MH21 research consortium of Japan conducted off-shore drillings to survey gas hydrate in Tokai-oki and Kumano-nada area in early 2004 and recovered drill cores of gas hydrate bearing sediment successfully. Mechanical properties of sandy core samples containing natural gas hydrate have been investigated by tri-axial compression test. The increase in tri-axial compressive strength against pore saturation of gas hydrate has been confirmed. The trend of strength increase shows good agreement with the previous results obtained from Toyoura sand containing laboratory-formed synthetic gas hydrate. The secant elastic modulus at 50% of peak strength of natural samples, however, lowered comparing with synthetic gas hydrate sand. No significant relationship has been confirmed between Poisson's ratio and pore saturation of gas hydrate. This relationship can be supported by the same results which have been obtained from the test of Toyoura sand containing synthetic gas hydrate with regard to Poisson's ratio.
Discovery of gas hydrate in a global scale has attracted publicity, while the prospect of commercial production of gas hydrate is yet uncertain technically due to the lack of real understandings of gas hydrate and hydrate bearing sediments in nature. We have conducted many tri-axial compression tests so far employing sand specimens containing laboratory-formed synthetic gas hydrate. Mechanical properties of those synthetic hydrate sand have been confirmed approximately from laboratory tests (Masui et al., 2005). However, discussion on essential difference between natural gas hydrate sediment and synthetic hydrate sand has remained unsolved in our experimental results. MH21 research consortium of Japan conducted off-shore drillings to survey gas hydrate in Tokai-oki and Kumano-nada areas of Nankai Trough in early 2004 and recovered drill cores of gas hydrate bearing sediment successfully. Marine gas hydrate has been confirmed mainly in sandy drill cores retrieved from mud and sand alternation strata.