Finite volume modeling of coupled thermo-hydro-mechanical processes with application to brine migration in salt

Abstract

The disposal of heat-generating nuclear waste in salt host rock generates a thermal gradient around the waste package that may cause brine inclusions in the salt grains to migrate toward the waste package. In this study, a dual-continuum model is developed to analyze such a phenomenon. In this model, fluid flow in terms of advective and diffusive fluxes in the interconnected pore space and diffusive and thermal-diffusive fluxes in the salt grains is considered. Due to the very distinct behavior of fluid flow in the interconnected pore space versus in the salt grains, this process is simulated based on a dual-continuum model. In the dual-continuum model, the mass balance of salt and water in the two continua is separately considered, and the coupling between the two continua is represented by flux associated with brine migration in one medium and out of another. The energy balance is simulated assuming thermal equilibrium among different components and phases in the whole system. For mechanical analysis, a new formulation (extended finite volume method, XFVM) is proposed and is applied with a Voronoi tessellated mesh. The coupling between the hydraulic and mechanical fields in terms of pore-volume effects is consistent with Biot’s theory, while thermal and mechanical fields are linked in terms of thermal expansion. The resulting fully coupled THM model is capable of modeling strongly nonlinear features, involving salt concentration effects on fluid mass associated with advection, and thermal effects on brine migration. A Newton-Raphson iteration formula is used to generate the linearized equations for this nonlinear problem. The model was verified step by step for each component of the coupling terms, including thermal-hydraulic (TH) and hydro-mechanical (HM) couplings, and was applied to analyze diffusion in single continuum and dual continua, small-scale brine migration, and large-scale brine migration induced by thermal gradient. The results show that the model is able to quantify brine under different conditions and thermal gradients, making it a valuable tool for performance assessment for nuclear waste disposal in salt.