Influence of the grain structure on solute transport in constructed inhomogeneous media from pore-scale simulations

Grain structures widely exist in porous media and play a significant role in solute transport in media. One homogeneous and three inhomogeneous grain structures were constructed, and the finite element method (FEM) was adopted to simulate conservative solute transport in the porous media. The velocity field and plume evolution were analysed. The breakthrough curves (BTCs) and residence time distributions (RTDs) were calculated, and the continuous time random walk (CTRW) inverse model was applied to fit the BTCs to reveal the influence of the grain structure on solute transport quantitatively. The flow velocity and plume evolution are homogeneously distributed in the monolayer homogeneous grain structure, but they are heterogeneous in multilayer inhomogeneous grain structures with non-Fickian characteristics. These results occur because the heterogeneity of the grain structure forms advantageous channels and the plume is divided into dominant branches with an “early arrival” character, which is consistent with the analysis of the BTCs. Conversely, the “long tail” characteristics occur because the main driving force is diffusion, which is consistent with the analysis of the RTDs. After further confirmation by the CTRW inverse model, it can be concluded that both the particle and layer thickness can influence solute transport. This paper proposed a new method for the construction of the grain structure and researched its influence on solute transport, which may improve our understanding of the mechanism of solute transport through inhomogeneous media at the pore scale.