The impacts of mechanical stress transfers caused by hydromechanical and thermal processes on fault stability during hydraulic stimulation in a deep geothermal reservoir

We performed a series of 3D thermo-hydro-mechanical (THM) simulations to study the influences of hydromechanical and thermal processes on the development of an enhanced geothermal system, strongly influenced by a network of short fault zones. The model we developed was calibrated by comparing the simulated THM responses to field observations, including ground-surface deformations, well pressure, and microseismic activity. Of particular importance was the comparison between the observed temporal and spatial distribution of microseismic activity, and the calculated shear reactivation of preexisting fractures inferred from simulated elasto-plastic mechanical responses in the short fault zones. Using this approach, we could identify when fault zones were reactivated (as manifested in the field by a surge of local microseismic activity within the fault zone), and we could back-calculate the in situ stress field as being close to the stress conditions required for shear reactivation. Our results show that the main mechanisms of inducing seismicity are related to injection-induced pressure increase and cooling. During injection, the reservoir expansion caused by the pressure increase led to mechanical stress transfer through the reservoir, which prevented or delayed the reactivation of preexisting fractures. After injection stopped, there was an inversion of the mechanical stress transfers that favored shear reactivation, which may explain why microseismic activity occurred after the cessation of the injection.