Variations of physical and mechanical properties of granite with different cooling treatments: An experimental study

The temperature gradient plays a critical role in the evolution of pore structure and mechanical properties of rock, but the underlying mechanisms remain unclear. This study conducts experiments on granite to investigate the effects of temperature gradients on pore structure and rupture propagation under different media, specifically liquid nitrogen and air. The pore structures and granite's transverse relaxation time (T2) are quantified using nuclear magnetic resonance (NMR). Then, Brazilian disk experiments are performed to explore the mechanical properties of granite following heating-holding treatments. Under the same temperature gradient, cooling with liquid nitrogen significantly influences the pore structures and mechanical properties of granite compared to air cooling. As the heat treatment temperature gradient increases, the mass loss rate, volume expansion ratio, and NMR porosity of granite increase, while the density, longitudinal wave velocity, and peak stress decrease. Splitting failure is caused by the constant propagation of the prominent cracks generated from the loading end to the load-bearing end. With increasing temperature in the thermal treatment, the secondary cracks increase and propagate toward the direction of the main cracks under the loading effect. These research results can provide a reference for developing high-temperature reservoirs and the technology for fracture initiation in such reservoirs.