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 CO₂ 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 CO₂ solubility under reservoir conditions. In this work a generalised Søreide-Whitson model is presented to calculate the solubility of CO₂ 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 Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, and NO₃⁻. The modified model introduces generalised expressions for the α-function and phase-dependent binary interaction parameter k_ij^AQ based on individual concentrations of ions rather than salts. This approach significantly improves the accuracy of CO₂ 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.