Molecular dynamics simulation of the interfacial properties of methane-water and methane-brine systems

In this study, a series of molecular dynamics (MD) simulations are performed for methane-water and methane-brine systems in the range of 263.15 K∼283.15 K, 1 MPa∼10 MPa with the ion mass fraction up to 25 wt%. The interfacial tension (IFT) and density profile, relative adsorption, charge distribution, ions hydration, interfacial thickness are predicted, and the interfacial features are analysed at a molecular level. The results show that the IFT decreases with pressure and is enlarged with the presence of ions, which is positively correlated with ion concentration. Moreover, the methane accumulation and ions depletion at the interface are observed. The relative adsorption of methane and ions at the Gibbs dividing surface and the interfacial thickness are estimated quantitatively. It is shown that the methane relative adsorption is positive and increases at a higher pressure and lower temperature, while the relative adsorption of ions is negative. For the two systems, the interfacial thickness increases with pressure and ion concentration, but the variation trend with temperature is related to the pressure. The interfacial features found in the present study may help understand the interaction among gas, water, and ions, as well as the phase transition behaviour of natural gas hydrate.