Extending the Søreide-Whitson equation of state to multi-ion brines
Abstract
The Søreide-Whitson (SW) equation of state (EoS) is widely used in the oil and gas industry to model the solubility of gases and light hydrocarbons in water and brine. SW introduces two modifications to the Peng-Robinson EoS: (1) an alternative α-function for water, which is fitted to experimental vapour pressure data, and subsequently, to the vapour pressures of aqueous mixtures of NaCl; and (2) phase-dependent interaction parameters, of which the interaction parameter for the aqueous phase 
is also a function of the salt molality.
One potential drawback of the SW EoS is that these modifications are expressed in terms of NaCl molality. There is therefore no guarantee that systems containing other salts/ions will be described correctly using this EoS, unless conversions such as Duhring-lines are available.
This study aims to extend the SW EoS to systems containing multiple ions by modifying the approach to account for ion molality rather than salt-species molality. First the α-function for water was revisited and adapted to include the effects of different ions. The modified α-function was fitted to vapour pressure data for pure water and for various salts.
Subsequently, the phase-specific binary interaction parameters for CO2 solubility in brine were refitted, introducing temperature-dependent expressions. The new parameterisation was validated against experimental data for CO2 solubility in single and mixed salt brines, showing significant improvements in accuracy. The results indicate that the modified SW EoS provides a robust and accurate model for predicting the solubility of CO2 in complex brine systems, with potential applications in various industrial processes.