Geothermal reservoir stimulation through hydro-shearing: an experimental study under conditions close to enhanced geothermal systems

Injection-induced shear stimulation (commonly known as 'hydro-shearing') is often implemented in Enhanced Geothermal Systems (EGS) to enhance the reservoir permeability. Hydro-shearing occurs when a critically stressed pre-existing fracture (or fault) slips at an injection pressure below the minimum principal stress in the reservoir, which opens the fracture due to self-propping of asperities on the fracture surface. In this study, six triaxial flow through tests were conducted on rough fracture and saw-cut granite specimens at temperatures of 20 degrees C, 65 degrees C, and 130 degrees C in order to investigate the impact of hydro-shearing on fracture response to coupled thermal-hydrological-mechanical-chemical (THMC) processes. Hydro-shearing results in a permanent shear-slip in a rough fracture, which opens the fracture due to self-propping of asperities and contributes to retainable permeability enhancement. Thermal stress induced shear-slip was observed in case of saw-cut fracture tested at 130 degrees C. Thermal stress at elevated temperature could have contributed to the mechanical crushing of asperities and weak fractures, which resulted in higher amount of gouge particles. Increase in temperature also resulted in an enhanced mineral dissolution rate in post-hydro-shearing stage compared to pre-hydro-shearing stage. X-Ray micro CT image showed new micro-cracks propagated from rough surface during hydro-shearing at 130 degrees C, which could contribute to the fracture permeability during post-hydro-shearing stage of the specimen. Gradual decrease in fracture permeability observed during post-hydro-shearing affects the long-term production of the reservoir. The result of the experiments presented here helps to strengthen the understanding of the fracture response to coupled THMC processes during EGS stimulation through hydro-shearing.