Studies revealed that gas hydrate cages, especially small cages, are incompletely filled with guest gas molecules, primarily associated with pressure and gas composition. The ratio of hydrate cages occupied by guest molecules, defined as cage occupancy, is a critical parameter to estimate the resource amount of a natural gas hydrate reservoir and evaluate the storage capacity of methane or hydrogen hydrate as an energy storage medium and carbon dioxide hydrate as a carbon sequestration matrix. As the result, methods have been developed to investigate the cage occupancy of gas hydrate. In this review, several instrument methods widely applied for gas hydrate analysis are introduced, including Raman, NMR, XRD, neutron diffraction, and the approaches to estimate cage occupancy are summarized.

The air micro-and nanobubbles on a silicon wafer surface, generated by ethanol−water exchange method in THF solution, are found with anomalous small contact angles on the gas side due to the pinning effect. As the pinning effect is only with the limited region of a bubble and varies with bubble size, the difference in contact angle between the microbubbles and nanobubbles is recognized. With a high-resolution atomic force microscopy, in situ direct observations of THF hydrate nucleation are performed in the presence of air micro-and nanobubbles in solution. On the basis of the observations, the sizes of the hydrate crystallites along the bubble edge are much larger than those in nonbubble regions, which can be explained by the lower nucleation barrier at the contact line region as to the classical nucleation theory. The growth of hydrate crystals at the bubble contact line experiences the competition for THF molecules, probably through Oswald ripening process, resulting in the spaced distribution of THF hydrate crystallites along the bubble edge.

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The knowledge of the mechanism of clathrate hydrate formation is crucial for studying the formation of a natural gas hydrate and developing hydrate-based technologies. In this study, low-field nuclear magnetic resonance (NMR) is applied to observe the processes of tetrahydrofuran (THF) hydrate formation from the solutions with different THF concentrations, and from the analysis of the NMR transverse relaxation time (T 2) of hydrates and solutions, the formation mechanism of THF hydrate is discussed. A quantitative relationship between the NMR T 2 spectral area and the mass of THF hydrate is established, and it is applied to estimate the hydrate amount in the formation process of THF hydrate. The results obtained show that T 2 of THF solution is controlled by the temperature and THF concentration: longer at a higher temperature and shorter at a higher THF concentration. Moreover, the initial THF concentration does not affect the final concentration of the remaining solution after hydrate crystallization, and the formation of THF hydrate from solution follows the thermodynamics of solution reaction, with the THF concentration of the solution ending at the phase line after hydrate crystallization. The kinetics of THF hydrate formation is affected by the temperature and THF concentration: a higher formation rate in the solution with stoichiometric composition (19.06 wt % THF) of THF hydrate than either THF-rich or water-rich solution and a lower formation rate at a higher temperature.

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The crystallization process of a droplet of tetrahydrofuran (THF) solution on a silicon wafer is studied based on the in situ observation with a highresolution atomic force microscope. On the basis of the results obtained, the crystallization process of the THF clathrate hydrate can be separated into two stages, namely, the ripening stage in the beginning of hydrate formation and the steady growth stage. In the ripening stage, hydrate crystals undergo the Ostwald ripening process, in which the large central hydrate particle grows slowly, and the small hydrate crystals experience dynamic formation-dissociation. In the steady growth stage, hydrate crystals grow at a relatively higher rate, and special features such as dendrites and protrusion along grain boundaries are observed at the surface of hydrate crystal as caused by the supercooling of the experimental system.

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