ABSTRACT

A project to extract methane hydrate (herein after referred to as MH), which is expected to be an energy resource in the next generation, from the deep sea-bed has been advanced (Research Consortium for Methane Hydrate Resources in Japan). MH is composed of a methane molecule and water molecules and exists as an ice-like crystal under low-temperature and highpressure condition. In the MH extraction project, a well is drilled into the sea floor from a marine platform. Then, fluids in the well are either heated or depressurized to induce MH dissociation and the dissolution of methane gas is collected in-situ. During MH production, there are concerns about the settlement of the seabed and the possibility that landslides will occur due to change in effective stress induced by drilling, water movement due to depressurization, dissociation of MH, methane gas generation and thermal change, which are all inter-connected. In addition, rebound of the ground and possible landslides caused by the reduction in effective stress accompanying the water pressure recovery after the end of production are also important issues. Therefore, an analysis of soil behavior is required to estimate the resulting ground deformation as a result of the extraction process

The authors (2005) have performed triaxial compression tests on MH bearing-sand and undisturbed soils under high-pressure and low-temperature, and have developed a constitutive model which can express deformation behavior (2008). In this paper, ground deformation in the vicinity of MH exploitation is predicted using a soil-water-gas-heat coupled finite element method with the developed elasto-plastic constitutive model.