Gas diffusion property of compacted GMZ bentonite with consideration of saturation and gas pressure

Abstract

During the long-term operation of a deep geological repository, hydrogen, methane and carbon dioxide, etc. could be generated and accumulated in bentonite around canisters, threatening integrity and safety of engineering barrier systems. In this work, self-designed test apparatuses were developed. Gas diffusion tests were conducted on GMZ bentonite specimens under both rigid and flexible boundary conditions with consideration of initial saturations and gas pressures. After experienced the gas diffusion tests, the specimens were cut and submitted for the mercury intrusion porosimetry (MIP) tests. Results revealed that gas diffusion coefficient was obviously influenced by the initial saturations and gas pressures. As gas pressure increased from 1 to 4 MPa, a decrease of 55.3% ~ 58.8% and 17.7% ~ 73.4% in the diffusion coefficient were recorded for the specimens tested under rigid and flexible boundary, respectively. Compared to the rigid boundary conditions, the effective pore volume for gas molecule diffusion was further compressed under flexible boundary, resulting in a relatively poor pore connectivity and a lower gas diffusion coefficient. Meanwhile, the diffusion coefficient decreased with increasing initial saturations for the flexible boundary specimens, while an opposite trend was recorded for the rigid boundary ones. Explanations to this observation could be that under the rigid boundary conditions, specimens with higher initial saturations have larger diameter macro-pores, facilitating the gas diffusion.