After 20 years of research and development on the geothermal reservoir, a 1.5 MWe power plant has been designed, built and tested at the EGS site of Soultz-sous-Forêts.
One of the major environmental concerns of the project has been for several years the induced microseismicity. During the early phases of the project, the most intense activity occurred during hydraulic stimulation experiments and, to a lesser extent, during chemical stimulation tests. It really became a worrying issue when several earthquakes of magnitude larger than 2 were felt on surface by the population. As the power plant is expected to operate continuously for years, it is of highly importance to study the seismic response of the geothermal reservoir in circulation conditions.
Five circulation tests performed in 2005, 2008 (twice), 2009 and 2010 offered the opportunity to observe the occurrence of microearthquakes in these conditions and will be presented in this study. They were carried out with different experimental setups: 2, 3 or 4 wells involved, artesian or pump-assisted circulation, different durations. The main result is that several hundreds of microseismic events were recorded in all the tests. Among them, earthquakes of magnitude ranging between 2 and a maximum of 2.3 occurred, which were likely to be felt by the population. Location of microseismic activity showed that almost the same zones of the geothermal reservoir were seismically active during all the tests. Moreover, correlations between the observed microseismic activity and hydraulic parameters of the circulation are performed in order to better understand the generation of microearthquakes in relation to hydraulics and to find circulation schemes that minimize the occurrence of microseismic activity for the future exploitation of the power plant.

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