Advances in modelling of hydro-mechanical processes in gas migration within saturated bentonite: A state-of-art review

Abstract

Bentonite is a key material for the construction of Engineered Barrier Systems (EBSs) in deep geological repositories (DGRs) for radioactive waste. Modelling of gas migration in saturated bentonite, as well as the accompanied hydromechanical (HM) processes is crucial for conducting a performance assessment for the safety of a deep geological repository (DGR). The objective of this paper is to review the state-of-the-art of coupled HM models for simulating the gas migration behaviors in saturated bentonite. To this end, the paper has reviewed and discussed the coupled HM models from several aspects, including governing equations, HM constitutive models, fracture theories and related numerical approaches. Moreover, these models are discussed in terms of their merits and limitations to simulate the observed experimental behaviors. It is found that the previous HM models were generally established in the framework of Biot's consolidation theory or mixture theory. The adopted mechanical models include linear/nonlinear elastic models, elastoplastic models, damage models and swelling models. The incorporation of plastic and damage models into the coupled HM framework has enabled to account for the effect of the development of preferential pathways. However, these mechanical models don't enable to explicitly simulate the preferential pathways. The gas flow in saturated bentonite has been generally modelled by the generalized Darcy's law. Visco-capillary two-phase flow model with enriched intrinsic permeability (gas-pressure-based model, porosity-based model, strain-based model, damage-based model, Embedded Fracture Model, EFM) is the most used hydraulic model to simulate the gas flow. The EFM and its variations are the most widely used permeability models to simulate the gas migration process in saturated bentonite. The development of preferential pathways has been commonly considered in an implicit way by using the embedded fracture model as intrinsic permeability model or by adopting plastic or damage models to describe the mechanical behaviors. Current numerical models that can explicitly simulate the gas-driven fracturing process in saturated bentonite were not frequently reported. The existing ones are not complete enough to simulate all the key experimental behaviors associated with the preferential pathways. Advanced numerical approaches, such as XFEM or DEM based approaches, and proper fracture theories, such as the cohesive zone model, need to be employed to explicitly describe the development of preferential pathways. At the end of this paper, conclusions and recommendations for future modelling studies on gas migration in bentonite barrier are given. • Review and discussion of existing coupled HM models for simulating gas migration in saturated bentonite • The merits and limitations of previous coupled HM models are provided • Explicitly simulating the development of preferential pathways is critical to evaluate the sealing ability of bentonite • Cohesive zone model is a promising fracture theory to simulate the gas-driven fracturing process in bentonite