Dissociation kinetics of methane hydrates was investigated by using in situ Raman spectroscopy at temperatures just below the melting point of ice. Measurements of decomposition rates were performed using finely powdered hydrate samples with a diameter range of 100-250 µm. It was found that the dissociation rate of methane hydrate at 0.1 MPa is considerably faster than that at 0.25 and 0.5 MPa. A kinetic model using non steadystate approximation and a diffusion-controlled regime is presented for describing the decomposition behavior of methane hydrates. This approach would be effective for modeling the dissociation of methane hydrate in the early stages of dissociation. The diffusion coefficients of methane molecules through the hydrate surface coated with ice are estimated to be 1.7 × 10 -13 , 6.7 × 10 -14 , and 2.9 × 10 -14 m 2 /s at 272. 65, 271.15, and 268.15 K, respectively.

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under 2.0-77.0 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ X-ray diffractometry revealed distinct changes in the compressibility of the axial ratios of the host framework with pressure. Raman spectroscopy showed a split in the C-H vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the compressibility change. The results indicate the following: (i) the orientational ordering of the guest methane molecules from an orientationally disordered state occurred at high pressures and low temperatures; and (ii) this guest ordering led to anisotropic contraction in the host framework. Such guest orientational ordering and subsequent anisotropic contraction of the host framework were similar to that reported previously for filled ice Ic hydrogen hydrate. Since phases with different guest-ordering manners were regarded as different phases, existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray study. In addition, above the pressure of the guest-ordered phase, another high-pressure phase developed in the low-temperature region. The deuterated-water host samples were also examined, and the influence of isotopic effects on guest ordering and phase transformation was observed.

This paper presents the measurement of the thermal constants of natural methane hydrate-bearing sediments and mud layer samples recovered from wells. Core samples were recovered from the Tokai-oki test wells (Nankai Trough, Japan) in 2004. The thermal conductivity, thermal diffusivity, and specific heat of the samples were simultaneously determined using the hot-disk transient method. The thermal conductivity of natural hydrate-bearing sediments decreased slightly with increasing porosity. In addition, the thermal diffusivity of hydrate-bearing sediments decreased as the porosity increased. Moreover, we also used simple models to calculate the thermal conductivity and diffusivity. Estimations of the distribution model (geometric mean model) were relatively consistent with the measured results, suggesting that sand grains and hydrates should be independently distributed for hydrate-bearing sediments, which exhibit a pore-filling pattern. The measurement results were also consistent with the thermal diffusivity, which was estimated by dividing the thermal conductivity obtained from the distribution model by the specific heat taken from the arithmetic mean. Finally, our estimate of the thermal conductivity of silt soil was much lower than that for sand soil in hydrate-bearing sediment, which suggests that the small grains influence thermal conductivity.

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High-temperature and high-pressure experiments were performed under 2-55 GPa and 298-653 K using in situ Raman spectroscopy and X-ray diffraction combined with externally heated diamond anvil cells to investigate the stability of methane hydrate. Prior to in situ experiments, the typical C-H vibration modes of methane hydrate and their pressure dependence were measured at room temperature using Raman spectroscopy to make a clear discrimination between methane hydrate and solid methane which forms through the decomposition of methane hydrate at high temperature. The sequential in situ Raman spectroscopy and X-ray diffraction revealed that methane hydrate survives up to 633 K and 40.3 GPa and then decomposes into solid methane and ice VII above the conditions. The decomposition curve of methane hydrate estimated by the present experiments is >200 K lower than the melting curves of solid methane and ice VII, and moderately increases with increasing pressure. Our result suggests that although methane hydrate may be an important candidate for major constituents of cool exoplanets and other icy bodies, it is unlikely to be present in the ice mantle of Neptune and Uranus, where the temperature is expected to be far beyond the decomposition temperatures.