Thermodynamic Modelling of Hydrogen in Hydrothermal Fluids

Hydrothermal fluids, produced through interactions between brine and ultramafic rocks at high temperatures, contain hydrogen generated via serpentinization of ultramafic rocks. 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 mixtures starting from a symmetric approach for vapor liquid equilibria to the asymmetric approach. The asymmetric approach is more popular for studying gas-brine systems because it allows speciation calculations for reactive transport. At conditions near the critical point of water, however, 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 the salting-out effect. The Akinfiev and Diamond (2003) and Plyasunov and Bazrakina (2018) models predicted solubilities and Henry’s coefficients more accurately compared to the HKF model. As expected, the models were not able to capture the salting-out effect that is of significant impact in the single-phase super-critical region compared to super-critical vapor because salts do not partition in significant amounts to the vapor phase. Our analysis revealed that a Sechenov coefficient between 0.4 to 0.6 captures the salting-out effect up to 1 molal salinity. Additional experiments, however, are required at higher salinities to evaluate salting-out effect at super-critical conditions. Accurate modeling of thermodynamic properties in the H2-H2O-NaCl system is crucial for hydrogen exploration and evaluation of redox conditions in the earth’s crust and upper mantle. Also, improved understanding could enhance the economic feasibility of energy extraction systems from geothermal reservoirs and vent fields.