Enhanced CO ₂ Replacement Extraction of Natural Gas Hydrates through Depressurization Stimulation

The CO2 replacement method, which enables simultaneous CH4 recovery and CO2 sequestration, is a highly promising green technology for natural gas hydrate (NGH) extraction. However, the replacement efficiency remains relatively low under conventional conditions. To overcome this limitation, the promoting effect of depressurization stimulation on the replacement process was systematically investigated using a laboratory-scale hydrate extraction apparatus. The results indicate that depressurization stimulation can effectively enhance CH4 recovery, primarily by disrupting the mixed hydrate layer on sediment particle surfaces and driving the replacement front deeper into the reservoir. While early depressurization favors CH4 recovery, later depressurization benefits CO2 sequestration. Increasing depressurization magnitude enhances CH4 recovery but contributes minimally to CO2 sequestration and is detrimental to geomechanical stability. Under five-stage depressurization, the CH4 recovery increased from 35.7% in the control experiment to 66.1%. Moreover, a threshold effect was observed for multistage depressurization, beyond which CH4 production primarily stems from hydrate dissociation, and replacement efficiency plateaus. These findings provide important experimental evidence and engineering guidance for the synergistic extraction of marine NGH coupled with geological CO2 sequestration.