The Impact of Injection Protocol and Stress Preconditioning on the Hydro-Mechanical Response of the Crystalline Rock

Hydraulic stimulation has been extensively utilized in the geothermal industry as the primary technique to create and develop an efficient heat exchanger in a low-permeable reservoir rock. This technique entails high-pressure fluid injection under various injection schemes to perturb the local stress field at both borehole and reservoir scales, leading to permanent permeability enhancement of the stimulated volume through shear dislocation and dilation. These stress disturbances can also potentially trigger and/or induce seismicity in the reservoir and beyond. Understanding how different injection protocols serve as a preconditioning tool and their impact on the hydro-mechanical response of the stimulated volume would enhance our understanding of geothermal reservoir enhancement and induced seismicity mitigation. In the preparatory phase of the FEAR (Fault Activation and Earthquake Rupture) project in Bedretto Underground Laboratory (Switzerland), various injection protocols were utilized to understand the hydro-mechanical responses of the stimulated volume as well as earthquake rupture processes such as nucleation and premonitory slip. The adopted injection protocols include a) constant pressure injection for a specific time followed by step-rate injection and b) constant pressure withdrawal for a specific time followed by step-rate injection. In both protocols, an approximately equal amount of water (~3000 liters) was injected over the stimulation phase. Each injection protocol was associated with the pre- and post-characterization tests such as HTPF (hydraulic tests on pre-existing fractures) tests. Hydro-mechanical response of the host rock during these tests was monitored using various pressure, strain, and acoustic emission sensors in the injection and monitoring boreholes. At first glance, there appears to be no significant difference in the hydro-mechanical responses as well as the seismicity pattern of these two injection protocols, yet deeper investigation mostly based on the strain data reveals that strategy a) produced more heterogeneity in strain rate on the fiber-optic array whereas b) produced a more homogenized response. Numerical modelling and an experimental campaign in the laboratory are now underway to better understand the underlying mechanisms producing this response with the aim to best select a proper injection protocol for the goal of the FEAR project.