Experimental Investigation of Injection-driven Shear Slip and Permeability Evolution in Granite for EGS Stimulation

Abstract

Permeability enhancement through shear slip has been considered as standard treatment of engineered geothermal systems (EGS). The so-called “hydroshearing” process reactivates pre-existing fractures to slip and dilate using fluid pressures below the minimum principal stress and can also cause fracture propagation in the shear and tensile modes creating secondary cracks, resulting in increased permeability for economic flow rates. Control and optimization of hydroshearing stimulation can be achieved by studying how fracture permeability evolves with shear slip and dilation. However, most experimental studies that have considered fracture slip and permeability evolution have used force-driven shear tests or have manually displaced the specimens to represent fracture slip. A few studies have considered fluid injection-driven slip but only using saw-cut smooth joints. In this work, we have conducted shear slip test by water injection on rough fractures. Water was injected into a granite sample containing a single tensile rough fracture to induce shear slip under triaxial condition. Flow rate during shear slip was measured to investigate fractures’ permeability evolution. In addition, the effects of confining pressure, differential stress, and injection pressure on stress-dependent permeability of the granite fractures were characterized. We tested three separate samples using different methods. Non-shear flow tests were conducted on sample Sierra White granite sample #1 (herein after SW #1) under both hydrostatic and triaxial conditions to characterize fracture permeability. We observed a linear relationship between flow rate and injection pressure, and an exponential relationship between flow rate and confining pressure. In addition, fluid injection-driven shear tests were performed on samples SW #2 and SW #3 using constant stress mode and constant displacement mode, respectively. Shear rates observed during the constant stress test were ~10-3 m/s and yielded up to 3 orders of magnitude increases in flow rate while the constant displacement mode caused ~10-5 m/s sliding rate and 20 times increase in flow rate through the fracture.