Solubility of H2 in water and NaCl brine under subsurface storage conditions: Measurements and thermodynamic modeling

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In the context of Underground Hydrogen Storage (UHS), deep saline aquifers offer great potential due to their availability and high storage capacity. In these porous media, the direct contact between gas (H2), brine and rock can lead to biotic and/or abiotic reactions (e.g. Hydrogen-induced calcite dissolution, (bio)methanation, Acetogenesis, etc.). These reactions take place in the aqueous phase between dissolved species (gas and minerals), hence the need for accurate high-pressure solubility data of hydrogen in brine. At high-pressure conditions, only few data on the phase equilibria of H2/brine systems are available in the open literature, and with significant inconsistencies among themselves. Hence, in this work, new solubility data for H2 in NaCl brine were obtained under subsurface storage conditions using a volumetric method, at temperatures between 298 and 373 K, at NaCl molalities between 0 and 4mol/kgw, and at pressures up to 200 bar. Interestingly, the data measured in this work lies between the two existing data sources which exhibited noticeable discrepancies. These data were used to parameterize and evaluate three well-known thermodynamic models based on different approaches: e-NRTL activity coefficient model, Duan-type (non-iterative) model and Søreide and Whitson's equation of state. It has been found that these models, of which the advantages and limitations of each have been discussed, are able to provide highly accurate predictions of the phase equilibria of the H2–NaCl brine system with an average absolute deviation of less than 3%. The resulting models and parameters can be used in reservoir simulators to evaluate dissolution losses and geobiochemical reactivity associated with UHS in porous media. Moreover, these models can be applied to estimate the saturation moisture content of gases coming out of UHS facilities, or in other applications (PEM electrolyzer, electrochemical H2 compressors, etc.).