A Pore Scale Model for Migration of Brine Inclusions in Salt Crystals in Thermal Gradients

Natural salt deposits have low permeability and are thus excellent candidates for hosting heat-emanating wastes (e.g., used nuclear fuels). However, salt may still contain a certain amount of water, either deposited with the salt or emplaced during some secondary process. A form this water may be present in salt formations is as intracrystalline inclusions. In the thermal gradients generated by the waste, because the solubility of salt increases with temperature, salt dissolves at the interface of the inclusion closest to the heat source, diffuse through the brine, and precipitate at the interface furthest from the heat source. Thus, the inclusions tend to migrate toward the heat source. Migration of brine inclusions in thermal gradients could negatively impact the safe disposal of heat-generating nuclear waste. In this contribution, we discuss the development of a pore scale model to simulation of the migration of brine inclusions in salt crystals in thermal gradients. In the pore scale approach, each point in space is assigned either to the solid or fluid phases; thus, there is no need to consider bulk properties for the porous medium as in the traditional Darcy-scale continuum approach. Rather, the size and shape of brine inclusions, and thus the solid-fluid interfaces are considered explicitly. Pore scale modeling is constrained by experimental observations. Specifically, we use the observed rate of migration to calibrate model reaction rates and to explore the effect of the magnitude of the temperature gradient as well as the effect of multiple minerals with different solubilities. We show that pore-scale modeling can provide a useful tool to inform constitutive relationships applicable at the continuum scale.