Fracturing of granite rock with supercritical water for superhot geothermal resources

Abstract Fracture patterns induced by supercritical-water fracturing show complex fracture networks with branched and isolated fractures. Such fracture patterns are expected to effectively increase permeability and significantly improve the heat exchange surface area. We clarify the mechanism of complex fracture induced by supercritical water using a coupled model and discuss the performance of the supercritical-water fracturing under geothermal environments. The numerical approaches to the coupled model are the pore-scale network method and finite element method with cohesive elements. Supercritical-water fracturing in granite at the laboratory scale was simulated and compared with the experimental results. The simulation results of fracture patterns are in good agreement with the experimental results. The results suggest that low viscosity of supercritical water leads to a strong seepage effect in granite, which reduces the effective stress over a wide area and makes it easier for shear failures to occur. We also found that differential stress and large confining stress induce more complex cloud-fracture networks. Our results clearly show that fracturing with supercritical water is effective not only for the geothermal resources that are considered so far but also for the development of the area at even deeper depths, which can contribute a leap forward in geothermal development.