Experimental observations of mechanical dilation at the onset of gas flow in Callovo-Oxfordian claystone
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.
A four-stage model has been postulated to explain the experimental results. Stage 1: gas enters at
the gas entry pressure. Gas propagation is along dilatant pathways that exploit the pore network of
the material. Around each pathway the fabric compresses, which may lead to localized movement of
water away from the pathways. Stage 2: the dendriticflow path network has reached the mid-plane of
the sample, resulting in acceleration of the observed radial strain. During this stage, outflow from the
sample also develops. Stage 3: gas has reached the backpressure end of the sample with end-to-end
movement of gas. Dilation continues, indicating that gas pathway numbers have increased. Stage 4:
gas-fracturing occurs with a significant tensile fracture forming, resulting in failure of the sample.
Both studies clearly showed that as gas started to move through the COx, the sample underwent
mechanical dilation (i.e. an increase in sample volume). Under in situ conditions, the onset of
dilation (micro-fissuring) is a necessary precursor for the advective movement of gas.