Raman spectroscopic determination of hydration number of methane hydrates

Hydration numbers of methane hydrates with various water/gas ratios of the sample were measured by Raman spectroscopy. Artificial methane hydrates were formed at temperatures ranging from 273.2 to 278.4 K, and pressures from 3.0 to 7.0 MPa, with a stirring rate of approximately 500 rpm. Under such nonequilibrium conditions, the obtained hydrate sample shad a water/gas ratio ranging from 6.4 to 17.4, which was determined by mass measurements. The spectroscopic analysis revealed, however, that the crystallographic hydration number was almost constant at 6.2 +/- 0.2, which corresponds to the hydration number estimated from the thermodynamic model. The same measurements were carried out on natural gas hydrate samples obtained from the deep-sea floor of Blake Ridge. The coincidence of the hydration numbers between natural and artificial samples indicated that the crystallographic hydration number is independent of the formation conditions and the large variation in the water/gas ratio of samples is described by the ratio between pure hydrate and free water.