Methane hydrate crystal growth in a porous medium filled with methane-saturated liquid water

Abstract

The nucleation, growth and ageing of methane hydrate crystals were observed visually in a porous medium filled with liquid water presaturated with methane. The pore space dimensions of the porous medium were 1.0 × 10² µm. The pressure−temperature conditions at which hydrate formation was initiated corresponded to system subcoolings of 3.4 K, 6.7 K, 12.3 K and 14.1 K, respectively, where the system subcooling denotes the difference of the system temperature from the triple methane−hydrate−water equilibrium temperature under a given pressure. Faceted (skeletal) hydrate crystals grew and bridged the pore spaces without intervention of a liquid water layer when the subcoolings were equal or smaller than 6.7 K. The faceted crystals may form a physical bonding with the walls of the porous medium. At the higher subcoolings, the dispersive formation of dendritic crystals and subsequent morphological change into particulate crystals were observed. The bridging of the dendritic crystals is unlikely in the absence of a large amount of additional methane supply due to the dispersive spatial distribution of the dendritic crystals that have dimensions smaller than those of the pore spaces. As a result of the interpretation of the observed variation in the crystal morphology of the methane hydrate formed in liquid water, the dependence of the crystal morphology on the magnitude of the mass transfer of methane molecules in liquid water observed in the porous medium was consistent with that previously observed in a bulk methane–water system.

Acknowledgements

This work was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21) and the National Methane Hydrate exploitation program planned by Ministry of Economy Trade and Industry (METI) and by a Grant-in-Aid for JSPS-Fellows (Grant 16-2627) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). We would like to thank Katsunori Matsushita (Hokkaido Branch, Suzuki Shoko Co., Sapporo Japan), Junji Ito (Ito Keiki Kogyosho, Co., Sapporo), Yoshie Takahashi (National Institute of Advanced Industrial Science and Technology, Sapporo), and Shinya Ito (National Institute of Advanced Industrial Science and Technology, Sapporo) for their help in maintaining the apparatus and in the experimental work. One of the authors (D. Katsuki) would like to thank the Japan Society for the Promotion of Science for providing a postdoctoral fellowship.