Apparent permeability for liquid transport in nanopores of shale reservoirs: Coupling flow enhancement and near wall flow

Abstract

Multiple mechanisms of oil transport in inorganic and organic nanopores of shale oil reservoirs are still unclear and possibly more complex than those of gas transport in nanoporous media, due to differences of molecules free path and fluid-solid molecular interactions. The accurate apparent permeability model considering oil transport mechanisms and different pore types is important for macroscale modeling in shale oil reservoirs development. Based on studies of molecular dynamics simulations (MDS), liquid flow through carbon nanotubes (CNTs) and theoretical analysis, a unified apparent permeability model of liquid hydrocarbon flow in the shale is derived coupling different transport mechanisms in inorganic and organic nanopores. The model of oil-wet organic nanopores considers liquid-solid adsorption, while the model of water-wet inorganic nanopores incorporates near wall flow and velocity slip. We then introduce complicated structural parameters including the tortuosity, porosity and total organic carbon (TOC) to develop models from nanotubes into porous media. After that, the proposed model is validated by MDS and experimental results, and the total apparent liquid permeability (ALP) as well as contributions of different mechanisms are studied. The results indicate that, flow enhancement should be considered in the characterization of oil transport in nanopores, and the velocity of oil in inorganic nanopores much faster than that in organic nanopores in this work. For pore radii under 10nm, the total ALP is much larger than intrinsic permeability, and adsorption effect as well as velocity slip in organic matter (OM) and inorganic matter (IM) influence the total ALP slightly when the pore radius is larger than 100nm. In addition, the greater slip length in IM results in greater contributions of oil transport in IM to the total ALP if slip length is less than 10nm. Moreover, the ratio of the total ALP to intrinsic permeability decreases as TOC increases when TOC is larger than 20%. This work focuses on enriching the theoretical research of oil transport in nanopores and provides a unified ALP model for macroscale modeling study in the shale reservoirs development.