EXPERIMENTAL STUDY OF METHANE HYDRATE FORMATION AND DECONPOSITION IN FROZEN SOILS

对冻土中甲烷水合物的形成和分解进行了研究。这里使用的样品是硅酸盐砂，膨润土粘土和多孔玻璃粉。采用孔径为50nm和10nm的两种多孔玻璃粉。样品在水饱和和非饱和条件下制备。在后一种情况下，只有孔隙内部被水饱和。这意味着水被高度束缚在小毛细血管的一侧。实验步骤如下：首先将样品放入压力箱中。用真空泵抽走细胞内的空气，将加压的甲烷放入细胞内。然后将样品温度降至0℃以下，使样品内部形成甲烷水合物。甲烷水合物形成完成后，以恒定升温速度对样品进行升温，分解甲烷水合物。在实验过程中，对样品温度和细胞压力进行了监测。用压力与温度的关系与无土纯水中甲烷水合物的相平衡进行了比较。饱和水土壤样品中甲烷水合物分解的P-T曲线与纯水中甲烷水合物分解的P-T曲线吻合较好。这可以解释为在水饱和条件下可以获得自由水。非饱和条件下甲烷水合物分解的P-T曲线向低温高压区域偏移。在6 MPa压力下，孔径为50 nm的样品比在纯水中分解温度低1.2 K，孔径为10nm的样品比在纯水中分解温度低6 K。这一结果是由于孔隙水高度结合在小毛细管的一侧。这些数据与核磁共振技术获得的未冻水含量数据进行了比较。

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.