Depressurization-Assisted CO₂/N₂ Replacement for Methane Recovery from Hydrate-Bearing Clayey Sediments

Natural gas hydrates (NGHs) are regarded as a secure and efficient renewable energy resource. Recovering methane (CH4) by injecting carbon dioxide (CO2) into NGHs is an innovative method for energy production and greenhouse gases control. However, the replacement method by injection of CO2 for CH4 recovery was constrained by mass transfer and permeability limitations, leading to a poor replacement efficiency and rate. Herein, a depressurization-assisted CO2/N2 replacement method was proposed to enhance CH4 recovery and CO2 sequestration. In this study, a coupled experiment integrating CO2/N2 mixed gas injection and staged depressurization was conducted in CH4 hydrate-bearing clayey (montmorillonite) sediments by using a custom-designed triaxial pressure reaction apparatus. The effects of replacement pressure, temperature, depressurization pressure, and time were systematically analyzed. The results indicate that there is substantially enhanced CH4 recovery and CO2 sequestration compared to the condition without depressurization assistance, with a remarkable increase from 21 to 85.7% and 45.4 to 68.3%, respectively. The integration of depressurization effectively overcame the physical barriers between the gas and hydrate phases, resulting in a 67.6% enhancement in CH4 replacement efficiency and significantly accelerated postdepressurization replacement rate. The study results provide insight for exploring low-carbon energy production and CO2 sequestration.