EXPERIMENTAL STUDY OF METHANE HYDRATE FORMATION AND DECONPOSITION IN FROZEN SOILS

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The experiments were performed to study methane hydrate formation and decomposition in frozen soil. The samples used here are silicate sand, bentonite clay and porous glass powders. Two types of porous glass powders whose pore diameters are 50nm and 10nm are used. The samples were prepared under water saturated condition or unsaturated condition. In the latter case, only inside of pores are saturated with water. It means that water is highly bound in side of small capillary tubes. The experimental procedures are follows : First sample were put inside the pressure cell. The air contained in the cell was removed with vacuums pumpand pressurized methane was put in the cell. Then the temperature of sample was lowered below 0ﾟC to form methane hydrate inside the sample. After the methane hydrate formation was completed, the temperature of the sample was warmed up at a constant warming speed to decompose the methane hydrate. During the experiment, the temperature of sample and pressure of cell ware monitored. The relationship between pressure and temperature was compared with the phase equilibrium of methane hydrate in pure water without soil. The P-T curves of methane hydrate decomposition in water saturated soil sample are coincided well with the P-T curve of methane hydrate decomposition in pure water. This can be explained by that free water is available under the water saturated condition. The P-T curve of methane hydrate decomposition in unsaturated condition shifted toward low temperature and high pressure region. At the pressure of 6 MPa, the decompose temperature was lower than that in pure water by 1.2 K for 50 nm pore size sample and 6 K for 10nm pore size sample. This result is due to that fact that the pore water is highly bound in side of small capillary tubes. These data were compared with the data of unfrozen water content obtained by NMR technique.