Presentazioni e Articoli Tecnici

Bentonite based materials (BBM) are considered as ideal buffer materials for deep geological repository (DGR) for nuclear wastes because of several desirable properties, such as low permeability, high adsorption capacity and proper swelling ability (Guo and Fall 2018). However, gas generation and migration in the DGR may have detrimental effects on the desirable properties of the BBM. Previous experimental studies have shown that the gas migration within the BBM is characterised by the development of preferential pathways which is driven by the highly pressurised gas (Graham et al. 2012). This paper proposes a new coupled hydromechanical (HM) model that considers the phase field method to simulate the preferential gas flow in initially saturated and heterogeneous bentonite material.

To explicitly model the preferential pathways, the phase field (PF) method is incorporated into the coupled HM framework for porous media. The governing equation for PF is implemented into COMSOL Multiphysics^® simulation software using the Coefficient Form PDE interface. In addition, the combination of a Domain ODE and a Previous Solution node is used to record the historical maximum value of elastic tensile strain energy. The coupled HM framework for porous media consists of a two-phase flow process and a deformation process. The two-phase flow process is modelled in COMSOL Multiphysics^®  by using two Darcy’s Law interfaces (one for water flow and the other for gas flow). Moreover, the rate of volumetric strain is included in the source term of each Darcy’s Law interface to account for the mechanical effects. The deformation process is modelled by the Solid Mechanics interface, in which the constitutive model for the Linear Elastic Material is modified in the Equation View to account for the damage effects caused by the PF. The average pore pressure is included in the Solid Mechanics interface by using the node of External Stress. The developed coupled HM-PF equations are solved in segregated steps by the time dependent solver. In the developed HM-PF model, no models in the Application Libraries were used.

Numerical simulation results show that the developed preferential pathway primarily passes through the area of low resistance to gas flow and fracturing, which is consistent with the physical nature and experimental results. Moreover, the developed pathway presents some tendencies to branch, which demonstrates the good ability of the PF method to simulate fracture propagation. As a summary, the developed HM-PF model has been successfully implemented into COMSOL Multiphysics^® simulation software and can satisfactorily capture the preferential gas flow in initially saturated bentonite.

References:

Graham, C.C., Harrington, J.F., Cuss, R.J., and Sellin, P. 2012. Gas migration experiments in bentonite: implications for numerical modelling, Mineral Mag 76(8): 3279-3292.

Guo, G., and Fall, M. 2018. Modelling of dilatancy-controlled gas flow in saturated bentonite with double porosity and double effective stress concepts, Eng Geol 243: 253-271.