The pore saturation of natural gas hydrate in sediments is a key parameter for estimating hydrate resources in a reservoir. For a better understanding of gas hydrate distribution, the experimental study of the pore saturation of methane hydrate in sediments from a hydrate reservoir in the Eastern Nankai Trough have been carried out. In total, eleven samples, comprising sand, silty sand, silt, and representative of the main sediment types identified in the Eastern Nankai trough, were tested. The results obtained clearly indicate a particle size and clay content dependent trend: almost 100% of pores were saturated with methane hydrate in sand when little silt and clay were present, decreasing to ~ 13% in silty sand (sand 54%, silt 41% and clay 5%), and ~ 4% in clayey silt. These results are generally consistent with NMR logging results for high-saturation samples, but somewhat different for samples with medium or low saturation levels.

In this study, we investigated the molecular characteristics of hydrates which were synthesized from gas mixtures containing the two isomers of butane, or the pentane isomers neopentane and isopentane, in excess methane. Thereto various techniques, including Raman spectroscopy, powder and single crystal X-ray diffraction and 13C NMR spectroscopy were employed. It turned out that shape and conformation of the guest molecule and hydrate structure both influence each other. In case of the mixed butane hydrate it could be confirmed that n-butane is enclathrated in its gauche conformation. This was verified by Raman spectroscopy, single crystal X-ray diffraction and calculated data. While isopentane is known as a structure H former, our results from powder X-ray diffraction, 13C NMR and ab initio calculations show that it can be also incorporated into structure II when the hydrate is formed from a neopentane/isopentane/methane gas mixture.

Among the three common gas hydrate structures, structure-H (sH) hydrate has been regarded as forming only in the laboratory since it was first reported in 1987. However, natural gas hydrate samples obtained from the Cascadia margin showed that sH hydrate can form naturally. Not only was the sH hydrate found in natural samples, but it was also discovered that n-alkanes such as n-pentane and n-hexane, considered to have too large molecular size to be sH hydrate formers, can act as co-guests of sH hydrates in mixtures with other sH hydrate formers. In this study, thermodynamic measurements and spectroscopic analysis of powder X-ray diffraction and 13C solid-state NMR methods, were performed for synthetic hydrate samples in order to identify the accommodation of n-alkanes with five or more carbon atoms. In addition, some new hydrate guests were found to form sH hydrates. From the present results, it is clear that, so far, our understanding of gas hydrates and guest enclathration needs to be revised and expanded in order to explain new findings.

Crystallization of water or water-encaged gas molecules occurs when nuclei reach a critical size. Certain antifreeze proteins (AFPs) can inhibit the growth of both of these, with most representations conceiving of an embryonic crystal with AFPs adsorbing to a preferred face, resulting in a higher kinetic barrier for molecule addition. We have examined AFP-mediated inhibition of ice and clathrate hydrate crystallization, and these observations can be both explained and modeled using this mechanism for AFP action. However, the remarkable ability of AFPs to eliminate „memory effect‟ (ME) or the faster reformation of clathrate hydrates after melting, prompted us to examine heterogeneous nucleation. The ubiquitous impurity, silica, served as a model nucleator hydrophilic surface. Quartz crystal microbalance-dissipation (QCM-D) experiments indicated that an active AFP was tightly adsorbed to the silica surface. In contrast, polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap), two commercial hydrate kinetic inhibitors that do not eliminate ME, were not so tightly adsorbed. Significantly, a mutant AFP (with no activity toward ice) inhibited THF hydrate growth, but not ME. QCM-D analysis showed that adsorption of the mutant AFP was more similar to PVCap than the active AFP. Thus, although there is no evidence for „memory‟ in ice reformation, and the structures of ice and clathrate hydrate are distinct, the crystallization of ice and hydrates, and the elimination of the more rapid recrystallization of hydrates, can be mediated by the same proteins.

Certain organisms, including some bugs (both insects and microbes) are able to survive low temperatures by the production of either ice nucleating proteins (INPs) or antifreeze proteins (AFPs). INPs direct crystal growth by inducing rapid ice formation whereas AFPs adsorb to ice embryos and decrease the temperature at which the ice grows. We have also shown that certain AFPs can inhibit the crystallization of clathrate hydrates and eliminate more rapid recrystallization or “memory effect”. Here we examine several bacterial species with iceassociating properties for their effect on tetrahydrofuran (THF) hydrate crystallization. The bacteria Chryseobacterium sp. C14, which shares the ice recrystallization inhibition ability of AFPs, increased induction time to THF hydrate crystallization in isothermal experiments. In an effort to understand the association between AFPs and THF hydrate we have produced bacterially-expressed AFPs as probes for hydrate binding. Although the structure of hydrates is clearly distinct from ice, the apparent potential for these products to perturb clathrate hydrate growth compels us to explore new techniques to uncover “green inhibitors” for hydrate binding.

Using cryogenic SEM, we investigated the physical states of gas-hydrate-bearing samples recovered by drill core from several localities including the SE India margin (NGHP Expedition 01), Cascadia margin (IODP Leg 311), Gulf of Mexico (RV Marion Dufresne 2002), and Mackenzie River Delta (Mallik site, well 5L-38). Core material with a significant fraction of preserved hydrate has only been obtained for cryogenic SEM investigation from relatively few sites worldwide to date, yet certain consistent textural characteristics, as well as some clear differences between sites have been observed. Gas hydrate in cores recovered from Cascadia, Gulf of Mexico, and Mallik often occurs as a dense substrate with typical grain size of 30 to as large as 200 μm. The hydrate often contains a significant fraction of isolated macropores that are typically 5–100 μm in diameter and occupy 10-30 vol. % of the domain. In fine-grained sediment sections of marine samples, gas hydrate commonly forms small pods or lenses with clay platelets oriented sub-parallel around them, or as thin veins 50 to several hundred microns in thickness. In some sections, hydrate grains are delineated by a NaCl-bearing selvage that forms thin rinds along hydrate grain exteriors, presumably produced by salt exclusion during original hydrate formation. Preliminary assessment of India NGHP-01 samples shows some regions consistent with the observations described above, as well as other regions dominated by highly faceted crystals that line the walls or interior of cavities where the hydrate grows unimpeded. Here, we focus on gas hydrate grain morphology and microstructures, pore characteristics and distribution, and the nature of the hydrate/sediment grain contacts of the recovered samples, comparing them to each other and to laboratory-produced gas hydrates grown under known conditions.

Cyclopentane hydrate interparticle adhesion force measurements were performed in pure cyclopentane liquid using a micromechanical force apparatus. Cyclopentane hydrate adhesion force measurements were compared to those of cyclic ethers, tetrahydrofuran and ethylene oxide, which were suspected to be cyclic ether-lean and thus contain a second ice phase. This additional ice phase led to an over-prediction of the hydrate interparticle forces by the capillary bridge theory. The adhesion forces obtained for cyclopentane hydrate at atmospheric pressure over a temperature range from 274-279 K were lower than those obtained for the cyclic ethers at similar subcoolings from the formation temperature of the hydrate. The measured cyclopentane interparticle adhesion forces increased linearly with increasing temperature, and are on the same order of magnitude as those predicted by the Camargo and Palermo rheology model.

It has recently been recognized that abundant gas hydrates occur in localized zones of upwelling fluids, with concentrations much higher than in regional distributions associated with bottomsimulating reflectors (BSRs). We report a study of multi-channel seismic reflection data across such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few BSRs. The structures are commonly up to several km across and a few hundred meters in depth extent, and are characterized by reduced reflectivity and bowed-up sediment reflectors on time-migrated sections. The seismic pull-up mainly results from higher velocities, although physical deformation due to folding and faulting is not ruled out. Some of the features extend upward close to the seafloor and others only partway through the gas hydrate stability zone. The base of gas hydrate stability zone (BGHSZ), calculated assuming a regional average constant heat flow of 110 mW/m2, is confirmed by the presence of gas inferred from reduced instantaneous frequencies and high instantaneous amplitudes, and from a decrease in seismic velocities. The vents are fed by upward migrating free gas or gas-rich fluids through near-vertical conduits probably due to regional, upward fluid flow caused by tectonic compression of the basin.

Gas hydrates bind immense amounts of methane in marine sediments. If produced cost effectively, they can serve as a stable energy supply. No viable technologies for extracting gas hydrates from deep ocean deposits have been developed to date. Due to the shallow depths, low hydrate concentration, low permeability of the gas hydrate stability zone, lack of driving pressure and the slow melting process, low productivity is anticipated for gas production from gas hydrates in marine sediments. Therefore, only a large number of low cost wells can support an offshore production facility and pipeline transport to shore. The method of harvesting natural gas from sea floor gas hydrates presented in this paper is a combination of several new concepts including electrically adding heat inside hydrate rich sediments to release gas, using an overhead receiver to capture the gas, allowing gas to form hydrates again in the overhead receiver, and lifting produced hydrates to warm water to release and collect gas. This approach makes the best use of the nature of hydrates and the subsea pressure and temperature profiles. Consequently, it leads to a simple and open production system which is safe, economical, energy efficient, environmentally friendly, and without significant technical difficulties. Basic analyses and calculations on the feasibility and heat efficiency of the proposed method are presented and discussed.

Hydrate phase equilibria for the CO2, CH4 and natural gas in silica gel pores of nominal pore diameters 6, 30 and 100 nm were measured, and compared with the calculated results based on van der Waals and Platteeuw model. At a specific temperature, three-phase hydrate–water-rich liquid–vapor (HLV) equilibrium curves for pore hydrates were shifted to the higher pressure condition depending on pore sizes when compared with those of bulk hydrates. The activities of water in porous silica gels were modified to account for capillary effect, and the calculated results were in good agreement with the experimental data. To investigate the formation kinetics of each system, the isobaric method was applied. It was found that there were no difference in structure between hydrate in silica gel pore and that in bulk free state. Results showed that hydrate formation in the silica gel pores indicated significantly faster rates, intensively reduced induction times, increased gas consumption and conversion of water to hydrate as compared to hydrate formation in bulk free water or fine ice powder. Utilizing these superior characteristics, formation of hydrate in porous material is expected to present the process on gas separation or storage.