Coupling depressurization and flue gas flooding to enhance CH4 recovery and CO2 sequestration in hydrate-bearing sediments: Pilot-scale experimental evaluation at marine conditions

The industrialization of natural gas hydrate (NGH) extraction faces significant challenges due to low gas production capacity, necessitating new methods to improve. This study investigates the CH4 recovery from water-saturated pilot-scale hydrate-bearing sediments (HBS, V = 22 L) via coupling depressurization with semi-continuous flue gas flooding. Key factors, including the timing, rate, and composition of flue gas injection, were analyzed for their effects on CH4 recovery, NGH dissociation, and CO2 storage. Injecting flue gas (CO2/N2 = 15/85) after 60 % NGH dissociation during depressurization yielded the optimal performance, increasing CH4 recovery ratio from 70.8 % to 86.4 %, boosting recovery and NGH dissociation rates by 36.4 % and 25.5 %, respectively. Lower injection rate delayed gas breakthrough, improving CH4 recovery. Water-alternating-gas injection, however, lowered the produced gas-to-water ratio and resulting CH4 recovery efficiency. Following CH4 extraction, CO2-rich flue gas injection induced CH4/CO2/N2 hydrate (Mix-H) formed and gradually grew toward the production well. Gas composition in depleted HBS influenced Mix-H kinetics more than residual gas saturation. The maximum CO2 storage ratio in hydrates reached 30.1 %, with a hydrate restoration ratio of 28.7 % for HBS. The total stored CO2 provides a carbon offset of up to 99.4 % for CH4 production, demonstrating the potential for carbon-neutral NGH extraction.