Experimental investigation of gas diffusion in claystones: The potential of gas uptake measurements as a means to assess diffusivity in water saturated porous media

This study investigates sealing properties of Jurassic claystones, specifically Lias and Dogger mudstones of the Lower Saxony Basin, in the context of the disposal of high-level radioactive waste. Within the repository, the occurrence of various mechanisms such as corrosion, radiolysis, and microbial degradation may lead to generation of gases including hydrogen, methane and carbon dioxide. It has been reported through experimental studies that claystones possess hydraulic conductivities ranging from 10^-12 to 10^-15 m/s, indicating that the primary mechanism to dissipate the generated gases away from the repository is diffusive transport. Excessively high diffusion coefficients may compromise seal integrity, while overly low diffusion coefficients could result in localized overpressure zones, free gas formation, and interconnected gas transport pathways. Therefore, it is imperative to conduct a comprehensive investigation of gas diffusion in the context of nuclear waste disposal.

In the present study we assess the use of the pressure decay technique to measure gas diffusivity through water-saturated claystones. The experimental approach involved conducting radial gas diffusion into the pore space of core plugs that were sealed at both ends. Gas diffusivity was determined using a mathematical model that couples the analytical solution of the diffusion equation with the real gas law. Experiments were conducted on deionized water and water-saturated Boom Clay to determine hydrogen and methane diffusion coefficients, and the results are highly consistent with literature data obtained from other techniques. Currently, the measurement of hydrogen, carbon dioxide, and methane diffusion coefficients through water-saturated Lias and Dogger rock specimens is underway.