NMR-based comparative study of gas permeability and pore structure of GMZ bentonite

The gas permeability of buffer/backfill materials is one of the key scientific issues in deep geological disposal. It is closely related to the pore structure. The current study investigates the pore structure and its effect on gas permeability by combining gas permeability tests and NMR tests on GMZ bentonite. It was found that the gas permeability decreases with the confining pressure. The variation in gas permeability is the result of changes in the pore structure. The initial pore size distribution of the compacted GMZ bentonite sample with a dry density of 1.70 g/cm3 is dominated by micron pores. After being subjected to certain confining pressures, the pores become composed of two parts: nanoscale and micron-scale pores, with nanoscale pores predominating, pores develop more discretely and connectivity becomes poor. As the confining pressure increases, the pore space tends to develop towards a uniform distribution, while the pore structure undergoes irreversible changes. The response of the pore structure to stress has a certain hysteresis, when the stress is fixed, the internal pore structure would undergo a certain time adjustment to reach a stable state.