Microscopic Insights and Optimization of the CH₄-CO₂ Replacement in Natural Gas Hydrates

Using the CO₂ replacement method to exploit natural gas hydrates and store CO₂ has great significance in energy access and environmental protection. Herein, the molecular dynamic method is utilized to analyze and evaluate the CH₄-CO₂ replacement at different constant temperatures and pressures. For optimization, various temperature oscillations are introduced in the CH₄-CO₂ replacement. It illustrates that increasing the temperature can improve the amounts of CH₄ escape and CO₂ capture but is unfavorable to the long-term CO₂ 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 CO₂ in the higher temperature stage and the rapid rebuilding of CO₂ 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 CO₂ capture and hydrate stability. This study can help understand the mechanisms of CH₄-CO₂ 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.