Implementation and validation of pressure-dependent gas permeability model for bentonite in FEM code Thebes

Abstract

In an Engineered Barrier System of a nuclear waste repository, gas migrates through: a) diffusion/advection of dissolved gases, b) two-phase continuum flow, c) dilatant pathway flow and d) single-phase gas flow through macro-fractures in the soil. The gas production rate and the corresponding gas pressure accumulation affect the clay material behaviour and its properties such as air entry value. For the safe design of the EBS system, computational models need to account for the identified transport mechanisms. This study presents an enhancement in the finite element code Thebes [1, 2] that replicates the observed increase in permeability at higher gas pressures, e.g. due to pore dilatancy and gas fracture as proposed by Xu et al. [3]. The formulation links permeability to gas pressure and threshold/critical pressure. For model validation, the study utilizes a gas injection experiment carried out in IfG (Institute for Rock Mechanics, Germany) on Opalinus Clay [4]. The results show a good fit against the measurements while giving insight into gas flow through clays.