Experimental and modeling investigations into hydrate shell growth on suspended bubbles considering pore updating and surface collapse

Abstract

Modeling of hydrate behaviors on the bubble surface is important for accurately evaluating the global budgets of greenhouse gas released from the deep ocean. Current mass-transfer models for hydrate shell growth mainly consider the inward and outward hydrate growth. However, the pore updating inside the hydrate shell and hydrate surface collapse may also play a significant role in hydrate growth. In this work, sustained visual observations on the morphological variation of hydrate shell on suspended methane bubbles in supersaturated water were conducted. The observed new insights and data processing results indicate that the dynamic growth of hydrate shell has successively experienced three stages: rapid development, continuous collapse and growth stagnation. Based on the surface characteristics of hydrated bubbles at different stages, a new mechanistic model of hydrate shell growth on bubble surface is developed, which considers the intercoupling of multiple hydrate-forming reactions, multiphase mass transfer processes, and hydrate collapse mechanical properties. This model not only characterizes the hydrate growth on both sides of hydrate shell, but also first presents a quantitative description of capillary pore updating inside the hydrate shell. Moreover, the relationship between the critical collapse pressure of hydrate shell and the dynamic pressure inside the hydrated bubble was investigated. Combining with experimental measurements from the literature, the efficiency of gas diffusion through the hydrate shell was determined, and correspondingly the time evolution of intergranular pore structure at different supercoolings was also performed. Using the proposed model, the variation rules of hydrate growth characterization parameters in different initial conditions were simulated and analyzed. Furthermore, the influence of the surface collapse behavior on hydrate shell thickening growth was also discussed. This work provides further insights into the growth characteristics of hydrated bubble accompanied with interphase mass transfer and surface morphological change.