Thermodynamic Modelling of Hydrogen in Hydrothermal Fluids

Abstract

Hydrothermal fluids, produced through interactions between brine and ultramafic rocks at high temperatures, contain hydrogen produced via serpentinization. Understanding hydrogen partitioning between vapor and liquid phases in high salinity fluids is crucial to estimate hydrogen production in these systems. We reviewed several frameworks for thermodynamic evaluation of hydrogen starting from symmetric approach for vapor-liquid equilibria to the asymmetric approach. The asymmetric approach is more popular for studying gas-brine systems since it allows speciation calculations for reactive transport. However, at conditions near the critical point of water, the asymmetric approach needs to be extended to capture the physics of that region accurately. We compared the performance of three models, including: Helgeson–Kirkham–Flowers (HKF), Akinfiev and Diamond (2003), Plyasunov and Bazrakina (2018) against datasets at near and super-critical conditions and with brine to evaluate salting-out effect. The Akinfiev and Diamond (2003) and Plyasunov and Bazrakina (2018) predicted more accurate solubilities and Henry’s coefficients compared to the HKF model. Also, as expected the models were not able to capture salting-out effect, which was more pronounced in the single phase super-critical region compared to super-critical vapor since salts do not partition in significant amounts to vapor phase. Our analysis revealed that a Sechenov coefficient between 0.4 to 0.6 can capture salting-out effect up to 1 molal salinity. However, additional experiments are required at higher salinities to evaluate salting-out effect at super-critical conditions. Accurate modeling of immiscibility in the H2-H2O-NaCl system is crucial for hydrogen exploration in the crust and seafloor. Also, it could enhance the economic feasibility of heat extraction systems such as geothermal reservoirs and vent fields.