Mixture flow of water and methane through shale nanopore by molecular dynamics simulation

Abstract

The coexistence and flow of water and methane is important during hydraulic fracturing process in shale gas. In this work, the mixture flow of water and methane inside the kaolinite nanopore under shale reservoir conditions is investigated using nonequilibrium molecular dynamics simulations. The kaolinite pore has rectangular cross-sectional shape with inner pore surface area of basal surface and B chain edge surfaces. The B chain edge surface is determined by Periodic Bond Chain theory and the partial charge of edge oxygen atoms are computed by bond valence coordination method. The force field is CLAYFF which is specifically for clay minerals. The external body force field is applied to drive the water gas to flow. The results indicate that clay surface is hydrophilic and methane is hydrophobic due to the affinity difference between water and methane. Methane has more affinity with siloxane oxygen atoms of kaolinite surface than water molecules. The nanoscale confinement makes the water channel in the cuboid pore exhibit quasi-round shape, which is due to the potential overlap from the pore corner atoms. The fluid flow reaches stable in one nanosecond, indicated by the stabilized flux. The flow of methane exhibits Poiseuille flow feature in the round pore. The calculated number flux of methane is about two times as large as water, and the slip velocity of water and methane increase from 0.4 m/s, 1.6 m/s to 9 m/s, 15.2 m/s respectively as external pressure changes from 10 to 400 MPa. The viscosity components of water along different planes show significant anisotropy due to the anisotropic pore surface chemistry and irregular pore shape. In addition, the water gains much larger viscosity (11.4 mPa∙s) when confined in the kaolinite nanopores, significantly higher than its bulk state viscosity of 0.47 mPa∙s at 333 K, while the viscosity of methane changes relatively small compared with its bulk state.