Fracture Network of the EGS Geothermal Reservoir at Soultz-sous-Forêts

Chrystel Dezayes (1) , Albert Genter (1) , Sylvie Gentier (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

In the framework of the EGS geothermal project at Soultzsous-Forêts, characterizing the fracture network provides a basis for understanding both the underlying mechanisms and results of hydraulic stimulation, and the fluid circulation within the underground heat exchanger. The Soultz site is located within the Rhine Graben, near its major western boundary fault. Five boreholes have been drilled since 1987: two relatively shallow exploratory wells (GPK1 and EPS1) and three for reservoir development and heat exploitation (GPK2, GPK3 and GPK4), all of which were drilled to depths of approximately 5,000 m, where temperatures reach 200°C. One of the exploration wells was cored; the others were partially or totally logged with borehole image logs (UBI or Ultrasonic Borehole Imager). Analysis of cores and downhole logs provides a basis for characterizing the fracture network at Soultz. The reservoir is a fractured massif of granite overlain by 1,400 m of sedimentary cover. In a first step, the fracture network within the whole granite massif has been evaluated based on the analysis of the image logs. Fracture orientations are quite persistent in the entire massif: two main conjugate sets of nearly vertical fractures occur with a N170°E direction in all the wells, with dips both to the east and the west. This orientation is consistent with the regional tectonic framework, and the changes in dip direction may relate to major structures within the graben. In a second step, we characterized the main permeable structures within the deep reservoir between 4,580 and 5,100 m. To evaluate the main features of these permeable structures in the open hole sections of GPK2 and GPK3, we simultaneously analyzed several sets of logs, including UBI and spectral gamma ray logs collected at the time of well completion, and temperature and flow logs collected during hydraulic experiments. For the new borehole GPK4, completed in April 2004, the hydraulic tests have not yet been carried out yet and the geophysical logging data are not available. In GPK2, we have identified seven fracture zones that appear to be participating in the fluid circulation. The most important zone is located at 4,780 m. However, the orientation of these zones could not be determined because there are no data from image logs available in this section. We analysed in detail the GPK3 well, which is fully documented, to improve our knowledge about the fracture network involved in the hydraulic experiments in the deep wells. In GPK3, UBI and spectral gamma ray logs are available for the whole borehole section. The analysis of flow and temperature logs measured during the hydraulic tests shows eight main fracture zones

Mots clés: EGS; fractures; fluid flow; granite; borehole image logs

Domaines: Sciences de la Terre

Chrystel Dezayes, Albert Genter, Sylvie Gentier. Fracture Network of the EGS Geothermal Reservoir at Soultz-sous-Forêts. Transactions - Geothermal Resources Council, 28 (213-218), 2004. ⟨hal-03739961⟩