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An accurate knowledge of cage occupancy of methane is central forunderstanding the physical-chemical properties of gas hydrates, the actualinventory of natural gas in hydrate deposits and the description of gasexchange processes. Here we report the absolute cage occupancies, the cageoccupancy ratios and hydration numbers of the synthetic CH4-H2O and CH4-D2Ohydrates formed from the ice-gas system under different pressures andtemperatures. The results were obtained from Rietveld refinement usinghigh-resolution synchrotron X-ray powder diffraction patterns and from Ramanspectroscopic measurements. The small-cage occupancies of methane in thedeuterated hydrates are found to be slightly higher than in the hydrogenatedform, likely due to their different lattice constants. The CH4 occupancy in thesmall cages agrees fairly well with the predictions of CSMGem at the formationpressure of 3.5 MPa, but with the increasing formation pressure thedisagreement grows up to 11 percent. While some deficiency of the predictionmodel cannot be excluded, the observed discrepancy may well be due toexperimental difficulties of reaching true equilibrium at higher pressures. Theexperimentally determined large-to-small cage occupancy ratios of the syntheticand natural CH4 hydrates formed from the water-gas system are consistentlyhigher than the results of CSMGem calculations. Possible reasons for thesediscrepancies will be discussed.

Cage occupancies of methane hydrates: Results from synchrotron X-ray diffraction and Raman spectroscopy