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 N45E 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 1800-2000m TVD (True Vertical Depth) is highly permeable. At 3000-3400m 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 modeling of geothermal reservoir life time submitted to various thermo-hydro-mechanical and chemical processes generated by hydraulic or chemical stimulations and hydraulic circulations. RésuméLe réservoir géothermique de Soultz est constitué par des zones de fracture majeures connectées à un réseau dense de fractures secondaires.Les méso-fractures sont pratiquement verticales et la direction majeure est à peu près N-S. Cette direction diffère de la direction régionale du fossé rhénan qui est localement à dominante N20°E à N45°E dans le Nord.Un total de 39 zones de fracture a été identifiées et caractérisées dans six puits entre 1400 et 5000 m de profondeur. Ces structures sont réparties en trois clusters suivant la profondeur. Le premier cluster à 1800-2000m TVD (profondeur verticale) est très perméable. A 3000-3400m TVD, le cluster intermédiaire apparaît comme un réseau plus dense dans un milieu plus altéré et constitue le réservoir supérieur. Dans la partie inférieure des puits, le cluster profond apparaît comme un réservoir fracturé développé dans une matrice très peu perméable. 2La caractérisation des zones de fracture représente un élément important à prendre en compte dans la modélisation de la durée de vie du réservoir géothermique soumis à des processus thermo-hydromécaniques et chimiques engendrés par les stimulations hydrauliques et chimiques et les circulations de fluide. Key words: Rhine graben, fractures, fracture zones, cores, borehole images, Enhanced GeothermalSystem. Mots-clés : Fossé rhénan, fractures, zones de fractures, carottes, image de paroi, Système GéothermalAmélioré. IntroductionSince 1980 [27; 28], the EGS project at Soultz (France) goals to experiment and develop a new geothermal technology. After an initial Hot Dry Rock (HDR) concept of artificial fractures creation in a homogeneous rock by hydraulic fracturing, the concept at Soultz has progressively evolved to an Enhanced Geothermal System (EGS) where reservoir development involved the reactivation of the preexisting fractures in the granite [16; 29]. Thus, a good knowledge...

Abstract. In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.

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A vertical profile of maximum horizontal principal stress, sHmax, orientation to 5 km depth was obtained beneath the swiss city of basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (Ot2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (bs1) imaged entirely within the granite from 2569 m to 4992 m. stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DItFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DItFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. the mean sHmax orientations from DItFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean sHmax orientation averaged over several hundred meters depth intervals is evident. this mean sHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10-15 km within regions immediately to the north and south of basel, and with the t-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DItFs and breakouts identified in Ot2 above and below the sediment-basement interface suggest that a change in sHmax orientation to N115°E ± 12° within the rotliegendes sandstone occurs near its interface with the basement. the origin of the 20-30° change is uncertain, as is its lateral extent. the logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of basel suggest a mean orientation of N-s, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of basel. thus, the available evidence supports the notion that the orientation of sHmax above the evaporites is on average more N-s oriented and thus differs from the NW-sE inferred for the basement from the bs1/Os2 wellbore failure data and the earthquake data. changes in stress orientation with depth can have significant practical consequences for the development of an EGs reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.