Methane hydrate has been formed synthetically in two types of hostspecimens, ice-sand and/or water- sand mixture, which were confined in a tri-axial pressure vessel under the deep-sea condition equivalent to approximately 800m deep. The shear strength of synthetic sediments increased due to methane hydrate formation. However, synthetic sediment from ice-sand mixture showed no significant increase in the strength at less than 25% of the saturation degree of methane hydrate, whereas the sediment strength by water-sand mixture rose successively with the methane hydrate saturation. Moreover, the effects of saturation degree of methane hydrate on cohesion and elastic properties were confirmed.
Research Consortium for Methane Hydrate Resources in Japan conducted exploration drillings in Nankai Trough off the coast of Japan recently. Methane hydrate has been confirmed in core samples recovered from marine sediments. However, the exploitation of methane hydrate needs more technological developments due to distinctive characteristics of methane hydrate. Because of so-called self-preservation effect, promising methods for dissociation of methane hydrate are thought to be "depressurization", "thermal injection" and "usage of inhibitors" in production wells. AIST(National Institute of Advanced Industrial Science and Technology) joined the consortium and has been undertaking research projects to develop technologies for exploitation of methane hydrate in the near future. Development of simulation code for methane hydrate production is one of the main purposes of the projects and our research group in particular is currently engaged in consolidation modeling of methane hydrate bearing sediments. Dissociation of methane hydrate in marine sediments may cause seabed subsidence and/or deformation of methane hydrate sediment strata. Such incidence will directly affect productivity of production wells if it is not properly estimated prior to the exploitation. Deformation behavior and change in shear strength of the sediments are not clearly understood on the occasion of methane hydrate production.