Effect of Ionic Composition on Methane Hydrate Formation Kinetics in Natural and Artificial Deep-Sea Seawater

On the cold seep seafloor, methane hydrate is an important form of carbon storage. The stability of methane hydrates and mechanisms of ionic effects on hydrates are closely related to seafloor methane release and carbon cycling. Cold seeps are endowed with abundant and high-quality hydrate resources under the environmental conditions of low temperatures and high pressures. Methane hydrates are seen as a potential source of renewable energy and a possible form of energy storage, as the global demand for clean energy increases. Although studies have focused on the formation and dissociation processes of methane hydrates, the influence of seawater ions on the kinetics of hydrate formation is still unclear. In this study, we studied the formation kinetics of methane hydrate in ion systems, analyzed the effects of various main ions, and compared them with in situ seawater composition. The phase equilibrium point of methane hydrate in the in situ seawater system was experimentally determined by us to better understand the stability. The degree of inhibition on the methane hydrate formation kinetics in natural seawater is more closely to that of CaCl2, MgCl2, and SrCl2, as indicated by gas consumption during methane hydrate formation. However, NaCl exhibits a greater inhibition effect than that of CaCl2, MgCl2, and SrCl2. The thermodynamic behavior of in situ seawater is consistent with the inhibition degree of SrCl2 on methane hydrate formation. In addition, morphological characteristics of hydrates possessed dense powdery particles and ice particles on the surface of the seawater–methane system, which combined with both hydrate morphologies, the pure water–methane system (ice particles), and the saline ions–methane system (powdery particles). The findings of this study may provide a reference for further exploration of methane hydrate formation in cold seep environments and provide a basis for an in-depth understanding of submarine methane release and carbon cycling.