Kinetics of methane hydrate formation and dissociation in sand sediment

Methane hydrate is being considered as a potential future energy source but may at the same time constitute a considerable geo-hazard. In the present study, methane hydrate bearing sand sediment was created by pressurizing methane gas into previously moistened, then chilled, packed sand specimen (excess gas method). The excess gas was then replaced by water at high pressure. Afterward, a heating/cooling cycle was applied under undrained conditions, in order to completely dissociate gas hydrates and then recreate them inside the specimen. Finally, the pore pressure was reduced to the atmospheric pressure to dissociate gas hydrates. The whole process was performed in a magnetic resonance imaging (MRI) system, allowing the determination of water and/or gas and hydrate quantity (and spatial distribution) at various times. The MRI signal was finally analyzed to interpret various processes in sand sediment: initial hydrate formation, heating-induced hydrate dissociation, cooling-induced hydrate re-formation, and depressurizing-induced hydrate dissociation.