Gas flow in Callovo-Oxfordian claystone (COx): results from laboratory and field-scale measurements

Abstract

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.

Experimental observations of mechanical dilation at the onset of gas flow in Callovo-Oxfordian claystone

“…For the BGS test the sample was orientated with the long axis parallel to the bedding direction, whereas the LAEGO study was performed perpendicular to bedding. Despite the difference in core orientation, gas injection testing reported by Harrington et al (2012) also gave a high gas entry pressure similar to that reported here for a sample of COx orientated perpendicular to bedding. Therefore bedding direction does not have a major role in determining gas entry pressures and does not influence the underlying physics governing gas flow.…”

“…Figure 7 shows the interpretation of the onset of gas migration in COx. A three-stage interpretation has been postulated, which fits all the observations of the BGS and LAEGO, and also previously published results (Horseman et al 1996(Horseman et al , 2004Angeli et al 2009;Harrington et al 2009Harrington et al , 2012.…”

“…There is now a growing body of evidence (Horseman et al 1996(Horseman et al , 2004Angeli et al 2009;Harrington et al 2009Harrington et al , 2012 that in the case of plastic clays and in particular bentonite, classic concepts of porous-medium two-phase flow are inappropriate. Flow through dilatant pathways has been shown in a number of argillaceous materials (Horseman & Harrington 1994;Harrington & Horseman 1997Ortiz et al 1996Ortiz et al , 2002Gallé & Tanai 1998;Horseman et al 1999;Autio et al 2006;Angeli et al 2009).…”

Gas Breakthrough Pressure (GBP) through Claystones: Correlation with FIB/SEM Imaging of the Pore Volume

“…In general, at the laboratory scale, the initial conditions are a fully water-saturated medium subjected to a gas pressure of given amplitude, until gas passage is detected by varying means. However, no consensus exists as the exact phenomena involved in the gas migration (or breakthrough) through water-saturated clayrocks [3,20,22,[24][25][26][27][28][29]. Rather, several mechanisms are considered.…”

Long-term impact of temperature on the hydraulic permeability of bentonite

“…Cylindrical test specimens with an external diameter of 60 mm were manufactured by a combination of hand-trimming using a tubular former with a sharpened leading edge (Horseman & Harrington 1994) and machine lathing (Harrington et al 2012). Sample lengths were 53.9, 60.0 and 59.7 mm, respectively, for samples CRT-1, CRT-2 and CRT-3.…”