Effects of Salinity on the Macroscopic and Microscopic Evolution Characteristics of Methane Hydrate Films on Bubble Surfaces

The formation and evolution characteristics of hydrate films on methane bubble surfaces play a crucial role in influencing the solidification and resequestration of methane in deep-sea methane seepage environments, which is pivotal in affecting whether methane from seabed seepage can reach the shallow layers of the ocean. Differences in seawater and pore water salinity among different marine regions are the key factors influencing hydrate formation. However, the effects and mechanisms of salinity on macroscopic and microscopic evolution characteristics of methane hydrate films on bubble surfaces remain unclear. In this study, the formation and evolution characteristics of the hydrate film on the surface of methane bubbles were investigated at the macroscopic and molecular scales using a combination of Raman spectroscopy and microscopy. It was found that although the driving force for methane hydrate formation was consistent in all systems, the lateral growth rate of the hydrate film on the bubble surface slowed with increasing salinity, and the initial hydrate film gradually became rougher with enlarged hydrate particles. The lateral growth rate of the methane hydrate film in the 10 wt % salinity system was 50 times slower than that in the pure water system. Additionally, the salinity significantly affected the thickening process of the methane hydrate films. The obvious difference in pressure inside and outside the bubbles under different salinity conditions resulted in overall gas consumption phenomena in the system different from those of hydrate formation on the bubble surface. Mechanism analysis suggests that the observed phenomena primarily stem from the decrease in efficiency of methane and water molecule transfer caused by increased salinity as well as the formation of a porous and loose hydrate film structure. The results of this work can provide valuable insights into CH4 capture and sequestration in deep-sea cold seeps and offer essential fundamentals for estimating hydrate resources in the marine environment.