Predicting CO2-water interfacial tension under pressure and temperature conditions of geologic CO2 storage

Storage in subsurface geologic formations, principally saline aquifers, is currently under development as a major approach to counter anthropogenic CO2 emissions. To ensure the stability and long-term viability of geologic carbon storage, injected CO2 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 CO2 phase being the primary control. However, published experimental CO2-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 (5-45 MPa) and temperature (298-383 K) 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 CO2-H2O interaction. Through molecular dynamics simulations, we compared the quality of three CO2 models and two H2O models (SPC/E and TIP4P2005) in predicting IFT under the pressure and temperature conditions relevant to geologic CO2 sequestration. Interfacial tension at fixed temperature simulated via molecular dynamics decreased strongly with increasing pressure below the critical CO2 pressure of 7 MPa, then leveled off, in agreement with experiment, whereas increasing temperature from 300 to 383 K 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 CO2-H2O interaction exerts a major influence on IFT. The CO2 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 CO2 model underestimated IFT by similar to 10 mN/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 CO2-H2O IFT at 383 K over the entire pressure range examined. These pressure variations will have the dominant effect on IFT -especially at pressures lower than the CO2 critical pressure (similar to 7 MPa)-and, therefore, on the CO2 storage capacity and sealing integrity of a subsurface reservoir. (C) 2011 Elsevier Ltd. All rights reserved.