Modeling hydrogen–brine mutual solubility from surface to underground storage conditions: the thermodynamic framework development and validation

The global transition toward renewable energy systems has established hydrogen as a promising energy carrier, with Underground Hydrogen Storage (UHS) in geological formations emerging as a solution for large-scale storage. While understanding hydrogen behavior in geological formations is crucial, modeling mutual solubility in hydrogen-brine systems, particularly in complex brines under UHS conditions, remains challenging. This study presents an enhanced thermodynamic model for predicting mutual solubility in hydrogen-brine systems under deep geological formation conditions. Our approach combines the Peng-Robinson equation of state for non-aqueous phase fugacity calculations with the Pitzer model for aqueous phase activity determination, incorporating comprehensive interaction parameters and multi-phase equilibria under elevated pressure and temperature conditions. Model validation against experimental data demonstrates superior accuracy compared to existing approaches, especially in complex brine systems with multiple ionic species. Furthermore, the model extends predictions to temperature and pressure conditions beyond available experimental data. This work establishes a robust framework for understanding hydrogen behavior in geological formations, enabling accurate prediction of phase equilibria critical for assessing and mitigating risks like salt precipitation. These capabilities enhance evaluation of long-term hydrogen injectivity and storability, advancing the development of safe and efficient UHS systems.