Depressurization assisted CO2-CH4 replacement in hydrate Structure: Molecular mechanism and kinetic modeling

Abstract Methane hydrates as a source of energy and a medium for long-term storage of anthropogenic CO2 gained many attentions in the recent years. Methane hydrate deposits can secure the energy demand in the world for decades. CO2-CH4 replacement technique is a state of art for exploitation of methane hydrate deposits and simultaneously under-ground CO2 storage. This technique covers lots of shortages existing in conventional techniques for production from methane hydrate reservoirs. Due to the complexity of this process, there is not a clear insight from the mechanism of replacement. Additionally, slow rate of replacement makes this method technically and economically inefficient. Combining CO2-CH4 replacement and depressurization techniques is a novel rout to enhance the rate of methane recovery. Here we search for the molecular mechanism of CO2-CH4 replacement in depressurized CH4 hydrate in the presence of excess water by experiments conducted in a constant stirred hydrate crystallizer. A representative kinetic model is developed based on the confirmed molecular mechanism. According to the proposed mechanism, the replacement reaction includes two steps; partial breakage of methane hydrate cages, and exchange between CO2 and CH4 in damaged cages and reestablishing hydrate structure. In addition to the replacement reaction, a new mixed CO2 + CH4 hydrate will form from the existing free water, and some portions of the damaged methane hydrates decompose completely. Proportionality of the replacement rate with methane fugacity difference reveals that partial breakage of hydrate cages is the rate limiting step of the replacement reaction. Replacement kinetic parameters extracted from the proposed kinetic model and experimental data are correlated over a wide range of operational conditions.