Hydro‐Mechanical Modelling of Gas Migration in Bentonite: Formulation and Laboratory‐Scale Validation

Abstract

Gas generation and migration in bentonite buffer materials are critical processes affecting the performance of deep geological repositories for radioactive waste. This study presents a new hydro‐mechanical formulation for simulating gas migration in saturated, compacted bentonite, with particular emphasis on the role of non‐uniform stress fields at the laboratory scale. The objective is to establish and evaluate a mechanistically consistent numerical framework capable of reproducing key gas migration features observed in controlled experiments. The proposed formulation incorporates a stress‐dependent intrinsic permeability law in which permeability evolution is governed by the differential pressure between gas pressure and the minimum principal stress. This approach provides a continuous representation of the transition from background two‐phase flow to stress‐activated, dilatancy‐controlled gas migration. To account for heterogeneity associated with spatial variations in dry density, a random distribution of elastic properties is incorporated into the model. Gas transport is represented using a dual‐continuum framework in which preferential pathways emerge implicitly through constitutive coupling, without prescribing discrete fractures or explicit pathway geometries. The model is implemented and validated against gas injection tests on Mx80 bentonite conducted under constant‐volume. Simulated gas fluxes, pore pressures, and stress responses show good agreement with experimental observations. The results demonstrate that incorporating non‐uniform stress fields is essential for reproducing localized dilatancy and non‐uniform gas flow at the laboratory scale. The proposed framework provides a robust basis for modelling stress‐controlled gas migration in bentonite and forms a foundation for future upscaling to larger‐scale experiments and repository conditions.