Thermal treated granite brittleness-index and energy storage characteristics under two-dimensional compression

Abstract

In deep geothermal resource development, the brittleness index plays a key role in assessing the feasibility of hydraulic fracturing. However, understanding how rock brittleness evolves after high-temperature exposure remains a critical challenge. This study examines the mechanical response of granite subjected to biaxial stress following high-temperature treatment in the laboratory. We analyzed the mechanical properties, energy storage characteristics, and failure behavior using high-speed cameras. Integrating mineralogical properties, thermal damage mechanisms, and strain energy density theory, we evaluated granite’s brittleness under thermal stress. The results reveal that wave velocity and porosity change significantly with increasing temperature, especially beyond 400 °C. Both peak and residual stresses increase with temperature and confining pressure, while peak strain decreases. The pre-peak energy storage coefficient first rises, then declines with temperature, with confining pressure enhancing storage, particularly between 400 and 600 °C. Calculations of elastic strain energy predict rockburst tendencies, with the palm face showing higher susceptibility, consistent with high-speed camera observations. Brittleness peaks at 400 °C under confining pressure above 10 MPa, with lower temperatures enhancing brittleness and higher pressures promoting plasticity. Failure modes shift from tensile and splitting cracks at 25–200 °C to shear cracks at 400–800 °C. At 800 °C, increased confining pressure promotes energy dissipation, leading to more cracks and altered failure modes.