Montmorillonite-Mediated Formation Kinetics of Methane Hydrate in Sandy-Clay Sediments: Inhibition or Promotion?

Methane is a major greenhouse gas; natural gas hydrates, along with methane reserves on the seafloor, play a critical role in the global methane cycle. As methane migrates from subsurface sediments to the seafloor, it can become trapped as solid hydrates in cold seep environments, preventing its release into the ocean and atmosphere. This study investigated how varying montmorillonite (MMT) content affects methane hydrate formation kinetics in sandy-clay environments. The results show that in sandy-suspension systems with low MMT-to-water mass ratios (0–30%), increased suspension salinity primarily drove MMT’s inhibitory effects on hydrate formation. Conversely, the MMT’s high surface area enhanced the methane transformation rate in sandy-clay systems with a high MMT mass ratio (200–400%). However, higher MMT content reduced water activity due to strong water adsorption, decreasing final methane conversion efficiency. Morphological analysis indicates that at low MMT content, hydrate formed around quartz sand as fine crystals that gradually aggregated into larger crystal grains. At high MMT content, hydrate formed in larger pores, causing clay layer fracturing. This work enhances our understanding of how clay minerals affect methane conversion into solid hydrates, providing a basis for assessing and predicting methane release and conversion in deep-sea cold seep environments.