Geomechanical interpretation of induced seismicity observed at Basel geothermal reservoir

摘要

Some of the induced seismic events had moment magnitude exceeded 2.0 at the hydraulicrnstimulation in Basel, Switzerland in 2006 and they were felt by the population in the town. Thernauthors analyzed seismic data including the large events observed during and after the hydraulicrnstimulation. We investigated the fundamental characteristics of the large events and have found thatrnthere are considerable differences in the characteristics of the large events by their hypocentralrnlocation and origin time. Based on previous results, we also estimated critical pore pressure for shearrnslip, which is considered as one of the triggers of the shear slip on existing fracture planes, using therninformation of the stress state. It has been revealed that many of the large events occurred with thernrelatively small increase in the pore pressure. These observations suggest that the large events werernnot triggered simply by the increase of the pore pressure. Following this observation, we investigatedrnthe effect of the stress redistribution induced by the other preceding events. We calculated static stressrnchanges caused by the shear slip of the seismic events using the information of fault plane and sourcernparameters. The results from the calculation showed that there was stress change which conduced tornfailure on the fault plane of the largest events. However, we did not observe any significant stressrnchanges before the occurrence of the largest events and the magnitude of the stress change was quiternsmaller than the increase of the pore pressure. We hence concluded that static stress change inducedrnby the shear slip of the preceding seismic events did not trigger the shear slip of the existing fracture.rnThe other phenomena to redistribute the stress, including thermal effect and volumetric change ofrnrock mass should be considered in our future study.