Molecular dynamics simulations study on equilibrium, transport, and interfacial properties of H₂S-brine systems under conditions typical of geological sequestration

YH Dehaghani and M Assareh and F Feyzi, ENVIRONMENTAL EARTH SCIENCES, 82, 541 (2023).

DOI: 10.1007/s12665-023-11246-x

Accurate predictions of equilibrium, transport, and interfacial properties of H2S-brine systems play a significant role in improving acid gas sequestration efficiency in saline aquifers. In the current study, molecular dynamics (MD) simulations were used to simultaneously predict the interfacial tension (IFT), mutual solubility, viscosity, and density of the H2S-brine solutions to compensate for the lack of experimental data on these crucial properties. The effects of temperature, pressure, salt type and concentration on the properties influencing the storage process are investigated in the ranges of pressures up to 30 MPa, temperatures of 323.15-393.15 K, and salinities of 1-3 mol/kg, which represent typical conditions of acid gas geological storage. We employed mixed brine systems with the most common monovalent and divalent salts to study the impact of different ions on the desired properties with detailed microstructural insight. A comprehensive validation was performed to verify the accuracy of MD model by comparison with experimental data of the H2S-water and H2S-NaCl solutions. The average absolute deviations percent (AAD%) of 4.27, 5.20, 3.74, and 4.93% were obtained for reproducing the H2S solubility in water, H2S-rich phase water content, density, and IFT of the H2S-water system, respectively, close to the experimental uncertainties. The increasing ion concentration results in a decrease in the mutual solubilities and a linear increase in IFT values because of forming contact ion pairs, which is more remarkable in the CaCl2-containing solution. However, the salting-out effect is more pronounced for the solubility of H2S than water content. IFTs of the H2S-NaCl + CaCl2 (aq) are greater in comparison with those of H2S-NaCl + KCl (aq) solution, while this trend is reversed for mutual solubilities. The H2S dissolution reduces the density values of brine solutions compared to fresh brine, which has an unfavorable impact on the density-driven convective process and has the higher opposite effect on the viscosity values. The most effective parameters on the density and viscosity values are salinity and temperature, respectively, under our studied operating conditions.

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