Salinity Effect on the Interfacial Tension of CO2-Brine: Estimation and Investigation by Molecular Dynamics Simulations

Abstract

AbstractWhile there are numerous experimental data on CO2-brine interfacial tension (IFT), few studies on salinity effects over a wide range of concentrations have been reported. Thus, there is room for further research in understanding the mechanism of the IFT change. The objective of this study is to estimate the effect of salinity on CO2-brine IFT by molecular dynamics (MD) simulations and to gain a better understanding of the phenomenon through microscale insight.IFT of CO2-brine was calculated for a wide range of salinity conditions from pure water to 5 mol/kg NaCl solution at temperatures and pressures from 298 to 473 K and 8 to 20 MPa. To calculate IFT, MD was performed by using the Nosé-Hoover thermostat and the Parrinello-Rahman barostat to keep temperatures and pressures constant.The calculated results show an increasing trend against salinity, which is in good agreement with experimental data from previous studies. For example, the IFT under typical reservoir conditions of 313 K and 10 MPa were 30.0 mN/m for pure water, while 31.9, 34.2, 36.9, 39.4, and 42.6 mN/m for 1, 2, 3, 4, and 5 mol/kg NaCl solutions, respectively. The density distribution of ions in the aqueous phase near the interface and in the bulk region captured the negative adsorption of ions. This enables us to interpret the mechanism of the increase of IFT in light of the Gibbs adsorption isotherm.IFT data calculated in this study is beneficial for estimating and modeling fluid behaviors of CO2-brine systems under a wide range of salinity conditions. In addition, atomic-scale insights would contribute to a better understanding of the interfacial phenomena regarding CO2-brine including high salinity regions.