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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...

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, 8 microseismic structures and 6 structures derived from vertical seismic profiles are represented in the 3D model using Discrete Fracture Network tools of the gOcad modelling platform (Paradigm TM , Earth Decision TM ). 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.

Abstract. Attributes of several thousand fractures were collected in three boreholes of 2.2, 3.6, and 3.8 km depth, penetrating the Soultz Hot Dry Rock reservoir (France). The fractures were sampled from cores and from several high-resolution imaging techniques such as borehole televiewer (BHTV), ultrasonic borehole imager (UBI), formation microscanner (FMS), formation microimager (FMI), and azimuthal resistivity imaging (ARI). A comparison was made between the data collected on cores and those provided by different imaging techniques. The comparison clearly establishes that the different wall-images are not as exhaustive as the core data and cannot provide a complete characterization of the fracture network. Discrete fractures thinner than 1 mm are not properly detected. This is also the case for discrete fractures closer than 5 mm, which appear only as single traces. The imaging techniques are, nevertheless, very powerful for characterizing altered fracture clusters. Whatever the technique used, the fracture strikes were correctly sampled with the different systems. This comparison allowed us to calibrate the fracture population data obtained from the imaging system in order to correct for the filtering effect introduced by the technique itself and by the alteration of the rock mass.

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