Three groups of hydrate-bearing sediment samples with/without underlying gas were prepared using a three-dimensional middle-size reactor to investigate the favorable conditions for methane recovery from hydrate reservoir with gaseous CO2. The experimental results indicated that hydrate reservoir with underlying free gas, high saturation of free gas and low saturation of water is appropriate for recovering CH4 with CO2. For the replacement mechanism, it is assumed that the CO2–CH4 replacement chiefly includes two parts: CH4 hydrate dissociation and mixed hydrate re-formation from the dissociated water and free water. In the re-formed hydrate, CO2 molecules mainly occupy the large cage and CH4 molecules mainly occupy the small cage. A kinetics model was constructed to describe the replacement process, which considers not only the kinetic reaction of CH4–CO2 replacement but also the reaction between CO2 and free water. The calculated results agree well with the experimental data. The influencing factors on the replacement rate were also discussed according to the developed model. It is found that the replacement rate and the mole fraction of CH4 in gas phase increase with the increase of initial CO2 mole fraction, the decrease of system pressure, and the increase of diffusion coefficient of CH4 in hydrate layer.
