Permeability evolution and crack characteristics in granite under treatment at high temperature

The evolution of permeability and microcrack characteristics of granite at high temperature has significant effect on the safe and stable operation of high-level nuclear waste disposal repositories. We measure the permeability, porosity and formation factors of granite specimens following thermal treatment including the use of NMR to certify observed response. The results indicate that the initial and residual permeability and porosity change little when T ≤ 300 °C, but increase rapidly when 300 °C ≤ T ≤ 600 °C before entering a stable phase when 600 °C ≤ T ≤ 750 °C. At T = 150 °C microcrack apertures and radius only slight increase while microcrack density and fraction of connectedness all slight decrease, but net causing porosity, the peak strength and elastic modulus to increase. The strength and elastic modulus decrease exponentially with fractional connectivity of microcrack while the initial compressibility of microcracks scale linearly with initial aperture. The formation factor for the granite increases near-linearly with effective stress with the rate of increase generally decreasing with increasing temperature. Thermal conductivity first increases rapidly before remaining constant with increasing effective stress. With increasing temperature, the thermal conductivity decreases and becomes more sensitive to the effective stress.