Deep-Seated Geology and Fracture System of the EGS Soultz Reservoir (France) based on Recent 5km Depth Boreholes

Abstract

In the framework of the EGS (Enhanced Geothermal System) European Soultz project (France), the recent drilling of the wells (GPK2, GPK3, GPK4) at 5km depth, where the bottom hole temperature reaches 200?C, allows to revisit the deep-seated geology of the granite reservoir.
The deep geology of the reservoir was studied from cutting observation and well logging data (spectral gamma-ray, caliper, drilling parameters). By using a fine resolution image log, the Ultrasonic Borehole Imager, the fracture network was evaluated in terms of fracture density and orientation between 1500 to 5100m depth.
Comparison between the geological data has pointed out different granite intrusion, which has been differentiated and characterised within the basement. The deeper granite intrusion corresponds to a two-mica granite, being the youngest, which intrudes within the shallower porphyritic biotite granite.
The small-scale fracture direction is mainly N-S, which is roughly parallel to those of the present-stress field. At larger scale, some fractured zones develop a hydrothermally altered halo, which represent some potential candidate for fluid circulation through the granite reservoir.
Hydraulic tests, carried out in the open hole sections, confirm that some connections occur between the boreholes 600m away, at 5km depth, throughout the natural fractures of the granite basement. A updated conceptual model of both the deep geology and the fracture network of the Soultz reservoir were reexaminated in the light of these recent investigations.