Prediction of fluid topology and relative permeability in imbibition in sandstone rock by direct numerical simulation

•Fully coupled visco-capillary simulation, using a free-energy based lattice Boltzmann approach, correctly predicts fluid connectivity in imbibition.•The results highlight the close connection between relative permeability and fluid topology and suggest that topological measures are much more meaningful validation criteria for pore-scale simulation.•Direct simulation approach described in this paper, using appropriate initial and boundary conditions, correctly predicts Darcy-scale effective properties. Pore-to-Darcy scale upscaling of multiphase flow is one of the major unresolved problems in many fields of porous media research. While this problem involves very fundamental aspects, there are many practical and application-driven challenges as well, such as the accurate prediction of Darcy-scale multiphase effective properties, e.g., relative permeability by pore-scale flow simulation on the basis of the imaged pore geometry, e.g., via X-ray computed micro-tomography. Validation of pore-scale modeling methods against experimental data by comparison of measured against simulated relative permeability curves has proven to be insufficient. Comparison of the fluid topology, in particular, the non-wetting phase topology, is a much more reliable criteria as relative permeability shows a very strong correlation with connectivity. While percolation-based quasi-static modeling approaches operating in the capillary limit have proven moderately successful in drainage, they largely fail to predict fluid connectivity in imbibition. We show that a fully coupled visco-capillary simulation, using a free-energy based lattice Boltzmann approach, correctly predicts fluid connectivity in imbibition, which was not predicted correctly by a quasi-static approach in a previous study on the same dataset. The respective relative permeability data shows a close match with results from Darcy-scale core flooding experiments while there is a major mismatch from quasi-static approach. The results once more highlight the close connection between relative permeability and fluid topology and suggest that topological measures are much more meaningful validation criteria for pore-scale simulation. The results also show that the direct simulation approach described in this paper, using appropriate initial and boundary conditions, correctly predicts Darcy-scale effective properties.