Replacement of Methane from Hydrates in Porous Sediments with CO2-in-Water Emulsions

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.

Experimental Study on the Distribution Characteristics of CO2 in Methane Hydrate-Bearing Sediment during CH4/CO2 Replacement

CH4/CO2 replacement is of great significance for the exploitation of natural gas hydrate resources and CO2 storage. The feasibility of this method relies on our understanding of the CH4/CO2 replacement efficiency and mechanism. In this study, CH4/CO2 replacement experiments were carried out to study the distribution characteristics of CH4 and CO2 in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH4/CO2 replacement process could be divided into two stages: fast reaction and slow reaction, representing CH4/CO2 replacement in the hydrate-gas interface and bidirectional CH4/CO2 diffusion caused replacement, respectively. After replacement, the CO2 content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO2 consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO2 hydrate by gaseous CO2 and water almost runs through the whole experimental process. Thus, the pore scale CH4/CO2 replacement process in sediments can be summarized in the following steps: CO2 injection, CO2 diffusing into sedimentary layer, occurrence of CH4/CO2 replacement and CO2 hydrate formation, wrapping of methane hydrate by mixed CH4-CO2 hydrate, continuous CO2 hydrate formation, and almost stagnant CH4/CO2 replacement.