The swelling of coal and shale as the result of gas adsorption has been widely studied in the context of important projects, such as coalbed methane (CBM) production, carbon capture and storage (CCS), and shale gas production. However, there are limited studies about the gas-adsorption-induced swelling of the argillaceous rocks, which have long been selected as the privileged candidate host rock for nuclear waste repository in many countries, such as Callovo-Oxfordian argillite (in France), Boom clay (in Belgium), and Opalinus clay (in Switzerland). In the long term sealing process, the possible leakage gases from the radioactive waste, which more or less have adsorption capacity, will penetrate into the host work and be adsorbed in clay minerals. Considering the existence of swelling minerals in clay, the gas adsorption can lead to volumetric expansion followed by the adsorption-induced swelling pressures. These swelling pressures can not only potentially change the pore structure, but also change local stress regimes. These evolutions will possibly affect the gas transport property, micro- and macro- mechanical properties of the host rock. Therefore, in order to ensure the sealing efficiency and safety of the geological repositories, it is important to study the gas-adsorption-induced swelling of the argillaceous rocks.

The main work of this contribution is to study the potential swelling capacity of COx argillite induced by the gas adsorption. A series of gas injection tests was carried out firstly by three kinds of gases with different adsorption capacities, such as helium (non-adsorbing gas), nitrogen (weakly adsorbing gas) and CO2 (strong adsorbing gas). The results show the strain caused by CO2 was larger than the strain caused by helium and nitrogen under the same gas pressure. This evidences the existence of the potential gas-adsorption-induced swelling pressure in argillite. In addition, the swelling strains exhibit an obvious anisotropic character, with larger deformation in direction perpendicular to bedding plane than those parallel to the bedding planes. This could be explained by the transverse isotropy structure of argillite. A series of gas permeability tests was also handed with the three gases. The results show that K(CO2) is extremely low, about 10-20m², compared to K (helium) about 10-18-10-19m² and K (nitrogen) (about 10-19m²). This phenomenon could be further attributed to the swelling induced by gas adsorption which has narrowed the pore channel and caused the decrease in the gas permeability.