Modeling of flow and transport in multiscale digital rocks aided by grid coarsening of microporous domains

Many subsurface porous media such as soils, carbonate rocks, and mudstones possess multiscale porous structures that play an important role in regulating fluid flow and transport therein. A pore-network-continuum hybrid model is promising for numerical studies of a multiscale digital rock. It is, however, still prohibitive to the REV-size modeling because tens of millions of microporosity voxels may exist. In this work, we develop a novel and robust algorithm for coarsening microporosity voxels of a multiscale digital rock. Then, we combine coarsened microporosity grids with the pore network of resolved macropores to form efficient computational meshes. Furthermore, a pore-network-continuum simulator is developed to simulate flow and transport in both a synthesized multiscale digital rock and a realistic Estaillades carbonate rock. We show that the coarsening algorithm can reduce computational grids by about 90%, which substantially reduces computational costs. Meanwhile, coarsening microporosity has a minor impact on the predictions of absolute permeability, gas production curves, and breakthrough curves of solute transport. We illustrate the mechanisms of flow and transport in multiscale porous media induced by microporosity. Finally, the efficient hybrid model is used to predict the absolute permeability of an Estaillades digital rock. The numerical prediction matches well with the reported experimental data. We highlight the importance of characterizing mean pore-size distributions in microporosity for the prediction of rock permeability and local flow fields. The developed pore-network-continuum hybrid model aided by grid coarsening of microporosity serves as a useful numerical tool to study flow and transport in multiscale porous media.