Numerical Modelling of Gas Flow in a Compact Clay Barrier for DECOVALEX-2019

Tamayo-Mas, Elena (British Geological Survey) | Harrington, Jon (British Geological Survey) | Shao, Hua (Federal Institute for Geosciences and Natural Resources) | Dagher, Elias (Canadian Nuclear Safety Commission / University of Ottawa) | Lee, Jaewon (Korea Atomic Energy Research Institute) | Kim, Kunhwi (Lawrence Berkeley National Laboratory) | Rutqvist, Jonny (Lawrence Berkeley National Laboratory) | Lai, Shu-Hua (National Central University) | Chittenden, Neil (Quintessa Ltd.) | Wang, Yifeng (Sandia National Laboratories) | Damians, Ivan (Universitat Politecnica de Catalunya) | Olivella, Sebastia (Universitat Politecnica de Catalunya)


The processes governing the movement of repository gases through engineered barriers and argillaceous host rocks can be split into two components, (i) molecular diffusion (governed by Fick's Law) and (ii) bulk advection. In the case of a repository for radioactive waste, corrosion of metallic materials under anoxic conditions will lead to the formation of hydrogen. Radioactive decay of the waste and the radiolysis of water are additional source terms. If the rate of gas production exceeds the rate of gas diffusion within the pores of the barrier or host rock, a discrete gas phase will form (Wikramaratna et al., 1993; Ortiz et al., 2002; Weetjens and Sillen, 2006). Under these conditions, gas will continue to accumulate until its pressure becomes sufficiently large for it to enter the surrounding material. In clays and mudrocks, four primary phenomenological models describing gas flow can be defined, see Figure 1: (1) gas movement by diffusion and/or solution within interstitial fluids along prevailing hydraulic gradients; (2) gas flow in the original porosity of the fabric, commonly referred to as two-phase flow; (3) gas flow along localised dilatant pathways, which may or may not interact with the continuum stress field; and (4) gas fracturing of the rock similar to that performed during hydrocarbon stimulation exercises.