Phase Equilibria of CO2, CH4, N2, H2, C2H6, C3H8, and Mixtures in Multisalt Brines: Robust Stability and Flash Computations with an Ion-Specific Electrolyte Cubic-Plus-Association Equation of State

In a recent study, we presented an ion-specific parametrization 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 six-ion multisalt brines (Na+, Cl–, K+, Ca2+, Mg2+, SO42–). Here we develop the computational methods that make ms-eCPA usable in compositional reservoir simulation, and we extend it to additional gases (N2, H2, C2H6, and C3H8). In contrast to the common multisalt pairing of an aqueous activity-coefficient model with a separate gas-phase fugacity model, both phases here derive from a single residual Helmholtz energy. This energy delivers fugacities, phase densities, and stability analysis on one thermodynamic surface. A tangent-plane-distance stability test classifies every feed, and an accelerated K-value flash computes the phase split. The nonvolatile ions enter the Rachford–Rice equation in closed form, and each phase keeps its own inner Newton solve (three unknowns in the aqueous phase, two in the gas phase) with complex-step Jacobians. Convergence and efficiency are demonstrated across 151,632 states spanning the calibration window (283–623 K, 1–1000 bar, eight salt configurations). We reproduce the validation data of that study at a global CO2-solubility average absolute relative error of 6.55%. We also predict the CH4–CO2–H2O ternary mutual solubilities and CH4/CO2 selectivity in brine with no parameter fitted to ternary data. Below the critical temperature of a condensable gas we extend the framework to three coexisting phases (vapor, brine, and a gas-rich liquid). The two systems for which external data exist are salt-free binaries, and we reproduce both. The CO2–H2O locus tracks the reference CO2 saturation curve within 0.2–1.1 bar. The measured propane–water three-phase pressure is matched to within 1.5% from 289 to 366 K. Along the locus, propane in the water-rich phase is reproduced to 2.2% and water in the vapor to 4.3% on average, while water in the liquid propane is overpredicted by 21–63%. No experimental three-phase data exist for gas–brine systems, so the three-phase results in brine are reported as predictions. New gases require few new parameters: N2 from a salt-free binary fit alone, H2 adopted from a published parametrization, and C2H6 and C3H8─for which the CH4 transfer breaks down─with one binary fit plus two ion-energy scales each, calibrated against evaluated Setschenow data. Their brine validation is confined to NaCl (N2, H2) and to atmospheric-pressure Setschenow constants in NaCl and KCl (C2H6, C3H8), so their multisalt behavior is a prediction. With the parametrization and the present framework, we provide a unified thermodynamic engine for subsurface CO2 sequestration, CH4 production, and N2/H2 storage in brines of mixed ionic composition.