Phase-equilibrium characteristics of methane hydrate in clay mineral suspensions: Differential scanning calorimetry experiments and density functional theory studies.

The investigation of methane hydrate equilibrium conditions is crucial for comprehending the occurrence of methane hydrate in marine sediments. In this study, the liquid-hydrate-vapor equilibrium condition of methane hydrate in montmorillonite and kaolinite suspensions in the presence of glycine was investigated through differential scanning calorimetry experiments. The results indicated that glycine inhibited the phase equilibrium of methane hydrate. The phase equilibrium conditions of methane hydrate in kaolinite suspension closely resembled those in pure water. In contrast, calcium montmorillonite hindered the phase equilibrium of methane hydrate owing to the presence of Ca2+. The phase equilibrium conditions of methane hydrate in kaolinite suspension with the addition of glycine were similar to those in glycine solution. The inhibitory effect of calcium montmorillonite on the phase equilibrium condition of methane hydrate intensified with the addition of glycine. Furthermore, density functional theory simulations indicated that glycine significantly reduced the binding energy between montmorillonite layers and Ca2+, potentially mitigating the inhibitory effect of Ca2+ on methane hydrate formation under suitable glycine concentrations. The diverse equilibrium conditions of methane hydrate, influenced by the types of clay minerals, salt ions, and organic matters, may play a critical role in the formation and occurrence of natural gas hydrates in marine environments, warranting exploration in future studies.