Development of a 3D hydrogeological and geomechanical model of an Enhanced Geothermal System using micro-earthquake and ground deformation data from a 1-year injection program

In this study, integrated coupled processes modeling and field observations are used to build a three-dimensional hydrogeological and geomechanical model of an enhanced geothermal system (EGS) at the northwestern part of The Geysers geothermal field, California. We constructed a model and characterized hydraulic and mechanical properties of relevant geological layers and a system of multiple intersecting fault zones. The characterization was conducted through detailed coupled modeling of a 1 year stimulation injection with simultaneous field monitoring of reservoir pressure, microseismic activity, and ground surface deformations. The structural reservoir properties were characterized through a dynamic analysis of the microseismic activity recorded during the injection. The analysis of ground surface deformations were found to be particularly challenging as the subtle ground surface deformations caused by the injection at >3 km depth are intermingled with deformations caused by both tectonic deformations and seasonal ground surface effects associated with rainfall. However, through a detailed analysis of the field data we isolated local surface deformations associated with injection. Using the coupled fluid flow and geomechanical analysis of reservoir pressure responses in a number of monitoring wells and microseismic activity around the injection well, we back-calculated the hydraulic and mechanical properties of relevant rock mass layers and faults. The results show (1) the main cause of induced seismicity is an injection related pressure increase sufficient to trigger detectable seismic events, (2) the extension and shape of the microseismic cloud is strongly influenced by the pre-existing tectonic structures, (3) the critical importance of considering the fault system, including faults that compartmentalize the EGS system, and more permeable faults that provide conduits for injection water and fluid pressure diffusion that can extend up to one kilometer from the injection well and deep down into a granitic intrusion at a depth of approximately 4.0 km.