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Understanding the mechanisms controlling the advective movement of gas and its potential impact on a geological disposal facility (GDF) for radioactive waste is important to performance assessment. In a clay-based GDF, four primary phenomenological models can be defined to describe gas flow: (i) diffusion and/or solution within interstitial water; (ii) visco-capillary (or two-phase) flow in the original porosity of the fabric; (iii) flow along localized dilatant pathways (micro-fissuring); and (iv) gas fracturing of the rock. To investigate which mechanism(s) control the movement of gas, two independent experimental studies on Callovo-Oxfordian claystone (COx) have been undertaken at the British Geological Survey (BGS) and LAEGO-ENSG Nancy (LAEGO).The study conducted at BGS used a triaxial apparatus specifically designed to resolve very small volumetric (axial and radial) strains potentially associated with the onset of gas flow. The LAEGO study utilized a triaxial setup with axial and radial strains measured by strain gauges glued to the sample. Both studies were conducted on COx at in situ stresses representative of the Bure Underground Research Laboratory (URL), with flux and pressure of gas and water carefully monitored throughout long-duration experiments.

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To understand the fate and impact of gas produced within a repository for radioactive waste, a series of laboratory and field scale experiments have been performed on the Callovo-Oxfordian claystone (COx), the proposed host rock for the French repository. Results show the movement of gas is through a localized network of pathways, whose properties vary temporarily and spatially within the claystone. Significant evidence exists from detailed laboratory studies for the movement of gas along highly unstable pathways, whose aperture and geometry vary as a function of local stress, gas and porewater pressures. The coupling of these parameters results in the development of significant time-dependent effects, impacting on all aspects of COx behaviour, from gas breakthrough time, to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures are indicative of microstructural heterogeneity which exerts an important control on the movement of gas. The localization of gas flow is also evident in preliminary results from the large scale gas injection test (PGZ) where gas flow is initially focussed within the excavation damaged zone (EDZ), which acts as a preferential pathway for gas. Numerical models based on conventional two-phase flow theory are unable to adequately describe the detailed observations from laboratory tests.