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

An accurate knowledge of cage occupancy of methane is central for understanding the physical-chemical properties of gas hydrates, the actual inventory of natural gas in hydrate deposits and the description of gas exchange processes. Here we report the absolute cage occupancies, the cage occupancy ratios and hydration numbers of the synthetic CH4-H2O and CH4-D2O hydrates formed from the ice-gas system under different pressures and temperatures. The results were obtained from Rietveld refinement using high-resolution synchrotron X-ray powder diffraction patterns and from Raman spectroscopic measurements. The small-cage occupancies of methane in the deuterated hydrates are found to be slightly higher than in the hydrogenated form, likely due to their different lattice constants. The CH4 occupancy in the small cages agrees fairly well with the predictions of CSMGem at the formation pressure of 3.5 MPa, but with the increasing formation pressure the disagreement grows up to 11 percent. While some deficiency of the prediction model cannot be excluded, the observed discrepancy may well be due to experimental difficulties of reaching true equilibrium at higher pressures. The experimentally determined large-to-small cage occupancy ratios of the synthetic and natural CH4 hydrates formed from the water-gas system are consistently higher than the results of CSMGem calculations. Possible reasons for these discrepancies will be discussed.