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Ionic Effects on Supercritical CO2–Brine Interfacial Tensions: Molecular Dynamics Simulations and a Universal Correlation with Ionic Strength, Temperature, and Pressure

Zhao, Lingling, Ji, Jiayuan, Tao, Lu, Lin, Shangchao
Langmuir 2016 v.32 no.36 pp. 9188-9196
aquifers, capacitance, carbon dioxide, carbon sequestration, cations, engineering, ionic strength, magnesium chloride, molecular dynamics, salinity, sodium sulfate, surface tension, temperature
For geological CO₂ storage in deep saline aquifers, the interfacial tension (IFT) between supercritical CO₂ and brine is critical for the storage security and design of the storage capacitance. However, currently, no predictive model exists to determine the IFT of supercritical CO₂ against complex electrolyte solutions involving various mixed salt species at different concentrations and compositions. In this paper, we use molecular dynamics (MD) simulations to investigate the effect of salt ions on the incremental IFT at the supercritical CO₂–brine interface with respect to that at the reference supercritical CO₂–water interface. Supercritical CO₂–NaCl solution, CO₂–CaCl₂ solution and CO₂-(NaCl+CaCl₂) mixed solution systems are simulated at 343 K and 20 MPa under different salinities and salt compositions. We find that the valence of the cations is the primary contributor to the variation in IFT, while the Lennard-Jones potentials for the cations pose a smaller impact on the IFT. Interestingly, the incremental IFT exhibits a general linear correlation with the ionic strength in the above three electrolyte systems, and the slopes are almost identical and independent of the solution types. Based on this finding, a universal predictive formula for IFTs of CO₂–complex electrolyte solution systems is established, as a function of ionic strength, temperature, and pressure. The predicted IFTs using the established formula agree perfectly (with a high statistical confidence level of ∼96%) with a wide range of experimental data for CO₂ interfacing with different electrolyte solutions, such as those involving MgCl₂ and Na₂SO₄. This work provides an efficient and accurate route to directly predict IFTs in supercritical CO₂–complex electrolyte solution systems for practical engineering applications, such as geological CO₂ sequestration in deep saline aquifers and other interfacial systems involving complex electrolyte solutions.