In Situ Raman Analysis on the Dissociation Behavior of Mixed CH4-CO2 Hydrates

Previous studies have indicated that CH4-CO2 hydrate replacement is closely related to the crystal dissociation and reformation processes. To further elucidate the replacement mechanism, the melting processes of mixed CH4-CO2 hydrates were characterized using in situ Raman spectroscopy and compared to that of pure CH4 and CO2 hydrates. Analysis results from powder X-ray diffraction showed that the crystalline structure of the mixed CH4-CO2 hydrates is strucutre I. The current study suggested that the hydrate crystal units collapse as an entity with no clear dependence upon gas distributions within the hydrate phase. The dissociation time was found to be stoichastic, consistent with the polycrystalline nature of the hydrate particles. Interestingly, for hydrates containing CH4, the Raman peaks of both CH4 and CO2 in the hydrate phase showed a temporary rise during the hydrate dissociation processes, suggesting the reformation of hydrates below the hydrate dissociation surface. A dissociation inside the hydrate crystal together with the self-preservation effect was assumed to be responsible for such a phenomenon.