Gas permeability evolution of unsaturated GMZ bentonite under thermo-mechanical effects

The variation in the gas permeability of unsaturated GMZ bentonite caused by coupled thermo-mechanical effects is essential in evaluating the long-term safety of deep geologic repositories. Gas permeability tests were conducted on samples containing different water contents under different confining pressures and real-time heating processes to investigate their effects. Samples with different water contents were obtained by equilibrating the samples at different relative humidity (RH) conditions under unrestricted conditions. The results show that the gas permeability of the unsaturated GMZ bentonite varies from 10−15 to 10−20 m2 under coupled thermo-mechanical effects. It is significantly influenced by temperature and confining pressure, and the extent of change is related to the initial RH conditions. Gas permeability generally decreases with temperature. When the temperature reaches 80 °C, a sudden increase in gas permeability occurs in samples with high RHs (94.7% and 97.3%) as gas pressure pushes out moisture. The rapid increase in permeability remains dependent on the confining pressure, with it exceeding the value in the initial 20 °C under low confining pressures and falling below that under high confining pressures. The exploration of real-time water loss also reflects the phenomenon of the gas migration process carrying water vapor out as the temperature increases, which is responsible for the sudden change in gas permeability. The external stress condition is the key to ensuring its sound sealing performance. Microscopic pore structure observation results further reflect that pore closure due to thermal effects is the primary reason for the temperature-dependent decrease in gas permeability during the loading process. The cracking of samples with high RHs under thermal effects and the decrease in water-content capacity are potential endogenous causes of the sudden gas permeability increase. The study results provide reliable data to support the construction of the Beishan geologic repository in China.