In the context of the Cigéo project, the excavation of underground galleries in the Callovo-Oxfordian claystone induces a network of fractures around the excavated zone. This fracturing exhibits a typical profile of propagation in mixed I/II mode, which originates from the instantaneous discharge of the claystone after excavation. The topology and spatial extent of the induced network of fractures has been widely studied by ANDRA[1] around the excavations of the Laboratoire Souterrain de Meuse/Haute-Marne (LSMHM). Over long time, this network of fractures is expected to experience additional loadings by gas (mostly H2) originating from the corrosion of metals used in the envelop of nuclear waste packages and from water radiolysis. The accumulation of these gases induces a pressurization that can affect the fractured zone. ANDRA has been investigating the risks associated with gas pressurization since 2003. In particular, gas-fracturing tests have been performed in a vertical well drilled from the surface and in wells drilled from the LSMHM[2]. These test have revealed a fracturing induced by the injection of gas[3]. The interpretation of these tests is non trivial, for instance with a high sensibility to the injection kinetics or a crack propagation at gas pressures below the average in-situ stress. The process of gas injection involves many phenomena in the porous medium coupling transport (diffusion/convection), poro-mechanics and cracking/damage. Moreover, this process occurs in the complex geometry of the pre-existing fracture network due to the initial excavation. Accordingly, an approach by numerical simulation taking into account these different couplings appears necessary to evaluate the risk of fracturing under gas pressurization.
Enpc code crack
We propose to set up the numerical simulation based on Disroc[4]. Discroc is a finite element code dedicated in particular to the simulation of the fractured media. Cracking is accounted for by cohesive zone models, which are well adapted to the modeling of brittle and quasi-brittle media. Disroc has already been used for the study of fracturing in similar contexts. In particular, the propagation of fracture under hydromechanical couplings has been addressed in the PhD thesis of Z. Ouraga [5] (IFPEN) dedicated to the natural fracturing of sedimentary formations (oil and gas reservoirs), and of T. D. Vo [6] dedicated to the desiccation cracking of soils. A more recent research work [7] investigated the propagation of fractures under the effect of a fluid injection (cf. figure). The study of fracturing due to gas pressurization requires extending these works, dedicated to the case of a single incompressible fluid, to the case of a multi-phase fluid (liquid + compressible gas) in an unsaturated medium with possible effects of adsorption associated with bound water [8],[9].
The numerical model developed in this work will aim at predicting the cracking induced by a gas pressurization in order to better understand the roles of the various couplings involved and help interpret the in-situ tests. This modeling will take into account the various gas transport mechanisms in the porous medium, the presence of a damaged zone around the excavation and the influence of the different geological layers. The parameters of the numerical tool will be calibrated so as to reproduce the behavior of the Callovo-Oxfordian claystone. In this respect, various experimental results available in the literature or obtained in other ANDRA projects will be considered. The behaviors of the non-adsorbed fluids (gas, free water) will be accounted for with usual models (ideal gas, incompressible, or empirical equation of states if these two limit cases prove inappropriate in the range of temperatures-pressures of interest). The behavior of the bound water will be obtained by crossing experimental data of the THM couplings and molecular simulation data available in the literature[10].
The applicants must hold a Master of Science or equivalent in the field of mechanics and physics of (geo)-materials, with a strong taste for numerical approaches. Interested applicants are invited to send a CV, a motivation letter and their transcripts to L. Brochard (laurent.brochard@enpc.fr) or A. Pouya (amade.pouya@enpc.fr).
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