Advances in research and developments on natural gas hydrate extraction with gas exchange

Natural gas hydrate (NGH) is a source of energy that has attracted significant interest for the secure and effective extraction of CH4 gas from permafrost and marine regions. CH4 gas can be produced and greenhouse gases (GHG) sequestrated by injecting flue gas into NGH, minimizing environmental threats. This paper provides a complete summary of the advancement of CH4 recovery from NGH by CO2 replacement. The CH4-CO2 exchange mech-anisms are examined in different types of hydrates concerning their cage structure and formation in geological layers. The gas composition of CO2 injected with H2, N2, and air, thermodynamic pressure and temperature, hydrate and water saturations, and concentration of CO2 are factors influencing the efficiency of the CH4-CO2 exchange. The structural integrity of the sedimentary film throughout the exchange process is evaluated, and the evaluation indicates that the hydrate formed by CO2 gas is stronger than the layer formed by CH4 gas and that the mechanical integrity of the reservoirs can be maintained after the exchange process. The replacement effect of CH4-CO2 combined with depressurization, thermal stimulation, and other factors is discussed. Restrictions and further studies of the CH4-CO2 exchange process are also analyzed in light of current findings. This article offers information for future NGH extraction and CO2 storage applications by providing an overview of recent advances in CH4-CO2 replacement.