Effect of water behaviour on the oil transport in illite nanopores: Insights from a molecular dynamics study

Abstract

Both of the water injected by hydraulic fracturing and connate water can coexist with oil inside shale nanopores, which leads to different oil/water mixtures behaviors compared to those with only oil and water. However, the behavior of oil/water mixtures in shale pores is not clear. In this study, molecular dynamics simulations are applied to investigate the behavior of oil/water mixtures in a rectangle-shaped illite nanopore that has a hydrophilic internal surface. The behavior of oil/water mixtures is much more complex than that of gas/water mixture because water can dissolve in oil at particular pressures and temperatures. The results of the calculation show that illite confinement has a strong impact on both oil distribution and water adsorption. A structure of the oil droplets being wrapped by water can be identified in the illite nanopores which may be contributed by the hydrophilic surfaces. Besides, water bridges may form due to the dispersion of unabsorbed water into the oil and could increase flow resistance in the pore. Viscosity and self-diffusion of fluids are increased and decreased separately in the nanopores compared to those without the illite confinement which is called the bulk condition in this study. In the bulk condition, increasing the water concentration will reduce the shear viscosity of fluids, while an opposite phenomenon appears in shale nanopores. The results of this study may help to deepen the understandings of water and oil transport in nanopores and provide some guidelines in extracting shale resources in an aqueous environment.