The stress field at Soultz-sous-Forets from focal mechanisms of induced seismic events : cases of the wells GPK2 and GPK3

The stress field at the EGS geothermal site of Soultz-sous-Forets has been the subject of many studies, because it largely controls the response of the reservoir to fluid injection. The analysis of borehole logging data, especially breakouts and drilling-induced tension fractures, in the four geothermal wells define an average SHmax orientation ranging between 170 degrees and 180 degrees down to 5 km depth. It also reveals strong heterogeneities in several depth intervals. In this paper, the inversion of double-couple source mechanisms of seismic events induced during GPK2 and GPK3 stimulation tests is performed to retrieve the orientation and shape factor of the stress tensor, using the Slickenside Analysis Package of Michael (1984, 1987a, 1987b). The results indicate a well-determined orientation of Shmin in GPK2 and in GPK3; in GPK3 Shmin is clockwise rotated by about 10 degrees. The stress tensor defines an uniaxial extension. The results from both methods, analysis of borehole logging data and inversion of focal mechanisms, are only slightly different; the discrepancy may be due to the larger reservoir volume covered by the focal mechanisms, which can include strong stress heterogeneities