In Situ Investigation on the Morphology and Formation Kinetics of a CO₂/N₂ Mixed Hydrate Film

The influence of N2 molecules on the morphology and growth kinetics of CO2 hydrate films under different gas compositions and subcooling degrees (ΔT) was studied using a gas bubble suspended in water and in situ Raman spectroscopy, and some perspectives about the CO2/N2-CH4 replacement were put forth in this work. The evolution morphologies and formation of hydrate films were studied via optical microscopy and in situ Raman spectroscopy, respectively. The results demonstrated that the size of the hydrate single crystal on the hydrate film was related to the N2 concentration and ΔT; the roughness of the hydrate film was inversely proportional to the N2 content and ΔT; and the hydrate formation rate was highest in the 80% CO2 + 20% N2 gas system, in which the function of N2 on the hydrate formation may primarily be to improve the hydrate film’s roughness and enlarge the gas pore size on the film, which makes molecular mass transfer easier to carry out, and to stabilize the CO2 hydrate structure. Raman results showed that the amount of CO2 stored in the hydrate phase was still greater than that of N2 molecules due to CO2 molecules being more stable in the hydrate phase, despite the fact that N2 molecules were more competitive for mass transfer during the evolution of hydrate films because of their smaller size. With an increase in N2 concentration and ΔT, the amount of CO2 molecules trapped in the hydrate phase decreased, which is partly due to the low concentration of CO2 in the gas mixture as well as the fact that the hydrate film was much dense and impeded mass transfer of CO2 molecules. The experimental results implied that the morphology of the hydrate film and the composition of the CO2/N2 mixed gas are important to achieve better replacement efficiency during the exploitation of natural gas hydrates via the CO2/N2-CH4 replacement method.