Induced earthquake potential in geothermal reservoirs: Insights from The Geysers, California

Geothermal reservoir production and associated induced seismicity may experience pronounced attention in the near future, given the ambitious plans for reducing greenhouse gas emissions toward a carbon-neutral economy and society. At some geothermal sites, the occurrence of hazard- and risk-prone induced earthquakes caused by or associated with reservoir stimulation has resulted in project shutdown (e.g., Pohang, South Korea, and Basel Deep Heat Mining, Switzerland). At other geothermal sites, the maximum event magnitudes were successfully maintained below a threshold defined by local authorities (e.g., Helsinki St1 Deep Heat project in Helsinki, Finland). In this study, we review some of our results from seismological and geomechanical reservoir characterization at The Geysers geothermal reservoir in California, USA, the largest producing geothermal field worldwide. We relate our findings to other geothermal sites to better understand the variability of reservoir behavior. In particular, we obtain a constant and relatively low seismic injection efficiency at The Geysers, which is interpreted to be related to the large energy dissipation through thermal processes and additional dissipation through aseismic slip, the latter now being considered to play a fundamental role in earthquake nucleation. We discuss some characteristics of the seismicity from The Geysers that suggest stable reservoir seismic injection efficiency and possibly low potential to rupture into large induced earthquakes, reducing the associated seismic hazard.