Nucleation of Instability on Granitoid Faults Under Hydrothermal Conditions: Implications for Fluid Injection-Induced Seismicity

ABSTRACT: Typical in the development of enhanced geothermal systems (EGS), fluid injection for hydraulic stimulation and fracturing hosts the potential for induced seismicity by reactivating deep faults. We use granite cores from the Pohang EGS site in low velocity shear experiments on simulated fault gouges at confining stresses of 110 MPa, temperatures of 100-250 °C and fluid pressures of 21-80 MPa. These experimental conditions are representative of deep geothermal reservoirs and allow the influence of variations in temperature and effective stress to be explored on the nucleation of instability. Our observations document a transition from velocity-strengthening to velocity-weakening response when both the temperature was raised from 100 to 250 °C and the effective confining pressure was reduced from 89 to 30 MPa. Shear deformation localizes in a narrow shear zone with this increased temperature and a transition from fault compaction to dilation evolves with reduced effective stress – congruent with the transition to velocity weakening. These results suggest that high temperature and low effective stress both favor nucleation of earthquake ruptures during fluid injection. Thus, controlling fluid overpressures and being aware of anomalously high thermal gradients potentially enable important controls to be applied in mitigating such injection-induced seismic risks. 1. INTRODUCTION Fluid injection into the deep subsurface is capable of activating nearby faults and triggering seismicity. This phenomenon has been widely reported in numerous underground projects - including for enhanced geothermal systems (EGS), wastewater disposal and shale gas development (Elsworth et al., 2016; Grigoli et al., 2018; Walsh Bao Niemeijer & Collettini, 2014). Thus, detailed characterizations of the evolution of fault strength and stability under hydrothermal conditions are crucial in understanding and mitigating such seismic risk for induced earthquakes.