A continuous Approach for Modelling Fracture Formation and its Application to Gas Fracturing of Callovo-Oxfordian Claystone
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The Callovo-Oxfordian argillite beds located some 500m beneath the surface of the Meuse plain have been chosen as a site for the storage of nuclear material, particularly spent fuels and high level wastes. Determining the medium to long term mechanical response of the argillite to boring, changes in saturation arising during stockage and following sealing is essential to planning the repository. The presence of metals in the components of the repository conducts to the production of gases and to a pressure build up which are being taken into account through experiments to measure the resistance of the argillite subjected to hydraulic stresses. Recent advances in a model [1] implemented in the Cast3m finite elements code couple poroelasticity, plastic deformation, damage through cracking at the micro-scale, reversible opening of macro-scale cracks, diffusion of water and air with associated changes in degree of saturation and pore pressure, and swelling of clay minerals in a single, continuous model. Realistic simulations of argillite, incorporating spatial variations of properties such as arise from bedding planes, can be easily prepared and the model applied to them. Since the mechanical properties of argillite change from a relatively soft and plastic response near saturation to more a brittle response when dried, the variation of mechanical properties with the degree of saturation are an intrinsic part of the model. Simulations of the hydraulic fracturing experiments using the model reproduce the behaviour observed in the laboratory.
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