The three-phase coexistence curve of methane hydrate + saturated water + saturated fluid CH_4 was investigated in the temperture range from 305 to 321 K and pressure range from 98 to 500 MPa. The equilibrium curve increases monotonically on a T-p diagram at these experimental conditions. The Raman spectra of the C-H symmetric vibration mode in the methane hydrate split into two peaks, while a single peak is detected in the fluid CH_4 and wate rphases. The split of the Raman peak indicates that two kinds of hydrate cages are occupied by the CH_4 molecules. The peak intensity ratio of two types of CH_4 molecules is almost independent of pressure in the range up to 500 MPa; that is, the cage occupancy ratio is constant. The Raman spectrum for the intermolecular vibration mode (O-O stretching) of the water molecules changes linearly with pressure from 207 to 228 cm~(-1), and the Raman shifts of the C-H vibration mode in the S-cage and in the water phase vary linearly with pressure from 2915 to 2919 cm~(-1) and from 2910 to 2916 cm~(-1) ,respectively. On the other hand, the Raman shift of C-H vibration in the M-cage is nearly constant at the lower frequency. That is, the hydrate cage shrinkage is caused by a pressure increase; however,the M-cage contains adequate vacant volume for the CH_4 molecule.
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