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[1] The process of porosity and permeability creation in rock masses through increased pore pressure is important in many areas of geoscience, particularly for engineered geothermal sysytems. In this paper, we analyze an unusually complete data set to determine the hydraulic and mechanical changes that occurred about a 3.6 km deep borehole in previously undisturbed granite because of massive fluid injections. The hole is open for 750 m and intersects a relatively transmissive fault near the bottom at 3.5 km. The equivalent porous medium permeability of the rock mass in the 650 m above the fault was very low ($10 À17 m 2 ), and focused at 17 naturally permeable fractures that lay within hydrothermally altered zones. During injection, some 95% of the flow entered the rock mass at just 10 ''major flowing fractures,'' most of which were naturally permeable. Following the injections, the transmissivity of the section above the fault increased 200-fold, and the number of permeable fractures increased to $100, the distribution being clearly organized, with major flowing fractures each surrounded by clusters of weakly-flowing, newly permeable fractures. These zones of permeability creation/enhancement correlate with the presence of hydrothermal alteration, which in turn reflects the intersection of the borehole with extensive, hydrothermally altered, cataclastic shear structures. Thus permeability creation/enhancement occurred primarily within these structures, the major flowing fractures representing the core of the structures and the clusters of newlypermeable fractures denoting the damage zone. Comparison of sonic televiewer logs run before and after the injections showed that all permeable fractures had suffered damage and major flowing fractures had suffered dislocations of millimeters to centimeters.Citation: Evans, K. F., A. Genter, and J. , Permeability creation and damage due to massive fluid injections into granite at 3.5 km at Soultz: 1.

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S U M M A R YA high-rate injection of 20 000 m 3 of water into granite between 2.8 and 3.4 km depth at the Soultz hot dry rock (HDR) test site in France in 1993 September led to a 200-fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0.5 km wide, 1.2 km long, 1.5 km high and oriented 25 • NW. The resulting data set is unusually complete and well suited to studying permeability creation/enhancement processes in crystalline rock and the utility of microseismic data for revealing them. Although the microseismic cloud defined using joint hypocentre determination (JHD) locations was diffuse and showed little structure, application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently. One prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement. High-resolution microseismic mapping techniques (i.e. multiplets and clustering) showed it to be a subvertical, NNW-SSE striking, fracture zone of width 10-20 m. The strike and scale of the structure identifies it as a member of a family of hydrothermally altered, cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass, both under ambient and forced fluid flow conditions. The microseismicity occurred on subvertical, smallscale fractures within the cataclastic shear zone whose azimuths scatter within 22 • of parallel to the parent structure. Although the structure is likely to have been naturally permeable to some degree, its permeability appears to have been significantly enhanced as a consequence of the injection. The most likely mechanism of permeability enhancement, which is in accord with the strong preference for the microseismicity to grow downwards, involves strike-slip shearing, which produced the opening of vertical tubes at along-strike jogs in the fault (the so-called Hill mesh). Seismic moment release averaged over the structure suggests shear displacements of at least 0.3 mm occurred, which are sufficient to generate aperture changes that are hydraulically significant. The preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement (i.e. stimulation) within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m.

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S U M M A R YThe European Enhanced Geothermal System (EGS, formerly Hot Dry Rock, HDR) programme of Soultz-sous-Forêts is organized around three wells drilled to a depth of about 5000 m. Hydraulic stimulations were performed in the wells in 2000 (GPK2 well), 2003 (GPK3 well) and 2005 (GPK4 well). The stimulation of GPK2 induced more than 700 seismic events with a magnitude greater than 1.0. The seismicity depicts a dense, homogeneous cloud, without any apparent structure. Medium-size earthquakes represent more than 80 per cent of the cumulative seismic moment. The b-value of the Gutenberg and Richter law is larger than 1.2. The injectivity has been increased by a factor 20. These characteristics indicate that the stimulation reactivated a 3-D dense network of fractures. The stimulation of GPK3 induced only about 250 events with a magnitude greater than 1.0 but with a greater proportion of large events, up to 2.9. The hypocentres form clear structures identified as large faults, the b-value is about 0.9 and the large events (M > 2.0) account for the greater part of the cumulative seismic moment. The injectivity of the well, which was already high before the stimulation, remained unchanged. The stimulation of GPK4 was achieved in two stages. This stimulation produced even less induced events, making the interpretation difficult. The differences between the seismic response of GPK2 and GPK3 are due to the presence of large faults cut by GPK3 or in its close vicinity and reached by the injected water. Once a seismic event occurs on a fault, a sequence of earthquakes is triggered and the seismicity behaves, for a large part, independent of the injected flow rate. The stimulations also show some evidence that creeping could be a major source of deformation, if not the main one. The future EGS programme will have to drill wells in zones free of large faults to avoid poor hydraulic performance and inconvenience to the population.

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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...