Predicting CO ₂-water interfacial tension under pressure and temperature conditions of geologic CO ₂ storage

Storage in subsurface geologic formations, principally saline aquifers, is currently under development as a major approach to counter anthropogenic CO ₂ emissions. To ensure the stability and long-term viability of geologic carbon storage, injected CO ₂ must be kept in place by an overlying cap rock of very low permeability. Capillary forces in the cap rock act to prevent upward migration and escape of the stored supercritical fluid, with interfacial tension (IFT) between the aqueous brine phase and the CO ₂ phase being the primary control. However, published experimental CO ₂-water IFT data vary widely, mainly because of inadequate experimental protocols or inappropriate use of bulk-fluid properties in computing IFT from experimental observations. Only two published data sets were found to meet all criteria of merit for an accurate measurement of IFT over the entire range of pressure (5-45MPa) and temperature (298-383K) pertinent to geologic carbon storage. In such circumstances, molecular simulations can enhance the utility of limited data when used to validate assumptions made in their interpretation, resolve discrepancies among data, and fill gaps where data are lacking. Simulations may also be used to provide insight into the relationship between IFT and fundamental properties, such as the strength of the CO ₂-H ₂O interaction. Through molecular dynamics simulations, we compared the quality of three CO ₂ models and two H ₂O models (SPC/E and TIP4P2005) in predicting IFT under the pressure and temperature conditions relevant to geologic CO ₂ sequestration. Interfacial tension at fixed temperature simulated via molecular dynamics decreased strongly with increasing pressure below the critical CO ₂ pressure of 7MPa, then leveled off, in agreement with experiment, whereas increasing temperature from 300 to 383K at fixed pressure had little effect on IFT, which is also consistent with experimental data. Our results demonstrated that the strength of the short-range portion of the CO ₂-H ₂O interaction exerts a major influence on IFT. The CO ₂ model that best represented the attractive part of this interaction for randomly-oriented water molecules also best captures the experimental pressure dependence of IFT when combined with either water model. When combined with the SPC/E water model, this CO ₂ model underestimated IFT by ∼10mN/m, which approximately equals the amount by which the SPC/E water model underestimates the surface tension of pure water. When combined with the TIP4P2005 water model, this model accurately captured the pressure dependence of the CO ₂-H ₂O IFT at 383K over the entire pressure range examined. These pressure variations will have the dominant effect on IFT-especially at pressures lower than the CO ₂ critical pressure (∼7MPa)-and, therefore, on the CO ₂ storage capacity and sealing integrity of a subsurface reservoir.