Investigation of CO2-CH4 hydrate replacement using continuous CO2 injection in a dual-well injection-production mode

CO2 replacement is a promising method for natural gas hydrates (NGHs) exploitation, and it shares the advantages of CO2 sequestration and energy recovery. In this work, CO2-CH4 hydrate replacement by continuous CO2 injection in the dual-well injection-production mode was investigated at various CO2 injection rates. It is found that compared to the static replacement, CH4 production and CO2 sequestration were greatly enhanced with the increase of CO2 injection rate, peaking at 0.210 ml/min of the CO2 injection rate. Both CO2 hydrate formation and CH4 hydrate dissociation during the replacement were promoted by the continuous CO2 seepage flow, beneficial to the nondestructive and highly efficient exploitation of NGH. However, a massive NGH decomposition was induced and CO2 sequestration was reduced when increasing the CO2 injection rate to 0.420 ml/min, which was detrimental to the stratum stability. The CO2/CH4 fugacity difference remains larger under continuous CO2 injection and increases with the injection rate, providing higher driving forces for the replacement. Moreover, it is found that CO2 seepage flow accelerated both the fast and sluggish stages with the CO2 injection rate ranging from 0.104 to 0.210 ml/min, but the promoting effect declined at 0.420 ml/min. The results reported will provide insights into the improvement of exploitation strategies for NGHs recovery and CO2 sequestration.