Microscopic kinetic model of gas hydrate and the effect of brine: a case study of natural gas hydrate from the seabed off the Tokachi coast

Abstract

Crystallization and dissociation are among the most prominent phenomena in condensed matter science; however, their microscopic understanding, such as with impurities, remains fragmentary. In this study, an experimental approach was used to investigate the microscopic mechanisms of crystallization and dissociation using structure I (sI) natural gas hydrates sampled from the seabed off the coast of Tokachi, Japan. Detailed information on the structural signature was obtained using multiscale imaging methods, including two synchrotron X-ray computed tomography (CT) methods, for samples with a hydrate mass fraction of 65 wt% and a salinity of 0.1 wt% in an icy sample. It was revealed that the crystallization step was driven by CH₄ gas bubbles rising in the subsurface and that the pseudo-spherical structure, which was subsequently converted to gas hydrate, was retained. After cryopreservation of the sample, the dissociation front at the time of recovery was preserved by self-preservation; however, hydrate dissociation proceeded from the inner grain boundary, even at subzero temperatures. These results clearly indicate that the formation and dissociation processes of gas hydrates are affected by the salinity of seawater. Quantitative observations of natural gas hydrate crystals over time contribute to our understanding of the crystallization and decomposition mechanisms under more complex conditions.