Injection tests at the EGS reservoir of Soultz-sous-Forêts. Seismic response of the GPK4 stimulations

•Induced seismicity during hydraulic and acidified injections at Soultz-sous-Forêts.•Double difference re-location and improvement of the hypocenter parameters.•Temporal and spatial evolution of the seismicity during and after the injections.•Seismic response of the geothermal reservoir under perturbed stress conditions. The European Enhanced Geothermal System (EGS) program of Soultz-sous-Forêts is organized around three wells (GPK2, GPK3, and GPK4) drilled to a depth of about 5000m. Hydraulic stimulations were performed in order to increase the injectivity of the reservoir and the connectivity among the wells. The stimulation of GPK4 was carried out in two stages, in September 2004 and in February 2005, followed by an acidification test performed in March 2005. The stimulations produced fewer induced events than those of the other wells, with interpretation remaining difficult. In this work we present some new observations on the seismicity of the GPK4 stimulations after a complete review of the seismic bulletins collected in 2004 and 2005. Furthermore, the events were relocated using the double difference method. The new images of the seismicity are presented in temporal sequences according to the main variations of the injection parameters. The seismic events that occurred during the 2004 stimulation are grouped in a dense cloud and centered on the well open-hole section, while in 2005 seismicity depicts a specific pattern suggesting that the “natural” stress field in the reservoir was not completely restored. Finally, the events recorded during the acidified test show that the reservoir behaved differently from the previous injections, which suggests that a different mechanism has controlled the induced seismicity.