Method for recovering methane gas from natural gas hydrate by injecting CO2 and air mixed gas

A method for recovering methane gas from a natural gas hydrate layer, said method comprising:

• injecting a gas mixture comprising carbon dioxide and air into a natural gas hydrate, the natural gas hydrate exhibiting a solid state, above a critical methane concentration, at which chemical potential of methane of the solid natural gas hydrate equals chemical potential of gaseous methane in a vicinity of the natural gas hydrate, such that gaseous methane is decomposed from the natural gas hydrate and, simultaneously, the carbon dioxide from the injected gas mixture replaces the methane in the solid natural gas hydrate to maintain a cubic crystal structure thereof and to free the replaced methane for recovery, at which point chemical potential of gaseous methane equals chemical potential of gaseous methane in a vicinity of the natural gas hydrate;
• wherein continued injecting of the gas mixture into the natural gas hydrate achieves a methane concentration below the critical methane concentration such that gaseous methane is decomposed from the natural gas hydrate without maintaining the cubic crystal structure thereof.