Improved lattice Boltzmann method to simulate liquid flow in nanoporous media: Coupling molecular dynamics simulations and theoretical model

Understanding the mechanisms of nanoconfined liquid flow in nanoporous media is crucial for many scientific and engineering applications such as enhanced shale oil recovery and water purification. However, due to molecular interactions which cause slip boundary and heterogeneous viscosity/density in nanoscale space, the conventional continuity equations are no longer applicable. Besides, the pore size in popular molecular dynamics simulation (MDS) is too small to meet the engineering application, and the pore-scale lattice Boltzmann method (LBM) is difficult to accurately consider molecular interactions in complicated nanoporous media. In this work, we summarize and establish four theoretical models for liquid flow in nanopores, which are region-separation model, effective viscosity model, apparent viscosity model, and apparent slip length model. Then, through comparative analysis, we indicate that the apparent viscosity model is most suitable for coupling LBM. Based on apparent viscosity model which comprehensively considers slip boundary and heterogeneous viscosity/density, we present a local-apparent-viscosity LBM (LAV-LBM) to simulate liquid flow in nanoporous media. According to LAV-LBM simulations, the effects of molecular interactions, porous media geometry, and wall wettability on apparent permeability of water flow in nanoporous media are discussed. Finally, we couple MDS results with the proposed model to calculate apparent permeability of shale oil flow in inorganic and organic media, which provides a basic model for the upscaling method from molecule-scale to pore-scale simulations.