MOLECULAR MODELING OF CARBON DIOXIDE-WATER MIXTURES UNDER GEOLOGIC SEQUESTRATION CONDITIONS

The success of geologic carbon sequestration depends largely on the ability of the caprock to prevent the escape of CO₂. Capillary breakthrough overpressure—the pressure above which CO₂ may leak through the caprock—in turn depends upon the interfacial tension (IFT) between supercritical CO₂ and brine. Molecular dynamics (MD) simulations are widely used to probe the interactions that lead to fluid-fluid properties such as IFT. Among the leading models of CO₂-water interactions, three-site parametric approaches are favored for their accuracy and ease of implementation. We used MD simulations to compare three of these models [EPM2 (Harris and Yung, 1995), PPL (Panhuis et al., 1998), and DZ (Duan and Zhang, 2005, 2006)] for their ability to reproduce IFT and solubility data in CO₂-SPC/E H₂O mixtures over the range of P and T relevant to geologic sequestration. Long-range interactions in the models are characterized by Coulomb potentials between partial charges localized on three sites in CO₂ and H₂O, while short-range interactions are characterized by Lennard-Jones (LJ) potentials with two parameters, potential well depth (ε) and hard-sphere radius (σ). Each model differs significantly in one or more of these parameters as applied to the CO₂-H₂O interaction, enabling us to examine the effects of varying ε, σ, or q on IFT and solubility. None of these models perfectly reproduces experimental IFT or solubility data. For example, the PPL model under-predicts IFT and over-predicts CO₂ solubility as functions of P, while the DZ and EPM2 models track IFT reasonably well but under-predict CO₂ solubility. The best agreement overall was obtained with the EPM2 model, which may be improved to reproduce solubility more accurately by increasing its CO₂ ε-parameter.