Phase Equilibria of CO2 and CH4 in Multisalt Brines: Parametrization and Validation of an Ion-Specific Electrolyte Cubic-Plus-Association Equation of State

Abstract We present an ion-specific parameterization of the electrolyte Cubic-Plus-Association (eCPA) equation of state, which we refer to as ms-eCPA, for phase equilibria of CO2 and CH4 in multisalt aqueous brines containing Na+, Cl–, K+, Ca2+, Mg2+, and SO42–. The model includes a Soave–Redlich–Kwong physical term with a Huron–Vidal mixing rule, Wertheim 4C association of water with CO2 cross-association, and Debye–Hückel plus Born electrostatic contributions on an ionic basis; the static permittivity is closed self-consistently through the Onsager–Kirkwood relation. The parameterization is hierarchical─each level of added physical description (nonpolar fluids, the associating solvent, ions in the solvent, gases over brine) introduces only parameters fitted to that level’s own data─with the top-level gas–ion interaction energies and ion volume translations calibrated against 1,931 experimental CO2–brine VLE points in five salts (T = 283–473 K, P = 1.3–1200 bar, ms = 0.09–6 mol kg–1), 581 CH4–brine points in six salts (T = 298–598 K, P = 4–1379 bar, ms = 0.25–6 mol kg–1), and 905 high-accuracy brine densities (T = 283–473 K, P = 9–686 bar). A colligative verification of the inherited ion parameters against the Pitzer osmotic-coefficient standard uncovers, and corrects, two defects in the published ion set (the Ca2+ parameters and the Mg2+ polarizability). Across the parametrized range, at salt molalities to 6 mol kg–1, the model predicts CO2 solubility to within 4.3–11.1% AARE per salt and CH4 solubility within 1.6–10.2% AARE per salt, reproduces brine density across the full salinity and temperature range through a temperature-dependent ion-specific Péneloux volume translation with a single ion-additive coefficient set, predicts CO2 solubility in mixed-salt brines with no mixed-brine tuning, and predicts measured calorimetric enthalpies of CO2 dissolution in water and NaCl brines to 0.6 kJ mol–1 with no adjusted parameters. The parameterization provides the thermodynamic basis for subsurface CO2 sequestration and CH4 production in realistic formation brines. In a separate study, the model is paired with robust two- and three-phase stability and flash computations and extended to N2, H2, C2H6, C3H8, and CH4–CO2 gas mixtures.