Molecular Dynamics Investigation on the Coupled Effects of Temperature, Pressure, and Salinity on the Growth and Structural Evolution of H ₂ S Hydrates

In this study, molecular dynamics (MD) simulations were employed to investigate the coupled effects of the temperature, pressure, and salinity (NaCl concentration) on the nucleation and growth kinetics of H₂S hydrates. A liquid-solid two-phase system was constructed based on the sI-type H₂S hydrate unit cell, and the structural evolution under various thermodynamic and saline conditions was systematically simulated. Multiple indicators─including potential energy, radial distribution function (RDF), mean square displacement (MSD), molecular number density distribution, and F₄ order parameter─were used to comprehensively evaluate the crystallization rate and structural ordering of the hydrates. The results revealed that a low temperature (250 K) and high pressure (15 MPa) significantly promote the nucleation and growth of H₂S hydrates, enhancing the compactness and crystallinity of the system. In contrast, increasing NaCl concentration markedly inhibits the formation of ordered hydrate structures, with nearly no crystal formation observed under 13 wt % salinity. The F₄ order parameter analysis further confirmed the findings from RDF and MSD, indicating that a high pressure can partially mitigate the negative impact of ionic interference. This work provides valuable theoretical insights into the microscopic mechanisms of H₂S hydrate formation and the thermodynamic behavior under complex geological conditions.