Methane hydrate is being considered as a potential future energy source but also a considerable geo-hazard. In this study, methane hydrate bearing sand sediment was firstly created by pressurizing methane gas into already chilled moistened 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 zero to dissociate the 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.

15In this study, methane hydrate-bearing sand (MHBS) was created in the laboratory following 16 two methods in order to obtain two types of gas hydrate morphology in sandy sediment. The 17 hydrate morphology in the sediment was assessed by measuring the compressional wave 18 velocity combined with models to predict the wave velocities of the sediment containing gas 19hydrates. The mechanical properties of the MHBS were investigated by triaxial compression 20 tests. The results obtained by the compressional wave velocity show that after saturating the 21 MHBS sediment (created by the excess gas method) with water, the methane hydrates are partly 22Key words: methane hydrate, sand, mechanical behavior, pore habit, rock physics model. 33 34

Understanding the mechanisms involved in the formation and growth of methane hydrate in marine sandy sediments is crucial for investigating the thermo-hydro-mechanical behavior of gas hydrate marine sediments. In this study, high-resolution optical microscopy and synchrotron X-ray computed tomography were used together to observe methane hydrate growing under excess gas conditions in a coarse sandy sediment. The high spatial and complementary temporal resolutions of these techniques allow growth processes and accompanying redistribution of water or brine to be observed over spatial scales down to the micrometre—i.e., well below pore size—and temporal scales below 1 s. Gas hydrate morphological and growth features that cannot be identified by X-ray computed tomography alone, such as hollow filaments, were revealed. These filaments sprouted from hydrate crusts at water–gas interfaces as water was being transported from their interior to their tips in the gas (methane), which extend in the µm/s range. Haines jumps are visualized when the growing hydrate crust hits a water pool, such as capillary bridges between grains or liquid droplets sitting on the substrate—a capillary-driven mechanism that has some analogy with cryogenic suction in water-bearing freezing soils. These features cannot be accounted for by the hydrate pore habit models proposed about two decades ago, which, in the absence of any observation at pore scale, were indeed useful for constructing mechanical and petrophysical models of gas hydrate-bearing sediments.