Confinement facilitates wetting liquid slippage through mixed-wet and heterogeneous nanoporous shale rocks

Molecular Dynamics (MD) simulation has been helpful to study liquid transport through simple pore structures, e.g., single straight pores. However, to study the overall flowing capability through a porous medium, e.g., shale rocks, heterogeneities should be considered at the Representative Elementary Volume (REV) scale. We propose an analytical permeability model for liquid through a nanoporous REV by accounting for the heterogeneity, tortuosity, and wettability features. We model oil slippage and adsorption in hydrophobic pores and hydrophilic pores to investigate the apparent slippage phenomenon in the mix-wet porous media. The fractal theory is applied to characterize REV-scale heterogeneities including pore size distribution, pore-throat tortuosity, and pore surface roughness. We particularly modify the classical slippage factor by a fractal tortuosity to study liquid slippage through tortuous pores by using MD data for straight nanotubes. The proposed model gives an insight to liquid transport mechanisms in nanoporous and heterogeneous porous media and contributes to understanding hydrocarbon production in tight reservoirs at field scales. The results show two competing impacts of pore confinement. 1) The apparent oleic slippage is the result of liquid-solid interactions, and more importantly, the pore confinement's effect. Oil can slip in hydrophobic organic pores evidently, quantitatively comparable to oil slippage in hydrophilic inorganic pores, due to a higher pore-throat tortuosity and a smaller pore size. 2) The apparent oil permeability is the result of intrinsic permeability and apparent slippage. Despite a comparable slippage factor in both organic and inorganic pores, the apparent permeability in organic matter is restricted by stronger pore confinement.