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 N2 or H2 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 N2 or H2.

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 N2/H2 hydrates Raman peaks were obtained throughout the experimental process, which indicates that the N2/H2 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. H2 has a more significant effect on the decomposition of CH4 hydrate than N2. This is probably due to the larger diffusion coefficient of H2 in the hydrate phase than that of N2.

This kind of promotion effect is beneficial to the production of CH4 gas from hydrate.