A Unified Multiscale Model for Pore-Scale Flow Simulations in Soils

Pore-scale simulations have received increasing interest in subsurface sciences to provide mechanistic insights into the macroscopic phenomena of fluid flow and reactive transport processes. The application of pore-scale simulations to soils and sediments is challenging, however, because of the characterization limitation that often allows only partial resolution of pore structure and geometry. A significant proportion of the pore spaces in soils and sediments is below the spatial resolution, forming a mixed medium with pore and porous regions. The objective of this research was to develop a unified multiscale model (UMSM) that can be used to simulate fluid flow and transport in mixed media containing pore and porous regions. The UMSM modifies the classic Navier-Stokes (N-S) equations by adding a Darcy term to describe fluid momentum and uses a generalized mass balance equation for saturated and unsaturated conditions. A series of simulations of water flow in pore, porous, and mixed pore and porous regions were performed to evaluate the UMSM by comparing with other numerical approaches. A water imbibition experiment was conducted in a soil column to compare theoretical predictions with experimental measurements. The results indicated that the UMSM is numerically equivalent to the N-S equations in pore regions, becomes Darcy's law in porous regions, and is equivalent to a model coupling the N-S and Darcy's law in a mixed medium containing pore and porous regions. The UMSM-simulated water imbibition also matched well with experimental measurements in the soil column, with its pore structures characterized from X-ray tomography. The UMSM approach allows direct simulation of fluid flow at the voxel resolution of characterization in realistic soils and sediments under both saturated and unsaturated conditions.