Investigating fault reactivation potential for the Californië geothermal field (the Netherlands) by addressing uncertainties with probabilistic modeling of structures and in situ stress.

Developing geothermal energy projects require a clear understanding of seismic hazard potential in the subsurface, specifically, fault reactivation and induced seismic events of societal significance. The Californië geothermal field in South Eastern Netherlands is one such project where concern for disruptive seismicity has stalled development. Evaluating seismic hazards of structurally-controlled geothermal systems must include a clear understanding of subsurface geometries, specifics of the current stress field, and rock properties at depth. At Californië, although considerable subsurface data is available, the extent and specific geometries of local faults and fault topologies, including paleo-fault structures, stratigraphic formations, as well as the stress field at reservoir depths are not all well understood. This study addresses these uncertainties with a probabilistic approach to three-dimensional structural and geomechanical modeling, qualifying primary observations of structural features and evidence of the in situ stress state with a cumulative measure of their uncertainties. A number of structural geometric realizations are derived from these probabilistic uncertainties and analysed using the finite element method to evaluate slip tendency, dilation tendency, and fracture susceptibility. The results of these calculations provide meaningful distributions for fault stability considering uncertainties of in situ stress, structural geometries, and frictional properties to inform development and operational parameters and enable a finer evaluation of seismic hazards and further geothermal development.