Fluid injection in enhanced geothermal systems (EGSs) frequently triggers earthquakes by reactivating pre-existing faults, posing a hazard to the development of geothermal energy. We conducted direct shear experiments on fault gouges derived from the granites of Gonghe geothermal reservoir, northwest China. These gouges are representative of materials infilled within natural fault zones and sampled at different depths. Experiments were conducted under normal stresses of 10-50 MPa and under varied humidity to capture the evolution of frictional strength and stability, together with healing behavior. Our results reveal a strong dependence of fault slip behavior on mineralogy. As tectosilicate-rich gouges exhibit frictionally strong but velocity-weakening behavior, indicative of potential unstable slip, while phyllosilicate-rich gouges are weaker but velocity-strengthening, favoring stable aseismic creep. Furthermore, reducing normal stress results in a transition from velocity-strengthening to velocity-weakening friction in tectosilicate-rich gouges, which suggests that earthquake ruptures can be facilitated at low stress. In addition, rates of frictional healing in tectosilicate-rich gouges are higher but decrease with increasing normal stress, whereas in phyllosilicate-rich gouges, it remains largely insensitive to stress variation. Therefore, tectosilicate-rich faults near the GR1 injection well of the Gonghe reservoir are prone to induce seismicity during failure, in contrast to phyllosilicate-rich faults, which may buffer stress through aseismic creep. Understanding the process-based feedback between mineral composition and imposed stress is essential in evaluating the potential of fault reactivation and mitigating seismic hazard during EGS operations.
Cao et al. (Fri,) studied this question.