Molecular dynamics simulations of natural gas-water interfacial tensions over wide range of pressures

Natural gas-water interfacial tension (IFT) plays an important role in gas production and transportation as well as hydrate formation. While experiments can measure IFT from macroscopic perspective, the interfacial phenomena remain less clear at molecular level. In this work, we use molecular dynamics (MD) simulations to study hydrocarbon-water IFT up to 5000 bar at various temperature conditions. At each temperature, we study the IFT of C1-H2O system, C1 + C2 mixture-H2O system, C1 + C3 mixture-H2O system, and C1 + C2 + C3 mixture-H2O system. We find that IFTs decrease with increasing temperature at low pressure conditions, while the differences become insignificant at high pressures. Addition of C2 and C3 can lower IFT in line with the previous experimental findings, while C3 has a more pronounced effect than C2. However, after a certain pressure, IFT becomes similar for various hydrocarbon mixture-H2O systems. As pressure further increases, IFT gradually increases. At low and intermediate pressures, hydrocarbons can form adsorption layers on gas-water interfaces but become less significant at high pressures. We find that IFT decreases when the relative adsorption obtained from density distributions is positive but increases for negative relative adsorption at high pressures. This finding agrees well with previous molecular simulation work on C1-water interfacial tension. At low and intermediate pressures, relative adsorption becomes more significant when the heavier components (C2/C3) are added, resulting in a more pronounced IFT reduction effect. Our study provides fundamental understandings about the interfacial phenomena between hydrocarbon and water, and important insights into the energy prospection and flow assurance problems.