Effectiveness of CO2-N2 injection for synergistic CH4 recovery and CO2 sequestration at marine gas hydrates condition

CH4 hydrate have been considered as the future clean energy resource. Recovery of CH4 from natural gas hydrates by CO2 replacement is a promising method for energy harvest and CO2 sequestration. Field production tests have proved the feasibility of gas production by CO2 replacement in the permafrost region. However, most of the hydrate reservoirs are located at marine conditions where pressure and temperature conditions are not favorable for the replacement process. In this work, we identified a possible marine condition, where mixed hydrates could potentially form. We investigated the technical feasibility of CH4 recovery by a depressurization process followed by the injection of CO2-N2 gas. To quantify the dynamic behavior of mixed hydrates formation and dissociation, we developed a numerical method to estimate the phase partitioning during the replacement process. The effects of mixed gas compositions in the reactor on the replacement efficiency were investigated. Our experimental results showed that rapid co-growth of mixed hydrates occurs when the injected gas composed of a lower N2 concentration (<25.2%). On the other hand, continuous CH4 hydrate dissociation was observed when the injected gas was composed of a higher N2 concentration (28.5%). During the prolonged replacement stage, the amount of CH4 hydrate decreased and the amount of CO2-N2 hydrates increased only slightly, which indicated that possible replacement occurred but at a very localized level. The results proved the technical feasibility of CH4 production by gas injection at marine conditions, but the compositions of the mixture gas injected need to be well controlled.

Automated summary

The study demonstrates that recovery of CH4 hydrate at marine conditions through CO2-N2 gas injection is technically feasible, but careful control of the composition of the mixed gas is required to ensure replacement efficiency.