Effect of ion concentration and multivalence on methane-brine interfacial tension and phenomena from molecular perspectives

Abstract

Gas-brine interfaces play an important role in natural gas recovery, which can be negatively affected by capillary pressure, induced by the interfacial tension (IFT) between gas and brine phases. Therefore, understanding about the gas-brine IFT is of great importance for natural gas production. While experiments can measure the IFT, the underlying mechanisms and interfacial phenomena remain less clear at molecular level. Previous molecular simulation works are limited to narrow salt concentrations and types. In this work, we use molecular dynamics (MD) simulations to study IFT between methane and brine containing various monovalent and divalent cations (Na+, K+, Mg2+, and Ca2+) over wide range of salt concentrations (∼0.05 to ∼4.53 M, i.e. between ∼3.0 and ∼23.6 wt% in terms of NaCl solution) under reservoir conditions. We find that methane accumulates at interfaces, resulting in the decrease of IFT as pressure increases, but becomes insignificant at high pressures. On the other hand, pressure has minor effects on water and ion distributions. Cation type has a negligible effect for given anion (Cl−) concentrations, indicating that the charge molarity is the dominant factor to determine the gas-brine IFT. In addition, while both cations and anions deplete from the gas-brine interfaces, divalent cations are more devoid from the interface than monovalent cations, showing strong electrical double layers. The electrostatic potentials on the gas and brine sides are positive and negative, respectively. Our study should provide fundamental understandings on the gas-brine interfacial properties, and important insights into natural gas production.