Laboratory Investigation of Permeability Evolution in Shear Stimulation of Granite Fractures for EGS

摘要

Shear stimulation has been recognized as one of standard treatments for engineered geothermal systems (EGS). The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. Control and optimization of shear stimulation can be achieved by studying how fluid flows through fractures as the stresses (shear and normal) change and how fracture permeability evolves with fracture slip. However, the fundamental mechanism of permeability evaluation in shear stimulation was not well understood, and most current experimental methods have certain drawbacks. In this work, we have conducted both stress-dependent permeability test and injection-driven shear test on tensile induced granite fractures. In the non-slip hydrostatic and triaxial flow test, the stress-dependent behavior of water flow through fractures under various stress conditions were characterized. We observed a linear relationship between flow rate and injection pressure, and an exponential relationship between flow rate and confining pressure. In addition, flow rate tends to linear decrease with the increase of effective confining pressure and differential stress. In the injection-driven shear test, significant fracture shear slip (with ~10~(-5) m/s slip rate) was noticed and 2 orders of magnitude increases in flow rate was induced by fracture slip. Furthermore, fracture permeability tends to linearly evolve with the shear slip and dilation during shear stimulation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding.