Variability of the Seismic Response of the Rittershoffen Geothermal Reservoir to the Series of GRT-1 Stimulations

Between April and June 2013, the GRT-1 well at the Rittershoffen geothermal site in the Upper Rhine Valley (France) underwent three distinct stimulation phases: first a thermal stimulation, then a chemical stimulation, and finally a hydraulic stimulation. These fluid injections significantly enhanced the injectivity index of the well, rendering it suitable for economic exploitation. Throughout these operations, a local surface seismic network continuously monitored the site, recording thousands of unfelt seismic events.

This study builds upon and refines the findings of Lengliné et al. (2017), who focused solely on the hydraulic stimulation of GRT-1, and Maurer et al. (2020), whose interpretations were constrained by uncertain absolute locations of seismic events, particularly in terms of depth. By employing an improved template matching technique and a relative location method, we established a comprehensive seismic event catalog comprising over 3,000 events.

This reliable catalog enables precise tracking of the reservoir’s seismogenic response to the successive yet distinct stimulation types, with high spatial and temporal resolution. Consequently, it allows for an investigation into the potential seismic interplay between these stimulations. Our analysis examines the evolution of key characteristics, including event distribution and clustering, b-value, and seismic injection efficiency across the stimulation phases. The observed differences prompt critical questions regarding the reliability of using responses from prior stimulations to forecast seismogenic behavior during subsequent operations, even for the same site.