CH4 recovery and CO2 sequestration from hydrate-bearing clayey sediments via CO2/N2 injection

Abstract Injecting carbon dioxide (CO2) into natural gas hydrates (NGHs) to replace methane (CH4) is a prospective technique to simultaneously realise energy resource exploitation and greenhouse effect reduction. Tremendous amounts of NGHs occur in clayey reservoirs. However, the replacement of CH4 with CO2 in clayey sediments has received little attention. To study CH4 recovery from and CO2 sequestration in clayey NGH reservoirs, injection of CO2/N2 (with a molar ratio of CO2 to N2 of 1:4) into CH4 hydrate-bearing clayey sediments was experimentally researched. CH4 hydrates were formed in the sediments with different clay minerals and contents to simulate NGH clayey reservoirs. The experimental results show that gas molecule exchange between the CO2/N2 gas mixture and CH4 hydrates could occur in clayey sediments. It was found that the presence of montmorillonite in sediments had a much greater inhibition effect on CH4 recovery and CO2 sequestration than that of kaolinite or illite. In hydrate-bearing sediments with kaolinite and illite, the extent of formation of mixed hydrates was significant in the early stage of the replacement. In contrast, the strong water adsorption of montmorillonite led to the gas molecule exchange dominating the whole replacement process in the sediments. In addition, the increase of hydrate saturation resulted in the reduction of CH4 recovery efficiency in montmorillonite-bearing sediments, whereas it was conducive to CH4 recovery efficiency in hydrate-bearing sand/sandstone. These results provide theoretical support for the exploitation of NGHs in clayey reservoirs by CO2/N2 injection.