Pressure oscillation controlled CH4/CO2 replacement in methane hydrates: CH4 recovery, CO2 storage, and their characteristics

Abstract The recovery of CH4 from natural gas hydrates via CH4/CO2 replacement is a highly attractive method for achieving the recovery of CH4 and storing CO2. However, although extensive simulation and experimental studies have been conducted, applying CH4/CO2 replacement is currently hampered by a lack of in-depth understanding about the replacement mechanism and the insufficient replacement efficiency and rate achieved. Therefore, to enhance CH4 recovery and CO2 storage, we propose a method that uses pressure oscillation during CH4/CO2 replacement. The effects of depressurization pressure, depressurization time, and depressurization frequencies were analyzed in this study. The characteristics of pressure and temperature were analyzed during the depressurization process, and the decomposition and regeneration of hydrates relating to the oscillation pressure were observed on a macro level. The results show that this newly proposed combined method effectively improves CH4/CO2 replacement, and the maximum amounts of CH4 recovered and the CO2 storage efficiency are nearly double than those achieved without pressure oscillation. CH4/CO2 replacement is promoted by the pressure oscillation mechanism by breaking the balance of the hydrate layer, which results in the dissociation of CH4 hydrates and finally forms a CO2/CH4 mixed hydrate layer with free water. The results of this study provide a valuable method that can be used to enhance CH4 recovery and CO2 sequestration.