Characteristics of Methane Dissolution and Hydrate Formation during Methane Bubble Seepage at Different Pressures

Widespread methane seepage from the seafloor plays a critical role in regulating marine and global carbon cycling. The migration and transformation characteristics of methane after its release into the water column are key factors determining its ultimate fate. Within the water column hydrate stability zone, seeping methane bubbles may form hydrate films, which can slow the bubble dissolution and prolong their ascent lifetime. However, existing research predominantly focuses on static or single-bubble systems, with few systematic studies having been conducted on the specific influence of pressure (as a proxy for depth) on methane seepage and hydrate formation characteristics under constant, deep-sea relevant temperatures. This study experimentally investigated the dissolution behavior and hydrate formation evolution of methane bubbles under various pressures corresponding to different water depths. The results demonstrated that supersaturated dissolved methane was the core driving force for hydrate growth in the liquid phase and its evolution was significantly influenced by the presence or absence of methane bubble seepage. Conversely, the pressure did not exhibit a significant linear effect on the methane mass transfer coefficient. Methane consumption after hydrate formation was not positively correlated with the pressure driving force, possibly due to dual mass transfer limitations caused by the nonuniform distribution of flocculent hydrates in the solution. Hydrate coverage reduced the velocity of rising methane bubbles by 13.7–19.6%, significantly prolonging their retention time in the deep-seawater column. Hydrate nucleation became difficult at pressures of ≤8 MPa, indicating that seeping methane bubbles are unlikely to form hydrates at shallower depths. The findings of this study provide fundamental data and mechanistic insights into the pressure-dependent dissolution, transformation, and hydrate formation behaviors of methane bubbles. These findings contribute to the understanding of bubble fate in deep-sea, low-temperature environments.