Contactless probing of polycrystalline methane hydrate at pore scale suggests weaker tensile properties than thought

Abstract

Methane hydrate is widely distributed in the pores of marine sediments or permafrost soils, contributing to their mechanical properties. Yet the tensile properties of the hydrate at pore scales remain almost completely unknown, notably the influence of grain size on its own cohesion. Here we grow thin films of the hydrate in glass capillaries. Using a novel, contactless thermal method to apply stress, and video microscopy to observe the strain, we estimate the tensile elastic modulus and strength. Ductile and brittle characteristics are both found, dependent on sample thickness and texture, which are controlled by supercooling with respect to the dissociation temperature and by ageing. Relating the data to the literature suggests the cohesive strength of methane hydrate was so far significantly overestimated.

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

“…Most filaments were spiralled, with a typical pitch of 10-15 µm, some completing a dozen or more turns before terminating in a club head or mass of small turns. Interestingly, the halo of gas hydrate, commonly observed on glass substrates [35,45], was not prominent at this stage. Instead, circular hydrate halos appeared that were centred on, and radiating around, the points of contact of the filaments drooping onto the glass floor of the cell (Figure 4b,c).…”

“…The grains were covered in a few minutes by a dense growth of filaments of methane hydrate extending into the gas, similar to the observations by Servio and Englezos [33], who observed filaments growing into the gas from a hydrate crust on a droplet of water under large enough supercooling. (See also Atig et al [45], who observed MH filaments growing on the methane side from the hydrate crust separating the water and gas compartments in a thin glass capillary). Within a few minutes, filaments with typical lengths of up to several hundred micrometers over curves-corresponding to a growth rate of a few micrometers per second-formed and grew from their extremity in the gas phase (see Discussion below).…”

“…All cold parts were enclosed in an ad hoc housing with a dry nitrogen flow to prevent condensation. See [45] for more details. Samples were observed in transmission on an inverted widefield microscope (Nikon Ti-Eclipse, equipped with an ORCA-4.0s CMOS camera, Hamamatsu).…”

“…Up to now, macrophotography and optical microscopy have been mainly used to observe gas hydrates growing at simple interfaces, like those of water drops on a flat substrate under the guest phase or guest-containing phase (e.g., a guest-saturated organic solvent [33][34][35][36][37][38][39][40]), or around a meniscus between the phases in capillaries or larger vessels [41][42][43][44][45]. As a rule, the hydrate forms a crust over the interface between the aqueous and the guest (or guest-containing) phase, often nucleated at the contact line with the substrate or sample cell [46,47].…”

Crustal fingering facilitates free-gas methane migration through the hydrate stability zone

“…2C, magenta circles; see also refs. 29 and 30) and is likely controlled by the local pressure difference and crust tensile strength (47)(48)(49)(50). The location of the thinnest crust corresponds to weaker tensile strength and, thus, is more prone to rupture.…”