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.

Gas hydrate is a very complicated reservoir system characterized of temperature, pressure, gas composition, pore-water geochemical features, and gas sources, gas hydrate distribution within the gas hydrate stability zone. Temperature, pressure and the gas composition of the sediments were suitable for gas hydrate formation in the gas hydrate reservoir system of Shenhu Area, north slope of South China Sea. The high-resolution seismic data and the gas hydrate drilling getting high concentrations of hydrate (>40%) in a disseminated form in foram-rich clay sediment showed that gas hydrate is distributed heterogeneously at all spatial scales in all drill holes, and the hydrate-bearing sediments ranged several ten meters in thickness are located in the lower part of gas hydrate stability zone (GHSZ), just above the bottom of gas hydrate stability zone (BGHSZ). It is likely seem that the methane to crystallize gas hydrate is from in-situ microbial methane.

Small-scale morphology of gas hydrate is important for understanding the formation of gas hydrate deposits, for estimating the concentrations of gas hydrate from geophysical data, and for predicting their response to climate change or commercial production. The recent use of borehole pressure coring tools has allowed marine gas-hydrate-bearing sediments to be recovered with centimeter to sub-millimeter gas hydrate structures preserved in their in situ condition. Once these sediment samples are recovered at in situ temperature and pressure, nondestructive analyses, including gamma density, P-wave velocity, and X-ray imaging, are used to examine the character of the gas hydrate relative to the structure of the surrounding sediment. Gas hydrate morphology from pressure core data is summarized from the recent national gas hydrate expeditions of India, China, and Korea, as well as from Ocean Drilling Program Leg 204, Integrated Ocean Drilling Program Expedition 311, and the Gulf of Mexico Chevron-Texaco Joint Industry Project. The most striking result is the variability of gas hydrate morphology in clay, ranging from complex vein structures to an invisible pore-filling matrix. Both of these morphologies have been observed in clay sediments at gas hydrate saturations equivalent to 30-40% of pore volume. A clear knowledge of detailed gas hydrate morphology will provide important data to help determine the mechanisms of gas hydrate deposit formation and also provide crucial data for modeling the kinetics of deposit dissociation, from both natural and artificial causes. The morphology also has large effects on sedimentary physical properties, from seismic velocities on a large scale to borehole electrical resistivities on a smaller scale, and gas hydrate morphology will therefore impact estimation of gas hydrate saturation from geophysical data. The detailed morphology of gas hydrate is an essential component for a full understanding of the past, present, and future of any gas hydrate environment.

Thermal properties of pure methane hydrate, under conditions similar to naturally occurring hydrate-bearing sediments being considered for potential production, have been determined both by a new experimental technique and by advanced molecular dynamics simulation (MDS). A novel single-sided, Transient Plane Source (TPS) technique has been developed and used to measure thermal conductivity and thermal diffusivity values of low-porosity methane hydrate formed in the laboratory. The experimental thermal conductivity data are closely matched by results from an equilibrium MDS method using in-plane polarization of the water molecules. MDS was also performed using a non-equilibrium model with a fully polarizable force field for water. The calculated thermal conductivity values from this latter approach were similar to the experimental data. The impact of thermal conductivity on gas production from a hydrate-bearing reservoir was also evaluated using the Tough+/Hydrate reservoir simulator (Revised version of ICGH paper 5646).

The paper presents results of the experimental research on the process of CO2 gas hydrates formation in the porous media of sediments under positive and negative temperatures. The subject of research were sediment samples of various compositions including those selected in the permafrost area. The research was conducted in a special pressure chamber, which allowed to monitor pressure and temperature. Using the monitoring results it was possible to make quantitative estimation of the kinetics of CO2 hydrates accumulation in the model sediments. In the course of the research it was demonstrated, that active hydrates accumulation occurred in frozen sediments under negative temperatures (about -4 оС). At the same time a comparative analysis of СО2 and СН4 hydrates accumulation was made in the porous media of the sediment under negative temperatures. The performed experiments enabled to estimate an influence of temperature, sediment composition and water content on kinetics of CO2 hydrates accumulation in porous media. Besides, we made an estimation of the amount of hydrates, which could be formed in hydrates containing sediments at freezing of the remaining pore water.

The equilibrium “pore water in sediment–gas hydrate-former–bulk gas hydrate” was experimentally studied. This residual pore water corresponds to a minimal possible amount of water in the sediment, which is in thermodynamic equilibrium with both gas and the bulk hydrate phase. This pore water can be defined as non-clathrated water by analogy to unfrozen water widely used in geocryological science. The amount of non-clathrated water depends on pressure, temperature, type of sediment, and gas hydrate former. The presence of residual pore water influences the thermodynamic properties of hydrate-saturated samples. The paper’s purpose is to describe a new experimental method for determining the amount of non-clathrated water in sediments at different pressure/temperature conditions. This method is based on measuring the equilibrium water content in an initially air-dried sediment plate that has been placed in close contact with an ice plate under isothermal, hydrate-forming gas pressure conditions. This method was used to measure the non-clathrated water content in kaolinite clay in equilibrium with methane hydrate and CO2 hydrate at a temperature of –7.5o C in a range of gas pressures from 0.1 to 8.7 MPa for methane and from 0.1 to 2.5 MPa for CO2. Experimental data show that at the fixed temperature the non-clathrated water in hydrate-containing sediments sharply reduces when gas pressure increases. The experiment demonstrates that the non-clathrated water content strongly depends on temperature, the mineral structure of sediment, and the hydrate-forming gas.

The presented work describes first principles for monitoring gas hydrate formation and dissociation in solution by evaluating state-responsive IR absorption features of water with fiberoptic evanescent field spectroscopy. In addition, a first order linear functional relationship has been derived according to Lambert Beer’s law, which enables quantification of percentage gas hydrate within the volume of water directly probed via the evanescent field. Moreover, spectroscopic studies evaluating seafloor sediments collected from a gas hydrate site in the Gulf of Mexico revealed minimal spectral interferences from sediment matrix components, thereby establishing evanescent field sensing strategies as a promising perspective for monitoring the dynamics of gas hydrates in oceanic environments.

Tetra-n-Butyl Ammonium Bromide (TBAB) forms semi-clathrate hydrates which can incorporate small gas molecules, such as methane and nitrogen at ambient temperatures and atmospheric pressure. Such favourable stability conditions, combined with ease of formation could make semi-clathrates particularly attractive for a large variety of applications. These hydrates have recently been investigated for their use in the separation of gases, and it is proposed that the same technology could potentially be used for storage and transportation of gases. To evaluate the feasibility of using TBAB hydrates for separation and storage purposes, an extensive test programme was conducted to determine: phase stability of the semi-clathrates, gas storage capacity, and composition of the stored gas. The results show that TBAB semi-clathrates have very favourable stability conditions. They can store considerable quantities of gas, and favour small molecules in their structures. These experiments suggest that semi-clathrate hydrates, such as TBAB, could have a significant potential as an alternative for industrial separation, storage, and transportation of natural gas.