In-situ Raman study on kinetics behaviors of hydrated bubble in thickening.

Natural gas leakage by means of bubbles in cold seep abundantly existed on the ocean floor, causing the change of ocean ecology and the increase of atmospheric temperature. Fortunately, hydrated bubbles as a way of methane sequestration can reduce the effect on the ocean ecology and the escape of gas bubbles from the ocean floor, and are getting attention. To know the growth mode and efficiency of gas hydrate sequestration on bubble, the thickening growth kinetics of hydrated bubble was studied in present work. In-situ Raman spectroscopy was used to analyze the evolution of gas pores and mass transfer channels in the sI CH4, sI CH4-C2H6 and sII CH4-C2H6 hydrate films on the hydrated bubble by the peak area ratio of Raman spectra. Three types of Raman spectra (a-, b-, and c-type), three texture structures of film (Large gas pore; Small gas pore; No gas pore) and two hydrate thickening patterns (filling of new hydrate within large gas pores; covering growth on the original hydrate lattice) were provided in the thickening of hydrated bubble. Results showed that the thickening of the hydrated bubble was a multi-stages growth, i.e., quick growth (stage I), slow growth (stage II), and no growth (stage III). The texture structures and the type and size of gas pore in hydrated bubble were critical for the kinetics growth rate of hydrated bubble in thickening. Especially, the theory of heterogeneous growth of hydrated bubble was proposed to apply the hydrate growth at the interface of two or multi- bubbles, accelerating the efficiency of carbon sequestration as the hydrated bubble. This study will provide a better theoretical basis for understanding the behaviors and efficiency of hydrated carbon sequestration on the surface of bubbles resulting from the gas leakage in the hydrate exploitation or the natural cold seep. SYNOPSIS: Hydrated bubble strongly modulates the emission of a potent greenhouse gas from the deep sea.

Self-preservation and structural transition of gas hydrates during dissociation below the ice point: an in situ study using Raman spectroscopy.

The hydrate structure type and dissociation behavior for pure methane and methane-ethane hydrates at temperatures below the ice point and atmospheric pressure were investigated using in situ Raman spectroscopic analysis. The self-preservation effect of sI methane hydrate is significant at lower temperatures (268.15 to 270.15 K), as determined by the stable C-H region Raman peaks and AL/AS value (Ratio of total peak area corresponding to occupancies of guest molecules in large cavities to small cavities) being around 3.0. However, it was reduced at higher temperatures (271.15 K and 272.15 K), as shown from the dramatic change in Raman spectra and fluctuations in AL/AS values. The self-preservation effect for methane-ethane double hydrate is observed at temperatures lower than 271.15 K. The structure transition from sI to sII occurred during the methane-ethane hydrate decomposition process, which was clearly identified by the shift in peak positions and the change in relative peak intensities at temperatures from 269.15 K to 271.15 K. Further investigation shows that the selectivity for self-preservation of methane over ethane leads to the structure transition; this kind of selectivity increases with decreasing temperature. This work provides new insight into the kinetic behavior of hydrate dissociation below the ice point.