Recovering CH4 from Natural Gas Hydrates with the Injection of CO2–N2 Gas Mixtures

Conversion of natural gas hydrates into CO 2 hydrates is well-known as an innovative and eco-friendly method of both future energy production and greenhouse gas control. To increase the total CH 4 production rate by using such a method, experiments were carried out by replacing CH 4 with gas mixtures containing CO 2 and N 2 from naturally occurring CH 4 hydrates at 273.9 K and a pressure ranging from 2.50 to 6.67 MPa. Raman spectroscopic analysis was performed to examine the structure and gas distributions in the hydrate phase. Results showed no structural transition in the hydrate phase. CO 2 was found to prefer replacing the CH 4 in the large hydrate cages, whereas N 2 was found to prefer replacing the CH 4 in the small hydrate cages. The total CH 4 production was found to increase with addition of N 2 . The experimental pressure showed little effect on the CH 4 recovery rate with pure CO 2 . However, with addition of N 2, the CH 4 recovery rate was found to decrease and became dependent on the experimental pressure. The transportation of CO 2 molecules in the hydrate phase was suggested to be the rate-limiting step, while N 2 captured in hydrate cages slowed the diffusion rate of CO 2 in hydrate phase, leading to a lower CH 4 recovery rate.