The fundamental properties of gas hydrates are reviewed; classification and type, structure and composition, formation and stability, and occupancy of guest molecules. Recent Raman studies on the properties of cage-occupancy are discussed by considering the C-H vibrations of their guest molecules for methane, methane-ethane mixed (structural phase transition), ethane, and cyclopropane (pressure-induced small cage occupancy) hydrates. Finally, I give an overview of very recent x-ray (neutron) diffraction and Raman scattering studies of phase transitions in the methane hydrate at high pressures, and discuss the inconsistent results by investigating the experimental details.
In this article, recent research works on the formation process of gas hydrate were reviewed. We describe how efficiently a solid-state 129Xe NMR technique was applied to monitor the formation processes of gas hydrates, together with a short review of 129Xe NMR. Especially, the formation process from an amorphous mixture of Xe and H2O was described precisely.
The elastic properties of structure I (sI) methane hydrate (MH) determined by high-pressure Brillouin spectroscopy up to 0.6 GPa at 23°C are reviewed by comparing them with those of ice-Ih. The pressure dependence of adiabatic elastic moduli of sI-MH is similar to that of ice-Ih except for C11. Elastic moduli and bulk modulus indicate that sI-MH is slightly more compressive than ice-Ih, and acoustic velocities show nearly isotropic behaviors with respect to the crystal orientation. The C44 of sI-MH is less sensitive to pressure and smaller than C11 and C12, which implies the sI-MH is becoming less stable against the shear stress under high pressures. These results are useful to investigate the dynamic stability and the estimated amount of MH in the deep-sea sediments.
The guest-host interactions and the rotational dynamics in the clathrate hydrates of acetone and THF are discussed in comparison with those in the supercooled aqueous solutions by measuring the 13C NMR chemical shift and the 2D spin-lattice relaxation time as functions of the temperature. When the temperature is decreased, the guest-water hydrogen bond is discontinuously reduced upon the clathrate cage formation. Due to the reduced friction with surrounding waters, the guest molecule reorients much faster in the clathrate cage than in the aqueous solution. On the other hand, the remaining guest-water interaction fluctuates the reorientational motion of the cage water. We also review the structural similarity between the clathrate hydrate and the aqueous solution.
Recent high-pressure studies using diamond anvil cell of methane hydrate are reviewed. Three high-pressure phases of methane hydrate were observed by x-ray diffraction and Raman spectroscopy up to 8 GPa in the present authors' study. The well-known cubic phase I decomposed into a hexagonal phase A and fluid at 0.8 GPa. The phase A transformed into an orthorhombic phase B at 1.6 GPa, and the phase B further transformed into another orthorhombic phase C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The bulk moduli, K0, for the phase I, phase A, and phase C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively. Comparison of the highpressure studies and the description of these high pressure structures were described in detail.
As a fundamental study of gas hydrates, which are key substances in the energy and environmental problems, the thermodynamic stability for the pure and mixed gas hydrate systems is investigated. The intermolecular vibration energy between the host water molecules and the intramolecular vibration energies of guest species are measured by means of laser Raman spectroscopy. The structural phase transition controlled on the equilibrium composition of the mixed gas is observed in the (methane + ethylene) mixed gas hydrate systems, while each pure hydrate has no structural phase transition in the whole range in this study.
CO2 ocean sequestration, which is thought to be a promising measure to mitigate global warming, can be classified mainly into the dissolution and the storage methods. On the evaluation of these measures, CO2 hydrate plays an important role, because CO2 becomes a hydrate in the ocean deeper than 500 m (North Pacific Ocean)∼900 m (North Atlantic Ocean). The solubility of CO2 shows a dual nature (solid and liquid solubilities) in the hydrate forming region. Two types of unexpected hydrate membrane strength just below the dissociation temperature and in a CO2 saturated solution, and the rebuilding process under stressed conditions greatly influence the above two CO2 ocean sequestration methods.
Exploration for natural methane hydrate was carried out in the Nankai-Trough offshore Japan at a water depth of 945 m over an 88-day period, from November 1999 to February 2000. This was a national project led by the Ministry of International Trade and Industry (MITI) to seek a new energy source. It was organized by Japan National Oil Corporation (JNOC) in collaboration with Japan Petroleum Exploration Co., Ltd. as the drilling operator. The Nankai-Trough wells were drilled with a deepwater semisubmersible rig. The location was selected where BSR (Bottom Simulating Reflector) is the clearest on the seismic section. Six wells were drilled through the BSR horizon and the hydrate rich formation was confirmed between 1135 m to 1213 m BMSL (below mean sea level) by LWD data, core samples and electric logging data.
Injection of liquid CO2 in hydrate layers of natural gas is a promising technique to recover methane simultaneously segregating CO2 from the biosphere. This work examined the rate of the conversion of CH4-hydrate immersed in liquid CO2 to CO2-hydrate in a temperature range of 274-281 K and a pressure range of 4-10 MPa. About 17% of the methane, utilizing CO2, was recovered after 300 hours at 10 MPa and 281 K, and the conversion was on going even at the end of every experiment. The solute mobility rose as the temperature increased; it decreased as the total pressure increased. The solute mobility of methane and CO2 in hydrate solid was determined for future feasibility studies. The configuration of the system should be investigated in a feasibility study, including the recovery system for CH4.
The volume compression of Bi, Pt and Au has been measured to megabar pressures by x-ray powder-diffraction experiments using a synchrotron radiation source. The EOS's of Pt and Au as the pressure scale have been crosschecked up to 145 GPa, and a considerable inconsistency between them is revealed in the megabar pressure region. The EOS of Au has been determined up to the megabar pressure range by the Pt scale. Stability of the high-pressure bcc phase of Bi is confirmed up to 222 GPa. The equation of state (EOS) of bcc-Bi is determined in a megabar pressure range with the bulk modulus, B0 = 35.45(26) GPa, its pressure derivative, B0' = 6.294(32) and the relative atomic volume, V/V0 = 0.8927(8) at atmospheric pressure, respectively.