ABSTRACT:

Permeability enhancement through shear slip has been considered as standard treatment of engineered geothermal systems (EGS). The process reactivates pre-existing fractures, making them slip and dilate using fluid pressures below the minimum principal stress resulting in increased permeability. It can also cause fracture propagation in the shear and tensile modes creating secondary cracks. Control and optimization of shear 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 conditions. 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 a fractured Sierra White granite sample (SW #1) under both hydrostatic and triaxial conditions to characterize stress-dependent 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 fractured 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. Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding.

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