| Abstract |
After the construction of the Soultz geothermal power plant completed in 2008, several circulation tests have been performed to test electricity production as well as downhole pumping technologies. Different production/injection strategies, involving 2, 3 or 4 boreholes were tested with different production/injection flowrates. Induced seismicity was continuously monitored with a surface seismological network of 8 permanent stations. In almost all cases, microseismic activity was observed, but at a moderate level as compared to the huge seismicity induced during stimulation experiment. In average, only a few microseismic events occurred per day. Some rare peaks of activity (up to 25 events/day) could be recorded, which usually follow variations of the hydraulic conditions. Several earthquakes reached magnitude higher than 1.7, the maximum recorded magnitude being 2.3. Fortunately, none of them was felt by the population. It has to be noticed that some of the largest magnitude events occurred during the shut-in period and more generally after sharp variations of the hydraulic regime. The location of the seismic activity is very similar from one test to the other. The events occurred mainly at great depth, between 5 and 5.5 km and 3 main zones are mostly active. However within these 3 zones and from one circulation test to the next, microseismic events tend to take place in previously inactive areas. Since 2011, it has been decided to split the reinjection between 2 wells in order to decrease the reinjection pressure. By maintaining the reinjection pressure below 20 bars, a spectacular decrease of the seismic activity was observed: in 2010, more than 400 microseismic events occurred over 11 months of circulation, whereas only 5 were recorded during 6 months of circulation in 2011. Thus as the strategy consisting in lowering the reinjection pressure by splitting the reinjected volume between several boreholes was very effective in 2011 in order to reduce the microseismic activity, it has been successfully applied in the following circulation tests. |