We investigated surface changes of methane and ethane hydrates during depressurization using optical and confocal scanning microscopes. The dissociation of methane hydrate above 242 K and ethane hydrate above 267 K resulted in the formation of clear ice sheets on the hydrate surface. The ice sheets that formed at 252 K were the thickest of those from 242 to 262 K, indicating that methane hydrate stability may be greatest around 252 K. Our results reveal that the formation of ice sheets is closely related to the mechanism of the self-preservation effect of methane hydrate and ethane hydrate. Practical transportation and storage of methane hydrate and ethane hydrate should be undertaken at 250 and 270K at atmospheric pressure. Dissociation control of methane hydrate by the self-preservation effect based on ice sheet formation may be helpful for research into methane hydrate resources, and thus our results are of value.
Gas hydrates are inclusion compounds that consist of an open network of water molecules that are hydrogen-bonded in a manner similar to ice and that interstitially engage gas molecules under high pressures and low temperatures. The actual gas storage density depends on the gas occupation fraction and the particular crystallographic structure of the hydrate (Sloan, 1998; Stackelberg and Muller, 1951; Muller and Stackelberg, 1952; Ripmeester, Tse, Ratcliffe, and Powell, 1987). A volume of gas hydrate contains more than 150 times the mass of gas as that for the same volume of pure gas at the standard temperature and atmospheric pressure. The most common compound is methane hydrate. Interest in these compounds has risen in recent years due to the discovery of large deposits below the ocean floor and in permafrost regions (Sloan, 1998; Stackelberg and Muller, 1951; Muller and Stackelberg, 1952; Ripmeester, Tse, Ratcliffe, and Powell, 1987).
