ANALYSIS OF MACROSCOPIC FRACTURES IN GRANITE IN THE HDR GEOTHERMAL WELL EPS-1, SOULTZ-SOUS-FORETS, FRANCE

An exhaustive analysis of 3000 macroscopic fractures encountered in th e geothermal Hot Dry Rock borehole, EPS-1, located inside the Rhine gr aben (Soultz-sous-Forets, France), was done on a continuous core secti on over a depth interval from 1420 to 2230 m: 97% of the macroscopic s tructures were successfully reorientated with a good degree of confide nce by comparison between core and acoustic borehole imagery. Detailed structural analysis of the fracture population indicates that fractur es are grouped in two principal fractures sets striking N005 and N170 degrees, and dipping 70 degrees W and 70 degrees E, respectively. This average attitude is closely related to the past tectonic rifting acti vity of the graben during the Tertiary, and is consistent with data ob tained from nearby boreholes and from neighbouring crystalline outcrop s. Fractures are distributed in clusters of hydrothermally altered and fractured zones. They constitute a complex network of fault strands d ominated by N-S trends, except within some of the most fractured depth intervals (1650 m, 2170 m), where an E-W-striking fracture set occurs . The geometry of the pre-existing fracture system strikes in a direct ion nearly parallel to the maximum horizontal stress. In this favorabl e situation, hydraulic injections will tend both to reactivate natural fractures at low pressures, and to create a geothermal reservoir.