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Pierre Gerard

Collaboration

Jon Harrington (British Geological Survey)

Frédéric Collin, Robert Charlier (ULg)

Description
Over time, after closure of a deep radioactive waste repository in an argillaceous formation, steel containers will corrode, water and organic material will be irradiated. The processes lead to the generation of hydrogen gas. As such, understanding the migration of gas in the host formation and in the engineered barrier systems are important issues, which may affect the safety function of the clay barriers. Indeed some laboratory scale experiments of gas injection on clay samples exhibit complex hydro-mechanical behaviour especially for initially water saturated conditions. Laboratory tests highlight that gas entry and breakthrough are often accompanied by the development of preferential pathways, which propagate through the sample.

Long-term gas migration tests have been performed by the British Geological Survey on the Callovo-Oxfordian argillite, the proposed host rock for the French repository. Results show a small emergent flux during the early stages of testing, before a spontaneous increase of discharge rate, which is interpreted as evidences of major breakthrough. The movement of gas is then through a localised network of pathways, whose properties vary temporarily and spatially within the claystone (Harrington et al., 2012).

 

gas injection test

 

Figure 1:Long-term gas injection test on Callovo-Oxfordian argillite (British Geological Survey)

 

In this study, this issue is addressed from the hydro-modelling of the gas migration tests with finite element code. A new model linking the permeability, but also the degree of saturation, with the mechanical behaviour has been formulated and provides an innovative explanation to the development of preferential gas pathways in clayey rocks. In the finite element code, this aperture is linked with the strains in the discontinuities. The model has been validated on a number of experimental datasets and shows that the hydro-mechanical coupling play an important role in a successful simulation of such gas flows. The gas breakthrough followed by a spontaneous gas outflow can be reproduced with the proposed hydro-mechanical couplings (Figure 3). Pathway permeability increases and air entry pressure decreases with the extension induced by the gas pressure raise (Figure 2).

 

pathway

Figure 2: Pre-existing fracture and distribution of the degree of saturation, gas permeability and gas pressure at different increasing gas injection stages

 

gas flow

Figure 3: Gas outflow: comparison between experimental and numerical results with HM coupling between pathway aperture, air entry pressure and permeability

   
References
  • Harrington, J.F., de La Vaissière, R., Noy, D.J., Cuss, R.J. and Talandier, J. (2012). Gas flow in Callovo-Oxfordian Clay (COx): Results from laboratory and field-scale measurements. Mineralogical Society special publication (in press).
  • Gerard P., Harrington J., Charlier R., Collin F. (2012) Hydro-mechanical modelling of the development of preferential gas pathways in claystone. Unsaturated soils: research and applications. In Mancuso C., Jommi C., D'Onza F. (Eds.), Springer, vol. 2, pp. 175-180.