High-pressure phase equilibrium and Raman microprobe spectroscopic studies on the methane hydrate system

The three-phase coexistence curve of methane hydrate + saturated water + saturated fluid CH4 was investigated in the temperature 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 CH4 and water phases. The split of the Raman peak indicates that two kinds of hydrate cages are occupied by the CH4 molecules. The peak intensity ratio of two types of CH4 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 CH4 molecule.