Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

Abstract

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.

Crystal Dewetting at the Water–Guest Interface in Macropores Sidesteps the Hydrate Mass-Transfer Bottleneck

“…Such comets are indeed observed on cooling cyclopentane-in-water emulsions formed by dissociation of the hydrate, , but there, the finite source of cyclopentanea single oil dropletrestricts the length of the comet to tens of microns vs tens of mm for the present tubes. Before closing, we note that the origin of the sparse, short hydrate fibers, sometimes seen growing into guest phases from crusts on sessile droplets or halos, , appears still undiscussed. In SI note 1.7 we propose that they too are pressure driven, but by a different mechanism, namely, transitory, crystallization-induced overpressure in water trapped between the crust and the cell floor.…”

“…Hydrate tubes could not be produced in the larger diameter glass capillaries available here, internal diameter 200 μm, in which curvature and the Laplace pressure differential across the water–cyclopentane interface are smaller. The differential is reversed for a sessile water drop under the guest phase, as in, e.g., refs , , where fibers indeed grow instead into the guest; see also SI section 1.7.…”

“…76−80 Returning to the gas hydrates themselves, hairs, limited filaments, or spikes of gas hydrate a few microns in diameter so far were observed mostly in the guest phase, sprouting from hydrate crust 43,81 or halo. 45 In common with the present hydrate fibers, they must be hollow, since growth into the guest phase implies a flow of water that, on grounds of interfacial energies, must be within the fiber. But they differ in other respects: sparsity vs dense growth, shortness, tens vs thousands of μm, and growth at the tip vs at the root.…”

“…Hydrate fibers or needles on crusts or halos suggest a way around the difficulty by growing into the third dimension. For example, hydrate fibers were reported growing into the guest phase from crusts of methane or carbon dioxide hydrate on sessile water droplets, as were methane hydrate fibers on some activated carbons , or on halos on glass or on sand particles in a model sediment . These fibers are necessarily hollow, lengthening by a flow of water through them to feed growth at the three-phase contact line at their tip.…”

“…For example, hydrate fibers were reported growing into the guest phase from crusts of methane or carbon dioxide hydrate on sessile water droplets, as were methane hydrate fibers on some activated carbons , or on halos on glass or on sand particles in a model sediment . These fibers are necessarily hollow, lengthening by a flow of water through them to feed growth at the three-phase contact line at their tip. Unfortunately, they typically are shorter and sparser than here, for reasons discussed below.…”