Effect of N2/H2 injection on CH4 hydrate decomposition

Gas injection, as a method for natural gas hydrate exploitation, has attracted much attention, because of its advantages of low energy consumption and non-destructive characteristics. The gas recovery can be significantly enhanced by adding small-molecule gases such as N-2 or H-2 to the injecting gas mixture. To evaluate the independent effects of the small-molecule gases on the CH4 hydrate decomposition behaviours, the Raman spectroscopy, gas chromatography, and a CCD camera were adopted. The experimental results show that the CH4 decomposition pressure was increased by injection of small-molecule gases such as N-2 or H-2. The addition of small-molecule gases in the gas phase led to the change of the hydrate phase equilibrium, which made the CH4 hydrate become more unstable. No N-2/H-2 hydrates Raman peaks were obtained throughout the experimental process, which indicates that the N-2/H-2 molecules did not enter the hydrate cages and did not participate in the hydrate formation. Moreover, it suggests that the injected small-molecule gases promote the decomposition of CH4 hydrate by changing the fugacity of CH4. H-2 has a more significant effect on the decomposition of CH4 hydrate than N-2. This is probably due to the larger diffusion coefficient of H-2 in the hydrate phase than that of N-2. This kind of promotion effect is beneficial to the production of CH4 gas from hydrate.