Modelling of dilatancy-controlled gas flow in saturated bentonite with double porosity and double effective stress concepts

Dilatancy-controlled gas flow is a dominant process of gas migration in saturated bentonite. In this paper, a fully coupled hydro-mechanical model, which incorporates the concepts of double porosity and double effective stress, is developed to simulate the gas migration process in saturated bentonite. Double effective stress is derived based on the first law of thermodynamics and the mixture theory. The volumetric strain which is work-conjugated to each effective stress level is explicitly included in the mass balance equations. Thus, the resultant mass balance equations are more robust from the viewpoint of thermodynamics. The developed model is successfully validated against the results of three laboratory tests conducted under different mechanical boundary conditions. In general, the model is able to well simulate the main experimental behaviors, including the gas breakthrough, the volume dilation, the matrix consolidation, the build-up of water pressure and the shut-in pressure. Two important processes that contribute to the development of preferential pathways, i.e., the dilation of the fractured porous media and the consolidation of the porous continuum, have been successfully identified by the model.