Macro and microscopic CH₄-CO₂ replacement in CH ₄ hydrate under pressurized CO₂

CH₄-CO₂ replacement in CH₄ hydrate with high pressure CO₂ was studied with in-situ laser Raman spectroscopy at 273.2 K and at initial pressures of 3.2, 5.4, and 6.0 MPa. Replacement rates increased with increasing pressures up to 3.6 MPa and did not change at higher pressures (∼6.0 MPa). These results showed that the replacement rates were dependent on pressure and phase conditions with the driving force being strongly related to fugacity differences of the two guest components between fluid and hydrate phases. When CH₄ hydrate was contacted with CO₂ under flow conditions, in-situ Raman measurements of the hydrate phase showed differences of cage decomposition rates between the Medium-cage (M-cage) and the Small-cage (S-cage) in the CH₄ hydrate with decomposition of the M-cage being faster than that of the S-cage. The van der Waals-Platteeuw model was applied to the measurements of the transient data and it is shown that the theory allows estimation of occupancies of each component during replacement.