Liquid CO2-CH4 Hydrate Replacement Reaction above 281.15 K: Implication for CH4 Recovery and CO2 Sequestration in Marine Environments

Temperature and pressure of natural gas hydrate (NGH) reservoirs influence CH4 recovery and CO2 sequestration through the CO2-CH4 hydrate replacement reaction. The typical temperature and pressure in marine NGH reservoirs are mainly 275.65-294.51 K and 8.41-22.25 MPa, respectively. However, the corresponding CO2-CH4 hydrate replacement reaction under the temperature above 281.15 K has been studied little in previous studies. In this work, six experiments were conducted at 281.15-289.15 K and 15-17 MPa using a self-developed experimental apparatus. These experiments utilized techniques such as general photography, microscopy, temperature and pressure measurements, computational analysis, and gas chromatography to study the dynamic changes in the morphology and distribution of CH4, CO2, and H2O under varying temperatures, pressures, and compositional conditions. The results showed that liquid CO2 replaced CH4 in CH4 hydrates or CH4-CO2 mixed hydrates. This replacement process was featured with multiple mechanisms during the soaking stage and governed by the CH4-CO2 mixed hydrate phase equilibrium, which was controlled by CH4-CO2 mixtures and their compositions. Moreover, the relative increases of the CH4 molar ratio in the CO2-rich fluid phase, the efficiency of CO2 sequestration, and the CO2 sequestration amount in comparison with the CH4 reserves reached 24.97%, 19.53%, and 2.17 times, respectively. CO2 injection pressure and the initial volume fraction of liquid water and the CO2-rich fluid were found to have larger impacts on the CO2-CH4 hydrate replacement reaction above 281.15 K than temperature and the initial volume of CH4 hydrate. Besides, controlling the composition of the CH4-CO2-H2O mixed system and depressurization were proposed to balance CH4 recovery and CO2 sequestration. This work may offer some references for CH4 recovery and CO2 sequestration under real settings.