Seismic interplay during the GRT-1 stimulation series of the Rittershoffen EGS reservoir

Between April and June 2013, the GRT-1 well, located at the Rittershoffen EGS geothermal site in the Upper Rhine Valley (France), underwent three distinct stimulations: a thermal stimulation, a chemical stimulation, and finally a hydraulic stimulation. These stimulations, in particular the hydraulic stimulation, have been previously explored by several studies. The current study addresses a feature that has been less analyzed: the possible seismic interplay between these stimulations. By applying improved template matching and a relative location method, a comprehensive catalog of more than 3000 seismic events was established, covering the three stimulations. This catalog allows tracking and comparison of reservoir seismogenic responses to successive, yet different, stimulation types with high spatial and temporal precision. We present the evolution of several indicators such as event distribution and clustering, b-value, and seismic injection efficiency across the stimulations. The observed differences raise the question of the relevance of using previous stimulation indicators to anticipate seismogenic behavior during subsequent stimulations. We also evidence simultaneous seismicity migration, decrease of the instantaneous injection index and decrease of the seismic efficiency at the end of the hydraulic stimulation but prior to the most energetic burst of seismicity that occurred 4.5 days after shut-in.