Mechanical Properties of Natural Gas Hydrate Bearing Sediments Retrieved from Eastern Nankai Trough

Abstract

Abstract 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. Introduction Public 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 method1). 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 sediments2). 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 from tri-axial compression tests on natural gas hydrate core samples can be presented focusing on strength, elastic modulus and Poisson's ratio in relation to pore saturation of gas hydrate. Moreover, these results have been compared with previous test results obtained from tri-axial compression tests on Toyoura sand containing synthetically formed gas hydrate.