Microscopic Insights and Optimization of the CH 4 -CO 2 Replacement in Natural Gas Hydrates.

Using the CO 2 replacement method to exploit natural gas hydrates and store CO 2 has great significance in energy access and environmental protection. Herein, the molecular dynamic method is utilized to analyze and evaluate the CH 4 -CO 2 replacement at different constant temperatures and pressures. For optimization, various temperature oscillations are introduced in the CH 4 -CO 2 replacement. It illustrates that increasing the temperature can improve the amounts of CH 4 escape and CO 2 capture but is unfavorable to the long-term CO 2 storage and hydrate stability. The effects of pressure are not as significant and definite as those of temperature. Appropriate temperature oscillations can achieve comprehensive improvements, which benefit from both the deep diffusion of CO 2 in the higher temperature stage and the rapid rebuilding of CO 2 hydrate within just nanoseconds caused by the memory effects in the lower temperature stage. The results also reveal that the optimal lower temperature duration and frequency should be moderate. Decreasing the lower temperature value can distinctly enhance CO 2 capture and hydrate stability. This study can help understand the mechanisms of CH 4 -CO 2 replacement under different temperature and pressure conditions, especially at temperature transitions, and proposes a potentially effective method to achieve large-scale carbon sequestration in the hydrate.