Thermal effects on gas migration in saturated bentonite under rigid boundary conditions

Abstract

The influence of temperature on gas migration behavior constitutes a critical consideration for both the design and operational safety of deep geological repositories for high-level radioactive wastes (HLW). In this study, water injection and subsequent gas injection tests were performed on specimens with dry densities of 1.3, 1.5 and 1.7 Mg/m³ at temperatures 20, 40 and 60 °C. Meanwhile, the specimens that experienced related gas injection tests were subjected to mercury intrusion porosimetry (MIP) tests. The results indicate that the effects of temperature on the effective gas permeability are dependent on both the injection pressure and the initial dry density. Under low gas injection pressures, an increase in temperature leads to a rise in effective gas permeability, while under high gas injection pressures, the temperature impact on the effective gas permeability depends on dry density. For specimens with high dry densities, the effective gas permeability positively correlates with temperature, while for low dry densities, the value at 40 °C exceeds those at 20 and 60 °C. Additionally, the gas breakthrough pressure decreases with increasing temperature. Higher dry density specimens are more likely to experience capillary breakthrough, while interfacial breakthrough more commonly happens in lower dry density specimens. According to the microstructural observations from the MIP tests, increasing temperature reduces the specimen pore space due to the shrinkage of the bentonite matrix. These findings indicate that gas migration in saturated bentonite is governed by a competitive mechanism between pore structure and the state of pore water, while both of which are influenced by temperature.