Molecular Dynamics Simulation Study on the Effects of Ion Type and Concentration on Methane Hydrate Phase Equilibrium

Methane hydrate, a potential clean energy resource, is widely distributed in seabed sediments and terrestrial permafrost regions. During exploitation in practical marine environments, the presence of salt ions significantly influences the hydrate’s phase equilibrium curve, leading to an increase in the dissociation temperature, thereby increasing the difficulty and cost of exploitation. Therefore, a systematic investigation of the effects of ion type and concentration on the phase equilibrium of methane hydrate is conducive to optimizing exploitation technologies under saline conditions. In this study, Molecular Dynamics (MD) simulations were employed to explore the impact of ion types (Na+, K+, and Ca2+) and their concentrations on the phase equilibrium curve of methane hydrate. Using the direct coexistence method, the dissociation temperature (T_diss) was determined by analyzing density profiles, potential energy curves, and Radial Distribution Functions (RDF). The results show that the phase equilibrium temperature of methane hydrate in a pure water system at 10 MPa is 297.43 K. For monovalent cations, as the concentration increases from 3.5 wt% to 10 wt%, the phase equilibrium temperatures for Na+ and K+ systems increase from 298.67 K and 297.36 K to 300.56 K and 297.94 K, respectively, with the magnitude of the shift for Na+ being greater than that for K+. Among ions of different valences, as the concentration increases from 3.5 wt% to 10 wt%, the phase equilibrium temperature for the Ca2+ system decreases from 300.67 K to 299.24 K; however, both values remain higher than the phase equilibrium temperature of the pure water system. This is primarily attributed to the order of ionic hydration strength (Ca2+ > Na+ > K+), which leads to differences in their attraction to water molecules. Ions with higher hydration strength bind more strongly to water molecules, meaning the thermal driving force of the pure water system is insufficient to disrupt this ion-water hydration. Consequently, a higher temperature is required to meet the new phase equilibrium conditions, which ultimately manifests as a shift in the phase equilibrium temperature. This research provides theoretical support for understanding the thermodynamic behavior of natural gas hydrates in saline environments and for their field exploitation.