Hydrothermally Altered and Fractured Granite as an Hdr Reservoir I N the Eps-1 Borehole, Alsace, France

As part of the European Hot Dry Rocks Project, a second exploration borehole, EPS-1, has been cored to a depth of 2227 m at Soultz-sous-For@ts (France). The target was a granite beginning at 1417 m depth, overlain by post-Paleozoic sedimentary cover. Structural analysis and petrographic examination of the 800-111 porphyritic granite core, have shown that this rock has undergone several periods of hydrothermal alteration and fracturing. More than 3000 natural structures were recorded, whose distribution pattern shows clusters where low-density fracture zones (less than 1 per meter) alternate with zones of high fracture density (more than 20 per meter). Vein a1 terati on, ascribed to paleohydrothermal systems, developed within the hydrothermally altered and highly fractured zones, transforming primary biotite and plagioclase into clay minerals. One of these zones at 2.2 km depth produced a hot-water outflow during coring, indicating the existence of a hydrothermal reservoir. Its permeability is provided by the fracture network and by secondary porosity of the granitic matrix resulting from vein alteration. This dual porosity in the HDR granite reservoir must be taken into account in the design of the heat exchanger, both for model1 ing the water-rock interactions and for hydraulic testing.