MH21 research consortium of Japan conducted off-shore drillings to survey gas hydrate in Tokai-oki and Kumano-nada areas of Eastern Nankai Trough in early 2004 and recovered drill cores of gas hydrate bearing sediment successfully. Marine gas hydrate was confirmed mainly in sandy drill cores retrieved from mud and sand alternation strata. Tri-axial compression test on those core samples has been conducted to ascertain mechanical properties of natural hydrate bearing sediments for the first time. The results from the tests show that with increase in pore saturation of gas hydrate, shear strength and elastic modulus tend to arise, whereas no significant tendency was found in Poisson's ratio in relation to hydrate saturation. IntroductionPublic attention has been drawn recently to gas hydrate or methane hydrate in common in Japan as potential energy resources since gas hydrate was located offshore Japan. In general, clathlate compounds trapping gas molecules in the cage of water molecules are called gas hydrate, which is stable at ceratin temperature and pressure in phase equilibrium. Gas hydrate exists stably, therefore, in a permafrost region and/or under sea floor in deep-water along continental margins where the equilibrium state can be met in natural. In the development of gas hydrate resources, conventional methods used for a current gas and oil production are not necessarily applicable, because gas hydrates should be dissociated before an accumulation of methane from a hydrate reservoir. Moreover, the endothermic reaction of gas hydrate in dissociation reducing temperature at surroundings will cause regeneration of gas hydrate and restrain the extension of a gasification path, which will be pecurior to the gas production from gas hydrate. Considering the unique characteristics of gas hydrate, pressure decrease and/or temperature increase are needed to destabilize the equilibrium state of gas hydrates. Field-scale trial tests by MH21 research consortium of Japan were conducted and more onshore field tests have been scheduled employing depressurization method 1) . At present, dissociation by depressurization is a promising application to a future production compared with thermal stimulation and inhibitors in addition. Meanwhile, dissociation of gas hydrate may reduce a mechanical strength of sediment strata, which will cause wellbore instability directly affecting a safety and stable gas production from gas hydrates in case. Since test samples of drill cores containing natural gas hydrate are quite limited in number, Toyoura sand containing methane hydrate which was formed synthetically in a tri-axial compression test has been used so far to ascertain the mechanical properties of gas hydrate bearing sediments 2) . MH21 research consortium conducted offshore drilling survey in Tokai-oki and Kumano-nada areas of Eastern Nankai Trough region in early 2004 as shown in Fig.1, and confirmed marine natural gas hydrates in drill cores recovered from mud and sand alternation strata. In this paper, experimental results f...

This paper presents a theoretically correct method for predicting the heat of dissociation of gas hydrates using a fugacity-based model. Based on the Clausius-Clapeyron equation, the proposed approach takes into account the difference in compressibility by phase transition and the solubility of a guest component in the liquid phase. It is found that, for specific gas hydrates, the effect of two correction factors on the prediction of the heat of dissociation is not negligible. All corresponding properties including the hydrate number, compressibility, solubility of gas in water, water content in the vapor phase, and heat of dissociation are calculated and compared with experimental values in part.

The dissociation kinetics of methane and methane-ethane hydrates was investigated under a variety of experimental conditions. Hydrates of pure methane or methane-ethane mixtures were prepared. The composition and structure of methane-ethane mixed hydrates were identified using Raman spectroscopy and gas chromatographic analysis of the hydrate phase. With these hydrate powders, pellet-shaped samples that mimic a naturally occurring hydrate in ocean sediment were prepared. The dissociation rates of gas hydrates were measured in pure water and a viscous fluid mixed to imitate drilling mud fluids under several isothermal and isobaric conditions. Gas bubbles generated by dissociation affected the dissociation rate, possibly because gas bubbles near the active surface resisted heat flux during dissociation. For methane-ethane mixed hydrates, the calculated time profile agrees well with the experimental results when the composition of the vapor phase is identical with that of the hydrate phase. It indicates that the free gas composition around the dissociation surface is determined by the kinetics of dissociation and not by thermodynamic equilibrium. The dissociation rates of gas hydrates in viscous fluids were essentially proportional to the concentration of fluid.

Methane hydrate is anticipated to be a promising energy resource of natural gas, since a large amount of reservoir exists in marine sediments or in permafrost regions worldwide. In order to evaluate a productivity of methane gas from methane hydrate reservoirs, it is necessary to develop a gas production numeric simulator. And, for precise assessment of long-term gas productivity, it is important to consider time-dependent properties of sediments containing methane hydrate. In this study, loading rate dependency, one of important time-dependent properties, of artificial sediment containing methane hydrate was experimentally examined. Results of triaxial compression tests of water-saturated Toyoura sand (saturated-sand specimen) and Toyoura sand containing synthetic methane hydrate (hydrate-sand specimen), at conditions of confining pressure 9MPa, pore water pressure 8MPa and temperature 278K, were presented. Loading was conducted at a constant strain rate or alternating strain rate. In constant strain rate test, specimen was loaded at strain rate of 0.1%/min, 0.05%/min and 0.01%/min. In alternating strain rate test, one specimen was loaded at a strain rate alternated between the lower rate C 1 and the higher rate C 2 at strain intervals Δε. The conditions of C 1 , C 2 and Δε were briefly examined in this study. Experimental results indicate that saturated-sand specimen shows very weak strain rate dependency suggesting that, in most cases, time-dependency is negligible small. Strain rate dependency of the peak strength for hydratesand specimen was considerably stronger than that of saturated-sand specimen. Referring to earlier works, the timedependency of hydrate-sand specimen seemed to be weaker than that of ice or methane hydrate and comparable with that of frozen sand.

No abstract