Numerical simulation of gas production from permafrost hydrate deposits enhanced with CO2/N2 injection

A new numerical simulator was developed from the widely used CH4 hydrate simulator TOUGH + HYDRATE to realize the simulation of hydrate exploitation by CO2/N2–CH4 replacement. Focusing on actual hydrate reservoir, CO2/N2 injection combined with depressurization in a practical continuous injection-production mode was applied for gas production. The influence of feed gas composition and injection pressure on CO2 sequestration and CH4 production was investigated. Moreover, we conducted a fair comparison and revealed the advantages of CO2/N2 injection over two traditional methods in gas/water production performance. During gas injection, a continuous CO2/N2 separation process under stratum environment was observed, and the whole gas replacement process can be roughly summarized as a continuous cycle of CH4 hydrate dissociation and CO2/N2 hydrate formation. Increasing N2 mole fraction from 30% to 50% significantly enhanced the CH4 production efficiency, while its increase from 50% to 100% mainly resulted in more N2 production and higher injection-production ratio. Raising the injection pressure from 4.5 to 5 MPa improved CH4 recovery by 1.5 times, while increase from 5 to 7 MPa reduced CH4 recovery by 8.3%. A favorable CH4 recovery with relatively low cost can be achieved by finding an appropriate balance between CH4 release and CO2 sequestration. •A new numerical simulator for hydrate exploitation by gas exchange is developed.•Field-scale hydrate exploitation enhanced with CO2/N2 injection is simulated.•Balancing CH4 release against CO2 storage benefits CH4 recovery and energy saving.•Gas production performance can be improved according to the controlling factors.