Abstract The main reserves of methane on our planet are in the form of gas hydrate and more than 90% of them are at the bottom of the oceans. Scientists in many countries are making active efforts to establish effective technology for the development of gas hydrate deposits and, primarily located in the sedimentary rocks of the ocean floor. However, the first experience indicates that the development costs of gas hydrate deposits are very high. One of the main reasons for the high cost of recovering methane hydrate is the high value of the decomposition's thermal effect. The heat of methane hydrate decomposition is 410 kJ/kg, that 30% exceeds, even energy intensive process, as the melting water ice. We have proposed and patented the way to develop marine gas hydrate deposits without the cost of thermal energy and using of any environmentally hazardous substances. The method consists in grinding of sediment filled of methane hydrate and its subsequent lifting platform by the pipeline in the form of a suspension in seawater. When lifting the hydrate-contained suspension crosses the line of phase equilibrium and goes out of his stable thermodynamic state. Decomposition of gas hydrate phase in this case will be provided at the expense of the sea water crystallization energy. In the pipeline the conversion of gas hydrate slurry in the slurry containing of water, ice particles and gas, released from gas hydrate happen. It should be noted that the methane gas released from the hydrate, creates a lifting force in the pipe, thus avoiding the energy costs for transporting hydrate-contained rock on the surface.

Abstract In order to obtain the dynamic character for hydrate-based gas separation, hydrates formation and dissociation were experimental investigated as gas mixture (CO 2 /H 2 ) flows through cooled glass beads using Magnetic Resonance Imaging (MRI). The experimental results showed that the hydrates blockage restricted the incomplete transformation of residual solution. The initial residual solution saturation is about 42.7%, and the highest hydrates saturation appears in the first cycle, which is higher than that of other two about 10%. The present study provided some important thermodynamic and kinetic data for hydrate-based technology in a flow system.

Abstract As the final goal of visualization of the dissociation front of methane hydrate in the sand layer, we are proceeding with a program of P-wave tomography monitoring. We describe a monitoring of the elastic wave propagation in the methane hydrate-bearing sand artificially formed in a giant pressure cell. First, we conducted a preliminary experiment to check the performance of the measurement system. In the stage of the methane hydrate generation, the P-wave velocity was estimated about 3000 m/s. In the dissociation stage, the travel time delay was not observed, but also the signal decreasing was observed with the dissociation.

Abstract In-situ dissociation of natural gas hydrate is necessary for commercial recovery of natural gas from natural gas hydrate sediment. Thermal stimulation is an effective dissociation method, along with depressurization. In this study, we examined the efficiency of electrical heating of the hydrate core for gas production. To ensure safety and to avoid explosions, we investigated electrical heating of xenon gas hydrate sediment rather than methane hydrate sediment. Depressurization and additional electrode heating of hydrate sediment saturated with electrolyte solution was confirmed to enable higher gas production from sediment with less electric power.

Abstract An original method of enhanced recovery by using a partial oxidation method was proposed for gas hydrates reservoirs. This method was based on the heat generation of the oxidation reaction by means of oxidizing materials, some kinds of catalytic agent and iron powders. This paper reports the results of numerical simulation of partial oxidation method as a method of enhanced recovery for gas hydrates. The fundamental data was obtained by a series of experiment of partial oxidation in a vessel simulated as a situation of marine sediments.

Abstract Recently there has been much research into Methane Hydrate as a developable material in the deep seabed in Nankai Trough. In this study, specimens were prepared to simulate the sediments in Nankai Trough, and a series of triaxial tests were performed on samples with varying density and fines content. From the results of Methane Hydrate bearing sand however, an increase in fines content tended to lead to an increase in stiffness and maximum axial deviator stress, with positive dilative behavior occurring during shearing.

Abstract Pump performance of a submersible centrifugal type is investigated experimentally using a multiphase vertical flow loop. Methane in water is tested under turbulent bubbly pipe flows. Some tests are conducted at a three-phase equilibrium point of methane-water-methane hydrate. The pump is operated at 0.1 MPa and 3.9 MPa with void fractions up to 0.048. It is found that the pump performance is improved at higher pressures under vapor-liquid system but it is not significantly affected by the phase transformations. The results indicate that the fluid properties under the three-phase equilibrium and vapor-aqueous conditions are not very different.

Abstract Methane Hydrate (hereafter referred to as MH) is being researched and developed in Japan. In order to investigate the effect of the characteristics of a host sand on the shear behaviour of MH-bearing sands, MH was artificially produced in specimens composed of four kinds of material. In this study, a high stress and low temperature triaxial plane strain testing apparatus was used. A marked increase in stiffness and strength was observed in all MHbearing materials. An increase in degree of MH saturation led to an increase in the angle of the shear band and the width of the shear band became narrower.

Abstract In order to give an optimal design of methane hydrate inhibitors for deep-sea drilling, the most popular thermodynamic hydrate inhibitors (THIs) including NaCl and glycol, kinetic hydrate inhibitors (KHIs) including poly (vinyl pyrrolidone) and poly (vinylcaprolactam), and the mixtures of these inhibitors were experimentally tested in drilling fluid under the seabed conditions at different water depths. Moreover, a high performance KHI was developed and was proved to be a better one than the typical KHIs. Based on the performance tests and compatibility tests, the optimal hydrate inhibitors were designed for drilling deep-sea wells located at different water depth.

Abstract Gas hydrate was successfully sampled again in DK-9 hole in Qilian Mountain permafrost. However gas hydrate occurrences are heterogeneous both in horizon and in section within a limited area. What geological constraint affects gas hydrate occurrences is not known yet. Since features of gas hydrate and other related geological information were well recorded in DK-9, a possible answer is put forward based on analyses of geochemical and geological data in this hole. In this paper, features of hydrocarbon in headspace gases from cores at various depths are compared with occurrences of gas hydrate, faults or fractures in DK-9 in the Qilian Mountain permafrost. The results show that gas contents are relatively clearly higher at the intervals of 180.26m~308.50m, 356.45m~399.32m, 458.55m~508.65m than other intervals. Interestingly gas hydrate and its related anomalies fall into the intervals with higher gas contents. In the meanwhile, hydrocarbon concentrations in headspace gases within the faults or fracture zones are higher than those above the faults or fracture zones. This may suggest that faults or fracture zones have great impact on gas hydrate occurrences. Further comparative analyses on spatial relation of gas hydrate occurrences to faults or fracture zones, reveal that faults or fracture zones serve as migration paths for gases in the deep and provide accumulation space for gas hydrate in the shallow in the Qilian Mountain permafrost. Actually geological data show that gas hydrate occurrences are limited within the belt controlled by faults of F 1 and F 2 in a plane and are mainly within the lower block of these faults in a profile. Hence it is preliminarily drawn out that faults of F 1 and F 2 controlled gas hydrate accumulation in Qilian Mountain permafrost.

ABSTRACT The depressurization-induced production behavior of 10-meter-scale hydrate-bearing sediments was experimentally investigated to explore the possibility of controlling the gas production behavior by adjusting pressure conditions. A 10-meter-scale apparatus was constructed to simulate the hydrate dissociation under depressurization. An unconsolidated sedimentary sample was prepared using artificial particles with a similar grain-size distribution of sandy layers to that in real hydrate fields. In the experiments, pressure, temperature, and gas production volume were measured. The results showed that the gas production behavior was dominated by the fluid flow and was strongly affected by the rate of depressurization, which contrasts with previous experimental results.

ABSTRACT A series of triaxial tests has been carried out to determine the mechanical properties and dissociation characteristics of sands with fines containing methane hydrate, using an innovative high pressure apparatus which has been developed to reproduce the in-situ conditions expected during proposed methane extraction methods. It was found that the strength of MH sand increased with MH saturation due to particle bonding and that the bonding effect was particularly dependent on the grain size of the host sand. A high pressure and low temperature plane strain testing apparatus was also developed for visualizing the deformation of methane hydrate bearing sand due to methane hydrate production. Using this testing apparatus, plane strain compression and methane hydrate dissociation by depressurization tests were performed with the measurement of localized deformation.

ABSTRACT Clathrate hydrates have been researched for their various applications such as natural gas hydrate production, energy gas storage, carbon capture and sequestration, and synthesis and fabrication of versatile energy devices. The structure H(sH) hydrate is important in the oil industry because its large cage(5 12 6 8 ) can accommodate larger molecules found in crude oil regions that cannot occupy the large cages of structure I(sI) (5 12 6 2 ) and structure II(sII) (5 12 6 4 ) hydrates. However, there is only limited research of sH hydrate, contrary to sI and sII. The major difficulties of understanding of sH hydrate are that 1) sH can be formed only in the presence of large guest molecules (LGMs) and 2) most of the LGMs are insoluble or sparingly soluble in water. Also, there have been no reports on sI-sII-sH whole-phase-behaviour. The aforementioned barriers can be overcome by exploiting hydrophilic LGMs such as hexamethyleneimine(HMI). These highly water-soluble LGMs can create a homogeneous solution, which guarantees uniform hydrate phases. Therefore, the kinetic behaviour of gas hydrate phases can be compared using HMI at a laboratory scale.

ABSTRACT This experimental study evaluated the performance of sand control screens used to inhibit the production of sand accompanying hydrate dissociation by applying depressurization. Experimental apparatus was developed that visualizes the phenomenon of the production of sand and fine particles during hydrate dissociation. The sediment samples were prepared using sand the size distribution of which was similar to that of sandy layers in real hydrate fields. The screen sample was the same specification as that of a commercialized product used for well completion. In order to evaluate both gas production behavior and the performance of the screen sample according to hydrate dissociation by the application of depressurization, the internal pressure of sediment samples, amounts of water and gas produced, and imaging and photos of the sand-production phenomenon were observed on a real-time basis. Hydrate production behavior showed different results according to whether free gas was present. The results of the screen performance showed that sand production was not visibly observed in the screen sample, and the amount of sand collected after the experiment was less than 0.012% of the total mass of the sediment samples.

ABSTRACT Gas production methods from gas hydrates deviate from the conventional gas production since the gas hydrate production involves phase transformations from solid phase to gas and liquid phases. These phase changes produce uncertainties during production processes. These uncertainties are experimentally explored in this study using both synthetic and natural gas hydrate-bearing sediments. The parametric study on the behavior revealed that the magnitude of subsidence during production mostly scales with the compression index of sediments and gas hydrate saturation.

ABSTRACT The replacement process in of pure sI methane clathrate powders exposed to CO 2 have been quantitatively followed by means of neutron diffraction at conditions relevant to sedimentary matrixes of continental margins. The exchange of methane with CO 2 within a crystalline lattice of gas hydrates is seen as a two-step process of (1) a fast interfacial reaction (2) followed by much slower diffusion-limited transport.

ABSTRACT We found that the amount of methane hydrates formed during synthesis can substantially increase, if trace alcohol vapor (methanol, ethanol or 1-propanol) is added to the Ice-Seed system during the "pressurization" stage. Among them, ethanol has the best promoting effect when the initial system temperature is around 270K. Nearly 91% of the ice seeds can be converted during this stage. Furthermore, the duration of the "heating" stage can be shortened while consuming the remaining ice. Ethanol also has similar catalytic effect for synthesizing carbon dioxide hydrate. This discovery may have practical value for the transportation and storage of natural gas.

ABSTRACT Abnormal methane population in natural gas hydrates (NGHs) recovered from natural deep sea sediment has been demonstrated. Highly increased CH 4 occupancy ratios of large to small cages were observed in the sediment rich component of NGH, indicating that various minerals and mobile ions included in deep-sea sediments may significantly inhibit the normal CH 4 cage occupancy in gas hydrate formation. Therefore, we suggest that the real methane amount contained in natural gas hydrate deposits should be more carefully evaluated from site to site with consideration of clay-based NGH.

ABSTRACT Methane-dominated gas hydrates deposits are considered as a potential hydrocarbon resource and storage medium for anthropogenic CO 2 by gas replacement. We establish Raman spectroscopy as a quantitative tool to determine the composition of mixed gas hydrates. Using our empirical ratios of Raman quantification factors, the cage occupancies, the bulk guest composition and hydration number of gas hydrates containing CH 4 , C 2 H 6 , CO 2 and N 2 molecules can be determined. In this way, we can gain insight into details of the CH 4 exchange reaction with CO 2 or CO 2 + N 2 and the preference of certain gas species for certain types of hydrate cages.

ABSTRACT Thermal conduction and phase transformation are physical-chemical processes during the dissociation of gas hydrate-bearing sediments. Heat transfer leads to the expansion of hydrate dissociation front and the weakening of soils accompanied by the seepage of fluids and the deformation of sediments. As a consequence, ground failure may occur which can damage engineering structures and lead to environmental disasters. Hydrate dissociation in sediments is investigated using tetrahydrofuran (THF) hydrate sediments under various thermal strengths, and the time-dependent development of a hydrate-dissociation front is elucidated. An axial-symmetrical theoretical model and a numerical method are proposed based on experimental observations and analysis of the physical processes. Numerical and experimental results for evolution of the hydrate-dissociation front are in good agreement.