Molecular Dynamics Insights Into the Phase Equilibria and Thermophysical Properties of CO2–H2S–Brine System During Acid Gas Sequestration in Saline Aquifers

Abstract

This work represents an extensive molecular dynamics (MDs) simulation study with the microstructural insight at the interface to simultaneously predict the phase equilibria, transport, and interfacial properties of the CO₂–H₂S–brine system within the range of temperatures 323.15–393.15 K, pressures up to 30 MPa, H₂S contents of 0–70 mol%, and salt molalities of 1–4 mol/kg, aiming to address the insufficiency of data under typical conditions of acid gas sequestration. The validation results demonstrate that the average absolute deviations (AAD%) for the predicted solubility of CO₂ and H₂S in water and in 2 mol/kg NaCl solution were found to be 5.45%, 6.34%, 5.78%, and 5.41%, respectively. Moreover, the AAD% for interfacial tension (IFT) and density were 6.74% and 3.70%, respectively, verifying the validity and performance of the applied force field parameters and computational methods. The simulation results indicated that H₂S solubility in brine is more sensitive to changes in the acid gas composition and temperature compared to CO₂ solubility. The presence of H₂S remarkably reduces the CO₂–H₂S–brine IFT, with the reduction degree depending on the H₂S content. Increasing the H₂S mole fraction in acid gas mixtures delays convective mixing by reducing the brine density. At about 64 mol% H₂S, the aqueous solution's density equals that of fresh brine, which is the highest H₂S content that can maintain the benefit of convective mixing in the dissolution trapping. The maximum acid gas column height that can be safely stored is most significant at lower temperature and H₂S content. On the basis of the results, pressure, temperature, and salt molality have a higher influence on the viscosity than density in the studied ranges. The new data generated by the current study can be utilized to develop predictive models of acid gas long-term behavior, which will reduce the uncertainty of real storage schemes.