An Investigation of Gas-transport Properties of COx Argillite During the Drying/Wetting Paths

We report laboratory experiments to investigate the evolution of gas-transport properties of COx argillite along drying and wetting paths, in the context of deep underground nuclear waste disposal. The gas injection technique was used to simultaneously measure the variations of gas accessible porosity (ϕ_ag) and gas permeability (K_g) under different confining pressures (P_c) and relative humidity (RH). Our results show that porosity (dry samples) and water saturation measured or derived by the gas injection technique are in good agreement with the conventional water vapor equilibration method, which confirms the quality and efficiency of the technique. Furthermore, the different changing magnitudes of ϕ_ag and K_g measured in the wetting and drying paths are attributed to the fact that the swelling deformation is greater than the shrinkage deformation. A coupling effect is observed between P_c and RH. Specifically, when the sample is in a dry (or quasi-dry) state, relative gas accessible porosity ϕ_rag does not (or negligibly) vary with P_c, whereas the relative gas permeability (K_rg) shows a strong sensitivity to P_c. This is due to crack closure, which increases the tortuosity of the pores and/or causes part of the pores to be “crack closing”. However, in the highly saturated case, both ϕ_rag and K_rg become more and more sensitive to P_c. This is caused not only by the crack closing but also by the softening of the material and the redistribution of saturation.