Oil diffusion in shale nanopores: Insight of molecular dynamics simulation

Abstract

Shale contains various minerals. The occurrence behavior and flow mechanism of gas and oil in organic matters, inorganic material and clay minerals have significant differences. In this study, the diffusivity of oil in shale nanopores composed by various mineral types were investigated. The effects of pore size, temperature, pressure, alkane types, mineral types and moisture content on shale oil diffusion were discussed. The results revealed that the diffusion coefficients of octane in shale nanopores were on the order of 10−9 m/s2. The adsorption amount of oil in organic matters was almost two folds larger than that in inorganic composition and clay minerals. However, the diffusivity of oil decreases in the order of organic matter, inorganic material and clay mineral due to the ultrasmooth structure of organic surface. The oil diffusivity increased with the increase of pore size and temperature. Long chain alkane presented a weaker diffusivity due to the larger molecular weight and intermolecular forces. The temperature effect on long chain alkane diffusion was less evident comparing with shorter chain alkane. The existence of water inhibited the adsorption and diffusion of oil in the pores composed by all kinds of materials. The water inhibiting effect of oil diffusivity was much more significant in organic pores since the water accumulated as cluster in organic slit, providing a piston-like process, while a water film was presented on the surface of inorganic materials. The findings of this study revealed the oil diffusion behavior in shale nanopores and provided the theoretical support for shale oil exploration and development.