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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.

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A three-scale in situ observation technique combining the optical microscope, confocal microscope, and Raman spectroscopy was proposed to gain insights into the thickening growth kinetic of hydrate film formed on a gas bubble suspended in water. The evolution of the morphology and mass transfer channels of the hydrate films formed from pure CH4 or the mixture of CH4 and C2H6 during thickening growth was investigated. Results demonstrated that the morphological evolution of hydrate films during thickening growth depended on the initial morphology of the hydrate film formed in the lateral growth. The sI CH4 and sI CH4–C2H6 hydrates had the same ultimate morphology (e.g., a large number of well-defined polyhedral granular hydrates), while the sII CH4–C2H6 hydrates appeared massive. It was clearly observed that the gas pores on the hydrate film gradually vanished with the growth of hydrate film, which suggested that the predominant mass transfer channels on the film changed from the gas pores to the lattices. This was confirmed by the Raman spectra, which was successfully used to analyze the evolution of mass transfer channels by evaluating the interference of the peaks of the free gas molecules to that of the gas molecules in the hydrate phase. It showed that the Raman spectra of hydrate were strongly interfered by free gas molecules moving in mass transfer channels during the initial stage, while this effect weakened at the end stage of thickening growth. To the best of our knowledge, the three-scale method presented in this work was for the first time used to shed light on the mechanism related to the morphology evolution and molecules motion during the growth of hydrate film on a gas bubble.