A multi-scale insight into gas transport in a deep Cenozoic clay

The migration of gases is crucial to ensure the long-term feasibility of argillaceous formations for the deep disposal of radioactive waste. This paper presents an experimental investigation with a multi-scale perspective on the response to gas transport of initially saturated Boom Clay (Belgium). Gas injection tests have been performed under oedometer conditions at different controlled-volume rates, constant total vertical stress and different sample orientations (flow orthogonal or parallel to bedding planes).
The results confirm soil expansion and consequent degradation during injection that has a significant impact on the aperture of localised gas pathways (fissures) and increases intrinsic permeability during the gas pressure dissipation stage. The analyses with complementary techniques (mercury intrusion porosimetry, field-emission scanning electron microscopy and X-ray micro-tomography) confirm the opening of fissures with different apertures and separations at the microstructural scale. Large-aperture fissures develop along the weaker bedding planes. These techniques allow the volume of fissures to be quantified, which does not significantly depend on gas flow direction, as also measured in the isotropic response of the gas effective permeability. A scalar damage variable derived from the fissured fraction has been used to assess the gas-entry pressure reduction and the intrinsic permeability increase after the gas tests in both directions.