Hydraulic Fracturing Characteristics and Fracture Mechanism of Granite under High Temperature and Pressure

Geothermal energy, as a widely distributed renewable resource, has become a crucial safeguard for humanity's future energy security. Enhanced geothermal systems (EGS), particularly those involving hot dry rock (HDR), hold significant exploitation value due to their vast reserves and characteristics of cleanliness, high efficiency, and renewability. Currently, the extraction of HDR typically requires hydraulic fracturing to create permeable flow channels for heat exchange. Therefore, research on the hydraulic fracturing characteristics and fracture mechanisms of reservoirs is of great importance for the efficient exploitation of HDR resources. This study focuses on granite, a common HDR reservoir, to investigate the hydraulic fracturing characteristics under varying confining pressures and temperatures. The evolution of acoustic emission (AE) characteristics during the fracturing process was analyzed, the fracture propagation patterns were elucidated, the influencing factors of hydraulic fracturing were clarified, and the fracture mechanisms and crack propagation criteria of hydraulic fracturing were revealed. The experimental results demonstrate that: (1) The hydraulic fracturing curves of granite under both room temperature and high temperature can be divided into four stages: borehole water filling, borehole pressure increase, hydraulic fracturing, and fracture propagation. (2) Post-fracturing cracks predominantly propagate along the borehole's depth direction. As the confining pressure increases, the crack length extends, and the fracture path becomes more complex. The fracture propagation modes include intergranular and transgranular fractures. (3) Hydraulic fracturing characteristics are significantly influenced by confining pressure and temperature. At a constant temperature, the breakdown pressure increases with higher confining pressure, exhibiting an approximately linear relationship. Under constant confining pressure, the breakdown pressure rises with increasing temperature, though this correlation is nonlinear. (4) The dominant fracture mechanism in granite hydraulic fracturing is tensile failure, though shear failure characteristics are also observed. The breakdown pressure is influenced by multiple factors, including confining pressure, temperature, and the material properties of the rock. The findings of this study can provide theoretical support for the fracturing design of granite-based geothermal reservoirs.