Experimental investigation on the effects of confining pressure and pore pressure on gas/water transport properties of COx argillite

As a preferred host rock for the disposal of high-level long-lived radioactive waste in France, Callovo-Oxfordian (COx) argillite inevitably undergoes gas and water intrusion during long-term sealing. In this study, a series of gas and water permeability tests have been performed on dry, partially saturated, and fully saturated COx argillite to investigate the effects of confining pressure and pore pressure on transport properties of this material. It is first found that gas permeability is very sensitive to the sample damage state for dry samples. Due to the presence of microcracks, the relationship between porosity and gas permeability is not evident, while the Klinkenberg effect is clearly observed in intact samples. Moreover, gas permeability of partially saturated samples is very sensitive to water saturation, especially for highly saturated samples. This indicates that the material has a good gas sealing capacity after the host rock is resaturated during the long-term sealing process. For both low- and highly saturated samples, confining pressure and gas pressure have very different effects on gas permeability of partially saturated argillite. Water permeability of argillite varies between 10 ⁻²⁰–10 ⁻²¹ m ² ( K _w), suggesting a good water-sealing capacity. The evolution of K _w is more sensitive to the change of pore pressure than to the change of confining pressure, indicating that the effective stress coefficient for water permeability is greater than 1. The intrinsic permeability of this low-permeable clayey rock is discussed at the end of this work.