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Understanding the natural fracture network is essential for geothermal-related investigations. However, the geometrical attributes depend on the scale of observation. Therefore, a multiscale characterization of the fracture network is essential to ensure that forward heat and flow simulations are based on stochastically generated discrete fracture network models representative of the natural fracture system observed. This was the goal of this work. Fracture data was collected from satellite imagery, outcrops and well cores to evaluate the scale effect and to study the impact of fracture size and density on the performance of engineered geothermal systems by numerical modeling. The numerical simulations highlighted that networks made of small fractures (0.08 to 27 m) tend to decrease the performance of the system compared to a network made of large fractures (22 to 1,437 m). However, thermal short-circuiting is easily reached in the latter scenario. Thus, the simulations suggest that the best-case network is made of fractures ranging between 1.57 to 135 m with fractures spaced by 5 m. This scenario provides the best compromise between heat extraction, water losses, hydraulic impedance and thermal drawdown. Despite the uncertainties, the fracture data used highlights the importance of multiscale fracture analysis for heat-flow simulations of geothermal reservoirs.

Deep geothermal energy represents an essential component of the future energy supply because the resources greatly exceed the demand, and the base load capability can compensate for temporal fluctuations in wind and solar power. By far, the largest amount of heat is contained in the crystalline basement, accessible almost everywhere through deep drilling. An interdisciplinary approach for a techno-economic resource assessment was applied to provide stakeholders with a more reliable basis for decision-making in the Northern Upper Rhine Graben. This approach incorporated data from various sources such as boreholes, outcrops, geophysical surveys, geomechanical models, and operating geothermal power plants. Emphasis was placed on resources tied to large-scale fault zones, exhibiting preferential hydraulic properties. Uncertainties in the calculations were quantified using Monte Carlo simulations. The resource base in the basement of the Northern Upper Rhine Graben is about 830 PWhth, of which about 8.2–16.1 PWhth could potentially be extracted with current technologies in enhanced geothermal systems. A comprehensive sensitivity analysis was carried out, examining the economic influence of nine parameters. Considering the reference power plant and the energy prices before the Ukraine war began in February 2022, the geothermal reserves are about 65% of the resources. However, given the massive recent increase in heat and electricity prices, a higher percentage is also possible. Finally, a socio-economic-environmental assessment shows that in the Northern Upper Rhine Graben, geothermal resources largely coincide with favorable conditions at the surface, making the region a preferred target for geothermal utilization.

Geothermal exploration concepts can be improved at an early stage of exploration by assessing the petrophysical rock properties of samples from surface outcrops. The results can be used to better evaluate the proposed geothermal resource and exploration strategies for the region. Our study provides fundamental knowledge on thermohydraulic rock properties to help identify the heat transfer mechanisms associated with geothermal resources in the extensional tectonic settings of the Ambilobe and Ambanja areas in North Madagascar. Thermostratigraphic units were defined according to the thermofacies concept. The geology of both areas is predominantly Quaternary volcanics (mainly basalt) and Permian–Triassic sediments (mainly shale and sandstone) overlying a faulted and fractured Proterozoic metamorphic basement of gneiss and amphibolite. Normal faults related to graben structures, along which hot springs occur, are characteristic of this extensional regime. Permian sandstone and mica schist have the highest porosities, with values up to 0.10–0.11. The matrix permeability of sedimentary and basement rock formations varies from less than 1.0 $$\times$$ 10–19 to 1.1 $$\times$$ 10–14 m2 (

Classification schemes for geothermal energy sources have recently been improved to include the concept of geothermal play. Our comprehensive review for the island of Madagascar aims to enhance the understanding of the factors controlling the country’s geothermal energy sources. The result is the first catalogue of geothermal systems in Madagascar. The geothermal systems of six prospective areas were categorized into three classes using the geological and tectonic context combined with the temperature of the potential geothermal reservoir. The three classes are (1) graben border-fault liquid-dominated moderate-temperature (Ambilobe and Miandrivazo areas), (2) fossil magmatic liquid-dominated moderate-temperature (Ambanja, Itasy and Antsirabe areas), and (3) sedimentary liquid-dominated low-temperature (Morondava Basin). The hydrothermal resources of Madagascar are commonly associated with extensional domains referred to as geothermal plays. A conceptual model was developed for each prospective area to better understand the factors controlling the heat source, formation permeability and fluid migration. Our results can be used to guide exploration strategies and have implications for assessing and quantifying Madagascar’s geothermal potential.

The crystalline basement of the Upper Rhine Graben presents an attractive target for deep geothermal projects due to its favourable temperatures and its high potential as a fractured and faulted reservoir system. It is already exploited at several sites, e.g. Soultz-sous-Forêts or Landau, and further projects are currently planned or under development. The crystalline units are furthermore the main source of radiogenic heat production and thus, together with the shallow Moho depth and convective heat transport along large fault zones, significantly contributing to the crustal temperature field. For these reasons, we developed the most detailed 3D geological model of the basement in the northern Upper Rhine Graben to date within the Interreg NWE DGE-ROLLOUT and Hesse 3D 2.0 projects. Due to the small number of very deep boreholes as well as seismic profiles reaching the basement beneath the locally more than 5 km thick sedimentary cover, we additionally used high-resolution magnetic and gravity datasets. In contrast to common deterministic modelling approaches, we performed a stochastic joint inversion of the geophysical data by applying a Monte Carlo Markov Chain algorithm. This method generates a large set of random but valid models, which enables a statistical evaluation of the results, e.g. concerning the model uncertainties. For a realistic attribution of the model, we used existing petrophysical databases of the region and measured the magnetic susceptibility of more than 430 rock samples. As a result of the inversion, high-resolution voxel models of the density and susceptibility distribution were generated, allowing conclusions about the composition and structure of the crystalline crust, which leads to a reduction of uncertainties and risks associated with deep geothermal drillings in the northern Upper Rhine Graben. Furthermore, our model will serve as a basis for realistic simulations of heat transport processes in the fractured basement and a meaningful assessment of the deep geothermal potential in the future.

Advances in new technologies and the desire to achieve a sustainable and safe energy supply, enable communities to transition from conventional to renewable resources, such as geothermal energy. Perception and acceptance amongst different audiences have a high impact on the feasibility of energy projects, which is an important aspect to analyze. For this reason, this study focuses on describing the level of awareness and acceptance of deep geothermal energy within an educated segment of the population in five European and American countries (Canada, Colombia, Chile, Belgium, and France) at different stages of geothermal development. This study was conducted through an online survey, which was targeted to post-secondary students and professionals. Some of the most significant conclusions are: 1) there is a high degree of awareness of geothermal energy among the respondents in Chile and Canada, a medium level in Belgium and France, and a low one in Colombia; 2) there is a favorable acceptance of a geothermal project in each country, even when hydraulic stimulation is considered; 3) environmental aspects and community safety are the most important issues that must be addressed to support a pilot geothermal project.

The Canadian off-grid communities heavily rely on fossil fuels. This unsustainable energetic framework needs to change, and deep geothermal energy can play an important role. However, limited data availability is one of the challenges to face when evaluating such resources in remote areas. Thus, a first-order assessment of the geothermal energy source is, therefore, needed to trigger interest for further development in northern communities. This is the scope of the present work. Shallow subsurface data and outcrop samples treated as subsurface analogs were used to infer the deep geothermal potential beneath the community of Kuujjuaq (Nunavik, Canada). 2D heat conduction models with time-varying upper boundary condition reproducing climate events were used to simulate the subsurface temperature distribution. The available thermal energy was inferred with the volume method. Monte Carlo-based sensitivity analyses were carried out to determine the main geological and technical uncertainties on the deep geothermal potential and risk analysis to forecast future energy production. The results obtained, although speculative, suggest that the old Canadian Shield beneath Kuujjuaq host potential to fulfill the community’s annual average heating demand of 37 GWh. Hence, deep geothermal energy can be a promising solution to support the energy transition of remote northern communities.

This paper deals with the design, acquisition and processing of vertical seismic profiling data collected in 2016 at the directional Thonex‐01 geothermal well, drilled in 1993 in the Geneva Canton (Switzerland). The aim of the study was to obtain additional information from existing‐abandoned well for downhole geophysical characterization of potential geothermal reservoirs in the Geneva area. The main results obtained by this study allowed an improved subsurface image by multi‐offset acquisitions near and around the deviated well. Specifically we were able to (1) improve the velocity model of the Geneva Basin, which, at present, is only constrained by one unique deep exploration well, located at the south‐west of the basin, far away from the study area; (2) image the transition between the Tertiary Molasse siliciclastic sediments and the Mesozoic carbonates; (3) define an acquisition strategy for future vertical seismic profiling surveys in support of the geothermal exploration campaigns in the area. This study has demonstrated the value of vertical seismic profiling method for the refinement of the subsurface geology at local scale, and its usefulness to assist the planning of second well associated with geothermal doublets.

The energetic framework of Canadian remote communities relies on fossil fuels. This has adverse environmental and energy security issues. In order to offset diesel consumption, the search for local, sustainable and carbon-free energy sources is of utmost importance. Unfortunately, in such remote regions, subsurface data to evaluate the geothermal potential is often nonexistent. This raises a key question: how to characterize geothermal resources associated to petrothermal systems based on surface data? Answering this question is the purpose of this work highlighting how outcrops can be used as deep subsurface analogues. The variability induced by laboratory methods to characterize thermophysical properties is further evaluated in the estimation of the present-day temperature at depth. The community of Kuujjuaq, Canada, is used as an example where guidelines are defined to evaluate the steady-state geotherm. Rock samples were collected and analyzed with a guarded heat flow meter and an optical scanner to determine thermal conductivity. Radiogenic elements concentration was evaluated with gamma-ray and mass spectrometry. 2D temperature models were built taking into account the regional geology and the results obtained from the different laboratory methods. A base-case temperature of 57–88 °C at 5 km is predicted below Kuujjuaq. This range is based on different methods used to evaluate both thermal conductivity and internal heat generation. The work conducted in Kuujjuaq shows that the combination of gamma-ray spectrometry and optical scanning gives lower base-case temperature predictions when compared to mass spectrometry combined with the guarded heat flow meter. Despite the nonexistence of deep temperature measurements in northern regions, the assessment of thermophysical properties from outcrops is shown to be a useful tool for a preliminary assessment of geothermal resources in remote areas facing critical energy issues.

As many ocean basaltic volcanoes worldwide, Piton des Neiges (Reunion Island) hosts a hydrothermal system that represents a potential geothermal resource. Despite several prospection campaigns over the past decades, this potential has never been confirmed because many aspects of fluid supply and circulation remain unclear. To track the origin and fate of hydrothermal fluids, we analyzed the geochemistry of thermal springs (water and gas), by combining conventional tracers (major-trace elements and O, H, C, Sr, Li isotopes) with Cl isotopes, a geochemical tool under development. Added to literature data, our new results allow us to compare the composition of thermal springs with cold waters, rocks, gas, and fumarolic deposits from La Reunion and elsewhere, and to untwine the complex history of fluid mixing that constitutes the geothermal system of Piton des Neiges. delta O-18(H2O) and delta D-H2O values of springs (-8.67 to -4.9 and -56.3 to -23.5 parts per thousand VSMOW, respectively) are positioned close to the local meteoric water line, showing that thermal waters are of meteoric origin. However, their depletion in heavy isotopes relative to cold waters suggests a recharge strongly influenced by cyclones, with a possible contribution from high altitude (>= 2000m) rainfalls. Their absence of O-18 enrichment relative to the local meteoric water line indicates that the isotopic exchange between rocks and water is very limited, and therefore suggests that their deep temperatures are relatively low and/or the water/rock ratios are high. delta C-13 values in thermal waters (-8.0 to +3.2 parts per thousand PDB) and gases (-6.7 to -5.3 parts per thousand PDB) confirm that carbon is essentially of magmatic origin, which we interpret as supplied by regional degassing of La Reunion hotspot. Major-trace elements and Sr-87/Sr-86 (0.704142 to 0.704336), delta Li-7 (+2 parts per thousand and +34.8 parts per thousand LSVEC), and delta Cl-37(-0.35 parts per thousand and +0.40 parts per thousand SMOC) compositions show that three additional processes contribute to the mineralization of thermal waters: 1) interaction with basalt at temperatures >= 50 degrees C, 2) very limited seawater contamination (< 3 mol parts per thousand), and/or 3) leaching of trachyte or scrubbing of fumarolic gas or condensates emitted by a shallow trachytic magma in the volcanic edifice. The geothermal system of Piton des Neiges is thus the result of meteoric water interaction with a trachytic heat source (as a solidified intrusion or a magma pocket), unrelated to the CO2 flux. We also emphasize that thermal waters at Piton des Neiges are exceptionally isolated from seawater compared to other geothermal systems in ocean island volcanoes worldwide, which implies a heat source located above the seawater/freshwater interface. These inferences are of critical importance for the future geothermal exploration on the island, as they suggest that the geothermal resource is within reach of relatively shallow drilling depths (

The crystalline basement of the Upper Rhine Graben is a major target for deep geothermal exploration due to the generally high reservoir temperatures and the increased radiogenic heat production. The geothermal potential is strongly dependent on the lithology because mainly the thermal but also the hydraulic properties are affected by the rock type. For this reason, the so far most detailed 3D model of the basement in the northern Upper Rhine Graben was developed based on existing structural models, in particular the Hesse 3D 2.0 and GeORG models. Since only a few boreholes fully penetrate the thick sediment cover, additional magnetic and gravity data provided valuable information on the geometry of the deep horizons. To interpret the Bouguer anomalies reasonably with respect to the crystalline basement, the regional gravity field and sedimentary effect were subtracted from the observed data. In comparison to the commonly applied deterministic modelling approaches, a stochastic joint inversion of the gravity and magnetic anomalies was performed that utilizes the principles of a Monte-Carlo-Markov-Chain simulation. For an appropriate attribution of the model units, existing petrophysical databases of the region have been used and in addition, the magnetic susceptibility of more than 430 rock samples was measured. High-resolution voxel models of the density and susceptibility distribution were generated as a result of the inversion, which allow conclusions about the crustal composition under the sedimentary formations. An interpretative map of the basement geology, derived from the inversion results, is presented.

Assessing the geothermal potential in remote northern regions is challenging. First, due to remoteness and, second, due to data gaps. Often, Earth heat flux is inferred from deep equilibrium temperature profiles. However, in remote areas where deep boreholes are nonexistent, an adapted methodology is needed. Therefore, this study presents a new versatile and reliable numerical approach that simulates climate events and infers heat flux from a shallow temperature profile (80 m depth). Thus, addressing the challenge linked with heat flux assessment in northern regions and offering an original contribution to advance geothermal research in remote areas facing energy development challenges.

Defining the fracture network and the active fluid pathways are fundamental to characterizing a fractured reservoir. With this goal, we chose the Saint Pierre Bois quarry (Vosges Massif, France) as a natural analogue for the Permian-Stephanian cover (arkoses)/Hercynian basement (granite) transition that serve as geothermal reservoirs, for example at Rittershoffen, Landau and Insheim, where geothermal plants exploit this kind of reservoir within the Upper Rhine Graben (URG). During our study, along walls of the quarry, we measured fracture orientation on multiple 1D scanlines, sampled fracture fills and associated wall rock, and measured porosity. Our interpretation of the fracture dataset and associated mineral deposits and alteration indicates a large unmapped fault zone (displacement around 10m) we infer to cross the quarry. This fracture zone comprises a core zone (>50 m thick), which is the most highly altered-fractured zone in the quarry. In the transition zone (20–25m thick), some E-W faults cross and displace the arkose/granite transition. The two major fracture sets (E-W and NNE-SSW strikes) are present in the damage zone (>100m thickness) and are typically randomly distributed, but sometimes are grouped in clusters (defined by high spacing coefficient of variation). The E-W fractures and faults are the result of local tectonic inheritance in relation to the Ville basin formation at the end of the Hercynian orogen. Based on rock alteration, these structures constitute good pathways for fluid, first of all for the meteoric fluid during the E-W fault zone creation associated to the Ville basin opening, and then for the hot brine, which clogged the system by quartz precipitation during the burial of the basin.

International audience; In order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate many customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120°C and 150°C. In a normal geothermal gradient (around 30°C/km) context, this temperature range is reached between 3 and 5 km depth and corresponds to the deep layers of sedimentary basins (aquifers more or less permeable, namely HSA) and the upper part of the basement, more or less fractured and altered, having potentially used EGS technologies. In such complex and diverse geological setting, we propose to set up and validate an integrated and scale-dependent exploration workflow that consists in building, step by step, more and more refined subsurface models based on larger scale models To achieve this, three questions arise:-What are we looking for? What are the key parameters and the key situations favorable for geothermal heat/fluid extraction? In order to optimize the exploitation, the three essential parameters to know are obviously temperature (the highest at the shallowest depth), the natural permeability (both from porosity and fractures), and the presence of brines and recharge waters. These key parameters are related to the geological patterns and subsurface phenomena, which are called key situations and constrain spatially the favorable areas.-Which methods are able to identify these key situations at the different scales? Standard exploration methods (e.g. active/passive seismics, electromagnetics, geothermometry and geology) have been tested on the specific case of basement and deep sedimentary layers to validate which techniques are relevant to identify key situations. Whereas some of these can be very capital-intensive (e.g. active seismics), the economic benefit to the project through its geological risk reduction can easily compensate for the upfront investment.

International audience; Within the frame of the ANR-funded CANTARE-Alsace project, we have undertaken a multi-scale and multidisciplinary approach to increase our knowledge of the transition zone between the sedimentary cover and the basement and provide fundamental knowledge for the assessment of its geothermal potential. In this paper, we report out the results of a study performed on an exhumed transition zone in the Ringelbach area in the Vosges Mountains, on the flank of the Rhine graben. In this analogue of a deeply buried transition zone of the Rhine Graben, a thin layer of Triassic sandstones is still present on the top of the fractured and altered granitic basement providing the opportunity to study in-situ the physical properties of this transition zone. In this paper, we focused on electrical and acoustic properties of the transition zone as they are the main physical parameters usually assessed with the help of geophysical methods during the exploration phase of a geothermal project.

International audience; Geothermal fluids are extracted through deep wells drilled in a geological reservoir, which can be represented by highly heterogeneous volcanic complex, sedimentary basins or deep basement rocks. The main objective of this project was to propose suitable methodologies and techniques for the characterization and modeling of fractured geothermal reservoirs, to ensure their sustainable exploitation and, therefore, ensure acceptation of this kind of energy by local communities. This goal was achieved with experiences from the volcanic complex of the Nevado del Ruiz (Colombia), the St. Lawrence Lowlands sedimentary basin (Quebec, Canada), the Carboniferous limestone reservoir in the area of Mons (Belgium), the Soultz-Sous-Forets power plant and the Ploemeur fractured rock hydrogeological experimental site (France), and various areas of interest for geothermal exploration in Chile. The work conducted provided new insights into the construction of conceptual and numerical models for geothermal reservoirs and allowed the creation of a solid research network between Europe and Americas, fostering participation in geosciences of young researchers, students, and women.

International audience; In this paper, we focus on paleocirculation at the Hercynian basement/sedimentary cover interface in the tectonic environment of the Upper Rhine graben. The goal is to increase our understanding of the behavior of the fracture-fault network and the origin of the hydrothermal fluids. We studied orientations, mineral fillings, and fluid origins of fractures that crosscut the Hercynian granitic basement and the Permo-Triassic formations in relation to the major tectonic events. Because the Mesozoic formations and the Hercynian basement on the graben flanks and inside the graben do not have the same evolution after uplift, our study includes 20 outcrops on both graben flanks and cores of the Soultz-sous-Forêts geothermal wells located inside the graben. The Hercynian granitic basement and Permo-Triassic formations were affected by several brittle phases associated with fluid circulation pulses related to graben formation during the Tertiary. We distinguished at least four stages: (1) reactivation of Hercynian structures associated with pre-rift tectonics during the early Eocene and descending meteoric waters, characterized by shearing/cataclasis textures and precipitation of illite and microquartz; (2) initiation of convective circulation of deep hot brines mixed with descending meteoric waters at the Hercynian basement/sedimentary cover interface during this first stage of Eocene rifting, characterized by dolomite and barite fillings in reactivated Hercynian fractures; (3) N-S tension fractures associated with rift tectonics just prior to uplift of the graben shoulders during Oligocene extension and descending meteoric waters, characterized by cataclastic textures and precipitation of quartz, illite, hematite, and barite; and (4) current convective circulation of deep hot brines mixed with descending meteoric waters at the Hercynian basement/sedimentary cover interface, characterized by calcite and barite fillings within the graben. This convective circulation is today present in deep geothermal wells in the western part of the Rhine graben.

International audience; Geothermal energy exploitation in the Upper Rhine Graben currently targets high-temperature anomalies in the crystalline Paleozoic basement at depths up to 5 km. However, at certain locations (e.g. Rittershoffen, France), geothermal installations are actively targeting resources at shallower depths where the Paleozoic granite transitions into the overlying Permo-Triassic sandstones. We here investigate the variation in physical properties—including porosity, P-wave velocity, permeability, uniaxial compressive strength, and thermal properties—in rocks that locally extend across the Paleozoic–Permo-Triassic boundary in four locations to the west of the Upper Rhine Graben. The lithologies investigated include sandstones, breccia, granites, metagranites, dolomite, and altered and unaltered volcanic rocks and represent the variety of lithologies at this transition. We note that while the porosity, permeability, thermal conductivity, and P-wave velocity of the Permo-Triassic sedimentary cover and Paleozoic crystalline basement rocks are consistent with values determined for rocks from exploratory and production boreholes at Soultz-sous-Forêts (France), the other lithologies (belonging to neither the sedimentary nor basement sequences) are conspicuously lower in porosity and permeability. Further, the attendant strength of these other lithologies could make them relatively unamenable to fracturing, reducing the possibility of fracture-controlled permeability in these units. Indeed, we conclude that in areas where these low-permeability and high-strength rocks act to cap the crystalline basement, hydrothermal convection may be curtailed and geothermal exploitation may be rendered untenable.

Face a une demande croissante en electricite et a une necessite de reduire les emissions de gaz a effet de serre, plusieurs pays envisagent le developpement des energies renouvelables. L’energie geothermique est une alternative particulierement interessante puisqu’elle permet une production de base qui n’est pas influencee par les conditions climatiques et dont l’empreinte au sol est relativement restreinte comparee, par exemple, a celle d’une centrale hydroelectrique. La revue de litterature presentee ici resume les principales caracteristiques du potentiel geothermique au Quebec, dans les bassins sedimentaires, et en Colombie, en particulier au niveau du complexe volcanique Nevado del Ruiz. Actuellement, dans ces deux nations, l’energie hydraulique domine la production d’electricite, mais il y a un interet semblable envers le developpement de centrales geothermiques. Les sites francais de Soultz-sous- Forets en Alsace et de Bouillante en Guadeloupe sont presentes respectivement comme exemple d’exploitation des systemes geothermiques ameliores (Enhanced Geothermal System; EGS) et des ressources en regions volcaniques. Le premier site constitue un modele pour un futur developpement de la geothermie profonde au Quebec, tandis que le deuxieme est un exemple pour la Colombie. Une description des impacts environnementaux lies a l’exploitation des ressources geothermiques profondes complete ce portrait. Abstract Because of an increasing demand in electricity and a necessity of reducing greenhouse gas emissions, several countries envisage the development of the renewable energies. The geothermal energy is a particularly interesting alternative because it allows a production of electricity which is not influenced by weather conditions and it requires relatively restricted surface areas compared, for example, to the area required by a hydroelectric power plant. The literature review presented here summarizes the main characteristics of the geothermal potential in Quebec, in sedimentary basins, and in Colombia, in the area of the Nevado del Ruiz volcanic complex. Currently, in these two regions, the hydro-electric power dominates the electricity production, but there is a similar interest to the development of geothermal power plants. The French sites of Soultz-sous-Forets in Alsace and Boiling in Guadeloupe are respectively presented as an example of exploitation of geothermal improved systems (Enhanced Geothermal System; EGS) and geothermal resources in volcanic regions. The first site constitutes a model for the future development of the deep geothermal exploitation in Quebec, whereas the second is an example for Colombia. A description of environmental impacts related to the exploitation of deep geothermal resources is presented at the end of this paper.

International audience; This paper is the first to present the main geochemical characteristics of the native brines collected from all the geothermal wells penetrating the granite basement underlying the sedimentary cover, in the Upper Rhine Graben. These deep wells (from 2580 to 5000 m) were only drilled in four geothermal sites (Soultz-sous-Forêts and Rittershoffen in France; Landau and Insheim in Germany). The Na-Cl geothermal brine samples collected from the granite returned TDS values ranging from 99 to 107 g/l with pH values close to 5, along with Cl and Br concentrations and delta D, delta 18O and delta 34S values that indicate a multiple origin with mixing between primary brine formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and dilute meteoric water, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to the Oligocene. Chemical, isotopic and gas geothermometers indicate concordant reservoir temperatures close to 225 ± 25°C for all the fluids, even though the maximum temperature so far measured on site is 200°C. An exhaustive literature review has indicated that only the geothermal brine from the deep Cronenbourg well (2870 m) ending in the Buntsandstein has similar chemical and isotopic compositions (apart from Br and Ca) to the fluids from the granite, with an identical estimation of reservoir temperature from geothermometry. Geothermal brine from the deep Bruchsal well (2540 m), drilled down to the junction of the Buntsandstein and the Saxo-Thuringian formations, has a higher TDS value (120-125 g/l) with its chemical and isotopic compositions giving a lower estimation of reservoir temperature (190 ± 25°C). By contrast, geothermal brine from the Bühl well (2655 m) ending in the Buntsandstein has an even higher TDS value of about 201 g/l and a lower temperature-at-depth estimation of 110 ± 25°C, close to the temperature measured on site (115°C). The above results indicate that the geothermal fluids collected from the granite probably originate from Triassic sedimentary formations located at great depth (≥4 km) with temperatures close to 225 ± 25°C in the centre of the Rhine Graben, but that their different TDS and Cl/Br values reflect the presence of several distinct geothermal reservoirs. Many discrepancies due to high-temperature water-rock interactions are revealed on comparing the chemical and isotopic compositions of the hot brines with those of cooler brines from Bühl and the Landau Eocene-Oligocene oilfield wells, among others. The hottest brines are much enriched in K, Ca, SiO2, Li, Rb, Cs, As, Sr, Ba, Mn, Nd, U and in metals such as Zn, Pb, Cu, Co, Cd, Sb, but are depleted mainly in Mg, SO4 and B and have much lower isotopic Li and B signatures. The He isotopic signatures of the gases associated with these fluids (R/Ratm. = 0.128 at Bruchsal and 0.252 at Insheim) confirm that the thermal anomalies are mainly crustal and not mantle-derived (1.46% and 2.88% of mantellic He, respectively, for the two sites). Thus it is concluded that the thermal anomalies are associated mainly with the convective circulation of hot fluids along probable NE-SW faults between the graben’s deep sedimentary centre and the fractured granite basement at its edges. Moreover, the western part of the Upper Rhine Graben (the Landau, Insheim, Soultz, Rittershoffen and Cronenbourg sites) seems to be hotter than the eastern part (Bruchsal and Bühl). According to the U-Th isotope system, the minimum transit time of these deep geothermal brines would be about 1000 years.

International audience; In order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120°C and 150°C. In this context, we propose to set up and validate an integrated exploration workflow that consists in building, step by step, models based on larger scale models and key situations identifiable by innovative and validated exploration methods. Therefore, three questions arise:-What are we looking for? What are the key parameters and the key situations that allow withdrawing the geothermal fluid?-Which methods are able to identify these key situations at the different scales?-How used these methods to find the best drilling place and reduce the financial risk of drilling? At the base, a geological and structural model has been done from public data and improves from geophysical and geological analysis. Thermal, Hydraulic and Mechanical models, independent or partially coupled, are built based on this geological model and from other data like temperature, fluid geochemical and stress data. Three scales have been considered in this pre-drill exploration workflow: continental scale (around 1000kmx1000km) considers temperature, heat flow and stress field that constrain regional scale; the regional scale (50 to 100km) covers a geological region considered as relevant; the local scale (10 to 25km) corresponds to the focus which is made at the last step. The way between the different scales is done by the analysis of the key situations defining the geothermal location. Finally, at the smaller scale, the best geothermal location is refined in terms of risk and cost for the first exploration well.

International audience; In order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120 • C and 150 • C. In a normal geothermal gradient (around 30 • C/km) context, this temperature range is reached between 3 and 5 km depth and corresponds to the deep layers of sedimentary basins (aquifers more or less permeable, namely HSA) and the upper part of the basement, more or less fractured and altered, having potentially used EGS technologies. In this context, we propose to set up and validate an integrated exploration workflow that consists in building, step by step, models based on larger scale models and key situations identifiable by innovative and validated exploration methods. Therefore, three questions arise:-What are we looking for? What are the key parameters and the key situations that allow withdrawing the geothermal fluid?-Which methods are able to identify these key situations at the different scales?-How used these methods to find the best drilling place and reduce the financial risk of drilling? At the base, a geological and structural model has been done from public data and improves from geophysical and geological analysis. Thermal, Hydraulic and Mechanical models, independent or partially coupled, are built based on this geological model and from other data like temperature, fluid geochemical and stress data. Three scales have been considered in this pre-drill exploration workflow: continental scale (around 1000kmx1000km) considers temperature, heat flow and stress field that constrain regional scale; the regional scale (50 to 100km) covers a geological region considered as relevant; the local scale (10 to 25km) corresponds to the focus which is made at the last step. The way between the different scales is done by the analysis of the key situations defining the geothermal location. Finally, at the smaller scale, the best geothermal location is refined in terms of risk and cost for the first exploration well.

International audience; The Rhine Graben is a major site of development for the geothermal heating production in France. Targeted geothermal reservoirs are in deep Hercynian granitic basement which is fractured dominated system, and more recently at the cover/basement interface.In this framework of geothermal exploration, a better understanding of the hydraulic behaviour of the fracture network and fluid/rock interactions is needed. For that fracture fillings inHercynian granitic basement and in the formations of the cover (Permian rhyolites, Permian and Triassic sediments) were studied for mineralogy, fluid inclusion microthermometry and (C, O, Sr) isotopes in order to trace paleocirculations at the cover/granite interface in the Rhine Graben. Data were acquired on fracture fillings in samples of the basement/cover interfaces from the EPS1 borehole at Soultz-sous-Forêtin the Rhine grabenat ~ 1417 meters depth, and from outcrops in quarries on the flanks of the graben (Waldhambach, Saint Pierre Bois, Windstein, Heidelberg...). Mineral sequences of polyphasedfillings were interpreted in relation with the geological contextincludinglate evolution of the Hercynian basement and major extensive tectonic events. Quartz, carbonates, sulfates and illiteare major minerals identified in fractures crosscutting Hercynian granites, Permian rhyolite (Waldhambach) and Permian and Triassic sedimentary cover. Although quartz being considered as a major mineral filling fractures, petrological observations showed that carbonates are also an important and probably underestimated phase of filling, and of interest for two reasons. Firstly, from a geothermal point of view, they contribute to the clogging of fractures. Secondly, from a scientific point of view, they are informative on the variations of fluid chemistry through geological times. Among carbonates, dominant dolomite with minor ankerite, Mn-bearing carbonates and siderite was identified by CL, SEM and EPMA in fractures. A same generation of dolomite was identified in fractures crosscutting Hercynian granite and in Permian and Triassic sandstones of the Rhine Graben (EPS1 borehole) but also on the flank of the Rhine Graben in Hercynian granite of Waldhambach, Heidelberg and Windstein quarries, and in Permian rhyolite at Waldhambach. This dolomite is Fe and Mn poor, formed at ~120-130°C, and has a 87Sr/86Sr of ~0.708-0.709. Barite is the major sulphate observed in fracture filling, already formed later than dolomite. Rare microthermometric data combined with strontium isotopes provide evidence of several generations of barite with one generation formed at ~130°C. Preliminary data on dolomite provide evidence of large fluid circulations at the cover/granite interface.This dolomite is observed at least at depth down to 1650 m of the granite in the EPS1 borehole, i.e. with a minimum penetration of 200 m into the granite. Alternating deposition of ankerite and dolomite in fracture corridors strongly suggest pulses of fluids. Such fracture fillings at 1641 meters depth were attributed to present-day fluid circulations. However similar generation of dolomite also observed in fracture corridorcrosscutting the Hercyniangranite and Permian rhyolite at Waldhambachon the flank of the Rhine graben demonstrate that the fluid circulations

The permeability of the granite geothermal reservoir of Soultz is primarily related to major fracture zones, which, in turn, are connected to dense networks of small-scale fractures. The small-scale fractures are nearly vertical and the major direction is about N0°E. This direction differs from that of the Rhine Graben, which is about N20°E to N45°E in northern Alsace. A total of 39 fracture zones, with a general strike of N160°E, have been identified in six wells between 1400 and 5000 m depth. These fracture zones are spatially concentrated in three clusters. The upper cluster at 1800–2000 m True Vertical Depth (TVD) is highly permeable. At 3000–3400 m TVD, the intermediate cluster in composed of a dense network developed in an altered matrix and forms the upper reservoir. In the lower part of the wells, the deeper cluster appears as a fractured reservoir developed within a low permeable matrix. Fracture zones represent a key element to take into account for predicting the geothermal reservoir life time submitted to various thermo-hydromechanical and chemical processes generated by hydraulic or chemical stimulations and by hydraulic circulation tests related to long-term exploitation.

A contribution to the definition of the structural pattern of the Soultz-sous-Forets EGS (Enhanced Geothermal System) is presented here. After reprocessing, the PHN84J seismic reflection profile highlights the tilted blocks of the Merkwiller-Pechelbronn oilfield. In the Soultz-sous-Forets horst, complex fault patterns are observed: the Hermerswiller normal fault flattens at depth and is rooted in decollements occurring in Triassic salt or clay series, while other steep normal faults affect underlying sedimentary formations and basement. Some methods for the exploitation of a seismic diffraction recorded by multi-component Vertical Seismic Profiling (VSP) are also illustrated to locate the diffractor without specific data processing. Polarisation and travel time analysis of a diffraction event recorded in the GPK1 borehole are analysed, and its exploitation combined with seismic reflection helps defining a tilted block geometry.

International audience; In the Upper Rhine Graben, geothermal projects are strongly under development, particularly for the exploitation of fluid in the deeper part of the basin.This type of reservoirs constitutes a fractured dominated system. However, the hydraulic behaviour of the fracture network is poorly known and its knowledge constitutes an important way to better target the exploration works. Combining data from structural analysis, mineralogical analysis of fracture filling from outcrops and drilling cores and fluid geochemical analysis from deep boreholes, we propose a new model of fluid circulation in the deeper part of the Rhine Graben.The present-day fluids collected from the basement and the Buntsanstein sandstones in the deepest wells have the same originand are totally different to those from Tertiary sediments. These deep fluidshave reacted with deep sedimentary rocks at about 4 or 5 km depth before migrate through the graben and probably diluted at small proportions with meteoritic freshwaters.These fluids circulate through a fracture network constitutes by different sets linked to the tectonic history of the Rhine Graben. In the basement, the Hercynian fractures, oriented NE-SW and NW-SE, show two successive stages of fillingsshowing two fluid circulation pulses: 1) asheared/cataclased phase associated with illite and quartz, which is prior to the graben opening;2) precipitation of dolomite in tension fractureslinked to theearly stage of the graben formation.An other main direction fracture set is present in the basement, probably reactivated, and in the Permo-Triassic sandstones with a N-S direction. On outcrops, this fracture set have no macroscopic mineralogical fillings, but in boreholes and for some large shear zone in the sandstones, a cataclasis phase associated to quartz and illite, and radial illite in tension fracture in the granite, and barite in the Buntsandstein sanstonesis present.This study suggests that the reactivation of old Hercynian structures, oriented NE-SW and NW-SE, in relation with the Tertiary tectonic history of the graben formation develops the fluid/basement interaction in deep temperature conditions. In the contrary, the N-S large structures favor recent circulation system and rather constitute a recharge drain

The petrographic model for the deep-seated geology at the Soultz-sous-Forets Enhanced Geothermal System (EGS) site in the Rhine Graben, France, was revised on the basis of data from 5 km-deep wells. Several types of granite and sub-facies were characterized through thin-section analysis of drill cuttings. Various datalogs were studied to correlate them with petrography. The new geological model of the Soultz site is made up of a massive porphyritic granite that shows some paleo-weathering at its top, overlain by about 1400 m of Mesozoic and Tertiary sediments. Between 2700 and 3200 m depth, the same granite is highly fractured and hydrothermally altered. Below this zone, the granite is less fractured and is very rich in biotite and amphibole. Starting at 4700 m depth is a younger, fine-grained, two-mica and amphibole-rich granite that intrudes into the porphyritic granite.

Background This study aims on investigating the regional flow field of the Soultz and adjacent geothermal fields located on the western side of the central Upper Rhine Graben and thus to provide insight into the origin of the 70% of the geothermal fluid coming from the regional inflow in the deep reservoir of the Soultz site. In an integrative approach, we consolidate conceptual models on fluid flow in the central Upper Rhine Graben. Methods Based on a 3D geological model and a new 3D temperature interpolation, we tackle the relation between tectonic structures and the occurrence of advection/convection along favourably oriented fault zones. Using sequential Butterworth filters, we study the distribution of negative residual anomalies in a pseudo-tomography down to a depth of about 6 to 8 km. Results We derived N-S-striking V-shaped negative anomalies that are consistent with the orientation of fault zones revealing major temperature anomalies to their east. Conclusions Following the concept of negative anomalies revealing zones of increased fracture porosity, and in agreement with fluid-chemistry, our findings suggest infiltration of meteoric water through the graben boundary fault and along preferential flow pathways that merge at intermediate depth. Up-flow of thermal water mixed most likely with brine from the deeper eastern part of the graben occurs along W-dipping typically rather steep structures.

The geothermal reservoir at Soultz-sous-Forets is targeted in a granite horst bounded by subvertical normal faults. Exploration and production results tend to show that the Buntsandstein formation, made of inter-bedded sandstones and argillite in the Rhine Graben, above the granitic basement, may also have geothermal potential. Indeed, temperature gradient anomalies observed in the Buntsandstein indicate that the Buntsandstein could be exploited to produce heat or even electricity. In order to determine the fluid flow pattern within the sandstone formation, we compare two temperature gradient logs: on the one hand, the observed gradient log derived from temperatures measured along a borehole, on the other hand, a calculated one based on thermal conductivities measured on borehole cores. The observe temperatures gradients anomalies, could be explained by fluid flow patterns, thus phenomenological study has been carried out using TOUGH2 to simulate fluid flow and heat transfer for various boundary conditions. These results allow building a conceptual model (980-1400 m) of the sedimentary reservoir above the granitic basement at Soultz-sous-Foret.

Microfractures appearing in thin section as fluid inclusion trails in quartz crystals were studied in four core samples of Soultz-sous-Forets granite. Their orientations in four series of three mutually perpendicular thin sections were estimated using a previously described apparent dip method and a new method involving measurements of strike and apparent dips. Three samples display three microfracture sets and one sample displays two sets. In all samples, one set is nearly vertical and strikes N–S. In two samples, one and two sets are nearly vertical and strike E–W. In two samples, two sets strike NW–SE: one is vertical, the other dips gently to the NE (or SW). Comparing microfracture and mesofractures sets in the same cores shows that (1) the N–S microfracture set is always dominant at both scales and (2) all other microfracture sets have no mesoscopic counterpart. The N–S microfracture sets could have been created during E–W extension of earliest Cenozoic age (Rhine Graben rifting). Differences between the two scales are explained by a σ1/σ2 switching which occurred at the crystal scale and generated mutually perpendicular cracks.

International audience; In the Upper Rhine Graben, geothermal projects are strongly under development, particularly for the exploitation of fluid within the top of the basement. This type of reservoirs constitutes a fractured dominated system. However, the hydraulic behaviour of the fracture network is poorly known, such as connexion, main fluid flow direction, role of the fault zone versus minor fractures, the deformation types (cataclase, fault, crack...). We propose two approaches to understand the fluid flow for a better geothermal exploitation in the Upper Rhine Graben. The first approach is to study the fillings of the fracture which plug partially or totally the current fluid flow. The second approach is to determine the nature of the palæo-circulations by studying fracture fillings, their textures and establish mineral sequence of fillings in relation to the tectonic phases and the geological history of the basement. And finally we compare chemistry of palæo-fluids with those of present-day fluids which flow in the deeper part of the Upper Rhine Graben. In this work, we propose a combined structural and mineralogical analysis of the fractures and their fillings based on samples collected on the EPS1 Soultz cores and outcrops on the both flanks of the Upper Rhine Graben. These mineralogical fillings have been sampled within fractures identified by measuring their orientation and defining their relative age. Petrological study (Optical microscope, Scanning Electron Microscope noted SEM, cathodoluminescence noted CL) completed by chemical analyses (Electron Microprobe Analyses, noted EPMA) have been performed in order to determine the filling minerals and their relationships. Among filling minerals, quartz, carbonates and illite are the major ones. Different types and generations of carbonates (calcite, dolomite, ankerite, Mn-bearing carbonates and siderite) have been highlighted at the scale of the major fracture zones in the upper part of granite, using CL and EPMA. Combined mineralogical, textural and structural data allow establishing a chronology of the palæo-circulations relative to the tectonic phases and the geological history of the basement. We identify the major phases which plug the fractures and, consequently, the set of fractures which remain more permeable to the current fluid flow.

International audience; Two different approaches have been employed to investigate the localization of fault zones and natural permeability at regional and local levels in extensional tectonic settings with potential for EGS technology. Regional distribution of potential permeable fault zones has been analyzed dynamically using a new 2D geodynamic model. Numerical simulation of instability of a viscoelastic-plastic lithosphere require a lower crustal viscosity in the order of 5*1021 to 1022 Pa s to match the deformation pattern of the Upper Rhine Graben (URG). Models clearly show a localization of deformation at regional level, but do not explain the internal fault distribution on the horst level as it can be seen at the Soultz. This has been investigated further using geophysical means. The high-resolution 3D structural regional model of the Soultz area is used as a basis for new geophysical interpretation and inversion of gravimetric and magnetic data. Several gravimetric and magnetic anomalies can be linked to thermal anomalies, which in general the Upper Rhine Graben are known to be linked to free convection and thus, linked to porosity and permeability changes.

A method for localization of hot fluid circulation zones is proposed, based on thermal gradients analysis. Thermal gradients can be calculated, with the knowledge of heat flow density and thermal conductivity measurements, using the Fourier's law. They co

Fluid circulation at fractured basement/cover unconformity is of first importance for geothermal energy exploitation especially in continental rifting as the Upper Rhine Graben. This unconformity presents lot of natural fractures which constitute a large permeable fracture network driving brine movement. That constitutes a reservoir of hot water, which is exploited by several geothermal power projects at great depth with flow rate between 50 and 80 l/s of water produced. On the French territory of the Upper Rhine Graben, about 6.800 kilometres of seismic reflection have been acquired since 1954 to the 80's for the petroleum exploration. Raw seismic data which were recovered from oil and seismic companies were retreated by modern processing techniques. However, as the acquisition parameters of these old seismic profiles aimed to better image shallow petroleum reservoir from Tertiary to Jurassic sediments, the deepest levels were not imaged properly. In the Rhine Graben, the base of the basin is constituted by lower Triassic Buntsandstein and locally by Permian sediments. These both levels are clastic sediments yielded by the degradation of the hercynian massif and are constituted by fluvial deposits in an arid climatic environment (red sandstones) (Bourquin et al., 2007). These levels could not be distinguished from each other on seismic profiles because they have similar origin and composition. They overlain the Palaeozoic basement constituted by different granites, gneiss and metamorphic rocks, volcanic and volcano-sedimentary rocks (Edel and Fluck, 1989). Some seismic profiles, acquired in 70's and 80's, have been recently retreated to improve the seismic images, particularly the continuity of the horizontal reflectors, faults and deep zones (Capar et al., 2009). On these seismic profiles, the basement/cover unconformity has been mapped systematically in the northern part of the Rhine Graben, from Wissembourg to Selestat. However, few boreholes are deep enough to identify with certainty the reflector corresponding to the basement/cover unconformity. Moreover, , we tried to find a relation between the signature of the seismic reflector of the basement/cover unconformity and the nature of the underlying basement. It appears low magnetic rocks, such as granites, or Palaeozoic sediments, show a more continuous reflector than high magnetic rocks like gabbro, diorites, ... Three recent seismic profiles have been retreated with a different signal process, the post stack time migration or PSTM, in order to improve the image of faults and their dip. The interpretation of these seismic profiles shows two types of faults, which cross-cut the basement /cover unconformity: 1- Major faults, which cross-cut the whole infill basin, except the Quaternary, and extend down-ward in the basement. Two different structural features have been identified: normal faults are bound to the early stage of the graben formation and negative flower structures, which reactive older fault systems in transtension during the recent tectonic stage. 2- Normal faults, which affect the top of the basement, the Buntsandstein and locally the Muschelkalk. This study shows that the base of the basin is highly affected by large-scale fractures and constitutes a good target for geothermal projects. However, former petroleum seismic profiles, even if they are retreated, are not really appropriate to explore in detail the deep-seated basin of the Rhine Graben (detail mapping of the top of basement, trend of the deep-deated fractures...). Thus, in the framework of deep geothermal exploration, dedicated acquisition of 2D or 3D seismic surveys would provide a better image of the underground and would contribute strongly to minimize the risks of future projects.

International audience; In a perspective of geothermal exploration in graben systems, we analyse the recently built consistent structural model of Soultz area (Dezayes et al., this issue) in order to understand the links between the regional graben fault system, the tectonic history of the Upper Rhine Graben (URG) and the local reservoir features (a low permeable naturally fractured granite). The previous geological models of the area lack constraints concerning the geometry of the structures at the regional scale. To overcome this situation, the new 3D regional geological model of the Soultz area is based on a set of 2D seismic profiles (including recently reprocessed seismic lines) as well as former structural interpretations for oil exploration in the 70-80's and numerous deep wells (Dezayes et al., this issue). A network of 26 faults could be constructed in a 30x20x6 km model of the sedimentary cover and the top basement representing the faults as surfaces. The orientation of the major and generally synthetic faults (i.e. dipping eastward) is N22.5±7.5°E, whereas a second set strikes N0±10°E (mainly dipping westward), and a third one N45±10°E. The average dip of the faults is 60°.The fault density is approximately 3.7.10-4 m2 of faults per cubic meter. The resulting 3D geological model confirms the tectonic situation of the Soultz site as a horst structure and indicates a maximum vertical offset of approximately 500m of the basement in the area of the Soultz geothermal anomaly. A comparison between the smaller scale 3D fractured reservoir model based on borehole imaging, microseismicity and vertical seismic profiling (Sausse et al., 2010) and our 3D regional structural model suggests a strong correlation between the orientation of the partly stimulated fracture zones observed at reservoir scale and the secondary set of faults observed in our regional model.

International audience; The results of a calcimetry performed on cuttings from the 3 wells of the Enhanced Geothermal System of Soultz-sous-Forêts (France) are compared to other available data (petrography, mineralogy, fracture zones, flow pathways, etc...). The relationship between flow ranking and calcite content for the fracture zones of GPK3 and GPK4 is opposite to the one of GPK2 (the better the fluid flow, the lower the calcite content). This suggests that the fracture zones of GPK2 are different from those of GPK3 and GPK4, and that the connectivity to the fracture network may be different too. The results of this study provide also some explanation for the effects of the chemical stimulations performed in the 3 wells, as well as some information for future chemical stimulations that could be aimed to improve the connectivity between the wells and the fracture network.

International audience; The quantification and modeling of fluid flow in fractured rocks are extensively studied to solve and predict numerous economic or environmental problems (hydrothermalism, geothermy, storages, etc.). Indeed, discontinuities such as faults and fractures are potential sites for fluid circulation and have important implications for the hydraulic properties of rocks. The matrix permeability of igneous rocks is generally small and, consequently, the global permeability is mostly controlled by the fault and fracture networks. Therefore, the quantification of the fractured rock hydraulic properties strongly depends on the knowledge of the geometrical parameters of fractures (orientation, extension, aperture, density) and of the final 3D modeling of the fracture network organization. In the specific case of the Soultz-Sous-Forêts geothermal reservoir, a new statistical analysis of the fault and fracture networks is proposed to precise the actual 3D structural model of the reservoir (Sausse et al., 2009; Dezayes et al., 2009). The statistical characterization of the fractures and faults is realized with the re-interpretation of the whole U.B.I. images database available at Soultz. 1800 fractures are determined along the three deep Soultz well paths, grouped into main conjugates fractures sets, showing a mean N-S orientation and a mean dip of 70°, consistent with the Oligocene N-S extension responsible of the formation of the French Rhine graben. A correlation between the geometric parameters of fractures, width W and extension L is proposed and follows a power-law type correlation of the form L = k . WD with k, a coefficient characteristic of the facies and D the fractal dimension of the fracture set. These parameters are used to determine the volumetric density of fractures (number of fractures/m3) at the wells scale. Finally, this density and the statistics of fracture properties are used to constrain stochastic simulation of a discrete fracture network (DFN) in the geothermal reservoir.

International audience; In EGS concepts like the one at Soultz, knowledge of the fracture network is essential to understand reservoir behavior and plan further stimulations and long term hydraulic circulation. At Soultz, the targeted reservoir is located at a 5km depth within the granite basement of the Upper Rhine Graben near the western border of France. This granite underwent a complex tectonic history including the Hercynian and Alpine orogeneses, leading to the currently observed fracturing. This fracturing is dominated by steeply dipping fracture sets which are distributed at various scales in the granite basement. This structural work presents the characterization of the fracture network at different scales in order to understand and model the hydro-thermo-mechanical behavior of the granite geothermal reservoir through hydraulic stimulation and subsequent circulation tests. The small-scale fracture network has been characterized based on several kilometers of high resolution borehole image logs, allowing the characterization of different sets. There was a focus on large-scale fracture zones, which are the major fluid pathways. A total of 39 fracture zones have been described in detail based on borehole data. They have been compared to large-scale geophysical investigations using Vertical Seismic Profile (VSP) and passive microseismicity in order to build an extended fracture network in 3D. Hydraulically active fractures have been investigated in more detail in the different open-hole sections based on image logs, flow logs and temperature logs. These near-well borehole fractures represent potential fluid pathway for connecting the borehole to the far-field geothermal reservoir. The fracture network in the granite Soultz reservoir is constituted by large-scale fracture zones, which are connected to a dense network of small-scale fractures. However, fluid pathways are more complex, with channelized structures, because only a limited number of fractures support fluid flow.

International audience; The objective of the GéoCarbone-PICOREF project was to select and characterize geological sites where CO2 storage in permeable reservoir could be tested at the pilot scale. Both options of storage in deep saline aquifer and in depleted hydrocarbon field were considered. The typical size envisioned for the pilot was 100 kt CO2 per year. GéoCarbone-PICOREF initially focused on a "Regional Domain", ca. 200 × 150 km, in the Paris Basin. It was attractive for the following reasons: - detailed geological data is available, due to 50 years of petroleum exploration; - basin-scale deep saline aquifers are present, with a preliminary estimate of storage capacity which is at the Gt CO2 level, namely the carbonate Oolithe Blanche Formation, of Middle Jurassic age, generally located between 1500 and 1800 m depths in the studied area, and several sandstone formations of Triassic age, located between 2000 and 3000 m; - several depleted oil fields exist: although offering storage capacities at a much lower level, they do represent very well constrained geological environments, with proven sealing properties; - several sources of pure CO2 were identified in the area, at a flow rate compatible with the pilot size, that would avoid capture costs. 750 km of seismic lines were reprocessed and organized in six sections fitted on well logs. This first dataset provided improved representations of: - the gross features of the considered aquifers in the Regional Domain; - the structural scheme; - lateral continuity of the sealing cap rocks. An inventory of the environmental characteristics was also made, including human occupancy, protected areas, water resource, natural hazards, potential conflicts of use with other resources of the subsurface, etc. From all these criteria, a more restricted geographical domain named the "Sector", ca. 70 × 70 km, was chosen, the most appropriate for further selection of storage site(s). The geological characterization of the Sector has been as exhaustive as possible, with the reprocessing of additional 450 km of seismic lines, and the collection of a complete well-data base (146 petroleum wells). At this scale a relatively detailed characterization of the sedimentary layers could be done, in particular the formations potentially rich in aquifer units. For the Middle Jurassic carbonates observations were made on analogue sediments outcropping 150 km to the east of the Sector. A geological and numerical 3-D representation of the whole sedimentary pile of the Sector area was built. It forms a basis for constructing grids used by codes able to simulate various processes induced by CO2 injection (displacement of the fluids, pressure build-up and release, mechanical deformation, mineral interactions, control of the parameters used to check the local sealing efficiency, etc.). In parallel with that work on aquifers, GéoCarbone-PICOREF has access to all the petroleum data, including production data and reservoir modelling, of the Saint-Martin de Bossenay oil field, localized in the eastern part of the Sector. This was an opportunity to apply a comparable methodology and to test the capabilities of modelling codes to the specific case of a depleted hydrocarbon field, and to show some of the advantages of such a context with respect to a pilot-scale CO2 injection

International audience; The previous termEnhanced Geothermal Systemnext term (EGS) of Soultz-sous-Forêts (France) is made of three boreholes (GPK2, GPK3 and GPK4). The hydraulic connection between the wells, which is crucial to get an efficient previous termgeothermalnext term exchanger, may be unfortunately hindered by the more or less complete sealing of fractures by hydrothermal neoformed minerals. This paper takes over the recent work of the authors that quantified the amounts of calcite on a random sampling within the three boreholes in order to assess the role of this mineral on the exchanger zone and in particular its influence on the permeability of the flow pathways. In the present study we focused the sampling on the fracture zones that have been identified as flow pathways. Now, all the three wells show high (above 10 wt.%) and moderate to low (between 2.0 and 7.0 wt.%) calcite anomalies. In GPK2, fracture zones with a high conductivity are characterized by high calcite content, and those with a low conductivity by low calcite content. The fracture zones of GPK3 show the opposite relationship: the most efficient flow pathways have a low calcite content, whilst the less conductive show a high calcite content. Fluid flow in GPK4 is mainly accommodated by the fracture zone at 4775 m MD that is not calcite rich. The other fracture zones of GPK4, which are interpreted with a similar and low conductivity, show low, moderate and high calcite contents. These data show clearly that GPK2 is different from GPK3 and GPK4, these two later being more comparable. It suggests that the fracture zones themselves may be of different type, and that the connectivity to the fracture network may be different too. This difference of behaviour is consistent with microseismicity data showing that GPK2 is characterized by a rather compact and well structured network of medium-scale fracture whereas GPK3 and GPK4 are altogether characterized by more localized and discrete fracture zones. Some issues remain such as the occurrence of calcite anomalies with no evident relationship with fracture zones, as well as the occurrence of identified flow pathways with no abnormal calcite content.

International audience; This study presents a new deterministic 3D model of the fracture zones observed in the granitic reservoir of the Soultz European geothermal project. The major fracture zones encountered around 6 wells (4550, EPS1, GPK1, GPK2, GPK3 and GPK4) consist in 53 main structures that are located and characterized in terms of size and orientation: 39 fracture zones, 8microseismic structures and 6 structures derived from vertical seismic profiles are represented in the 3Dmodel using Discrete Fracture Network tools of the gOcad modeling platform (ParadigmTM, Earth DecisionTM). This work illustrates the complexity of 3D fracture zone correlation and interpretation in crystalline rock masses characterized at meter scale (borehole) and at the reservoir scale (kilometer) thanks to geophysical imaging techniques.

International audience; Three deep wells (5000 m) have been drilled into a fractured granite basement at Soultzsous- Foreˆts, within the Tertiary Rhine Graben, in order to develop a heat exchanger and produce electricity after the creation of an EGS reservoir. Very few analyses representative of the deep geothermal fluids are available because of frequent contamination by drilling fluids or injected waters. These indicate similar chemical and isotopic compositions (NaCl fluids) and high salinities (about 100 g/l) suggesting a common sedimentary origin and identical water–rock interaction processes at equilibrium temperatures close to 230 8C in a sedimentary rather than a granite reservoir. The latter would be situated closer to the Graben centre where the Triassic Buntsandstein formation is deepest and hottest. Tracer tests conducted after 2000 show that the deep native geothermal brine is omnipresent in the fluids discharged during the production and circulation tests. Its natural convective flux was estimated at 1–1.2 m3/h.