Two closely spaced geothermal doublets were operated in the Californië geothermal field near Venlo, the Netherlands. The geothermal wells target the Dinantian Zeeland formation below 2 km depth. For several years, hot fluid was produced from the Tegelen fault, a regional fault in the Roer Valley rift system, until a felt M1.7 earthquake led to the suspension of geothermal activities. The Californië showcase provides a rare opportunity to retrospectively evaluate the assessment and the management of induced seismicity risks for a geothermal project. A seismic hazard assessment was conducted at several stages of the project, and seismicity was continuously monitored with a local station network.

In this paper, we report on the characteristics of the induced seismicity and evaluate the findings of the seismic hazard assessments conducted prior to the earthquakes. Seismic hazard assessments were based on numerical simulations of subsurface stress changes associated with geothermal operations. A geomechanical analysis indicated that the mapped faults in the subsurface are likely to be critically stressed. The largest hazard was inferred to result from thermo-elastic stresses, originating from cold water injection close to the Tegelen fault.

Subsequent earthquakes predominantly occurred near a production well after stopping or reducing production. We attributed this observation to a thermo-elastic stress load caused by cold water injection close to the Tegelen fault, combined with a counter-acting stabilisation of the fault due to pressure depletion during production. This mechanism was consistent with the dominating mechanism considered in the preceeding seismic hazard assessments. Although geothermal operations have not resumed yet, the geomechanical analysis indicates that re-locating one of the injection wells further away from the Tegelen fault could provide an efficient measure for mitigating induced seismicity risks at Californië.