Changes of Coulomb Failure Stress due to Dislocations during Stimulation of GPK2 at Soultz-sous-Forêts

Abstract

The European deep geothermal research project at Soultz-sous-Forêts (Alsace, France) has been developed since 1987. The geothermal reservoir is situated in a horst structure within the granite basement of the Upper Rhine Graben. During the project for developing an Enhanced Geothermal System (EGS) several well stimulations have been conducted and induced several tens of thousands of microseismic events. In the field of seismology of tectonic earthquakes, aftershock sequences produced by a large magnitude main shock have been successfully described by changes of Coulomb failure stress due to the dislocation (in the following denoted as ΔCFS) by the main shock (e.g. King et al. 1994 and Toda et al. 2003). We present 3D computations of ΔCFS using different geometries to approximate an elliptical slip distributions on circular fault planes. Based on this study we apply the method on a dataset of over 700 focal mechanisms (Dorbath et al. 2009) to compute evolving coseismic stress changes during the stimulation of the well GPK2. This allows us to conclude that the stress perturbation of all microseismic events during stimulation cannot be depicted by one single fault as has been assumed earlier. Furthermore analysis of this dataset allows us to estimate the influence of ΔCFS by coseismic slips over the total change of Coulomb failure stress by other processes like the increase of pore fluid pressure, thermal stresses, hydraulic response of the reservoir and coupling of these. The role of triggering of microseismicity by other micro-earthquakes during massive hydraulic stimulation is discussed. REFERENCES Dorbath, L., Cuenot, N., Genter, A. & Frogneux, M. 2009: Seismic response of the fractured and faulted granite of Soultz-sous-Forets (France) to 5 km deep massive water injections. Geophysical Journal International, 177(2), 653–75. King, G. C. P., Stein, R. S. & Lin, J. 1994: Static stress changes and the triggering of earthquakes. Bulletin of the Seismological Society of America, 84(3), 935–953. Toda, S. & Stein, R. 2003: Toggling of seismicity by the 1997 Kagoshima earthquake couplet: A demonstration of time-dependent stress transfer. J. Geophys. Res., 108(B12), 2567.