Direct measurement of methane hydrate composition along the hydrate equilibrium boundary

The composition of methane hydrate, namely n_w for CH₄·n_wH₂O, was directly measured along the hydrate equilibrium boundary under conditions of excess methane gas. Pressure and temperature conditions ranged from 1.9 to 9.7 MPa and 263 to 285 K. Within experimental error, there is no change in hydrate composition with increasing pressure along the equilibrium boundary, but n_w may show a slight systematic decrease away from this boundary. A hydrate stoichiometry of n_w = 5.81−6.10 H₂O describes the entire range of measured values, with an average composition of CH₄·5.99(±0.07)H₂O along the equilibrium boundary. These results, consistent with previously measured values, are discussed with respect to the widely ranging values obtained by thermodynamic analysis. The relatively constant composition of methane hydrate over the geologically relevant pressure and temperature range investigated suggests that in situ methane hydrate compositions may be estimated with some confidence.