Fast and Robust Phase Equilibrium Computations for CO2 + H2O + NaCl Mixtures Using the Electrolyte Cubic-Plus-Association Equation of State

Abstract We develop fast, robust phase stability and flash algorithms for CO2 + H2O + NaCl mixtures using the electrolyte Cubic-Plus-Association (eCPA) equation of state, covering T = 0–425 °C, P = 1–1500 bar, and ms = 0–6 mol kg–1 NaCl. Analytical Jacobian-based Newton–Raphson inner solvers reduce the per-iteration cost of CPA/eCPA to ∼3× that of a plain cubic EoS. A hierarchical flash combining Michelsen tangent-plane stability testing with accelerated successive-substitution iterations achieves 100% convergence over 9825 two-phase conditions. A precomputed three-dimensional solution table warm-starts the flash, reducing iteration counts by 3.2× for the ternary system. Validation against >1100 experimental data points yields average absolute relative errors of 6.5% for CO2 solubility under subsurface storage conditions, 6.6% for CO2 molality in NaCl brine, and 0.30% for aqueous-phase density versus the IAPWS-95 standard. The framework is demonstrated in a prototype reservoir simulator with no flash failures.