newly formed phases) and in discontinuities (fracture infilling, sealing of discontinuities).

The exploration of geothermal reservoirs is performed first in the field (geophysics, geological mapping, and structural geology). The second step occurs in drillings, when available, to look for discontinuities either on cores or on well logs (e.g. borehole imaging) and to study the nature of the rock (possible also if only rock-chips are produced by destructive drilling) including mineralogy of primary and secondary (newly formed) phases.
This lecture is divided in two parts:
1- Characterization of discontinuities
2- Description and interpretation of hydrothermal alteration.

Geothermal reservoir exploration and exploitation are challenging tasks given the extraordinary variety of parameters and uncertainties to be taken into account.

One of the main reasons for these challenges are to be found in the lack of geological data directly available in the field as the reservoirs are usually buried at depths greater than a 1000 meters. Geological knowledge can therefore only be gained using a limited amount of borehole data and geophysical investigations which in turn forces the engineers to sometimes oversimplify their simulations.

In the present note, we are going to present a way to circumvent these limitations by performing highly detailed geological studies on naturally exhumed geothermal reservoirs. We will first discuss the validity of the approach by defining the concept of a reservoir analogue. We will show that the point is not to find a perfect replication of a given potential exploitation site. We believe that the analogue is an object to be used in order to study at large scale with the maximum precision a given process or a combination of processes of primary importance in the gain of geothermal energy.

The MEET project includes a variety of analogues and these will be summarized with a special focus on an analogue to the Soultz sous Forêts (SsF) demonstration site situated in the Rhine graben, Alsace, France.

The chosen analogue is found at the southern termination of the Southern Death Valley National park, in the Noble Hills (NH), CA, USA. This range has suffered vast amounts of right lateral deformation as presented by Pavlis and Trullenque (Subm.). In spite of the distancing between SsF and NH areas a summary of the work performed by Klee et al 2020 will show several key geological similarities in terms of rock nature, tectonic regime and fluid circulation record. These findings lead to consider the chosen analogue as valuable for a range of reservoir key parameters. A striking analogy between the NH range and the SsF reservoir in terms of fluid rock interaction processes, deformation mechanisms and rock fracture processes are indeed observed.

The different analogies will be presented and explained using raw field data in order to convince the audience about the validity of the approach.

These data are planned to be used in the near future for reservoir exploitation simulations and this approach started by Trullenque et al. 2020 will then be introduced.

The material anisotropy refers by definition to the variations of properties along different axes. Geological materials and sedimentary rocks in particular are prone to exhibit anisotropy of properties, such as the sonic velocity, the thermal conductivity or the permeability. The origin of these axial variations in (meta-)sedimentary formations is frequently related to the sedimentation and compaction processes, but the anisotropy can be in turn be affected by other events such as the development of a tectonic fabric (e.g. cleavage, faulting). During this lecture, we will explore the different properties affected by anisotropy processes through a series of study cases. The latter will be illustrated through practical exercises. In the second part of this lecture the impact of material anisotropy on the geothermal reservoir behaviour will be further studied under the angle pro’s and con’s.

Required equipment/software: the participants must have Excel software installed (or similar). They will also need to use the R language and possibly to install some packages. Hence the participants are required to have the administrator rights on their computer.

Summary of the lecture:
- DMT structure and development.
- Applications of DMT.
- Oil to water conversion on existing oil extraction field.
- An approach for assessing different locations using DMT.
- Expected results and impacts on investors and other stakeholders.

We will consider the practical synergies of valorizing the heat produced on a so-called "mature" oil site, i.e. one that produces large volumes of water in addition to oil. Taking the example of the well portfolio of Vermilion, a French-based operator, we will see how we have inventoried the resource and then the local utilization projects. A review of subsoil risks will be presented on an identified high school heating project, in particular the optimization of flow rates and the evolution of the temperature of the deposit.

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Abstract:
The H2020 MEET (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials) project aims at developing Enhanced Geothermal System in various geological settings across Europe.
The study carried out at UniLaSalle focuses on geothermal reservoirs within Variscan crystalline basement rocks overprinted by post-Variscan extension. We characterise an analogue from the Soultz-sous-Forêts reservoir (Upper Rhine Graben, France) as analogue studies are an efficient way to understand fault structural interactions at depth by precisely visualizing fracture networks. It is aimed to create a 3D structural model and discrete Fracture Network (DFN) analysis which allow the simulation of flow and transport through fractured rocks. The finality is to understand the deep geothermal system. The chosen analogue site is situated in the Noble Hills (Death Valley, CA, USA) as the arid climatic conditions allow a total outcrop exposure.
We first present 3D realistic photogrammetric models using Structure from Motion Multi-View Stereo (SfM-MWS) photogrammetry, based on both Unnamed Aerial Vehicle (UAV) and aerial pictures. In both cases, models are constructed step by step, following a detailed methodology: (1) alignment of pictures, (2) creation of sparse point cloud and removal of high error points, (3) construction of dense point cloud. The georeferencing of the created point cloud is done using Ground Control Points (GCP). The multi-scale approach allows to best characterize the geological structures with a resolution ranging from centimetre to decimetre.
The second part of the workflow is conducted by the Geosciences team of UniLaSalle. The 2,5D models obtained from photogrammetry allow the extraction of planar features with a combination of both manual polyline extraction and automatic facet detection. According to the classification of the fractures, a methodology was implemented to re-scale their features to an estimated real size. In this study, a new approach has been developed to build the DFN’s models by combining deterministic and stochastic approaches. The constructed 3D DFN models are closer to reality. Finally, the hydraulic simulations will be realised in the second part of the MEET project to understand the fluid circulation in the geothermal system.
The 3,000 pictures and 18 GCP of the plane model allowed the creation of a global georeferenced 3D model. Then, 13 photogrammetric models were created along the major geological structures of the Noble Hills and referenced based on the previous large model. They are composed of hundreds of thousands to million points. On each model, between 318 and 7207 fractures were extracted. The fractures with a surface above 8m² were kept and all the smaller elements were extrapolated to reconstruct a real fractured reservoir.

Keywords: Enhanced Geothermal System (EGS), photogrammetry, point cloud, fracture network, analogue study, District Fracture Models (DFN)

The ORC or the Organic Rankine Cycle,is a thermodynamic cycle, allowing heat to electricity conversion. The name Rankine cycle come from William Rankine, a Scottish engineer who developed the theory of steam Machine. The ORC vaporize an organic fluid having a molecular mass higher than water, and a liquid-vapor phase change occurring at lower temperature than water. This machine works based on these 2 principles. The ORC can work at low temperature, between 30° to 300°C.

ORC applications are broad and depends on the organic fluid used in the cycle. Some examples of heat sources that can be used are industrial waste, biomass combustion, geothermal heat…

The purpose of this course is to give basic knowledge of ORC technology. At the end of this course, the attendee will know what an ORC is, how to perform an ORC sizing, the pros and cons of this technology. ORC application in geothermal context will be emphasized.

Course plan
1. Introduction
2. The Story of the ORC
3. How does an ORC work ?
a. Principle of the ORC
b. How to select the working Fluid ?
c. What is a Regenerative Cycle ?
d. What is an Expander machine ?
4. How to design an ORC ?
5. Heat recovery in the geothermal context

There is no prerequisite, but knowledge of thermodynamics will help.

For 30 years, optical fibers have been widely deployed for building telecommunication networks. This silica wire, guiding light in a core as small as 9 microns, has good properties (such as ElectroMagnetic Immunity or Durability and Elasticity of anamorphous silica) making it suitable for many applications. Fiber Optic Sensing is one of them because optical fibers are intrinsically sensitive to temperature and strain. Moreover, it can be encapsulated in order to measure many other parameters. The purpose of this course is to detail this technology in the area of subsurface applications, specifically geothermal reservoir monitoring.

In this short course, we will first present the basics of fiber optic technologies in general. Then we will show how this small element can become a sensor thanks to the insertion of a local element called Fiber Bragg Grating into the optical fiber. Finally, we will present the Distributed Fiber-Optic sensing techniques: Distributed Temperature Sensing (DTS), Distributed Strain Sensing (DSS). Distributed Acoustic Sensing (DAS). Those methods use the whole length of the fiber optic itself as thousands of sensors.

In this lecture we will review the developments of EGS from their early days. The Hot Dry Rock Tests at Fenton Hill, Rosemanowes and at Hijori and Ogachi paved the way for the eventual success at Soultz-sous-Forêts. The lessons learnt at these sites will be reviewed and summarized and to derive the key components distinguishing successful EGS projects from conventional hydrothermal developments with naturally high productivity.
The lecture will provide examples how fracture networks can be determined by field mapping and the scanline method in analogue sites, which serves as a basis for the calculation of a realistic DFN that can be used as a basis for the EGS design. Determination of the subsurface stress field and of the mechanical rock properties will provide the further criteria for the layout of the well and/or the stimulation concept. A recently developed workflow will illustrates these steps.

Each geothermal projects is embedded in a geoscientific, economic, technical, legal and social framework, which can be described by parameters and interlinked datasets. The latter are a fundamental base for taking strategic decisions in terms of resources, risks or costs, for classifying, for evaluating or for comparing geothermal projects as well as for defining the project readiness level.

In an early project phase, a wide range of datasets is already needed to find promising sites or to foster commencing initiatives. Even before the project start itself, the geothermal potential of a certain area or distinct region has to be determined by the analysis of spatial and non-spatial datasets. There are several types of geothermal potential definitions in use depending on the subjects that are taken into account. A comprehensive approach, for instance to define the economic or technical potential of an area, comprises broad subsurface and surface datasets. Furthermore, the identified potential has to be matched with the current and future user’s demand for heat and/or power and their location, as both are linked to a certain area.

The content of the course is aimed to give the participants insights into this complex context and to introduce pathways to data sources and tools as well as to critically consider and discuss the recent situation in terms of data availability in Europe.

In the first part, we will discuss the types of geothermal potentials and the common classification schemes, which are deployed to categorize geothermal projects. Different potential maps and visualization approaches will be compared to select appropriate ways to express the potential in 2D or 3D. Finally, we will identify and rate major and minor “geothermal parameters” and to figure out their typical data types, data formats and data producers.

In the subsequent group work, the participants will investigate in small teams the availability and quality of manifold parameters on a national and a European scale by accessing a wide range of platforms, databases and knowledge hubs. Additionally, the teams will test online tools that were designed to map or estimate the geothermal potential. The findings will be presented shortly to the audience by each team.

The joint outcome of all teams will be critically discussed and examined in terms of data types, homogeneity, coverage, accuracy and informative value to identify highly recommendable data sources and tools, but also to define gaps und inconsistencies. Based on these results, we will work out an appropriate potential estimation approach that can be applied to map resources throughout Europe and that will rest on reliable datasets.

Finally, we will sum up the recent situation by collecting benefits and favourable trends on the one hand side as well as drawbacks and bottlenecks on the other. At the very end of the course we will gather new ideas and solutions to delineate future steps, which might be projects, cooperation initiatives or communication activities.

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Abstract:
Hydraulic fracturing plays a central role in developing enhanced geothermal systems. To simulate the propagation of fractures, the J-integral has been a standard technique used in the field of fracture mechanics. Its superior accuracy at coarser resolutions make it particularly attractive, especially for reservoir-scale simulations. However, the extension of the J-integral to simulate fluid-driven fracturing has not received the same attention or success. In particular, while several studies have highlighted the capacity of the method in simulating viscosity-dominated propagation, detailed investigations are missing. In this work, a finite element analysis is conducted which sheds light on the sources of error which arise when applying the J-integral in the viscosity-dominated regime. The case is put forward that the inaccurate numerical solution for fluid pressure is exclusively responsible for the loss in accuracy of the J-integral. Consequently, the extent of hydraulic fracturing which occurs is overestimated. With this new understanding, the J-integral is reformulated to minimise its dependence on inaccurate fluid pressures, bypassing the aforementioned sources of error. The reformulation is both simple to implement, and general to the numerical method. Within the framework of finite elements, a propagation algorithm using the novel J-integral is subsequently shown to perform excellently in simulating deep hydraulic fractures in both the toughness and viscosity dominated regimes of propagation; a method especially relevant to enhanced geothermal systems. Furthermore, the novel J-integral is shown to be capable of extracting the velocity of propagation of the fracture. Consequently, an inherent advantage in evolving the fracture geometry is offered compared to the original formulation. To investigate the performance of the novel J-integral with relevance to reservoir scale simulations, finite element simulations are conducted at various levels of refinement. Finally, the potential to devise more efficient algorithms to evolve the fracture networks of geothermal systems is discussed.

Keywords: Hydraulic fracturing, J-integral, hydraulic fracture mechanics, finite element

Conclusion of the 3 and 1/2 days of geothermal courses.

Award for the best presentation.

Certificate of participation.

Farewell words of the MEET organization team.

The course will be dedicated to MEET activities around the demo-site of Soultz-Sous-Forêts. It will firstly present the EGS plant of Soultz, both the reservoir and surface facilities. It will then focus on MEET activities which consists in lowering the reinjection temperature in order to valorise more energy.

On surface facilities, we will discuss the knowledge gained on corrosion and scaling effects at this low temperature. We will also overlook the upcoming test of mobile ORC to valorise this low temperature water into electricity.

On subsurface part, the course will focus on the hydrothermal modelling approaches to assess the impact of colder reinjection on long-term operation and discuss the sustainability of such system.

Finally, we will discuss the environmental impacts of EGS power plant and additional measures set up in the framework of MEET to monitor them.

Short advertizing presentations of selected PhD students to attract the audience on their poster room

This presentation will discuss the workflow and methods to characterize and manage fluid circulation in geothermal reservoirs from a geomechanical point of view. We will in particular focus on the following key-points:
i) in-situ stress field characterisation,
ii) hydraulic stimulation design and monitoring,
iii) multi-scale reservoir geo-mechanical response.
Incorporation of uncertainties in engineering decision will be highlighted.

Energy transition, and with it the geothermal market development, regards not only the adoption, strengthening and integration of various energy technologies: it also implies a different way to embed energy matters in and for the society as a whole. Many renewable energy technologies are facing concern, and industrial development is sometime slowed down by social opposition, a problem which is felt in the geothermal sector but that in most cases originates by a lack of trust caused in very different contexts. Nowadays the transparency and openness of project developments, i.e. to the opening up of the project to as much stakeholders as possible, thus strengthening knowledge and trust and legitimising the activities carried out, is accompanied by the involvement of citizens, i.e. the engagement of citizens who are directly and locally affected by the project. New forms of debate and democratic procedures are therefore required for project development, while pursuing profits (shareholders’ interest) and creating value for all society (stakeholders’ interest). We will investigate how social engagement is pursued in different part of the world, will discuss if and how it is effective and will explore its link to policy in Europe.

Heat supply for the existing district heating system of the Göttingen University Campus is covered by a University-owned gas-based heat and power station. For the conversion into a sustainable heat supply, deep geothermal energy has the most promising theoretical potential. However, the geological setting below Göttingen is highly complex in terms of lithology and structure. In addition, the data base is quite weak. However, the complexity offers on the other hand the chance of multifaceted possibilities of target horizons and has the potential for the integration of different systems. After the introduction of the surface infrastructural and the subsurface geological setting including own recent seismic lines, we will develop exploration and exploitation strategies in several groups as an exercise. Finally, we will together discuss different approaches considering chances and barriers in view of the geological, engineering, legal, sociological, political and economic setting of the real-life lab – the Göttingen campus demo site. A challenge which needs unorthodox thinking on one hand and convincing stakeholders on the other hand – always having the risks in mind.

During the last 50 years, about 90 geothermal plants have been installed in the Paris basin, of which 90% are in the central region of Ile de France. The Dogger limestones are the key geothermal target with more than 80% of the reservoirs tapped compared to others geological formations made of various sands and sandstones (Albian, Neocomian and Triassic).

The first geothermal doublet was drilled in Melun l’Almont in 1969 and more than 100 wells have been drilled before 1988. The corrosion and scaling problems caused by the geothermal water has affected deeply the exploitation of the doublets with casing perforation and /or diameter reduction, electrical submersible pumps broken, surface equipment plugged…At that time many plants have been closed for both technical and financial problems. Strong efforts in R&D, supported by the BRGM and much new experimental equipment where tested to overcome this bottleneck. Many good achievements have been successful with new technologies to clean the wells, new bio- chemical approach of the phenomenon, installation of downhole line injecting inhibitors at the bottom of the production wells and evolution of the well design towards bigger well diameter. This learning phase for exploitation and management of new doublets has allowed the developers to restart the deployment 15 years ago using the improved new technologies and benefitting of the subsidies of the new Heat Fund administrated by ADEME. Since 2006, 25 new plants have been created and old one been revamped with the use of new tools such as, horizontal drilling, multi-drains in the reservoir and more frequent use of composite casings. The planned power to be installed in the next ten years, in Ile de France, is ambitious (240 MWth) following the PPE (Energy Programing of the Ministry of Ecological transition). This target is attainable, but the Risk Mitigation System managed by SAF Environment, to cover the geological and hydrogeological risk, remains even after 50 years of deployment, the main tool since 1980, in order to achieve the development of geothermal district heating network in Paris area.

Visit of pilot ORC on Chaunoy oil well installations and explanations

Hydraulic stimulation of geothermal reservoirs in basement or crystalline igneous rock can enhance permeability by reactivation and shear-dilation of existing fractures. The process is characterized by interaction between fluid flow, thermal transport, rock deformation and the fractured structure of the formation. For example, the flow is highly affected by the fracture network, which in turn is deformed because of thermo-hydro-mechanical stress changes caused by the fluid injection. This process-structure interaction is decisive for the outcome of a stimulation and, in analysis of governing mechanisms, physics-based modeling has potential to complement field and experimental data. Here, we show how recently developed mathematical models and simulation technology are valuable as tools to understand governing mechanisms of thermo-hydro-mechanical coupled processes and the reactivation and deformation of faults. The methodology fully couples flow in faults and matrix with thermoporoelastic matrix deformation and a contact mechanics model for the faults, including dilation because of slip.

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The production of geothermal energy from the Dutch subsurface incorporates the use of corrosion inhibitors to protect the steel well-parts from the corrosive production fluid. The injection of inhibitors leads to a risk of exposing the reservoir rock to these solutions. Up until now no data has been published to describe the effects of the corrosion inhibitor on the mechanical reservoir rock properties. This study aims to remedy this situation by performing compressive triaxial experiments on rock samples which have been saturated with inhibitor solutions. Two rock types, representative of geothermal reservoir rocks (Bentheim sandstone and Treuchtlinger marmor), were tested at confining pressures of 10, 20 and 30 MPa. We used a concentration of 3000 ppm for each inhibitor (a large dose compared to the usual injection rate of 4 – 15 ppm). We performed control experiments with deionized water. All experiments were conducted at room temperature. The sandstone experiments show no discernible difference between the different types of fluid. The lack of difference between the sandstone samples could be attributed to the inert nature of the constituents (mainly quartz). The limestone experiments do show several significant differences, described here in terms of Mohr-Coulomb failure parameters. Those saturated with inhibitor type 1 exhibited an increase in rock cohesion and a decrease in internal angle of friction. In contrast, those saturated with inhibitor type 2 exhibited a decrease in rock cohesion and an increase in internal angle of friction. Limestones are chemically more reactive, although the exact nature of the chemical processes which lead to the observed differences cannot be determined with this experimental dataset. However, the tested concentration is 200 times higher than wellbore concentrations. Hence, at the prevalent subsurface conditions we expect that the changes in limestone rock properties should have no negative effects for the production of geothermal energy.

Keywords : Corrosion inhibitor, triaxial compression test, limestone, sandstone

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The Bedretto Underground Laboratory for Geoenergies (BULG) in the Swiss Central Alps (http://www.bedrettolab.ethz.ch) provides a unique environment to study processes related to enhanced geothermal systems (EGS), such as hydraulic stimulation techniques and associated induced seismicity. The location in the Bedretto tunnel with an overburden of >1000 m and the relatively homogeneous granite as reservoir rock provide conditions close to realistic EGS systems in Switzerland.
By performing small scale hydraulic fracturing tests, so called mini-frac tests, in six short (30 m to 40 m long) and one long, deviated borehole (300 m long), we were able to measure the variability of the two horizontal stress components. Extended shut-in times up to 15 hours were used to obtain the local pore pressure. All boreholes were logged with acoustic and optical televiewers to identify natural and hydraulic fractures and their orientations. In five long, deviated boreholes, extensive fractures/fault zones and borehole breakouts were mapped from the image logs. Breakout properties were analyzed to obtain a second stress estimate.
The experiments revealed a heterogeneity of the minimum principal stress magnitude on the scale of meters to tens of meters. The stress regime lies between normal- to strike-slip faulting conditions (Shmin < SHmax ≤ Sv) with SHmax around N100°E [1]. Close to the laboratory, the Shmin magnitude follows a lower gradient than the gradient of deeper intervals. In addition, all inferred pore pressures are below the hydrostatic case (Pp ≈ 3-6 MPa). We attribute such discrepancies to stress perturbations and drainage in the vicinity of the tunnel and to compartmentalized hydro-structures connected to major fault zones. Based on breakout presence, the azimuth of SHmax and the inferred stress regime are confirmed. The breakout azimuths show a rotation, which indicates that the stress field is perturbed by the presence of fractures and larger fault zones.

Keywords : Underground Research Laboratory (URL), in-situ stress measurements, mini-frac tests, borehole breakouts

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The long-term sustainable exploitation of geothermal resources requires cautious planning and regulation. Overexploitation results in reservoir pressure decline, causing a decrease in production rates. Furthermore, the unsustainable management of geothermal reservoirs may lead to compaction and land subsidence. Understanding of such phenomena is critical for the assessment of societal-environmental risks, but can also be used for optimization by constraining reservoir processes and properties.
Excessive thermal water volumes have been extracted from porous sedimentary rocks in the Hungarian part of the Pannonian Basin. Thermal water production in Hungary increased significantly from the early 70’s. Regional-scale overexploitation of geothermal reservoirs resulted in basin-scale pressure drop in the Upper Pannonian sediments, leading to compaction and ground subsidence.
We investigated surface deformation at the Szentes geothermal filed, SE Hungary, where the largest pressure decline occurred. We obtained data from the European Space Agency’s ERS and Envisat satellites to estimate the ground motions for the periods of 1995-2000 and 2002-2010. We applied inverse geomechanical modelling to understand the compaction behaviour of the reservoir system and to estimate the subsurface properties. We constrained the model parameters using the Ensemble Smoother with Multiple Data Assimilation, which allowed us to incorporate large amounts of surface movement observations in a computationally efficient way. The model requires pressure time series as input parameters, therefore, the lack of regular pressure measurements in geothermal wells of Szentes resulted in significant uncertainties. Still, we managed to identify a potential delay in pressure drop and subsidence, implying a time-decay compaction behaviour of the reservoir system, and we reached realistic estimates for the compaction coefficient of the reservoir. The improved parametrization enables better forecasting of the reservoir behaviour and facilitates the assessment of future subsidence scenarios. This study thus demonstrates the effectiveness of InSAR-based ground motion data and inverse geomechanical modelling for the monitoring of geothermal reservoirs and the establishment of a sustainable production scheme.

Keywords : ground motion, InSAR, inverse geomechanical modelling, Szentes geothermal field (SE Hungary)

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Reinjection of water in geothermal reservoir is an essential process for a sustainability development of geothermal energy. It permits keeping the pore pressure at a level necessary for maintaining the resources during exploitation. However, in most of the cases, the produced water is not sufficient to close the production/injection cycle and, the use of other sources like water from rivers, lakes, underground is needed to ensure a total reinjection.

Nevertheless, this methodology might conduct to issues referred generally as thermal or chemical breakthrough which leads to a cooling down of the reservoir with consequent thermal contraction triggering fracturing and/or to a chemical variation in the reservoir water.
Regarding the latter, the literature produced so far does not provide enough information. Typically, surface deformation is ascribed to thermal effects or variation in effective stresses, however, a chemical weakening might be the cause of deformation observed in some geothermal fields, especially when a water out of equilibrium is reinjected. Such a process is generally known as water-weakening.

In this work, we conducted experimental tests on a microporous carbonate rock, the Obourg Chalk deposited in the Mons Basin (Belgium) during the Campanian age. Water injection tests under triaxial loading showed that water-weakening is well expressed in this chalk and, for a critically loaded sample, the amount of injected water necessary to induce failure is exponentially correlated to the axial stress (with respect to the confining pressure) applied on the sample.
Furthermore, a systematic study of uniaxial tests on chalk samples saturated with deionized water with different dissolved salts showed that the fluid chemistry also exerts a significant influence on the weakening.

Our experimental study suggests that a comprehensive study of reservoir sensitivity to fluid chemistry needs to be considered before planning any reinjection strategy.

Keywords: water-weakening, chemical degradation, mechanical tests

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Borehole beat exchangers (BHEs) are one of the best methods of obtaining geothermal energy from the rock mass. They can have various designs: coaxial, single U-tube, double U-tube. It is common practice to make installations in the form of U-pipes up to a depth of 150 m or even 200 m, while in deeper boreholes a coaxial system is used. The amount of heat exchanged by BHEs and the rock mass is mainly dependent by thermal conductivity of that rock mass. An important construction parameter in borehole heat exchangers is the sealing slurry with increased thermal conductivity. It provides better heat flow from the rock mass to the heat carrier and vice versa.
The sample base for the cement was CEM I 42,5 R and as the mixing liquid water has been used. Also, a various additives were added in different proportions to the cement, which were determined on the preliminary studies and observations. The samples were prepared in circle-shape moulds, and after structure setting, they were removed from the moulds, properly labelled, and placed in a water bath for 28 days. This was to simulate borehole conditions. Samples were measured using the thermal conductivity equipment FOX50. The results of research on hardened samples of sealing slurries in terms of their thermal conductivity was presented.

Keywords: cement slurry, borehole heat exchangers, thermal conductivity

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With around 50 heating networks today operating, the aera around Paris is the European region which concentrates the most heating network production units in terms of deep geothermal energy. In France, the energy-climate strategy plans to produce 6.4TWh in 2023, compared to 1.5TWh produced in 2016 [1]. Despite an exceptional geothermal potential, the current average development rate of 70MWh/year will not allow this objective to be achieved, it would be necessary to reach a rate of 6 to 10 times higher. The optimization of the use of deep geothermal energy is a major challenge for France, and in Ile-de-France, which has a population of nearly 12 million inhabitants. This project aims to reconstruct and simulate heat flows in the Paris Basin using an innovative methodology (1) to characterize, predict and model the properties of reservoirs (facies, porosity, permeability) and (2) simulate future circulations and predict the performance at a given location (sedimentary basin) on its geothermal potential. This study focuses on a high density area of well infrastructures around Cachan, (8 doublets, 1 triplet in 56 km2). A new sub-horizontal doublet concept has been recently (2017) drilled at Cachan to enhance heat exchange in medium to low permeability formations [2]. Nuclear Magnetic Resonance (NMR T2) logs have been recorded in the sub-horizontal well (GCAH2) providing information on pore size distribution and permeability. We integrated all logging data (gamma ray, density, resistivity, sonic, NRM T2) of the 19 wells in the area and 120 thin section observations from cuttings to derive a combined electrofacies-sedimentary facies description. A total of 10 facies is grouped into 5 facies associations coded in all the 19 wells according to depths and 10 3rd order stratigraphic sequences are recognized. The cell size of the 3D grid was set to 50 m x 50 m for the XY dimensions. The Z-size depends on the thickness of the sub-zones, averaging 5 m. The resulting 3D grid is composed of a total of nearly 8.105 cells. After upscaled, facies and stratigraphic surfaces are used to create a reliable model using the “Truncated Gaussian With Trends” algorithm. The petrophysical distribution “Gaussian Random Function Simulation” is used to populate the entire grid with properties, included 2000 NMR data, considering each facies independently. The best reservoir is mainly located in the shoal deposits oolitic grainstones with average porosity of 12.5% and permeability of 100 mD. Finally, hydrodynamic and thermal simulations have been performed using Pumaflow to give information on the potential risk of interference between the doublets in the area and advices are given in the well trajectory to optimize the connectivity and the lifetime of the system. NMR data, especially permeability, allow to greater improve the simulations, defining time probabilities of thermal breakthrough in an area of high density wells.

Keywords : Flow simulation, Carbonate, Modeling, Paris Basin, NMR

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In closed-loop shallow geothermal, there are many reasons to prefer a specific pipe size and configuration of the vertical ground heat exchanger, which can be related to very practical issues such as drilling equipment or installation methods. However, a correct design should also consider the heat exchange between the heat-carrier fluid and the surrounding subsoil [1-4].
Determining the critical radius of grouted and non-grouted exchangers is critical to enhance the thermal performance [5]. In cylindrical coordinates, adding grout or increasing the diameter of the exchanger also increases the external surface, which decreases the borehole-resistance. Furthermore, it may happen that a decrease in borehole-resistance due to increased surface may be more important than an increase in thermal resistance due to the thicker pipe or grouting layer. The thickness up to which the heat flow increases and after which it decreases is defined as the critical thickness, which determines the optimal radius of exchanger [6].
Although well-known analytical equations exist that independently define the different phenomena involved from the physical standpoint, their application as a whole is intricate and cannot describe all the interaction between the variables that contribute to the critical radius. In a simplified form, such variables are the operating temperatures, the thermophysical properties, the radii and thickness of pipe and grout, as well as the thermal properties of the subsoil.
In this research, a different approach based on statistical modeling is proposed, which takes into account the abovementioned variables organized as a factorial design [7].
Each variable takes on analytically calculated levels to obtain responses. Than, through multiple linear regression it is possible to mathematically describe the problem through coefficients allowing to make predictions for whatever chosen exchanger configuration. Furthermore, it is possible to select or screen out the few important main variables from the many less important ones, and this is essential for improving or designing new types of exchangers.

Keywords : ground heat exchanger, pipe materials, grout materials, thermophysical properties, design of experiment.

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The Los Humeros geothermal system is an operating steam dominated field with 65 wells (23 producing). With temperatures above 380 °C, the system is characterized as a super-hot geothermal system (SHGS). The development of such systems is still challenging due to the high temperatures and aggressive reservoir fluids, which lead to corrosion and scaling problems. Extensive geological, geochemical, geophysical and technical investigations have been performed within the scope of the GEMex project (EU-H2020, GA Nr. 727550) to improve reservoir understanding and modeling of the Los Humeros caldera complex. Detailed information on the petrophysical rock properties of the target formations is critical for i) processing and interpreting geophysical data, ii) for developing conceptual geological models and iii) for parameterizing 3D-reservoir models. Thus, outcrop and reservoir core sample studies were carried out in order to create an extensive rock property database and to define and characterize all relevant key units from the basement to the cap rock on different scales (outcrop analysis, representative rock samples, thin sections and chemical analysis).
More than 200 rock samples were taken from representative outcrops inside of the Los Humeros caldera, the surrounding area and from the exhumed ‘fossil system’ in Las Minas. Additionally, 66 borehole core samples covering 16 wells of the Los Humeros geothermal field were obtained. The samples were analyzed for petrophysical (e.g. density, porosity, permeability) and thermophysical properties (thermal conductivity, thermal diffusivity, heat capacity), magnetic susceptibility as well as dynamic (ultrasonic wave velocities) and static mechanical properties.
Based on the outcrops and petrological analysis, the unit’s geological heterogeneity, which controls the rock properties, can be addressed. An extensive rock property database was created comprising more than 30 parameters analyzed on more than 1600 plugs altogether. The statistical analysis of the results enabled the classification of different lithofacies types with distinct properties, which was used to define model units within a 3D geological-geothermal model. Hydrothermal alteration of different intensities was observed on the borehole core samples resulting in high heterogeneity in terms of sample appearance, chemical composition and rock properties. Detailed petrographic and chemical analyses (XRF, XRD and ICP-MS analyses) were applied to identify alteration facies and to relate the borehole core samples to different volcanic events allowing for an update of previous lithostratigraphic classifications and conceptual geological models.

Keywords: Super-hot geothermal systems, rock properties, hydrothermal alteration, reservoir modeling

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Geothermal fluids are often loaded with mineral and organic particles in suspension, dissolved organic and mineral compounds, various additives, bacteria and heavy metals, etc. The presence of these compounds poses significant problems, in the short term on the sustainability of production and the maintenance of injectivity and, in the long term on the stability and continuity of the resource.
In this work, we propose to study the kinetics and the damaging mechanisms of a porous medium by colloidal particles in the specific conditions of geothermal energy. More precisely, the aim is to describe the characteristics of permeability damage process, at the pore scale, mainly within microfluidic/micromodel devices, in order to bring elements of comprehension to the industrial problem of particles retention in geothermal wells during the reinjection of geothermal fluids. This approach allows the study of the kinetics, mechanisms and regimes of deposit formation at the pore scale. Several key elements will be at the center of the study with in particular the determination of the link between the previously mentioned characteristics, the hydrodynamic forces, the physico-chemical conditions, the petrophysical properties of the medium, the surface roughness of the pores as well as the nature, the size and the concentration of the particles present. Consequently, predictive transport-deposit-damage laws and adapted remediation solutions will be proposed in order to optimize the durability of the systems.
This work will allow a better prediction of the flows and the adaptation of remediation solutions to limit/avoid losses of injectivity/productivity of wells in the context of geothermal energy. It will also allow the improvement of the characterization process in the laboratory through a better interpretation of the experimental results as well as an optimization of the simulation tools through a relevant description of the macroscopic properties of the flow and transport on a large scale.

Keywords : Porous media, Injectivity, Fluids/rock interactions, Micromodels

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In recent years, geothermal energy has been increasingly considered as a major contributor in the future’s green energy mix. Reservoir stimulation techniques enable heat and power generation from conduction-dominated play types, found away from tectonic plate boundaries. In the UK, conduction-dominated plays of the hot dry rock and hot wet rock types can be expected around radiothermal granites and in sedimentary basins. A promising target of the latter type is the Lower Carboniferous Dinantian Carbonate play; a known hot spring aquifer with an assumed subsurface extent that spans areas of increased heat demand in the country. The confirmation of this stratigraphic unit as a regional geothermal play will be contingent on the successful identification of facies types with favourable porosity/permeability characteristics; or that are suitable for EGS development. Induced seismicity as one of the main risk factors associated with EGS should be an important focus during the development of the Dinantian play; considering it is underlain by crystalline basement and its proximity to densely populated areas. Currently, seismic risk mitigation in the UK is based on a traffic light system with prescribed threshold magnitude values at which a reduction of injection rate, or stopping of injection is prompted. A shortcoming of this system is highlighted by examples from different injection sites that experienced increasing seismicity after such mitigative measures. Injection protocols should be designed and adjusted with a focus on keeping the maximum induced magnitudes below a certain level. Here, a project is proposed that aims to help the development of best practice procedures to minimize seismic risks associated with the future EGS development of the Dinantian play.

Keywords : induced seismicity, enhanced geothermal systems, reservoir stimulation

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Fluid injection-induced seismicity occurs not only during hydraulic stimulation, but also after the injection termination[1]. Due to a lot of uncertainties on both time scale and spatial scale, further study to understand the post-injection earthquakes is necessary. In this study, a cohesive zone finite-element conceptual model is built to reproduce hydraulic stimulation including both the injection and shut-in stage. The hydraulic fracture and the pre-existing faults are modeled with cohesive elements[2], with opening-mode and shear-mode damage respectively. Our results reveal that fault reactivation is possible after shut-in under specific conditions, even the fault is isolated from the hydraulic fracture. Stress drop caused by fluid pressure diffusion is the main mechanism during the shut-in phase to destabilize the fault depending on the adjacent diffusivity. The fault characterization such as its geometry, friction coefficient affects greatly the delayed-activation, wherein the initial stress also cannot be neglected. Those factors also change the moment magnitude and seismic slip rate to varying degrees. The results also suggest some effective measures like lower-rate injection and cyclic stimulation could be taken to minimize the post-injection induced fault slip.

Keywords : hydraulic stimulation; fault reactication; induced seismicity; shut in effects

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One of the most alarming recent findings in geo-science, is the dramatic rice in the rate of human-induced earthquakes in the past decade. The latter is due to the fluid injection or extraction in deep reservoirs for hydrocarbon production, wastewater and CO2 storage and exploitation of geothermal resources which result in the reactivation of nearby faults. These reservoirs are often located 2-3 km depth (i.e. 30 MPa), and are hosted or covered by sedimentary carbonate layers. As carbonate undergoes a brittle-ductile transition increasing confining pressure from values of around 20 MPa, ductility plays an important role on the nucleation and propagation of earthquakes on carbonate faults. Here, we investigate the role of increasing ductility on frictional parameters. The research is achieved through the new biaxial apparatus installed at EPFL, the HighSTEPS (High Strain Temperature Pressure Speed) apparatus, able to measure frictional parameters in a high range of shearing velocities (10-6 m/s – 0.2 m/s) and under unique boundary conditions representative of the Earth’s crust, i.e., normal stress up to 100 MPa, confining pressure up to 100 MPa, pore fluid pressure up to 100 MPa and temperature up to 120°. We present tests’ results charting carbonate fault mechanical behaviour from low shearing velocity 10-6 m/s to high shearing velocity 10-1 m/s. Tests were performed under different values of applied confining pressure (1 - 60 MPa) and normal stress (1.5 – 90 MPa) on the faults, keeping constant the ratio among them (around 1.5). Moreover, experiments’ results are modelled with rate-and-state friction laws (RSFLs) to define rate and state parameters related to the critical conditions for fault stability and its dependence on the presence of ductility. These results shed new light on the nucleation and propagation of earthquake at a brittle to ductile transition in carbonate bearing rocks.

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Deep fault systems in heat-generating crystalline rocks provide pathways for flow of hot fluids and thus can be targeted for geothermal energy production, providing carbon-neutral energy. However, the sustainability and reliability of these systems depends on the stability of the fault transport properties throughout the lifetime of a system. Geothermal energy production requires the presence of fluids as a transport agent for heat from the deep subsurface to the surface. These fluids may chemically and mechanically alter the rock properties which, over longer periods of time, could significantly impact the system’s productivity and possibly result in a risk of induced seismicity.
This research project follows a multidisciplinary approach to investigate the effects of reactive fluid flow-through at high pressures and temperatures on granitic geothermal systems. Focus is on the evolution of the rock mineralogy, as well as their hydraulic and frictional properties. The project combines permeability, reactive flow-through and friction experiments with microstructural and geochemical investigations to identify the coupling between these. We will use unaltered as well as mineralogically altered granite samples collected in Cornwall, United Kingdom, believed to be representative of the granites at depth in the United Downs Deep Geothermal Power Project (UDDGP, [1]), which will utilise geothermal heat from a well drilled into a fault zone at 5200 m depth. The unaltered granites will be used to investigate the process of granite alteration in fractures in the laboratory using a flow-through reactor. Permeability measurements are conducted using the steady state method under varying pressure and temperature conditions, while a triaxial deformation apparatus capable of reproducing upper crustal conditions will be used for friction experiments. The experimental work is complemented by reactive transport and microphysical modelling, both supported by the experimental results. For the reactive transport modelling, PHREEQC (to be integrated with the industry code MoReS in future) is used to numerically simulate and predict mineralogical changes in deep geothermal granitic systems. Microphysical modelling will be based on the main mechanisms contributing to friction in granites as identified from microstructural observations of experimentally deformed samples (SEM and μCT). The resulting models can subsequently be used to predict mechanical data at reservoir conditions. The outcomes of this study will help to improve the understanding of how to maintain favourable hydraulic properties in granitic geothermal systems and how long-term operation of such geothermal projects utilising fault systems affects the risk of induced seismicity.

Keywords: fault systems, permeability, friction, reactive transport

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The goal of the H2020 European MEET Project (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials) is to enhance demonstration of geothermal energy production throughout Europe.
Dealing with geothermal demonstration, the reference in France is the Soultz-sous-Forêts (SsF) site located in the Upper Rhine Graben (URG). Geothermal wells penetrate the fractured granitic basement and crosscut permeable fractures corresponding to open fractures with a high flow rate.
Our aim is to understand fault zone evolution and fluid/rock interaction processes in order to estimate the reservoir evolution through time and to define the target exploitation zones. To investigate this, 3D subsurface/surface analogues are needed. Therefore, two analogues were chosen:
1) The Noble Hills range (NH) situated at the southern termination of the Southern Death Valley Fault Zone (SDVFZ, CA, USA). The desert climate conditions allow virtually 100% outcrop exposure. Both SsF and NH share a common transtensional tectonic setting, the main difference being a much higher amount of displacement along the SDVFZ compared to the URG border faults.
2) The URG crystalline shoulders themselves. Exceptional quality outcrops are found in abandoned mining galleries. Ore mineral exploitation took place within fracture corridors affecting gneissic basement rocks.
In both analogues, high strain zones linked to substantial shearing deformation are observed. Those zones are most of the time rich in clay minerals (mainly illite), meaning that a fluid has circulated within the system during deformation. Due to a high amount of deformation and the presence of clay minerals, the fracture ends up to be sealed. At the opposite, open fractures showing modest shear deformation present systematically geodic minerals of various types (oxydes, silicates, sulfates, native ore…). At SsF exploited site, productive zones are open fractures presenting mainly geodic quartz [1].
We believe that fluid/rock interaction processes assisted by deformation may play a key role in hydrothermal activity of a given system. We propose a conceptual model in which, incipient deformation allows the sudden arrival of fluids in newly opened fracture voids. Ongoing continuous deformation leads to the onset of cataclastic deformation and higher reactivity of the host rock with the fluid due to grain size reduction. This dynamic system contribute to the onset of clay mineral (mainly illite) formation which in turn drastically changes fault rock petrophysical properties and fault mechanical behavior. In our model, ideal productive stage for a fracture corresponds to a time period where fractures having suffered modest amounts of deformation remain open allowing fluid circulation and act as a drain.

Keywords: hydrothermal alteration, cataclastic deformation, fracture mechanics, geothermal fluid production

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In the context of geothermal exploration in sedimentary basins, it is crucial to assess the occurrence of cementation in fractures, which are known to be important conduit for fluid migration in low matrix permeability carbonate reservoirs. Outcrop analogues of potential subsurface reservoirs constitutes an excellent playground to analyze paleo-fluid properties (i.e. temperature, composition, pressure, timing), origins and pathways, and provide constraints to predict the present-day cementation conditions of the reservoirs to improve drilling designs.

This study focuses on the Upper Mesozoic (Jurassic and Lower Cretaceous) carbonate successions of the Geneva Basin (GB), which are considered as potential geothermal reservoirs. To provide insights about the geothermal fluids circulation in these units, a multidisciplinary approach is applied, which combines: 1) fault and fracture geometric analysis; 2) paleo-fluid origin characterization; and 3) fluid-rock interaction simulations. The fossil geothermal system at the Vuache Mountain Fault Zone and basin subsurface reservoirs are investigated and considered as potential analogues for the active system currently explored in the Geneva area.

Scanline analyses on outcrops highlighted the complex structural framework of the study area. Several fault and fracture families were identified being consistent with the main fault corridors cutting through the GB [1; 2].
Petrographic analyses of calcite-filled fractures (i.e. veins) revealed a large spectrum of crystal habitus and textures with hints of synkinematic precipitation. Carbon (C) and oxygen (O) stable isotope analyses showed that host-rocks and corresponding calcite veins have similar δ13C (-6.88‰ to 3.06‰ VPDB) indicating a rock-buffered fluid system. The δ18O values are consistently negative (-17,74‰ to -6,38‰ VPDB) suggesting calcite precipitation at burial conditions.
Preliminary microthermometry results on calcite fluid inclusions suggest low precipitation temperatures (below 60°C; [3]) and revealed at least two types of fluids varying from low salinity (0,2 – 2,4 eq. wt.% NaCl), meteoric origin to high salinity fluids (5,3 – 8,8 eq. wt.% NaCl) interpreted as connate waters.

Additional clumped isotope (Δ47) thermometry combined with U-Pb geochronology will better constrain temperature, composition and timing of mineralizations and help building a conceptual model for paleofluid origins and pathways.

Keywords: geothermal, paleo-fluid, fracture characterization

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Fracture permeability is an important aspect of both natural and enhanced geothermal systems. To develop these successfully and sustainably it is important to characterize how fracture networks vary spatially and how sealing by precipitation from circulating fluids occurs within them. Field studies of vein networks in the exhumed epithermal Caleta-Coloso transtensional duplex, part of the Atacama Fault Zone in Chile, provide insights into the geothermal fluid flow history, with the intention that these insights may be applied to similar contemporary geothermal systems. Through analysis of mechanical damage and mineralogical data collected via linear transects and outcrop surface mapping, we characterize structural controls on episodic flow in order to help identify where within a transtensional duplex may be most prospective in terms of permeability and fluid flux. Ongoing related work also aims to further understand sealing rates in fractured geothermal reservoirs.
Measurements of vein intensity suggest that areas within the duplex have undergone more brittle deformation than areas outside, with greater connectivity also suggesting higher paleo-permeability. There is a large degree of variability, however, with greatest paleo-permeability in a localised dilational jog near a major bounding fault. Vein thickness - which indicates the amount of sealing by precipitate and is used as a proxy for fluid flux - is generally consistently high within the duplex, except at greatest distances from duplex-bounding faults where lower levels of fluid flux are inferred to be interrupted by sporadic high-flux events. Outside of the duplex considerable variability suggests that fewer fractures remained open, but potentially for longer periods. Also considering mineralogy, multi-phase veins are common, indicating that the same structures are often reactivated during subsequent fluid flux episodes. Precipitate in the duplex centre (dominated by a small number of thick, single-phase calcite veins), suggests that low-temperature fluids capable of propping open fractures succeeded short-lived higher-temperature fluid flow (thin epidote veins). Elsewhere within the duplex (i.e. nearer the bounding faults), frequent reactivation as indicated by hydrobreccia and multiple crack-seal veins (mixed epidote-chlorite, epidote-prehnite and epidote-quartz) suggests that ongoing high-temperature, high-pressure hydrothermal fluid flux has occurred more frequently and over longer timescales in these areas.

Keywords: fluid flow, fractures, sealing, duplex

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Abstract:
La Réunion is a volcanic island hosting many indicators of a promising geothermal resource (an active volcano, elevated temperature gradients, multiple thermal springs, soil degassing of magmatic CO2…). However, despite decades of prospection campaigns, this geothermal potential has never been confirmed, mostly because of the complex geological structure of Réunion’s polygenic volcanoes. To help resolving this issue, we built a new 1/25000e geological map of Piton des Neiges from field acquisition, based on a revised chronostratigraphy of this volcano.
Our mapping reveals that 1) the caldera is actually much smaller (~30 km2) than previously estimated, and is restricted to the innermost part of the volcano. 2) Thermal springs are observed both inside and outside the caldera. 3) Thermal springs systematically occur within units of breccia of volcanic or gravitational origin, which fill the caldera or are intercalated into the volcanic pile. 4) In these breccias, springs localize along fractures or intrusions, most of them being differentiated. These results suggest that breccias represent more fractured and hence permeable levels in the volcanic edifice, allowing surface leaking of the hydrothermal system. Based on these results, we interpret the fluid circulation as running through the network of caldera faults and differentiated intrusions from the deep part towards these breccia levels. Discontinuities within breccias (faults and intrusions) should thus be considered as the main target for future geothermal explorations. Previous exploratory drillings were located several kilometers away from this optimal zone, which may explain their unsuccess in finding a geothermal resource.

Keywords : Réunion Island ; Geothermal exploration ; Structural geology

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Structural heterogeneities commonly behave as barriers, baffles or pathways for fluids in the upper crust: understanding their architecture, their structure, and their petrophysical properties remains crucial in many research domains such as geothermics.
Volcaniclastic rocks are composed of consolidated volcaniclastic and pyroclastic materials mainly made of volcanic glass, ashes and/or phenocrysts; such rocks are ubiquitous in geothermal areas, may be highly porous (up to 38%), and thus may have excellent potential for fluid storage and/or flow. This work focuses on fault development in volcaniclastic rocks and the implications of faults and fault-related structures on subsurface fluid-flow.
The studied area is located in the Milos Volcanic Island, which forms part of the Southern Aegean Volcanic Arc. The Calc-alkaline Pliocene to present-day volcanic edifices are the surface expression of the northward subduction of the African plate beneath the Aegean microplate. The complex subduction-related tectonics has caused poly-phasal and multi-directional extensional tectonism affecting the Aegean microplate, during which multiple types of structural heterogeneities have formed.
The studied normal faults of Milos cut Pliocene subhorizontal strata of white and porous felsic pumice at the Saranikiniko beach. The faults exhibit cm- to m-scale extensional offsets, and trend approximately N100-130E, with dips of approximately 80°. The faults are comprised of (i) mm-wide isolated anastomosed deformation bands (DB) or (ii) cm-wide clusters exhibiting strong positive reliefs due to their higher resistance to erosion compared to the surrounding host rock. Microstructurally, the faults and deformation bands are characterized by (i) rearrangement of volcanic glass particles (ii), pore collapse, and (iii) cataclastic –mechanism, which tends to be highly localized to narrow zones within each fault/deformation band.
Based on these microstructural observations we interpret that a progressive loss of porosity occurs during development of faults and deformation bands in these rocks. This further suggests that DBs clusters, representing a more advanced state of deformation band/fault formation, tend to drastically reduce the permeability of the porous media and most probably has a significant impact on fluid storage locations as well as the preferential direction of flow.

Keywords: Volcanoclastic rocks, Deformation bands, Fluids circulation, Geothermal reservoirs.

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Geothermal energy is one of the most promising techniques to exploit renewable energy resources from the Earth and to limit emissions of greenhouse gas. Deep geothermal exploitations are associated with long term fluid circulation and pressure perturbations at great depth, in fractured and faulted zones and are likely associated with a risk of triggering earthquakes and inducing seismicity [1]. Such earthquakes are usually interpreted as the reactivation of rapid (m/s) shear slip on critically stressed faults caused by fluid flow and poroelastic stress changes. A better understanding of the hydromechanical processes controlling fault slip is therefore crucial to mitigate seismic hazard associated with geothermal exploitation. In this framework, this study aims at constraining the influence of stress state and fluid injection rate on the reactivation of pre-existing faults through mechanical modelling of a set of laboratory experiments [2]. The experiments consist of fluid injection into a saw-cut rock sample loaded in a triaxial cell. Fault reactivation is triggered by injecting fluids through a borehole directly connected to the fault. This experimental setup is modelled by a Finite Element Method (FEM, ABAQUS) coupled with a solver of the fluid diffusion [3]. The sample fault is modelled as a contact surface obeying slip-weakening friction law. This approach allows to compute slip and stress evolutions, as observed during the laboratory experiment (Fig. 1). We show that fluid injection triggers a shear crack that propagates along the fault. This approach can be used to investigate the relationship between fluid front and slip front during reactivation, which is an important issue to control the effects of fluid injections at depth.

Keywords : fault reactivation, fluid injection, numerical model

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The GEO-URBAN project aims to explore the potential for low enthalpy geothermal resources in urban environments. In the frame of GEO-URBAN project, two low-enthalpy deep geothermal reservoirs, Dublin basin (Ireland) and Valles Basin (Spain), will be evaluated using geophysical exploration techniques.
In the Valles Basin area (Catalan Coastal Ranges, NE Spain) the thermal anomaly is located in the northeastern limit of the basin, where a highly fractured Hercynian granodiorite act as the geothermal reservoir. Nevertheless, the geological structure of this area, as well as the role of the Vallès major normal fault, is poorly understood.
Different geological and geophysical surveys made on the 80s determined the presence of geothermal anomalies in different parts of the Vallès Basin, although their resources were not exploited [1] [2] [4]. In our study we decide to focus on La Garriga-Samalús area (northeastern edge of the basin), where the fractured nature of the crystalline bedrock represents a geological challenge.
Several geophysical techniques are being applied in this area to understand the main structure, which seems to control the heat and the hot-water flow. Magnetotellurics (MT), gravity, electrical resistivity tomography (ERT), passive seismic (H/V) and control source electromagnetics (CSEM), have already been carried out, however, the models are not yet completed.
Although the geophysical study makes up most of the study, we are also improving the geological map of the area, based on a fractures study at different scales. We are working with DEM alignments analysis, and fractures study from outcrops, thin sections, and digital models of borehole samples.
The geological model will be built up using all the geophysical information, the surface data, and previous information, as six exploratory wells made by the Geological survey of Spain [3] [5] [6]. Moreover, it will include petrophysical data from the granodiorite rock, which would help us to correlate the geophysical results and the geology of the area, also being essential for the final flow model.
Our preliminary results show a strong gravity gradient in the NE-SW Valles half-graben system and the recent MT profiles image the main fault of that system (Valles normal fault).
Interpretations of the fractures study, together with geophysical data and models, have allowed the characterization of damage zones associated to the fault system which are directly related with the fluid flow and the hot springs. The nature of this damage areas could also be related to relay ramps, commonly regarded as efficient conduits for fluid flow. In contrast, the high conductivity zones related to the fault core, and its interpretation as permeable versus impermeable (fault gouge) unit is still unknown.

Keywords: Geophysical exploration, Fracture study, Method combination

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Within the Act of 17.08.2015 on energy transition voted by the French government, the Hauts-de-France region (Northern France) is committed to invest and develop massively renewable energies in the coming decades, especially low-temperature geothermal energy. The current research project aims at better characterizing the geometry of the main deep geothermal reservoir in the area of the Nord-Pas-de-Calais (NPC) coal basin: the Dinantian karstic and brecciated limestones (Lower Carboniferous, 360-330 Ma). The occurrence of a Dinantian regional geothermal resource has already been proven in Belgium in the Hainaut coal basin area where the temperature in three geothermal wells reaches about 70°C.
The Dinantian reservoir forms part of a large-scale transgressive carbonate platform developed onto the slowly subsiding southern Avalonian margin during the Mid-Upper Devonian to Lower Carboniferous. The Upper Carboniferous Variscan collision between Avalonia and Armorica-Gondwana led to the tectonic inversion of the Avalonian margin and the development of a north-vergent thrust system, whose front crosses northern France. The Dinantian platform was progressively buried under the NPC synorogenic molassic foreland basin developed along the Northern Variscan front.
Prior to our study, very little information was known on the structure of this reservoir under the coal basin. Its 3D-geometry has been investigated through the integration in a 3D model of a large database including 590 boreholes and 532 km of reprocessed and interpreted seismic reflexion profiles. First results of the 3D-modelling indicate that the Dinantian reservoir is continuous and extends over an area of at least 10750 km². It is present at 100-200 m depth in the Brabant foreland near Lille metropole. It strongly deepens southward and reaches approximately 7.5 km depth under the coal basin to the south of Cambrai. Major thickness variations (250-1000 m), often related to a complex structure involving deep variscan thrusts, have also been observed.

Keywords: 3D structural modelling, Dinantian geothermal reservoir, Northern Variscan thrust front

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The EU H2020 funded ‘MEET’ project (‘Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials’, grant agreement no:792037) assesses the four different geological settings of Variscan basement as reservoir rocks for potential EGSs. The study presented here, as a part of MEET project, specifically focuses on the Cornubian batholith in SW England chosen as representative of the crystalline Variscan basement not overprinted by younger tectonics.
With the ambition to increase the clean energy share in United Kingdom’s energy portfolio, Cornwall has been the focus of deep geothermal resource studies starting from 1980s with the Rosemanowes Hot Dry Rock project conducted in the region and continuing today with the United Downs Deep Geothermal Power Project (UDDGP), the first deep geothermal power project of the UK. The main reason of the high local geothermal potential is the heat flow anomaly arising from the radioactive decay of U, Th, K elements within the Cornubian batholith.
The present work aims to investigate the untapped geothermal potential of fractured Cornish granite in more detail. To do that representative samples from the Cornubian Batholith were taken from the outcropping Land’s End, St. Austell and Carnmenellis pluton. 47 granitic samples were collected from both fresh as well as fractured and/or hydrothermally altered areas of 23 outcrops as analogues of the fracture and fault zones targeted by the United Downs wells. 338 core samples with different diameters were drilled in the HydroThermikum Research and Teaching Laboratory of Technical University Darmstadt to do a comprehensive petrophysical and rock mechanical characterization. 47 samples were selected to prepare thin sections, required for a detailed petrographic analysis including degree of weathering or alteration and fluid inclusions.
Measurement of the petrophysical (grain density, permeability, bulk density and porosity, compressive and shear velocity, thermal conductivity, thermal diffusivity, heat capacity and radiogenic heat production) and rock mechanical properties (uniaxial compressive strength, Poisson’s ratio, Young’s modulus, bulk modulus and compressibility as well as tensile strength, shear strength, cohesion, coefficient of friction and shear modulus and finally Biot and Skempton coefficients) on outcrop analogue samples were performed. Although the petrophysical rock properties were analyzed at laboratory conditions and therefore deviate from in situ properties at reservoir conditions, the presented dataset enhances the knowledge of petrophysical rock properties within the study area for further geothermal applications. To predict the properties at the in-situ reservoir conditions, we accounted for the increasing pressure, temperature and salinity to calculate representative properties for simulated enhanced geothermal systems.

Keywords : Enhanced Geothermal Systems (EGS), Variscan orogeny, Cornubian batholith, petrophysical dataset.

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The increase in thermal water utilisation is foreseen by many European and Croatian strategic documents regulating energetics, tourism, environmental protection and sustainable development. Croatian Geological Survey wishes to establish a multidisciplinary group for hydrothermal systems research, which will contribute to responsible geothermal development in our country through a 5-year research project HyTheC which started in 2020.
Pannonian part of Croatia has favourable geothermal characteristics and natural thermal water springs emerge at two dozen localities, with temperatures up to 65 °C. These waters have been used for millennia, and in the past fifty years they are a basis for the development of tourism and health care centres [1]. As their water demand increased, higher quantities were abstracted and additional intake structures and wells were constructed.
Thermal springs are part of hydrothermal systems which include: recharge areas in the mountainous hinterlands of the springs; geothermal aquifers (in Croatia mostly Mesozoic carbonate rocks [2]) in which water resides and gets heated due to heat flow from the Earth; and discharge areas in places with favourable structural characteristics of higher permeability. The continuous functioning of such systems depends on a delicate balance between groundwater flow velocities, precipitation/dissolution processes and structural framework.
In order to maintain that balance and use thermal water resources in a sustainable manner, a system-level understanding is required. Multidisciplinary methodology (structural geology, hydrogeology, geothermal, hydrogeochemical and geophysical research and remote sensing) will be used to construct conceptual models of systems, perform 3D geological modelling, hydrogeological and thermal parametrisation of the geological units involved in the thermal fluid flow, and conduct numerical simulations of system functioning in undisturbed conditions and with different extraction scenarios.
This methodology will be tested in three pilot areas in Croatia where thermal water is used (Daruvar, Hrvatsko zagorje and Topusko). These three areas have a different level of initial data availability and it shall therefore be determined which methodology and order of application of different methods should be applied while researching the systems with considerable existing data, medium amount of data and very scarce data, respectively.

Keywords : hydrothermal system, natural thermal spring, multidisciplinary research, Croatia

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Hydrogen is an important energy source and could be an interesting alternative in the future. However, at present, this hydrogen is mainly produced by methane reforming, a process that emits CO2. Different alternatives exist: the use of a methane plasma torch, the electrolysis of green electricity and the production of natural hydrogen.
The geological contexts suitable to produce natural hydrogen are ophiolites, Proterozoic intracratonic basins and oceanic ridges. Indeed, high H2 concentrations have been measured in the hydrothermal vents of oceanic ridges. This gas is, in part, the product of the serpentinization of ultrabasic or basic rocks. This reaction involves the oxidation of ferromagnesian minerals associated with a reduction of water, optimally between 250 and 300°C. Through this reaction the coupling of high temperature geothermal energy and hydrogen production could be significant. However, the conditions in deep oceanic environment seem to preclude a profitable economic production. Nevertheless, there are geographical areas where the coexistence with a mantle plume allows these ridges to emerge, like in Iceland or in the East African rift.
In Iceland, geothermal energy is a major energy source. In the case of the East African Rift, the geothermal gradient is quite high too. In addition, in Iceland, the climate involves a regular meteoric water recharge while for the East African Rift and especially the Asal Rift (Republic of Djibouti) the climate is dryer, the recharge is made by the ocean. A high geothermal gradient at shallow depths, sufficient water recharge and a crustal production rich in ferromagnesian minerals could allow an interesting production of H2.
Thus, the objective of this study is to discuss the origin of this gas in the Asal Rift and in Iceland. Then to illustrate the interactions between geothermal energy and the production of natural dihydrogen in the two previous cases.

Keywords: Hydrogen, Geothermy, East-African rift, Iceland

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In places where no heat flow data are available, evidences of high geothermal gradients mainly relies on surface manifestations of hot hydrothermal fluids. We demonstrate that low temperature thermochronology combined with apatite REE analyses has the potential to be a new tool to reveal hydrothermal circulations at depth and can be used to explore new targets for geothermal production.
In the Eastern part of the Pyrénées, the Neogene Têt normal fault displays 29 hot springs along its surface trace with temperatures between 29°C and 73°C. Numerical modelling suggests the presence of a strong subsurface positive anomaly of temperature along-fault [1], governed by topography-driven meteoric fluid upflow through the fault damage zones [2]. The results demonstrate a correlation between AHe age resetting and apatite REE depletion for the samples affected by hydrothermal circulations. The intensity of both processes is more pronounced around Thuès-les-Bains, the hottest spring cluster [3]. Hydrothermal effects along the fault are spatially restricted to the fault damage zone, especially around the hot spring clusters [4]. This new exploration tool shows that the size of thermal anomalies along the Têt fault is more restricted than predicted by numerical models. AFT and ZHe ages obtained within the fault damage zone are not affected by hydrothermal disturbance and are consistent with all the thermochronological data recorded in the study area. This suggests that the temperature of hydrothermal fluids did not exceed 150°C in this area.
This study highlights that crustal scale faults adjacent to reliefs can provide geothermal perturbations of high amplitude and small wavelength and a strong heterogeneity of the hydrothermal flow pattern into the fault damage zone. This case study provides new constraints for geothermal activity in orogenic-belt related hydrothermal systems and opens new perspectives for the use of low-temperature thermochronology and REE analyses in geothermal exploration.

Keywords : Thermochronology, Orogenic-belt related hydrothermal systems, Geothermal exploration, Pyrénées

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The depth to the bottom of magnetic sources (DBMS) is widely related with the Curie point depth and used to study the thermal state of the crust. Nevertheless, the DBMS is not necessarily associated with a thermal boundary because it may be determined by lithology changes or convective heat transfer. For this study, the DBMS, calculated using the modified centroid method, is mapped applying the Sequential Gaussian Simulation (SGS) to define the spatial distribution of the DBMS and the spatial uncertainties. The SGS calculation follows the normal score transformation, calculation of the semivariogram, conditioning the simulation using kriging; the simulation is validated using the histogram and the semivariogram, and analysing the simulation results in e-type and standard deviation (uncertainty) maps. The resulting map shows good agreement with the measured heat flow values, with shallow DBMS in tectonic actives zones that could be interpreted as a proxy of high heat flow. In addition, deep DBMS occurs at zones without recent tectonic activity. Furthermore, the comparison of the DBMS map with borehole heat flow data is presented, and the theoretical curves that resulted from the conductive heat flow equation. The analysed data present scattering, nevertheless, most of the high heat flow data are related with shallow DBMS, and vice versa, low heat flow data is related with deep DBMS. In some cases, the DBMS is probably related with a lithological boundary, while heat sinks or water movement in the boreholes could affect the heat transfer mechanism. This could explain the scattering observed between the DBMS geostatistical map and the borehole data.

Keywords: Depth to the Bottom of Magnetic Source, Geostatistics, Heat Flow, Mapping.

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Open-loop Groundwater Heat Pump systems (GWHPs) currently represent one of the most energy efficient and environmentally friendly air-conditioning systems for densely urbanized areas in temperate zones such as the Italian peninsula. However, the increasing implementation of GWHPs can potentially cause, even in short terms, a significant environmental impact associated with thermal interferences especially in shallow aquifers.
Thermal perturbation produced by GWHPs must be predicted and constantly controlled, in order to guarantee plants’ long-term sustainable use and to avoid adverse effects on adjacent geothermal systems.

Transient geothermal plant-operating conditions can be successfully modelled by means of numerical modelling tools. As developing simulation models require the use of complex, expensive and timeconsuming numerical calculation software, an analysis about the potentialities in applying available analytical solutions to a real hydrogeological and plant context for predicting TAZ size was proposed.

The Politecnico di Torino open-loop geothermal plant was modelled by using the finite-element code FEFLOW® 6.2, performing heat-transport simulations over its annual operating period (from May to September) and estimating the average cooling thermal-load at the end of two different monitored seasons. Transient condition modelling results were then validated through a comparison with the groundwater temperature available data, also collected through a temperature-measuring chain installed in a 35m deep monitoring piezometer.
The validity of using steady state analytical solutions proposed by Banks, 2009 to predict the TAZ development was subsequently tested by reconstructing an equivalent stationary state.

For the considered hydrogeological system and the analysed geothermal plant of the Politecnico di Torino, the analytical solutions proposed by Banks, 2009 were found to be a possible alternative to the use of numerical simulation software for TAZ sizing at the end of the selected system-functioning seasons. The validation of the described approach for different GWHPs could represent the object of further future analysis works.

Keywords : Groundwater heat pumps, Thermally affected zone, FEFLOW, Analytical solutions

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High Temperature Aquifer Thermal Energy Storage (HT-ATES) is a technique to store sensible heat in aquifers in the subsurface, usually used to address the seasonal mismatch between the availability and the demand of heat. The applicability of subsurface aquifers for HT-ATES depends, among other aspects, on the heat losses that will occur and the extent of impact on groundwater resources. Although the presence of potentially suitable aquifers is known in most western countries, the specific hydrogeological characteristics that are relevant for HT-ATES performance is largely unknown and or uncertain. Characteristics that are of key importance for the feasibility and performance of HT-ATES systems are e.g. thermal conductivity, aquifer thickness, permeability, porosity and (effective) anisotropy. It is therefore important to reduce the uncertainty of these parameters to get a better understanding of the possible use of aquifers for HT-ATES. Taking in situ samples or drilling and testing wells is expensive, therefore, other methods are sought. As many wells have been drilled through these layers in the past and continue to be (e.g. targeting deeper layers for oil/gas and geothermal energy). This paper investigates the use of geophysical well logging to reduce the uncertainty of key parameters for HT-ATES performance.
Geophysical well logging has been mostly used for relatively deep layers to get information on the characteristics, presence and availability of oil/gas from layers of interest. This has led to the development of various geophysical logging techniques (e.g. Gamma ray, electrical resistivity, density, sonic, etc.). Each logging method directly and indirectly measures different parameters. Also different methods have been developed to combine geophysical logs to accurately determine the layering and presence of different materials (e.g. sand, clay, silt) in the subsurface. How geophysical logs can be combined to determine the parameters that are specifically of interest for HT-ATES has not yet been defined. Therefore, this research investigates the possibility of using a combination of common logging techniques and more experimental techniques to get insight in the suitability of aquifers for HT-ATES. The effect of the uncertainty of different parameters, are tested with numerical modelling and compared to the (un)certainty that the combination of geophysical well logs could potentially provide. The latter being investigated based on existing literature. The goal of this research is to provide practical and applicable methods that are ready to be tested and used in practice.

Keywords : HT-ATES performance, subsurface characterization, geophysical logging methods

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In France, heating networks are largely dependent on fossil fuels (42%), and deep geothermal energy represents less than 5% of the energy mix of heating networks. Deployment of geothermal energy in large cities is limited by a geological risk, that constitutes an obstacle to the future development of geothermal energy in the Ile-de-France region. The aim of this work is to develop a predictive 3D reservoir model of stratigraphic geometries, facies, porosity, permeability and temperature of the Middle Jurassic limestones. To create this 3D model, 266 wells, drilled over the last 60 years, were integrated over an area of 800 km2. We compiled more than 3000 porosity (phi) – permeability (k) values previously measured on core plugs. Sedimentology and stratigraphy were studied in detail along two reference wells in order to link sedimentary facies, logs and phi-k in a well-defined sedimentological framework. Fourteen 3rd order stratigraphic sequences were recognized from the Bajocian (jason Zone) to the Middle Callovian (zigzag Zone). Twelve surfaces from Bj3 to Ca4 corresponding to 3rd order Maximum Regressive Surfaces (MRS) allow to correlate all wells and to define stratigraphic geometries. A total of 10 facies are grouped into 4 facies associations (1) marls of lower offshore (facies association FA1), (2) marl-limestone alternations of upper offshore (FA2), (3) oolitic grainstones of the shoreface (FA3) and (4) lagoon micritic limestones and pack/grainstones (FA4). These facies associations were coded in all wells according to the log depths. The best reservoir qualities are mainly reached in the oolitic and bioclastic grainstones (FA3) with an average porosity of 12% and a permeability of 130 mD. The lagoon micritic facies also presents interesting properties with average porosity of 8.2% and permeability of 46 mD. The model has been meshed into 6.5 million of cells split on 64 vertical cell layers of 150 m x150 m x about 5 m (length ×width ×height) each bearing specific property information (facies, porosity, permeability). The final model shows a high variability in the facies distribution over the 14 depositional sequences. The maximum thickness of the oolitic reservoir is about 50 m in the western part of the study area between surface Bt2 and Bt4. By combining the isopach map of oolitic facies between surface Bt2 and Bt4 with porosity above 10%, permeability of more than 100 mD and temperature larger than 60°C, we locate areas of interest for geothermal development in the Paris Basin.

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Aquifer Thermal Energy Storage (ATES) systems are becoming increasingly popular across Europe in response to the Global effort to transition from fossil fuel energy sources to low carbon renewable energy sources. ATES technology has yet to be introduced to the UK and Ireland, but there are multiple geological opportunities for harmonisation into our energy supply matrix. The Sherwood Sandstone aquifer presents a suitable opportunity in the Belfast area to introduce ATES. However, this aquifer is also used for potable water supplies in the Belfast area, so impacts of ATES require investigation.
The thermal plumes discharged into the “warm well” of an ATES system are not only likely to reduce aquifer porosities & permeabilities by facilitating inorganic precipitation and secondary mineralisation but also by promoting microbial activity within the aquifer, with the additional biomass adding to the reduction of aquifer porosities. This in turn may affect the overall efficiency and sustainability of the duplet Geothermal installation and reduce the sustainable yield of the groundwater body for water supplies. The influence of thermal plume temperature ranges on sustainability requires investigation and modelling.
This research study will combine the baseline characterisation of the aquifer system by completing a series of active well geophysical measurements, hydraulic tests, hydro chemical sampling and microbial profiling with the long-term monitoring of experimental thermal injection tests at varying temperatures. Hydro-chemical and microbial sampling & analysis will be supplemented with down hole temperature monitoring using multi-mode fibre optical cable as part of a distributed temperature sensing system and bio geophysical monitoring of microbial activity using borehole magnetic resonance logging. Collected monitoring data will be integrated into heat transport models using FeFlow and LeapFrog to evaluate field-scale thermal properties of the aquifer and to better understand the impact of thermal plumes on aquifer properties.

Key words: ATES, thermal plume, heat transport model, Sherwood Sandstone

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In order to achieve the European and German CO2 reduction targets, of fundamental importance is the development of sustainable and renewable solutions for the heating sector, especially in the populated regions of North-West Europe (NWE) that largely rely on coal.

With this motivation, two projects were initiated: Geothermica HeatStore and NWE-Interreg DGE-Rollout. Within HeatStore, the development of a high temperature mine thermal energy storage (HT-MTES) pilot plant is planned. During the summer, solar thermal energy will heat up the water present in a small flooded mine situated at the premises of Fraunhofer IEG, in Bochum. As part of DGE-Rollout, during the heating season, the mine water will be the heat source of a high temperature heat pump (HTHP) that will be connected to the district heating (DH) grid.

Literature review on technological solutions of large scale HTHP’s for DH, along with possible refrigerants, was conducted. Market readiness and available products were analyzed.
Simplified computational models on different refrigerants performances under several source and sink working conditions, were developed. Energy balances to determine the heat pump specifications were performed.

Through a collaboration with delta h Ingenieurgesellschaft, flow and heat transport simulations on the mine water were carried out, in order to choose best locations for production, injection and monitory wells. The 3 wells were drilled by Fraunhofer IEG during summer 2020.

Major results include the knowledge on large scale HTHP, the determination of requirements for the needed technology, the mine water modeling, and the drilling.
Next steps will be pumping and tracer tests within the mine, eventual HTHP requirement updates, HTHP installation and testing.

With the development of the projects, it will be possible to prove the usage of abandoned mines as heat storage facilities, and coupling them with heat pumps will enable the energy conversion towards the CO2 reduction goals.

Keywords: high temperature heat pump, mine thermal energy storage, CO2 reduction

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This study proposes a workflow to examine potential reasons for low injectivity which is a major issue related to utilization of thermal waters. In order to completely understand and avoid the geothermal reinjection problems, potential problem sources acting on different scales should be taken into consideration. Thus, in our workflow, possible problem sources are considered after categorized into 1) effect of regional hydraulics (potential presence of overpressure and upward flow) 2) inadequate reservoir performance (low extension and low permeability) and 3) local clogging processes (fines migration, mineral precipitation, microbial activity). Hydraulic conditions are characterized by defining the pressure regime and the direction of vertical driving forces. The reservoir properties are given by determining the grain size and the length of the screened sections, as well as the permeability and the transmissivity of the reservoir and the capacity of the injector. Physical, chemical and biological processes are investigated by specifying the rock properties, clay content; by analysing the type, probability and amount of the scaling; and by evaluating the possibility of biofilm formation.
The workflow was tested on a geothermal site (Mezőberény, SE Hungary, installed in 2012) that had to even be stopped because of the unsuccessful reinjection. The potential reasons were investigated by processing the available dataset (geological, geophysical, geochemical and hydraulic), by modelling (hydrogeochemical, numerical), and by comparing the results to other operating geothermal systems. Based on the results, in the Mezőberény case injectivity decline is a consequence of several separate problems and their interaction: Reservoir properties are insufficient due to low permeability and transmissivity of the reservoir and the limited vertical and horizontal extension of the sandstone bodies. The active vertical length of the screened sections is reduced by inactive segments with lower grain size. Precipitation of carbonates, iron and manganese minerals is expected based on the hydrogeochemical modelling and the solid phase analysis. Microbial products are created due to the particularly high organic content of the produced thermal water that provides organic nutrition. Formation of bioprecipitation and biofouling is also possible. Injection problems due to hydraulic effects are not expected since the pressure regime is hydrostatic and ascending flow was not detected in the close surrounding of Mezőberény. Consequently, reservoir properties determine a low injectivity, which is further decreased to a critical level by the clogging processes.
The proposed workflow can contribute to the detailed reservoir and geothermal system analysis which is essential for a sustainable geothermal use.

Keywords : geothermal reinjection, low injectivity, workflow, modelling

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Heat losses occur in the exploitation of geothermal water, especially when it comes to deep geothermal wells. It happens due to the distance water crosses between the reservoir and the ground surface. Surface heat transfer is occurring between flowing geothermal water and the pipe wall, while radial conduction takes place between the pipe wall and the hardened cement slurry filling the annulus [1]. Using pipes made of low thermal conductivity materials is one of the practices in minimising heat losses. In my research, I am investigating additives that would decrease the thermal conductivity of the hardened cement slurries sealing the annular spaces.
Air is a good thermal insulator and is often introduced to increase the thermal resistance of cement slurries, especially in concretes used for construction. However, for deep geothermal wells, the use of aerated cement slurry is not always possible due to the required mechanical properties [2]. The high temperature of the geothermal reservoir also negatively impacts the mechanical properties of the cement, as the hydration products vary depending on temperature. Therefore, there is a need to identify cement additives that would decrease the thermal conductivity of the hardened cement slurry, while keeping mechanical properties suitable for use in deep boreholes. I would like to present the idea of cement slurries with a low thermal conductivity factor and the research results I obtained until now.

Keywords : geothermal, cement slurry, thermal insulation, deep well

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The efficiency of geothermal utilization depends heavily upon the behavior of the fluids that transfer heat from the subsurface to the power plant, which is determined by their chemical and physical properties. Regarding the chemical properties, there are still few data on the occurrence of organic compounds in geothermal fluids. Organic compounds play an important role and are frequently found in sedimentary basin fluids. They serve as nutrients for microorganisms and can affect fluid chemical properties by complexation, and fluid physical properties through decomposition with increasing temperatures. As part of the REFLECT project (Redefining geothermal fluid properties at extreme conditions to optimize future geothermal energy extraction), several fluids from different geothermal sites will be sampled. If possible, fluid samples are taken from the production well, right before the heat exchanger (production side) and after the heat exchanger (injection side). So far, four sites comprising Insheim (Germany), Bad Blumau (Austria), Krafla and Theistareykir (Iceland) were sampled. Analysis on Insheim and Bad Blumau samples were conducted first with IC (Ion Chromatography) to determine the main inorganic anions (chloride, bromide, fluoride and sulfate) and LMWOA’s (Low Molecular Weight Organic Acids) such as formate and acetate. To characterize DOC (Dissolved Organic Carbon) and DOC fractions, the samples were analyzed via LC-OCD (Liquid Chromatography – Organic Carbon Detection). For Insheim these first results show high chloride concentrations of approximately 77 g/L throughout all samples. However, DOC concentrations in the production side (2 mg C/L) are low compared to the injection side (13 mg C/L). Bad Blumau shows low chloride concentrations with approximately 4 g/L throughout all samples. DOC concentrations in the production side (2 mg C/L) are also lower compared to the injection side (6.9 mg C/L). In addition to the mentioned analytical methods, FT-ICR-MS (Fourier Transform Ion Cyclotron Resonance) analyses are also to be conducted to determine polar compounds with m/z range between 200 – 800. Furthermore, more sites are to be sampled in the near future (e.g. Heemskerk, Neustadt-Glewe, Tuzla, Germencek) to expand the data on organic compounds in geothermal fluids. The aim of this work is not only the characterization but also the interpretation of the data with respect to various site parameters such as reservoir formation, temperature, and depth. A better understanding of the origin of organic compounds in geothermal sites and their impact on geothermal fluid physical and chemical properties might contribute to future studies aimed at optimizing geothermal energy extraction.

Keywords : geothermal fluids, organic geochemistry, REFLECT, geothermal

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Among the renewable energy resources geothermal is specific to a local production and consumption. Next to the heat and energy production, geothermal energy presents with opportunities for rural and suburban areas linking sustainable development goals with expansion of new local economy sectors. Therefore, we assume that local exploitation of geothermal resources delivers a local added value other than energy generation. Moreover, a change of a municipality socioeconomic structure is presumed while geothermal establishments are introduced. The aim of this research is to fill in the gap in the literature and establish an empirical link between the use of the geothermal renewable and measures of local development. The assumed added value of the geothermal can be analysed as direct, indirect and induced effects on a municipality of exploitation. This research attempts to provide a methodology that measures the induced impact related to the socioeconomic development. The theoretical conceptualization underpinning this studies is reflected in endogenous growth theories as well as in regional economics approach.

This study attempts to establish a methodology that is able to measure geothermal resources impact on local development. Our approach is to implement the added value analysis of the locations where the renewable is used in multiple forms. Municipalities in Poland are chosen as the main object of the research because of particularly high geothermal potential and private sector experience in bringing up geothermal enterprises. The developed model is to demonstrate the statistical relation between exploited geothermal energy and parameters of local development. The versatility of this study lies in the fact that exercised on the Polish municipality cases our model is applicable for analysis to any geothermal municipality in the world.

Throughout the selected statistical methods we have already obtained significant results to justify our research assumptions. We produced studies where the competitive advantage of using geothermal resources in the local economies is econometrically illustrated. In addition, we observed a faster development of the geothermal municipalities compering to the referred regions. Moreover, we built a local development model with geothermal parameters, that we reckon is replicable in further studies about the local geothermal energy impact.

The research aims to deliver statistical argument for the expansion of geothermal resources extraction in Poland and worldwide.

Keywords : geothermal resources, direct use, local development, Multiple-criteria decision-making (MCDM )

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Turkey is located in the Alpine-Himalayan Orogenic Belt, which constitutes a major factor in having high geothermal potential. There are more than 350 geothermal fields at different temperatures. Geothermal fields located in western Turkey and have more than 100 °C have always been the field with high geothermal energy potential. Turkey's Western Anatolia graben systems are very suitable for geothermal resources such as Germencik Region where is located in the Büyük Menderes Graben system. This area has a complex geology with active tectonics and high geothermal potential. Germencik geothermal field has high salinity geothermal fluid characteristics. It has a geologically older geothermal reservoir.
The aim of this study is to evaluate the water chemistry and physical properties of the geothermal fluid with the 3D geological conceptual model of the geothermal field. Scaling problems, which is one of the most critical problems of geothermal power plants, have been examined. The result shows that stibnite scaling is the most crucial problem in this region. Stibnite starts to precipitate below 90 °C in the Germencik geothermal field.

Keywords: Scaling, geothermal energy, Turkey, saline geothermal fluid

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The aim of this study is to provide a better subsurface insight by using integrated conceptual models of the geothermal system and by subsequently creating a steady state model. It also evaluates chemical reservoir and surface feature behavior during production stages. Geochemical monitoring techniques will be developed and future production scenarios will be modeled to support a sustainable reservoir management and maximize the reservoir performance. Our example study site is the high-enthalpy geothermal system Lahendong in Sulawesi-Indonesia.
Implemented methods throughout the study include static geological modelling, natural state and production history matching and geochemical methods development and testing to monitor the reservoir evolution. Future scenario numerical modelling will be used to forecast reservoir behavior in the next development stages.

Keywords : Thermal-hydraulic modelling, natural state & production history, chemical monitoring, future scenario simulation.

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Modelling of geothermal reservoirs is complex due to the coupled flow and heat transport, the large domains that must be modelled and the range of length and time-scales associated with geothermal processes [1]. When using a fixed mesh, capturing all these scales can be very computationally expensive. In a range of fields of computational fluid dynamics, dynamic mesh optimisation has been applied, yielding a significant reduction of computational costs of simulations. Here we present a new approach for modelling of geothermal reservoirs, based on the use of unstructured tetrahedral meshes and dynamic mesh optimisation. Dynamic mesh optimisation allows the mesh to adapt in space and time to focus computational effort where required, based on an error metric and tolerance specified by the user. This allows the mesh to refine at sharp thermal fronts or around the wellbore, without any user intervention. A new approach to represent geologic heterogeneity is required for efficient application of dynamic mesh optimisation. We use parametric spline surfaces to represent key geological features such as faults and lithology boundaries [2]. The resulting 3D models are mesh free; a mesh is created only when required for numerical computations. Dynamic mesh optimisation preserves the surfaces and hence geologic heterogeneity. We apply this new modelling approach to two realistic and highly heterogeneous models: a reservoir comprising meandering fluvial channel sandbodies surrounded by low permeability floodplain mudstones and a reservoir containing interbedded layers of contrasting (high and low) permeability, offset across conjugate faults. In both these models analogous to deep sedimentary aquifers, production of hot water occurs via a doublet configuration. The flexibility of unstructured meshes allows to model with minimal number of elements both models, preserving the connectivity of the channels as well as the sharp permeability jumps across the faults. As the cold water is injected, the mesh progressively refines to capture the moving interface, whilst other areas of the reservoir are coarsened. Once cold water breakthrough occurs at the well and thermal fronts diffuse, the mesh is coarsened as less resolution is required to accurately resolve the solution fields. Overall, the use of mesh optimisation allows us to model flow and heat transport with an equivalent accuracy to fixed meshes, whilst reducing run times by up to 8 times.

Keywords : Reservoir modelling, dynamic mesh optimisation, geological heterogeneity

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Hydraulic fracturing plays a central role in developing enhanced geothermal systems. To simulate the propagation of fractures, the J-integral has been a standard technique used in the field of fracture mechanics. Its superior accuracy at coarser resolutions make it particularly attractive, especially for reservoir-scale simulations. However, the extension of the J-integral to simulate fluid-driven fracturing has not received the same attention or success. In particular, while several studies have highlighted the capacity of the method in simulating viscosity-dominated propagation, detailed investigations are missing. In this work, a finite element analysis is conducted which sheds light on the sources of error which arise when applying the J-integral in the viscosity-dominated regime. The case is put forward that the inaccurate numerical solution for fluid pressure is exclusively responsible for the loss in accuracy of the J-integral. Consequently, the extent of hydraulic fracturing which occurs is overestimated. With this new understanding, the J-integral is reformulated to minimise its dependence on inaccurate fluid pressures, bypassing the aforementioned sources of error. The reformulation is both simple to implement, and general to the numerical method. Within the framework of finite elements, a propagation algorithm using the novel J-integral is subsequently shown to perform excellently in simulating deep hydraulic fractures in both the toughness and viscosity dominated regimes of propagation; a method especially relevant to enhanced geothermal systems. Furthermore, an inherent advantage in evolving the fracture front is acknowledged due to the velocity-dependence of the novel method. The performance of the finite element implementation of the J-integral is explored at refinement levels relevant to the reservoir scale. Finally, the potential to devise more efficient algorithms to evolve the fracture networks of geothermal systems is discussed.

Keywords: Hydraulic fracturing, J-integral, hydraulic fracture mechanics, finite element

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Heat demand for space heating represents ~1/3 of the energy consumption in the UK. In 2017, most of this energy is supplied by natural gas, contributing to 19% of the carbon footprint of the country. Among new low-carbon heat sources required to decarbonize heating, mine-water heat recovery from abandoned collieries available across the UK has been of growing interest.
Although the temperature of mine-water is expected follow the local geothermal gradient, recent studies revealed the lack of correlation between the temperature measured in mine shafts and the measurement depth. The aim of this study is to investigate the impact of past mining activities on the temperature distribution in coal mines after closure and flooding of the mine.
We use the finite-element modelling software OpenGeoSys to simulate groundwater flow and heat transfers in 2D porous media representing mines of simple geometry. Based on the geology of the Coal Measures in UK Coalfields, we examine the perturbations of the geothermal gradient induced by long-term pumping through highly permeable layers and the time to return to equilibrium. A thorough analysis is made on the impact of the mine geometry, rock properties, production period and hydraulic boundary conditions on the extent of the perturbations.
Using temperature time-series, temperature profiles in shafts and energy calculations, we show the existence of permanent temperature perturbations around the mined seams and shafts. Preliminary results suggest that the thermal footprint of mining is mainly controlled by the distribution of the most productive seams and the hydraulic recharge to the mine. After flooding, temperature profiles in the pumping shaft display higher and lower temperatures above and below the mined seams, respectively, in accordance with the observations made in UK mine shafts. This suggests the importance of considering the mining history when evaluating the initial heat potential of flooded mines.

Keywords: Mine-Water Heat, Numerical Modelling, OpenGeoSys, Thermal footprint

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To accurately picture both the Bruchsal geothermal thermal and the power production cycle with focus on CO2 handling in the surface installations, a model was set up which allows to further investigate brine and operational parameters. The presence of gases in various amounts in geothermal systems can cause corrosion and decrease heat exchange efficiency. In Bruchsal we observe a two-phase flow of brine and gaseous CO2 in the surface pipeline system on the hot side. Therefore, the aim of this study is to picture and optimize the handling of CO2 in Bruchsal as well as for other binary geothermal systems conditions. Implementation of the CO2-solubility model of Duan et al. (2006) into a simulation tool for the Bruchsal power plant yields results that indicate single-phase flow at 22 bar at production temperature of 124 °C, 130 g/L total dissolved solids (TDS) and a CO2-content of 0.3 wt.%. This, however, is not in accordance with the observations in Bruchsal. Therefore, the simulation was repeated at a higher CO2-content of 0.85 wt.% which matches best the observations of two-phase flow. The simulation was compared to gas breakout pressure measurements, which indicate that single-phase flow was not reached until 31.7 bar. Using this pressure as a minimum value, the minimum CO2-content at single-phase conditions is estimated at 0.86 wt.%, which is in accordance with the results from the modified simulation tool.
Two different technical solutions were compared by simulation regarding their power output. Furthermore, CO2 content, TDS, pressure and temperature were varied to simulate not only Bruchsal but also other geothermal systems’ conditions. Fig.1 A and B show both regardless of the technical solution, that the power output decreases significantly by increasing the salinity and the CO2 content. This was expected as the heat capacity of salts and gas is significantly worse than that of water. For the Bruchsal conditions, the existing gas bypass is the better technical solution compared to the version B on the right which only yields a power output of about 200 kilowatts for that configuration.

Keywords : CO2 handling, CO2 solubility, Bruchsal, simulation

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The Canton of Geneva and the SIG (Services Industriels de Genève) launched the Geothermie2020 program in order to explore the potential for geothermal energy in the Geneva Basin and neighbouring France (Figure 1A), a foreland sedimentary basin associated with the alpine orogeny. Based on the results from the hydrocarbon exploration carried out since the early 60’s and subsequent geothermal drilling campaigns [1], the presence of five potential geothermal reservoirs (Lower Cretaceous, Kimmeridgian, Bajocian, Muschelkalk, Buntsandtein) have been confirmed [2].
This PhD focuses on the description and reservoir characterisation of karstified Lower Cretaceous, consisting mostly of low porosity carbonates, and the Eocene and Oligocene units, filling the karst, and exhibiting potential good reservoir characteristics due to their coarse texture.
An assessment of the Cenozoic sediments composed by the “Sidérolithique”, a quartz-rich sandstone facies, and the Gompholite, a polygenic conglomerate, is being carried out to understand their distribution, their reservoir quality and their geothermal reservoir potential.
For this purpose, the construction of a regional conceptual 3D facies model of the present-day architecture of these Cretaceous-Cenozoic transition units (Figure 1B), integrating stratigraphic and compositional data will be developed during this PhD project. Specifically, this model will be based on detailed stratigraphic, sedimentological, mineralogical, diagenetic geochemical and reservoir properties data acquired from both outcrops and boreholes present in the study area. Ultimately, this study will contribute to achieve a better understanding of the Cenozoic sediments depositional environment and thus improve the predictive capability of the current geological model for this specific, still poorly understood stratigraphic interval.

Keywords: 3D model, karst, sedimentary basin, reservoir

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The knowledge of subsurface temperature distribution is very important to identify potential geothermal regions and understand a variety of geologic processes, though it is one of the most poorly known geophysical parameters. This could be inferred through thermal modeling which requires the measurement of temperature and heat flow from boreholes. However, they are neither uniformly distributed nor consistently deep, and thus they are not enough in inferring deep thermal distributions. In this case, Curie temperature isotherm (580oC) model can provide information regarding the thermal structure of the crust. Curie temperature isotherm can be estimated from magnetic data using spectral methods. The Curie temperature isotherm can then be used as a constraint to build a thermal model by assigning to a lower thermal boundary condition (Dirichlet boundary) of a crustal model. The computed Curie temperature isotherm and thermal model integrating with gravity, heat flow, geological and well log data may provide a comprehensive geothermal potential model and allow identifying areas of higher temperature and heat flow and potential interest for geothermal exploration.

Keywords : thermal model, Curie isotherm, heat flow

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More and more territories and public authorities are interested in deep geothermal energy to increase the part of renewable energies in their heat networks. To reduce the risk of future deep geothermal energy operations, the prediction of thermal and hydrological evolution of the aquifers by allowing precise water and heat flow simulations are needed, which required a reliable and precise numerical geological model. In this study, we present a 3D geomodel that has been realised in this purpose as part of the development of geothermal energy in the Orleans metropolis, France (Orléans Métropole). The model is at a detailed scale (35 x 40 km) and focuses on the Triassic and the Dogger aquifers that are the potential targets for deep geothermal exploitation for Orléans Métropole.
The realisation of the 3D geomodel involved the integration of different data input such as seismic datasets, wells description and a pre-existing 3D geological model. 65 km of seismic lines from campaigns acquired in the 1960s and the 1980s by E&P companies were reprocessed and more than 365km were interpreted in total. During the seismic interpretation step (using the GVERSE Geographix software), a particular attention was paid to the geometry of the Sennely major fault because it crosses the study area; it has been newly interpreted as a relay fault segmented in three parts. The interpreted horizons were then converted from two-way time to depth (in meters) using interval velocities and integrated in the GeoModeller software (created and co-developed by the BRGM and distributed by Intrepid Geophysics) for the model development. Once computed, the geological model was adjusted where there were very few data thanks to the pre-existing geological model.
The resulting geomodel is quite more precise and reliable than the pre-existing one covering the study area and that has been used for the model development. Within the framework of the Orléans Métropole project, hydrological and temperature simulations have been then performed using the 3D geomodel, but it could also be used for other investigations or research projects, such as CO2 storage currently in progress.
The work was conducted as part of the “Contribution du BRGM au développement de la géothermie sur le Territoire d’Orléans Métropole” project co-funded by Orléans Métropole and the BRGM (French National Survey). Seismic line reprocessing was enabled through collaboration with the GEOCO2 project, conducted by the BRGM and co-funded by the Centre-Val de Loire (CVL) Region.

Keywords: Seismic interpretation, 3D Modelling, Deep geothermal energy, Paris Basin

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Geothermal reservoirs are often intersected by, or situated in close vicinity of faults. These faults provide migration pathways for reservoir fluids, which is desirable, however, geothermal operation alters the stress state on the fault plane, potentially leading to induced earthquakes. This is especially the case for enhanced geothermal reservoirs, where hydraulic stimulation treatments are applied. To achieve economically feasible production, hydraulic performance must be maximized, while fluid-injection-induced seismicity should be kept on a minimum level. This is particularly important in urban areas, where even minor felt induced events can lead to the cancellation of geothermal projects due to public intervention [1]. The proposed study aims to gain a better understanding of injection-induced permeability improvement and induced seismicity, then using this knowledge to provide best practice protocols for maximizing productivity and minimizing seismic risk during geothermal operations. To achieve this goal, a numerical code will be utilized and improved (e.g. GOLEM [2]), which allows the assessment of induced seismicity based on rate and state friction theory (RSF) [3]. The simulator will be capable of calculating seismic events (time, location, magnitudes) triggered either by mechanical, hydraulic, or thermal effects. In order to validate the simulation approach, hydraulic and seismic history will be matched on available laboratory-, mine- and field-scale data. A parametric study will be conducted to investigate the effect of different intrinsic geological parameters (fault architecture, host rock properties, hydro-mechanics, stress-field), and external parameters (geometry of wells, injection schemes, pressure perturbation) on induced seismicity and well performance. Finally, a ranking system will be defined for these parameters, so recommendations and best practices can be formulated to support the scientific and technical community, as well as the operators of geothermal plants. Here, only the problem statement and the proposed methods are presented.

Keywords : Thermo-hydro-mechanical modelling, induced seismicity, rate and state friction theory

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Geothermometry is used for reservoir temperature estimation since the 1960s. Different kind of solute, gas, and isotopic geothermometers has been evolved and further developed. We are focusing on solute geothermometery, using the multicomponent approach by Reed and Spycher [1] and combined it with an optimisation process suggested by Nitschke et al. [2]. Therefore, IPhreeqC by Parkhurst and Appelo [3] and Matlab were coupled. Thus, the geochemical output of IPhreeqC is numerically evaluated and optimised with Matlab. Sensitive parameters, e.g. pH-value, and aluminium concentration etc. are varied simultaneously to minimise the temperature difference between multiple mineral phases, used as geothermometer. MulT_predict with its implemented optimisation leads to more accurate temperature estimations with lesser variance of error.

Keywords : MulT_predict, multicomponent geothermometer, reservoir temperature estimation

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I present ToMaHoC a full GPU based Thermo-Mechanical-Hydro-Chemical porous media simulator. It can be used in one or multiple GPUs, making it flexible to be used in a workstation, laptop or computing clusters with many GPU's per computing node. Some of the features included are elasticity, perfect-plasticity model, plasticity with cohesion weakening and frictional hardening, perfectly matched absorbing boundary conditions, Darcy flow, Non-darcian Barree and Conway flow, stress-dependent porosity, isotropic porosity dependent permeability, anisotropic porosity dependent permeability, permeability change due to plastic failure, thermal stresses, mechanical and thermal pressurization, heat flow using local thermal equilibrium approximation, plastic, and shear heating, concentration-pressure-temperature dependent fluid density, viscosity, compressibility and heat capacity, and non-reactive solute transport. All these processes can be controlled, activated, deactivated, and mixed using an easy to use and intuitive graphical user interface (GUI). Complex geologies can be imported to the simulator and more simple geologies with multiple faults and horizontals layers can be directly constructed using the ToMaHoC GUI.
This work was developed in part in the CHYN of the University of Neuchatel.

Keywords : Poro-elasto-plasticity, GPU computing, graphical user interface

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Within the Act of 17.08.2015 on energy transition voted by the French government, the Hauts-de-France region (Northern France) is committed to invest and develop massively renewable energies in the coming decades. This objective requires the development of regional alternative solutions to fossil fuels, especially low-temperature geothermal energy. The present research project aims at better characterizing the geometry of the main deep geothermal reservoir in the area of the Nord-Pas-de-Calais (NPC) coal basin: the Dinantian karstic and brecciated limestones (Lower Carboniferous, 360-330 Ma). The occurrence of a Dinantian regional geothermal resource has already been proven in Belgium in the Hainaut coal basin area where the temperature in the geothermal wells of Douvrain, Ghlin and Saint-Ghislain reaches about 70°C.
The Dinantian reservoir forms part of a large-scale transgressive carbonate platform developed onto the slowly subsiding southern Avalonian margin during the Mid-Upper Devonian to Lower Carboniferous. The Upper Carboniferous Variscan collision between Avalonia and the Armorica-Gondwana accretion complex led to the tectonic inversion of the Avalonian margin and the development of a north-vergent thrust system, whose front crosses northern France. The Dinantian platform was progressively buried under the NPC synorogenic molassic foreland basin developed along the Northern Variscan front.
Prior to our study, very little information was known on the structure of this reservoir under the coal basin. Its 3D-geometry has been investigated through the integration and interpolation in a 3D model of a large database including 590 boreholes and 532 km of reprocessed and interpreted seismic reflection profiles. First results of the 3D-modelling indicate that the Dinantian reservoir is continuous and extends over an area of at least 10750 km². It is present at 100-200 m depth in the Brabant foreland near Lille metropole. It strongly deepens southward and reaches approximately 7.5 km depth under the coal basin to the south of Cambrai. Major thickness variations (250-1000 m), often related to a complex structure involving deep variscan thrusts, have also been observed.

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As part of the energy transition, the geothermal potential of the Earth need to be studied in order to cope with climate change. In this context, the Horizon 2020 program is funding the MEET project to investigate the development of Enhanced Geothermal Systems in different geological settings, including sedimentary basins and the conversion of oil wells for geothermal purposes. Within this European project, we are investigating a Triassic reservoir made of sandstones and dolomites in order to evaluate the co-production potential of geothermal and oil resources.
Our study is intended to provide insights on the interrelations between rock physics properties and microstructures in a newly defined stratigraphic framework. The main objectives are focused on the geological controlling factors of the transport properties. The studied rocks were accumulated on the western part of the Paris Basin during the Upper Triassic. Eight wells were characterized, combining sedimentary logging of 470 meters of rock cores and analyses of well-log dataset (Gamma Ray, Photoelectric absorption factor, Neutron Porosity, Density, Sonic). Lithofacies were defined based on core description, together with facies associations definition and depositional environment interpretation. Furthermore, a sampling of 159 plugs was done, for performing petrophysical analysis. Rock physics measurements were acquired on those plugs in order to complete an existing petrophysical dataset of more than 700 samples (permeability, κ and porosity, Φ). Furthermore, a petrographic work was conducted, including 250 thin-sections observations using conventional microscopy and 32 thin sections using cathodoluminescence.
Based on a systematic petrographic description, we show that the combination between sedimentary texture and diagenetic overprint may greatly impact the porous network of the studied rocks, conducting to the definition of 3 main rocks types displaying distinctive κ-Φ trends. Furthermore, using both sedimentary descriptions and well-log dataset, seven electrofacies corresponding to three main depositional environments were defined, namely alluvial fan deposiqts (conglomerates & sandstones), floodplain/lacustrine deposits (heterolytic clays) and dolomitic paleosols (dolomitization of siliciclastic deposits). Then, a high resolution correlation transect was designed using genetic stratigraphy principles, including seven main genetic sequences that show the spatial evolution of depositional environments. In addition, the integration of rock types within the correlation transect provides a high resolution definition of the reservoir compartmentalization. To go further, we may build a 3D static geological model that will serve as a basis for the development of future numerical simulations of fluid flow and heat transfer at the scale of the reservoir.

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In this work, the impact of clay minerals on formation damage of sandstone reservoirs is studied in order to provide a better understanding to the problem of deep geothermal reservoirs clogging due to fine particle dispersion and migration. Our study is currently carried out on cores from a Triassic reservoir in the Paris Basin (Feigneux, 60 km NorthEast of Paris). Our goal being to first identify the clays responsible for clogging, a mineralogical characterization of these natural samples was carried out by coupling X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The obtained results show that the stratigraphic interval studied contains mostly illites and chlorites. Nevertheless, several parameters, notably the spatial arrangement of the clays in the rocks as well as the morphology and size of the particles, suggest that the illites are more easily mobilized by the flow of the pore fluid than chlorites. Thus, based on the results of this first part, illite particles will next be used to carry out laboratory experiments in order to better understand the factors leading to the aggregation and deposition of this type of clay particles in geothermal reservoirs under variant physicochemical and hydrodynamic conditions. To do so, the stability of illite suspensions under geothermal conditions will be investigated using different characterization techniques including Dynamic Light Scattering (DLS) and Scanning Transmission Electron Microscopy (STEM). Various parameters such as the hydrodynamic radius, the morphology and surface area of aggregates will be measured. Then, core flooding experiments will be carried out in order to explain the permeability decline due to the injection of illite containing fluids in sandstone reservoirs.

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Within the Horizon 2020 MEET project (Multidisciplinary and multi-context demonstration of Enhanced Geothermal Systems exploration and Exploitation Techniques and potentials) is to bring about this potential in previously considered unconventional geothermal reservoirs, focusing specifically on crystalline and meta-sedimentary Variscan basement rocks. One of the demo sites is located at the University of Göttingen with the reservoir at 4km depth, within the Variscan sedimentary basement.
With no research well and little in the way of seismic interpretation, the focus has been put onto the use of outcrop analogues within the Rhenohercynian zone to determine the potential of such a reservoir. The focus of this study is primarily within the Clausthal Culm Fold Zone (CCFZ) in the Western Harz Mountains, comprised of NW verging folds of Lower Carboniferous greywackes and slates. The planned reservoir is relying primarily on the primary permeability of the fracture network related to the folding and thrusting caused during the Variscan orogeny. Ideally one might suspect the fold hinge and thrust zones to be target zones, but due to the drilling location being fixed due to the demo-site already being active, any future drilling will be blind. This means all reasonable bases should be covered in the analogue characterisation.
With the complexity of the structural and lithological attributes of the reservoir, along with limited subsurface data, a methodology has been established to characterise the main target areas within the CCFZ for the collection of fracture network parameters for use reservoir simulation models. By collecting field data in various forms such as drone images and 3D outcrop models, the structural and lithological situations can be characterised, and an overview of the changes of the fracture network characteristics can be quantified. Throughout the analogue site fracture parameters are collected, aerial intensity, connectivity, and orientation, at sites that correspond to the structural and lithological situations listed in Fig.1.
This method focuses on the fracture characteristics based on their relationship to the lithology and placement within a fold rather than physical location, allowing for a more holistic approach for a potential development reservoir permeability in cases where subsurface data is limited.

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The GEO-URBAN project aims to explore the potential for low enthalpy geothermal resources in urban environments. In the frame of GEO-URBAN project, two low-enthalpy deep geothermal reservoirs, Dublin basin (Ireland) and Valles Basin (Spain), will be evaluated using geophysical exploration techniques.

In the Valles Basin area (Catalan Coastal Ranges, NE Spain) the thermal anomaly is located in the northeastern limit of the basin, where a highly fractured Hercynian granodiorite act as the geothermal reservoir. Nevertheless, the geological structure of this area, as well as the role of the Vallès major normal fault, is poorly understood.
Different geological and geophysical surveys made on the 80s determined the presence of geothermal anomalies in different parts of the Vallès Basin, although their resources were not exploited [1] [2] [4]. In our study we decide to focus on La Garriga-Samalús area (northeastern edge of the basin), where the fractured nature of the crystalline bedrock represents a geological challenge.

Several geophysical techniques are being applied in this area to understand the main structure, which seems to control the heat and the hot-water flow. Magnetotellurics (MT), Gravity, Electrical resistivity tomography (ERT), Passive Seismic (H/V) and Control Source Electromagnetics (CSEM), have already been carried out, however, the models are not yet completed.
Although the geophysical study makes up most of the study, we are also improving the geological map of the area, based on a fractures study at different scales. We are working with DEM alignments analysis, and fractures study from outcrops and from thin sections.
The geological model will be built up using all the geophysical information, the surface data, and previous information, as six exploratory wells made by the Geological survey of Spain [3] [5] [6]. Moreover, it will include petrophysical data from the granodiorite rock, which would help us to correlate the geophysical results and the geology of the area, also being essential for the final flow model.

Our preliminary results show a strong gravity gradient in the NE-SW Valles half-graben system and the recent MT profiles image the main fault of that system (Valles normal fault).
Interpretations of the fractures study, together with geophysical data and models, have allowed the characterization of damage zones associated to the fault system which are directly related with the fluid flow and the hot springs. The nature of this damage areas could also be related to relay ramps, commonly regarded as efficient conduits for fluid flow.

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The work presented here is part of the European MEET project (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials), which aims to enhance demonstration of geothermal energy production throughout Europe. Nowadays, active reservoirs as the Soultz-sous-Forêts (SsF) granitic reservoir (Upper Rhine Graben, Alsace, France) are not fully understood because of limited data (few cores, cuttings, parcel information only for boreholes and no direct 3D vision). To improve the understanding of the reservoir connectivity, surface analogues of the SsF geothermal reservoir are studied: the Noble Hills range (NH, Southern Death Valley, CA, USA) and the Upper Rhine Graben shoulders (abandoned mines in the Vosges Mountains, France and in the Black Forest, Germany). We have here chosen to present results from the NH analogue consisting in a highly fractured and altered Mesozoic granite. The high level of fracturation of this geological object is to be related to its location along the Southern Death Valley Fault Zone (SDVFZ). The SDVFZ is a dextral shear zone being one of the two major faults responsible for the Death Valley pull-apart basin. We focus the study on 1) the alteration processes linked to fluid circulation and 2) the impact of shear deformation on the reservoir connectivity.
Analogue studies allow us to better understand fluid circulations in a granitic fractured reservoir and gain insights in fluid rock interaction processes as these can drastically modify rock and fractures properties through time. From outcrop to thin section scale, fractures can be filled by different mineral types (carbonates, oxides, barite and sometimes quartz). Those different infills indicate the occurence of different fluid circulation episodes through the whole granitic body. Petrographical analyses were performed on 78 samples to define the composition of the altered granite and to describe the alteration processes. Among the major minerals composing the granite, plagioclase (oligoclase mainly) is transformed into illite/kaolinite/(calcite) and biotite into illite (Fig. 1). Similar alteration processes are observed in the SsF granite. However, plagioclase in the SsF granite are locally transformed into tosudite instead of kaolinite.
Fieldwork has shown a strong deformation gradient induced by strike-slip movements linked to the activity of the SDVFZ. Along this gradient, mineralized veins and high strain zones were observed and investigated. Those highly deformed zones associated to fluid circulation are most of the time sealed by clay minerals (mainly illite), a feature also observed in SsF cores. It is suspected that the onset of clay mineral formation causes a change in deformation mechanisms from the brittle to the ductile domain which in turn can drastically modify fault mechanical properties (Fig. 2).

This work has shown that 1) analogue studies are essential for the understanding of a reservoir in its entirety, 2) the SsF and NH granites react similarly to the alteration processes and 3) deformation play a major role on fault petrophysical properties.

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Tanzania is one of the East African countries with large amount of geothermal potential that has not been used yet and has only been explored to a limited extentThe national power system relies greatly on hydropower and natural gas. Willing to propose an alternative, the government of Tanzania has made geothermal development a priority.
The area around the city of Mbeya, located at the junction of two branches of the East African rift system has been identified as a first priority region from which two specific targets have been defined, the volcanic region of Ngozi and the half-graben of Songwe. In this study focus is made on the Songwe basin.
In geothermal exploration and development, numerical modelling is essential to understand both regional and reservoir scale processes. Key factors determining geothermal systems potential are temperature and fluid flow. Numerical models allow to constrain reservoir geological properties and fluid flow behaviour to better plan the well path design and field production. In Tanzania, only geophysical (MT and TEM), geochemical studies and field geology have been done. A need therefore arises for numerical modelling to evaluate and assess the potential of the geothermal fields which is the purpose of this study.
In order to evaluate the potential of the geothermal system in Songwe, the study approach includes the reconstruction of the geological model to get the geometry, thermal modelling, hydrothermal modelling, laboratory testing of hot springs fluid to find the chemical properties, and laboratory testing of rock samples to obtain their physical and mechanical properties.
The study presents a reconstructed geological model with the deep structure, a realistic geometry that is essential for the thermal modelling and geothermal modelling work (Figure 1). Thermal modelling with different scenarios to understand how temperature is distributed in the subsurface at a steady-state is also presented in the study and temperature values compared (Figure 2).
The study will also incorporate the mechanical and physical parameters from the laboratory measurements and construct a geothermal model that will help in understanding and predicting the long-term geothermal production in Songwe through testing different production scenarios to identify the related impacting factors on fluid flow and thermal production.

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Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. In an effort to investigate the effect of injection rate on slip characteristics and strain partitioning, we conducted laboratory fluid injection experiments on permeable sandstone samples in a triaxial deformation apparatus equipped with a 16-channel acoustic emission (AE) recording system. We injected fluid in sawcut samples containing a critically stressed fault at different pressurization rates. Specifically, the fluid pressure is injected stepwise from 5 MPa to 29 MPa with a pressurization rate of 2 MPa and 0.5 MPa, respectively. The experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Episodic slow stick-slip events (peak slip velocity < 4 μm/s) are induced at high fluid pressurization rate while the fault creep (slip velocity < 0.4 μm/s) occurs in response to slow fluid pressurization rate. The strong weakening of the dynamic friction coefficient of the experimental fault is observed at elevated pore pressure, independent of fault slip mode. The polarity analysis of AE events indicates that shear failure is dominant (about 70% of all events) for both fault slip modes. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.

Fault slip induced by fluid injection is partitioned in aseismic and seismic moment release. By analysing the seismic moment release of AE events caused by fluid injection, we find that fluid-induced fault deformation is dominantly aseismic. The released total seismic moment is found to be related to total injected volume, independent of fault slip behavior. Seismic moment release rate of AE is related to measured fault slip velocity. In our experiments, the fluid pressure migration is faster than rupture propagation by about five orders of magnitude, resulting in induced fault slip fully confined within a homogenous pressurized zone. The relation between moment release and injected volume is affected by fault slip behavior, characterized by a linear relation for slip at constant rate and fault creep while a cubic relation for unstable and dynamic slip. Our experimental results allow separating a stable pressure-controlled injection phase from a run-away phase, when cumulative moment release with injected volume follows a nonlinear trend.

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The long-term sustainability of fractures in Variscan metamorphic rocks will determine whether it is reasonable to utilize such formations as potential unconventional EGS reservoirs. During long lasting fluid flow within fractures, dissolution, precipitation, and chemical reactions between the fluid and the rock matrix may alter the flow pathway structure and flow properties. Within the framework of the European Union's Horizon 2020 initiative "MEET (Multi-Sites EGS Demonstration)", we performed long-term fracture permeability experiments on saw-cut slate samples from the Hahnenklee drill site, Harz Mountains, Germany, under constant pressure and temperature conditions. Two experiments were performed using deionized water as pore fluid with intermittent flow for more than one month at 10 MPa confining pressure and 1 MPa pore pressure. Three sequential investigations were performed, including (ⅰ) an initial continuous flow tests at room temperature, (ⅱ) temperature cycles between room temperature and up to 70 °C or 90 °C, and (ⅲ) measurement of the time-dependent permeability evolution at 70 °C or 90 °C. During stage (ⅲ), the effluents were sampled in time intervals of 6 days and analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). The results show that (ⅰ) sample permeability first continuously decreases, but progressively converges within about three days, (ⅱ) increasing temperature leads to an additional permeability decline that is irreversible, and (ⅲ) the time-dependent permeability reduction is much more pronounced at 90 °C in comparison to that at 70 °C. The effluents are enriched with Na, Fe, K, Ca, Si, where the Na concentration is always an order of magnitude higher than the others. Except for Si, concentrations are progressively decreasing with time. During the entire experimental period, sample permeability was reduced by approximately 90% at 90 °C and 60% at 70 °C compared to their initial values. In contrast, both samples showed a negligible permeability decline over time at room temperature after cooling. Our results demonstrate that thermally-enhanced fluid-rock interactions lead to a permanent and at least partial closure of fracture aperture, which is unfavorable for geothermal exploitation. However, the degree of permeability reduction may strongly depend on initial fracture roughness, which remains to be investigated.

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The EU H2020 funded ‘MEET’ project (‘Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials’, grant agreement no:792037) assesses the four different geological settings of Variscan basement as reservoir rocks for potential EGSs. The study presented here, as a part of MEET project, specifically focuses on the Cornubian batholith in SW England chosen as representative of the crystalline Variscan basement not overprinted by younger tectonics.
With the ambition to increase the clean energy share in United Kingdom’s energy portfolio, Cornwall has been the focus of deep geothermal resource studies starting from 1980s with the Rosemanowes Hot Dry Rock project conducted in the region and continuing today with the United Downs Deep Geothermal Power Project (UDDGP), the first deep geothermal power project of the UK. The main reason of the high local geothermal potential is the heat flow anomaly arising from the radioactive decay of U, Th, K elements within the Cornubian batholith.
The present work aims to investigate the untapped geothermal potential of fractured Cornish granite in more detail. As surface samples; outcrop analogue samples were taken from the Land’s End, St. Austell and Carnmenellis plutons of Cornubian Batholith. 47 granitic samples were collected from both fresh as well as fractured and/or hydrothermally altered areas of 23 outcrops as analogues of the fracture and fault zones targeted by the United Downs wells. 338 core samples with different diameters were drilled in the HydroThermikum Research and Teaching Laboratory of Technical University Darmstadt to do a comprehensive petrophysical and rock mechanical characterization. 47 samples were selected to prepare thin sections, required for a detailed petrographic analysis including degree of weathering or alteration and fluid inclusions. As subsurface samples; drill cuttings were gathered from UD-1 i.e the production well of United Downs project. To understand the mineralogy and hydrothermal alteration at reservoir depth better, XRD, XRF and thin section analyses were done on drill cuttings.
Measurement of the petrophysical (grain density, permeability, bulk density and porosity, compressive and shear velocity, thermal conductivity, thermal diffusivity, heat capacity and radiogenic heat production) and rock mechanical properties (uniaxial compressive strength, Poisson’s ratio, Young’s modulus, bulk modulus and compressibility as well as tensile strength, shear strength, cohesion, coefficient of friction and shear modulus and finally Biot and Skempton coefficients) on outcrop analogue samples were performed. Although the petrophysical rock properties were analyzed at laboratory conditions and therefore deviate from in situ properties at reservoir conditions, the presented dataset enhances the knowledge of petrophysical rock properties within the study area for further geothermal applications. To predict the related properties at the in-situ reservoir conditions, the increasing pressure, temperature and salinity were accounted for.

Keywords : Enhanced Geothermal Systems (EGS), Variscan orogeny, Cornubian batholith, petrophysical dataset.

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With around 50 heating networks today operating, the aera around Paris is the European region which concentrates the most heating network production units in terms of deep geothermal energy. In France, the energy-climate strategy plans to produce 6.4TWh in 2023, compared to 1.5TWh produced in 2016 [1]. Despite an exceptional geothermal potential, the current average development rate of 70MWh/year will not allow this objective to be achieved, it would be necessary to reach a rate of 6 to 10 times higher. The optimization of the use of deep geothermal energy is a major challenge for France, and in Ile-de-France, which has a population of nearly 12 million inhabitants. This project aims to reconstruct and simulate heat flows in the Paris Basin using an innovative methodology (1) to characterize, predict and model the properties of reservoirs (facies, porosity, permeability) and (2) simulate future circulations and predict the performance at a given location (sedimentary basin) on its geothermal potential. This study focuses on a high density area of well infrastructures around Cachan, (8 doublets, 1 triplet in 56 km2). A new sub-horizontal doublet concept has been recently (2017) drilled at Cachan to enhance heat exchange in medium to low permeability formations [2]. Nuclear Magnetic Resonance (NMR T2) logs have been recorded in the sub-horizontal well (GCAH2) providing information on pore size distribution and permeability. We integrated all logging data (gamma ray, density, resistivity, sonic, NRM T2) of the 19 wells in the area and 120 thin section observations from cuttings to derive a combined electrofacies-sedimentary facies description. A total of 10 facies is grouped into 5 facies associations coded in all the 19 wells according to depths and 10 3rd order stratigraphic sequences are recognized. The cell size of the 3D grid was set to 50 m x 50 m for the XY dimensions. The Z-size depends on the thickness of the sub-zones, averaging 5 m. The resulting 3D grid is composed of a total of nearly 8.105 cells. After upscaled, facies and stratigraphic surfaces are used to create a reliable model using the “Truncated Gaussian With Trends” algorithm. The petrophysical distribution “Gaussian Random Function Simulation” is used to populate the entire grid with properties, included 2000 NMR data, considering each facies independently. The best reservoir is mainly located in the shoal deposits oolitic grainstones with average porosity of 12.5% and permeability of 100 mD. Finally, hydrodynamic and thermal simulations have been performed using Pumaflow to give information on the potential risk of interference between the doublets in the area and advices are given in the well trajectory to optimize the connectivity and the lifetime of the system. NMR data, especially permeability, allow to greater improve the simulations, defining time probabilities of thermal breakthrough in an area of high density wells.

Keywords : Flow simulation, Carbonate, Modeling, Paris Basin, NMR

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Abstract:
Numerical simulation of the heat extraction from geothermal reservoirs provides a tool to examine different operational scenarios in lower time, cost and safety issues in comparison to the experimental or field tests and results could be used to optimize the production plan. To obtain a better result, details should be considered as much as possible in a way that the simulation be feasible from the computational machines capabilities aspect. Hopefully, with increasing the power of the computational machines in recent years, now we can go deeper into details of the involved processes in numerical simulation studies by coupling different involving physics.

Now, we are in a position that we can simulate the coupled THM processes involved in heat extraction from geothermal reservoirs. In a part of the MEET project, the possibility and consequences of colder fluid injection (in comparison to the current fluid temperature) into Soultz-sous-Forêts reservoir is examined from the THM response and enhanced energy production aspects. During this study, large scale faults are considered discretely and the equivalent permeability field is assigned based on the small-scale fractures (which are in order of hundreds of thousands) for the background reservoir rock.

The numerical model is validated based on the well-known experimental tests in the literature (Figure 1) and a hydro-thermal test which is done on the Soultz-sous-Forêts reservoir. Heat loss during fluid movement alongside wellbore and daily temperature fluctuation are found as important factors which affect the field measured results. By considering these effects, a good match obtained between the simulation results and the field test.

To shed light on the complex behavior of the faults in the three-dimensional simulation, two-dimensional simulations are conducted based on the fracture network which are obtained from outcrop analysis in the Göttingen site (Figure 2). Poro- and thermo-elastic behavior of the system, fracture aperture changes, effect of the relative orientation between wellbores and fracture network, different boundary conditions and material properties, effect of the fracture aperture distribution are examined in details. Three -dimensional simulations are done in small-scale models with discrete fractures.

The numerical tools are provided to examine the possibility of the new fractures’ initiation and propagation. Furthermore, initial models are developed to examine the fault activation and possible seismic events in response to the cold fluid injection. During the remaining time of the project, these models would be scaled up to the field scale.

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Nowadays, geothermal waters have become an important form of energy and many researchers have put to the forefront the issue of “sustainability” of the geothermal reservoir development. “Sustainability” of the geothermal waters use is a problem of primary importance, the solution of which requires an integrated approach. Sustainable management of resource utilization is possible with an adequate development strategy as “… for each geothermal system and for each mode of production there exists a certain level of maximum energy production, below which it will be possible to maintain a constant energy production from the system for a very long time (100–300 years)…” (Axelsson et al., 2001).
Reinjection of the used fluid during geothermal reservoir exploitation is the most commonly used exploitation method. It allows minimizing the possible negative effect on the environment and maintaining the reservoir pressure and therefore the initial high flow rates. However, it has a major disadvantage – a gradual temperature decrease in an aquifer, which needs to be taken into account. The most effective methods assessing this disadvantage are based on numerical modelling, which has been actively implemented in all areas of science. Modelling is used to simulate the behavior of complex geothermal reservoir system under different exploitation conditions and predict temperature change. It helps to manage problems of geothermal water exploitation and achieve sustainability.
Modelling was used during the most recent Russian geothermal project – implementation of Khankala geothermal plant (Figure 1).
The Khankala geothermal field is located within the East Ciscaucasian artesian basin, which occupies 250 000 km2. In the beginning of 2016, the Khankala geothermal plant started operating on the basis of the XIII productive layer with a thickness of 40–60 m. The capacity of the facility is 22.8 GJ/h with a greenhouse complex as a consumer. Two wells were drilled to conduct exploitation by a “doublet” system, i.e., one production and one injection well with reinjection of all cooled geothermal water back.
Based on the results of modelling, a gradual decrease in temperature in the production well after 6–7 years of exploitation is predicted due to reinjection of used geothermal water (45 °C) (Figure 2).
One of the main advantages of the Khankala geothermal field is its multi-layered nature. Vertical extension of the cold front is therefore limited by impermeable layers, and so, reinjection in one of the main productive layers does not have an impact on others. At the same time, in the case of significant drop in the production well temperature after some period of XIII layer exploitation, there is a possibility to drill a new “doublet” at the same territory on the resource of the highly promising IV–VII, XVI, or XXII layers, so that the geothermal plant could continue working. The resource of the XIII layer could be used again after some shut-down period considering the relatively high speed of temperature recovery. The installation and the periodic use of two or more circulation systems can be beneficial and are the only true solution of the sustainability problem in long-term operation.
The sustainable use of geothermal waters is possible, but requires to choose adequate exploitation scenario and therefore numerical modelling, as it helps to forecast geothermal reservoir behavior under different conditions.

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Abstract:
This study provides a characterization about analysis on well developments taking place in exploited geothermal field in Indonesia. A combination between various well behaviour data reveal new knowledge of changes taking place in south reservoirs due to exploitation response associate with structural geology.
Methods implemented on this study include integrated analysis among the existing data and modification well development maps through ArcMap.
A combination between integrated analysis and interpretation of the well behaviour data reveal reasons to develop south reservoirs in initial exploitation. It is followed by changes in the reservoir behaviour at some wells due to dynamics exploitation in a long time. The changes associates with structural boundary could present controlling the hydrogeological setting around the wells. Some supporting evidences toward the presence of the boundaries are supported by the well data.
The changes can indicate the boundary may have strong controls either act as barrier or composite to manage the fluid flow in subsurface. By knowing characteristic of the boundary can be expected to suggest improvements of well strategy.

Keywords: exploitation, well behaviour, dynamics changes, reservoirs, structural hydrogeology

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Abstract:
The International Energy Agency (IEA) estimates that the upper kilometers of the earth's crust contain thermal energy that is several million times greater than the Earth's annual energy consumption. Heat increases as depth increases.

This presentation starts from the three basic premises: 1) Geothermal energy is generally considered ecological and does not cause significant amounts of pollution, 2) Geothermal reservoirs are naturally replenished and therefore renewable (resources cannot be depleted) and, 3) Excellent for generating electricity, heating and cooling - even small homes can benefit.
It is extended with the benefits and advances offered by the technology of the oil and gas industry to optimize operations and costs in the geothermal industry.

The oil and gas industry has a lot to offer the geothermal energy industry.
This operational competence, research and technology within the exploration, management and management of reservoirs, real-time monitoring, stimulation, drilling, completion and registration of wells and instrumentation, are transferred to the geothermal industry and thus optimize operations, costs and advance the opening early to production from geothermal fields and heat drainage from surrounding rocks.
Geothermal energy provides many businesses and opportunities for companies and service companies in the oil and gas sector, which in their activity plans already include in their energy portfolio, a high percentage of transition towards renewable energies including geothermal energy.

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Abstract:
As the world is heading to sustainability environment, gather and analyze valuable data to find and evealuate new getohermal projects has become relevant. In recent years many professionals have invested time and expertise to build tools that make us able to analyze and get professional results to evaluate Geothermal prospects. All these tools are spread in the web and it is important to have a summary than can give us a kick to start to use them. The intention of this paper is to classify, listed and describe this open source tools in a way it would be easier to choose the one could be useful in our work and to measure the complexity and subject needed to use them succesfully. GeothermalDataGenerator (a) pyodbc (b) GMT (c) Gppeval (d) OpenGeoSys (e) pygfunction (f)

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Abstract:
Down-the-hole (DTH) hammer drilling technology has been used successfully for many years in the mining as well as oil and gas industry to access hydrocarbon reservoirs [4]. Typical DTH hammers use a piston moved by hydraulic power or compressed air, applying alternating loads onto the drill bit to crush the rock at the borehole bottom and thus, eroding the rock [2]. Due to this alternating movement of the piston this technology is called hammer drilling. The internal components of any DTH hammer are called percussion system and include mainly the piston itself and a valve system controlling its movement. Currently, most DTH hammer systems have a maximum operating temperature of 150 C° and are limited to use compressed air or clean water as drilling fluid [5]. As a result, the wellbore control and cuttings transport are more difficult compared to conventional rotary drilling technologies using drill mud with tri-cone or PCD bits. Furthermore, the lifetime of hydraulic type DTH hammers depends much on the quality of the liquid being used, while the achievable rate of penetration (ROP) in hard rock formation is over 10 times higher compared to other rotary drilling technologies.
The more widespread use of DTH hammer technology towards geothermal reservoirs requires the use of more conventional type of drill mud with additvies, higher lifetime of the device and tool functionality at high-pressure and high-temperature (HPHT) conditions [1]. Therefor, a novel DTH hammer for deep reservoir exploration is being developed using a fluidic switch [3], instead of a mechanical valve system, and advanced surface coatings to increase tool lifetime.
The development of the novel prototype hammer is realized by experimental as well as numerical work (simulations), and iteratively optimizing each component of the percussion mechanism. For this purpose a numerical model of this mechanism is developed based on a mass-spring-damper approach. The model allows to evaluate the efficiency of numerous variations of the mechanism and, therefore, straightens the path of investigation.

Acknowledgement
The GEODRILL project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 815319.

Keywords: Drilling Technology, Down The Hole (DTH) hammer, Percussion System Modelling, Reservoir Exploration