Seismic and Aseismic Deformations Occurring During EGS Stimulation at the Geysers: Impact on Reservoir Permeability

Abstract

We use the Seismicity-Based Reservoir Characterization approach to study the spatiotemporal dynamics of induced microseismic cloud, monitored during the Northwest Geysers EGS Demonstration project (California). We highlight the development, over 270 days of injection, of a spatial domain seismically quiet around the injection well surrounding by a spatial domain seismically active. We defined a ‘cessation front’ to describe the limit between the seismic and aseismic parts of the reservoir. Then, we compare these observations with 3D thermo-hydro-mechanical simulations of the EGS, where changes in permeability will depend on the normal stress and on the plastic shear strain. The model we developed was calibrated by comparing the simulated THM responses to field observations. One of the most important output of our modelling is that: (1) the aseismic area is due to the presence of the injected cold water and to thermal processes. They cause a cooling-stress reduction, which prevent shear reactivation and favors the fracture opening increasing significantly the permeability. This process is accompanied by aseismic plastic shear strain, which can be interpreted as slow slip events. (2) In the seismic area, microseismicity is caused by the reactivation of the preexisting fractures due to injection-induced pressure increase. In this area, the permeability increases as a function of the effective normal stress and of the plastic shear strains.