Extended Søreide-Whitson equation of state for CO2-brine equilibrium using an ion-specific parameterisation

Carbon capture and storage (CCS) initiatives rely on understanding the behaviour of CO 2 in saline aquifers, where solubility trapping is a key mechanism. Experimental data, while critical for model validation, are resource-intensive to obtain. Thermodynamic models, such as the Søreide-Whitson framework, offer an efficient alternative for evaluating CO 2 solubility under reservoir conditions. In this work a generalised Søreide-Whitson model is presented to calculate the solubility of CO 2 in pure water and brine solutions on an ion-specific basis across a wide range of temperatures (273–473 K), pressures (up to 50 MPa), and salinities (up to 6 mol⋅kg -1 ). The updated model includes expressions for , , , , , , and . The modified model introduces generalised expressions for the α-function and phase-dependent binary interaction parameter based on individual concentrations of ions rather than salts. This approach significantly improves the accuracy of CO 2 solubility predictions in brines containing multiple salts, as demonstrated through comparisons with experimental data. The generalised model maintains its robustness for mixed-salt brines and is readily implementable in simulation tools, expanding its applicability for CCS evaluations. • A generalized Søreide-Whitson model is developed for a broad range of temperatures, pressures, and salinities. • The model incorporates ion-specific expressions for Na+, K+, Mg 2+ , Ca 2+ , Cl - , SO₄ 2- , and NO₃⁻. • The model predicts CO₂ solubility in brines with great accuracy. • The model is compatible with simulation tools, making it a valuable tool for CCS initiatives.