Insights into Cage Occupancies during Gas Exchange in CH<inf>4</inf>+CO<inf>2</inf> and CH<inf>4</inf>+N<inf>2</inf>+CO<inf>2</inf> Mixed Hydrate Systems Relevant for Methane Gas Recovery and Carbon Dioxide Sequestration in Hydrate Reservoirs: A Thermodynamic Approach

Carbon dioxide injection for methane replacement from hydrate reservoirs is considered as one of the effective techniques for both methane production and carbon dioxide sequestration. To understand the gas exchange process, a thermodynamic model based on the classical fugacity approach has been utilized to predict the phase behavior and the cage occupancies of both binary CH4+CO2 and ternary CH4+N2+CO2 mixed gas hydrate systems in three phase (L-H-V) equilibria. A total of 256 experimental phase equilibrium data points on binary CH4+CO2, and ternary CH4+N2+CO2 gas hydrate systems have been selected from various literatures. For the ternary system, various compositions of CH4+N2+CO2 gas mixtures with varying N2/CO2 gas ratio, i.e., 0.13, 0.19, 0.27, 0.33, 0.37, 0.5, 0.98, 2.6, and 4, and varying CH4 compositions, i.e., 20, 50, 60, 70, 80, and 90%, have been considered. The absolute average deviations (%AAD) in the predicted equilibrium pressures with the experimental results are observed to be within 2.9% and 6.4% for binary and ternary mixed hydrate systems, respectively. For CH4+CO2 mixed hydrate systems, the average distribution coefficient of methane has been found to be 2.06, which indicates that the CH4 molecules are selectively replaced by CO2 molecules preferentially from large cages. For the ternary CH4+N2+CO2 mixed hydrate system, the N2/CO2 ratio in small cages of the hydrate is found to be almost 20 times larger than that in the large cages revealing the capacity of N2 and CO2 molecules to replace most of the CH4 molecules from small and large cages of the hydrate, respectively. The N2/CO2 ratio in small and large cages is temperature independent at low N2/CO2 gas ratio, which becomes temperature dependent at higher ratios, i.e., 2.6 and 4. From this study, we conclude that the injection of N2/CO2 gas mixture having ∼1:3 ratio is a good choice for enhancing the production of methane gas and carbon dioxide sequestration deep into the hydrate reservoirs.