Processes Governing Advective Gas Flow in the Callovo Oxfordian Claystone (COx)

Data from this study demonstrates advective movement of gas in COx is accompanied by dilation of the samples (i.e. the formation of pressure induced micro-fissures) at gas pressures significantly below that of the minimum principal stress. Flow occurs through a local network of unstable pathways, whose properties vary temporarily and spatially within the claystone. The coupling of variables results in the development of significant time-dependent effects, impacting many aspects of COx behaviour, from gas breakthrough time, to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures may result from the arbitrary nature of the flow pathways (which vary spatially and temporally), microstructural heterogeneity which may exert an important control on the movement of gas, or, a combination of the two. Examination of other clays, mudrocks and shales indicate gas flow is accompanied by an increase in sample volume associated with a change in porosity caused by the formation of gas pathways. Under these conditions, data suggests gas flow is along pressure-induced preferential pathways, where permeability is a dependent variable related to the number, width and aperture distributions of these features. This has important implications for modelling the migration and containment behaviour of these materials.