High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the Methane Hydrate System

Abstract

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.