Abstract
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 deg and 180 deg 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),
(Michael, 1987a) and (Michael, 1987b). The results indicate a well-determined
orientation of Shmin in GPK2 and in GPK3; in GPK3 Shmin is clockwise
rotated by about 10 deg. 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.
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