Overview of Microseismic Activity Observed at the Soultz-sous-Foręts EGS Power Plant under Stimulation and Circulation Conditions

After 25 years of R&D, the Soultz-sous-Foręts EGS power plant has been built in 2008. Three 5 km deep boreholes were drilled and both hydraulically and chemically stimulated, to improve their hydraulic connection to the geothermal reservoir. Hydraulic stimulations induced a very high microseismic activity: several thousands of seismic events occurred and some of them were felt by the local population. Largest magnitude events (up to ML = 2.9) were mainly observed during the shut in phase. A moderate microseismic activity was recorded during chemical stimulations, which thus represent a good alternative to stimulate EGS reservoirs. Since 2008, several circulation tests have been performed, during which a moderate microseismic activity was still observed. But by progressively sharing the reinjection of the geothermal fluid between two wells and then limiting the injection pressure, we were able to decrease the induced seismicity to a very level. Applying this scheme during the long-term exploitation of the power plant will help increasing the acceptability of the geothermal site.