The stability of hydrogen clathrate hydrate was investigated using a classical Molecular Dynamic (MD) calculation code “MXDTRICL” as a theoretical approach. Arranging hydrogen molecules one by one into host-frame of the hydrogen hydrates, the inclusion energy of their system was evaluated, where Lennard-Jones potential and two types of TIP4P potentials were adopted on the MD calculations as intermolecular potentials. From the result, it is concluded that multiple molecules are included in both large and small cages so that the storage density could attain higher than 6wt% for any potential. Observation of the movement of H2 molecules in the cage under various conditions revealed that H2 molecules are not stable in the cage and a few part of the H2 molecules come in and go out of the cage through the center hole between hexagons.

Methane hydrate has a unique structure that the host water framework forms two kinds of cages, which contain one methane molecule each. Therefore, it has been expected that there may exist three kinds of translational modes of a methane molecule and also the distortion of translational mode of host water molecules compared with normal ice. We need information of the host and guest molecular dynamics over the wide momentum and energy transfer region for studying such dynamics. In this study inelastic neutron measurements were carried under 40 K with MARI spectrometer at ISIS in UK, TAS at JRR-3 and CAT at KENS in Japan. For the methane molecular motion we could confirm its freelike rotation by complementary use of MARI and TAS spectra. After the subtraction of the scattering intensity of the rotation evaluated by the free rotation model from the experimental data, three kinds of translation modes were identified at first experimentally. On the experimental spectra there still remains the excess intensity which could not explain the single mode excitation. The libration mode of the water framework shows the different momentum and energy transfer dependence with those of normal ice. The feature of the libration mode is resemble to ice-IX, that could be considered as a proton ordering of the cage structure appeared in ice-II, VIII and IX.

This paper presents results of a seismic tomography experiment carried out at the Bullseye cold vent site offshore Vancouver Island. In the experiment, a seismic air gun survey was recorded on an array of five ocean bottom seismometers (OBS) deployed around the vent. The locations of the shots and the OBSs were determined to high accuracy by an inversion based on the shot travel times. A three-dimensional tomographic inversion was then carried out to determine the velocity structure around the vent, using the localized source and receiver positions. The inversion indicates a relatively uniform velocity field around and inside the vent. The velocities are close to the values expected for sediments containing no hydrate, which supports previous claims that the bulk concentrations of gas hydrates are low at the site. However, the largest resolved velocity anomalies of + 25 m/s are spatially within the limits of the acoustic blank zone seen in multichannel seismic data near the Bullseye vent. The velocity inversion is consistent with zones of high concentration (15-20 % of the pore space) in the top 50-100 m of sediment.

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.

In this study, Tetrahydrofuran (THF) hydrates were formed in-situ in the Zetasizer Nano ZS90. With various concentrations of SDS, we attempted to characterize the SDS adsorption on the surface of the hydrate particles. In doing so, we tried to correlate the adsorption of SDS to THF hydrate induction times with respect to SDS concentration (0 – 3.47 mM), to determine whether the fast nucleation of THF hydrates is due to the adsorption of SDS. The measured ζ-potential for pure THF hydrates was -100 ± 10 mV, indicating anion adsorption. An adsorption curve was observed where there is saturation leveling. Correlating this data to the hydrate induction times, we see that when the saturation level is reached, a significant reduction in induction time can be seen.

Gas hydrates are crystalline solids (clathrates) in which gas molecules are encaged within lattices of hydrogen bonded water molecules. Hydrates are stable at low temperatures and high pressures; and dissociation takes place at temperatures and pressures outside the stability zone. Modeling the dissociation behavior of hydrates in porous soil media requires attention be paid to the geomechanics of hydrate dissociation. This paper addresses the issue of coupling the hydrate dissociation problem with the soil deformation problem and constructs the mathematical framework. Thermally stimulated dissociation process under undrained conditions is considered with conduction heat transfer.

The anti-agglomerating hydrate behavior observed for some crude oils has previously been related to crude oil composition and to surface adsorption mechanisms. Petroleum acids derived from some crude oils have been found able to convert systems with initially high risk of plugging into easily flowable dispersions. In this work, acid fractions are isolated from three oils with low tendency to form hydrate plugs and from two oils associated with high risk of hydrate plugging by using an ion-exchange resin. The extracts are further separated into four sub-fractions by solid phase extraction (SPE). The chemical composition of the fractions is studied by means of HPLC, GPC, FTIR- and UV/VIS spectroscopy and elemental analysis. The distribution of chemical compound classes in the fractions differs between the non-plugging and plugging oils, and the differences are most distinctive in one of the sub-fractions. The results imply that acid sub-fractions holding a significant proportion of more weakly polar compounds, like ester functionalities, are important for how the hydrate surfaces and the oil phase interact.

Nonadhesive hydrate slurries have been shown to exhibit low viscosities in a field-scale flow loop when formed under appropriate conditions. The factors that favor formation of low-viscosity hydrate slurries include high Reynolds Number and Capillary Number, and high mass transfer and heat transfer rates. High liquid loading and high superficial fluid velocities are found to be conducive to the formation of low viscosity hydrate slurries. Dispersed bubble flow has been observed to facilitate flowable hydrate slurry production. Alternatively, the formation of nonadhesive hydrates at moderate superficial velocity is possible when a static mixer is used upstream of the hydrate formation location. For certain fields, low-viscosity hydrate slurry technology could eliminate the need for insulation and hydrate inhibitor chemicals (revised version of ICGH 2008 paper 5818) .

It was proposed that erosion of subsea ridges on the Hikurangi margin may be linked to a fluctuating level of the top of gas hydrate stability in the ocean. Since publication of this hypothesis, three field campaigns were conducted in the study area. Here we summarize relevant results from these cruises. We found that water temperature fluctuations occur at lower frequencies and higher amplitudes than previously thought, making it more likely that temperature changes reach sub-seafloor gas hydrates. Dredge samples encountered numerous consolidated mudstones. We speculate that gas hydrate “freeze-thaw” cycles may lead to dilation of fractures in mudstones due to capillary forces, weakening the seafloor. Ubiquitous gas pockets beneath the ridge may lead to overpressure that may also contribute to seafloor fracturing.

Monitoring of the seafloor for gas hydrate dissociation around boreholes during hydrocarbon production is likely to involve seismic methods because of the strong sensitivity of P-wave velocity to gas in sediment pores. Here, based on geomechanical models, we apply commonly used rock physics modeling to predict the seismic response to gas hydrate dissociation with a focus on P-impedance and performed sensitivity tests. For a given initial gas hydrate saturation, the mode of gas hydrate distribution (cementation, frame-bearing, or pore-filling) has the strongest effect on P-impedance, followed by the mesoscopic distribution of gas bubbles (evenly distributed in pores or “patchy”), gas saturation, and pore pressure. Of these, the distribution of gas is likely to be most challenging to predict. Conceptual 2-D FD wave-propagation modeling shows that it could be possible to detect gas hydrate dissociation after a few days.