An Investigation of Hydrate Formation in Unsaturated Sediments Using X‐Ray Computed Tomography

Abstract

Physical properties of hydrate-bearing sediments are often correlated with hydrate saturation with little or no information on hydrate distribution uniformity in the specimens. This study focuses on water redistribution and sediment skeleton shift depending on various hydrate formation conditions in unsaturated systems, as well as on the resulting hydrate distribution patterns. Using X-ray computed tomography, we investigate the factors such as fines content and the pressure-temperature path on mass migration during carbon dioxide hydrate formation. The experiments show water migration, preferential hydrate formation toward the core periphery, localized patchy hydrate distribution, and sediment particle movement toward the core center. Sediment particle movement can be impeded in densely packed specimens. The overall mass migration due to hydrate formation can be significantly suppressed by adding 5% by mass of kaolinite. Hydrate formation initiated by pressurization and then cooling causes less mass migration than the cases where hydrate is formed using cooling followed by pressurization or pressurizing frozen cores followed by heating methods. Freezing can induce water migration and particle pushing in a similar manner as hydrate formation. Image analyses show that the pressure-temperature path and the rates of heat transfer during hydrate nucleation and growth govern the uniformity of hydrate distribution in sediments.

Microstructure Evolution of Hydrate‐Bearing Sands During Thermal Dissociation and Ensued Impacts on the Mechanical and Seepage Characteristics

“…This result confirmed that the secondary hydrate formation would change the sediment fabric. Significant movement and roatation of sand particles were also observed in previous CT study on hydrate‐bearing sands during formation (Lei, Liu, et al, 2019; Nikitin et al, 2020). In addition, the sediment matrix size and the excess gas/water condition could influence this effect to some extent (Lee et al, 2010).…”

An experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron X-ray computed tomography

“…Chen et al (2018), observed via XRCT Ostwald ripening-type changes of the spatial distribution in the pores space of sediments of Xenon hydrate formed following the excess-gas method, at both the pore and sample scales. Lei et al (2019b) observed similar water migration during the formation of carbon dioxide hydrate in sediments. However, their image spatial resolution was low, with a voxel size larger than 10 µm.…”

“…However, no water migration was observed. In the work of Lei et al (2019b), fast water migration was found after MH nucleation, became slower after 24 h and water remained stable after 81 h. In addition, progressive methane hydrate migrations were observed by XRCT scans over the whole duration of the test. Lei et al (2019a) observed CO 2 hydrates forming preferentially toward the periphery of the sample.…”

Dynamic in-situ imaging of methane hydrate formation and self-preservation in porous media

“…After hydrate decomposition, water has redistributed in the sample more uniformly again due to capillary forces. In our experiments we did not notice a tendency of preferable hydrate formation near the sample side walls as reported in (Lei et al, 2019a). The reason could be the difference in the material of the walls of the environment cell: in our case they were made of plastic (PEEK) as opposed to metal walls in (Lei et al, 2019a).…”

“…Water migration in the pore space during hydrate formation was first observed by (Gupta et al, 2006) and (Kneafsey et al, 2007). This observation was confirmed later by using a micro-focus X-ray CT apparatus Lei et al, 2019a). It can be explained by the cryogenic suction phenomenon, i.e.…”