The Impacts of Mechanical Stress Transfers Caused by Hydromechanical and Thermal Processes on Fault Stability during Hydraulic Stimulation in a Deep Geothermal Reservoir.

Abstract:

We present 3D ThermoHydroMechanical (THM) and HydroMechanical (HM) numerical simulations to study the mechanisms of induced seismicity and the role of mechanical stress transfer due to the injection of cold water within a geothermal reservoir strongly impacted by a network of minor faults. Modeling assumptions are based on the results of previous modeling of the Northwest Geysers EGS Demonstration Project in California. We used the data collected during a one-year injection program to calibrate the hydraulic and mechanical properties and the in situ stress field. To estimate the impact of the thermal effects, we compared the results of the THM simulations to those of the HM simulations; then, we conducted a sensitivity analysis on the initial stress conditions. We investigated the link between the direction of the maximum horizontal stress, the propagation of mechanical stress transfer, and the reactivation of the preexisting fractures. Our simulations suggest that the mechanism of inducing microseismicity is related to injection-induced pressure, and the stress reduction caused by the thermal effect promotes additional displacements along the fault during the rupture. During the injection, the reservoir expansion caused by the pressure increase led to mechanical stress transfer, which prevented or delayed the reactivation of preexisting fractures. After injection had stopped, there was an inversion of the mechanical stress transfers that favored shear reactivation. Thermomechanical processes affecting microseismicity are complex. Thermal processes lead to stress reduction, which may cause shear reactivation along the fracture network channeling the fluid flow, depending on the fluid-flow direction and the initial state of stress. Overall, our study shows the importance of considering the orientation of the stress field in relation to the main flow direction, which has a significant impact on the temporal and spatial evolution of the fracture network reactivation.