Development of a 3D Hydrogeological and Geomechanical Model of an Enhanced Geothermal System Using Microseismic and Ground Deformation Data from a 1-year Injection Program

Abstract

In this study, integrated coupled process 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 shear 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 more than 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. Finally, we discuss the causes of induced microseismicity and the influence of pre-existing tectonic structures on the EGS development.