We have developed a one-dimensional numerical computer model (simulator) to describe methane hydrate formation, decomposition, reformation, and distribution with depth below the seafloor in the marine environment. The simulator was used to model hydrate distributions at Blake Ridge (Site 997) and Hydrate Ridge (Site 1249). The numerical models for the two sites were conditioned by matching the sulfate, chlorinity, and hydrate distribution measurements. The constrained models were then used to investigate the effect of changes in seafloor temperature and sea-level on gas hydrate stability. For Blake Ridge (site 997), changes in hydrate concentration are small. Both the changes in seafloor temperature and sea-level lead to a substantial increase in gas venting at the seafloor for Hydrate Ridge (site 1249).

Many places where natural gas hydrate occurs have a regionally extensive, bottom-simulating seismic reflector (BSR) at the base of the gas hydrate stability zone (GHSZ). This reflection marks the top of an underlying free-gas zone (FGZ). Usually, hydrate recycling (that produces gas as the stability field moves upward relative to sediments) is invoked to explain the presence and properties of the sub-BSR FGZ. However, this explanation is not always adequate: FGZs are often thicker in passive-margin environments where hydrate recycling is relatively slow, than in convergent-margin environments where hydrate recycling is relatively fast (e.g. Blake Ridge compared with Cascadia). Furthermore, some areas with thick FGZs and extensive BSRs (e.g. west Svalbard) have similar rates of hydrate recycling to northern Gulf or Mexico, yet the latter has no regional BSR. Here we discuss a gas-forming mechanism that operates in addition to hydrate recycling, and which produces a widespread, regional, BSR when gas is transported upward through the liquid phase; this mechanism is dominant in tectonically passive margins. If the gas-water solubility decreases downward beneath the GHSZ (this occurs where the geothermal gradient and the pressure are relatively high), low rates of upward fluid flow enable pore water to become saturated in a thick layer beneath the GHSZ. The FGZ that this produces achieves a steady-state thickness that is primarily sensitive to the rate of upward fluid flow. Consequently, geophysical observations that constrain the thickness of sub-BSR FGZs can be used to estimate the regional, diffuse, upward fluid flux through natural gas-hydrate systems.

The hydrogen molecule dynamics in tetrahydrofuran-H2-H2O clathrate hydrate has been studied by high-resolution inelastic neutron scattering and Raman light scattering. Several intense bands in the neutron spectrum are observed that are due to H2 molecule excitations. These are rotational transitions, center-of-mass translational transitions (rattling) of either para- or ortho-H2, and combinations of rotations and center-of-mass transitions. The rattling of the H2 molecule is a paradigmatic example of the motion of a quantum particle in a non-harmonic three-dimensional potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets. Raman spectra show a similar splitting of the S0(0) rotational transition, due to a significant anisotropy of the potential with respect to the orientation of the molecule in the cage. The comparison of our experimental values for the transition frequencies to a recent quantum mechanical calculation gives qualitative agreement, but shows some significant difference.

The aim of this work is to model the thermal evolution inside a hydrate forming system which is submitted to an imposed steady cooling. The study system is a cylindrical thin film of aqueous solution at 19 Mpa, the methane is the hydrate forming molecule and it is assumed that methane is homogeneously dissolved in the aqueous phase. The model in this work takes into account two factors involved in the hydrate crystallization: 1) the stochastic nature of crystallization that causes sub-cooling and 2) the heat source term due to the exothermic enthalpy of hydrate formation. The model equation is based on the resolution of the continuity equation in terms of a heat balance. The crystallization of the methane hydrate occurs at supercooling conditions (Tcryst < TF), besides, the heat released during crystallization interferes with the imposed condition of steady decrease of temperature around the system. Thus, the inclusion of the heat source term has to be considered in order to take into account the influence of crystallization. The rate of heat released during the crystallization is governed by the probability of nucleation J(T ). The results provided by the model equation subjected to boundary conditions allow depict the evolution of temperature in the dispersed phase. The most singular point in the temperature–time curve is the onset time of hydrate crystallization. Three time intervals characterize the temperature evolution during the steady cooling: (1) linear cooling, (2) hydrate formation with a release of heat, (3) a last interval of steady cooling.

How to exploit and make use of natural gas hydrates in oceans will weigh much in the future researches. Unlike the oil or gas reservoirs, the distributions of natural gas hydrate are very complicated and don’t congregate massively in oceanic sediments. Besides, factors such as seafloor geohazards and climate must be taken into account, which makes it much more difficult and complicated to exploit oceanic gas hydrates than conventional oil or gas. Nowadays neither of such methods as thermal stimulation, depressurization, inhibitor injection, carbon dioxide replacement and mixing exploitation etc. is applied to exploit gas hydrates in marine sediments because of their disadvantages. This paper introduces a conception of combining solar and geothermal energy for gas hydrates exploitation. The model mainly includes five parts: solar energy transferring module, sea water circulating module, underground boiler module, platform and gas-liquid separating module. Solar cells and electric heaters are used to heat the formations containing hydrates. Because they become relatively more mature and cheaper, it’s the key of how to utilize the geothermy to exchange heat in developing this conception, which needs solution of fluid leakage, circulating passages and heat-exchange interface problems in building underground boiler. Probably it’s a feasible measure to use an effective hydraulic control system and hydraulic fracturing. The idea should be a good choice to exploit marine gas hydrates by combining solar and geothermal energy since this method has a great advantage either in terms of efficiency or cost.

Laboratory tests have been performed using a stirred cell where SI and SII gas hydrates have been formed under the presence of the kinetic inhibitor Poly Vinyl Caprolactam (PVCap) and INHIBEX. The latter is a mixture containing 50wt% PVCap 2k and 50wt% butyl glycol. The effect of PVCap is enhanced by the presence of butyl glycol; the latter acts as a synergist for the former. Dissociation temperatures were obtained and compared for hydrates formed 1) in presence of PVCap and 2) in presence of INHIBEX. The effect of INHIBEX concentration on the temperature of dissociation was also investigated. Systems containing INHIBEX dissociated at lower temperatures than the corresponding systems with only PVCap present. Furthermore, 3000 ppm INHIBEX mixtures were found to have higher dissociation temperatures than 1500 ppm INHIBEX mixtures.

Inhibitor containing systems were investigated for hydrate structures I and II. The kinetic inhibitor PVCap was added to the water phase for each hydrate structure. Dissociation temperatures were determined for various formation temperatures and PVCap concentrations. Obtained dissociation temperatures were compared with corresponding values calculated with CSMHYD. Differences between experimental and calculated values were compared for various formation temperatures and inhibitor concentrations. Comparison revealed that these parameters (formation temperature and concentration) had an effect on the dissociation temperature. Dissociation temperatures for hydrates formed at low degrees of subcooling were higher than for hydrates formed at large subcooling. The effect depended on the system pressure; apparently decreasing or vanishing with increasing pressure. Furthermore, the temperature of dissociation increased with the inhibitor dose.

The kinetic inhibitor Poly Vinyl Caprolactam (PVCap) was added as a kinetic inhibitor to the gas-water system. Different hydrate formers were used in order to obtain formation of the different hydrate structures (sI, sII and sH). All hydrate structures were formed with PVCap. The effect of applying different melting rates was investigated. The isochoric technique was used to obtain dissociation temperatures and corresponding pressures. The melting rate was found to be a parameter influencial for the dissociation temperature. Even for very slow melting rates such as 0.0125 Kelvin per hour, the final dissociation temperature was significantly higher that the dissociation temperature for the corresponding non-inhibited system.

Except for direct drilling and sampling of marine gas hydrates, the occurrence of gas hydrates has been identified generally by inference from indirect evidence, derived from geological, geophysical, and geochemical data. In this paper, we intend to discuss the geochemical anomalies of pore waters and their implications for gas hydrate occurrence in the northern continental slope of the South China Sea. The molecular concentration and isotopic composition of methane in sediments can provide clues to gas sources, whereas ionic and isotopic compositions of pore waters, such as steep SO42- gradients, shallow SMI (sulfate-methane interface) depths; decreasing pore water chlorinity, and heavy oxygen isotopic compositions, are used to identify gas hydrate occurrence and the distribution and thickness of sediment layers containing gas hydrates. Other good geochemical indicators include anions and cations concentrations such as Br-, I-, PO43-, NH4+, Ca2+, Mg2+, Sr2+, B3+, Li+, and Ba2+ in pore waters. We also found that the very negative carbon isotopic compositions of dissolve inorganic carbon (DIC) in pore waters can serve as good indicators for gas hydrate occurrence. In the South China Sea, three most promising target areas for gas hydrates include the Dongsha, Shenhu, and Xisha Trough.

The northern margin of South China Sea contains several favorable areas for occurrence of gas hydrate. In this study, we collected pore water samples in two piston cores (X-01 and D-01) from Xisha Trough and Dongsha area, respectively, and the concentrations of sulfate and carbon isotopic compositions of dissolved inorganic carbon (DIC) were measured. The results showed different geochemical characteristics in these two sites. The X-01 core shows relatively constant δ13C-DIC values and sulfate concentrations, which suggest that anaerobic methane oxidation (AMO) processes did not occur in this site. In contrast, very large variation in δ13C-DIC values and sulfate concentrations are revealed in D-01 core, and good linear correlations for sulfate gradients and δ13C-DIC values are observed. The calculated sulfate-methane interface (SMI) depth is 9.6 mbsf. These data indicate that an AMO process occurred in sediments with large methane flux from depth in the Dongsha area, which are comparable to other gas hydrate locations in the world oceans such as the Blake Ridge. We suggest that the Dongsha area is one of the most favorable targets for future gas hydrate exploration.