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This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (435663 and 512 cages, respectively), θM/θS, was estimated to be approximately 1.3, and the Raman shift of the symmetric C–H vibrational modes of CH4 in S- and M-cages was 2911.1 and 2909.1 cm–1, respectively. The phase-equilibrium conditions of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and tert-butyl methyl ether, TBME) at the same temperature revealed that the equilibrium pressure increased in the order NH < MCH < BrCH < TBME ∼ MCP < BrCP. The phase stabilities of sH hydrates can be determined by not only molecular geometry but also their polar properties, which affect guest–host interactions.

Recovery factor of methane hydrate in sandy sediments can be enhanced using the sensible heat of the hydrate-bearing sediments and the latent heat of ice formation by applying deep depressurization.

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On the basis of hypothetical particle‐level mechanisms, several constitutive models of hydrate‐bearing sediments have been proposed previously for gas production. However, to the best of our knowledge, the microstructural large‐strain behaviors of hydrate‐bearing sediments have not been reported to date because of the experimental challenges posed by the high‐pressure and low‐temperature testing conditions. Herein, a novel microtriaxial testing apparatus was developed, and the mechanical large‐strain behavior of hydrate‐bearing sediments with various hydrate saturation values (Sh = 0%, 39%, and 62%) was analyzed using microfocus X‐ray computed tomography. Patchy hydrates were observed in the sediments at Sh = 39%. The obtained stress‐strain relationships indicated strengthening with increasing hydrate saturation and a brittle failure mode of the hydrate‐bearing sand. Localized deformations were quantified via image processing at the submillimeter and micrometer scale. Shear planes and particle deformation and/or rotation were detected, and the shear band thickness decreased with increasing hydrate saturation.

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