Estimation of Methane Recovery Efficiency from Methane Hydrate by the N2-CO2 Gas Mixture Injection Method

Methane hydrate (MH) is the best-known unconventional energy resource and has begun to open its promising future, especially for Japan. In 2017, the second offshore gas hydrate field test was conducted in the Eastern Nankai Trough, Japan, where a depressurization technique was adopted for producing methane gas. Alongside the depressurization method, the replacement of CH4 from gas hydrates by a N-2-CO2 gas mixture was suggested and adopted to increase performance for both methane gas recovery and carbon dioxide sequestration. The amount of abundant MHs is estimated to exist in the permafrost area; therefore, this method, which does not require any heat source, can be used as a standard method for recovering methane. In this study, experiments to continuously inject a N-2 -CO2 gas mixture (59 mol % CO2) into hydrate-bearing cores with different MH saturations were conducted. Simultaneously, the numerical simulation model was constructed, and to express the gas exchange phenomenon of CH4 -(N-2 + CO2), the phase equilibrium analyses were used between the gas mixture and hydrates. Good agreements were obtained between experiments and simulations. At these experiments, the recovery factor of 40.8% and the exchange ratio of 8.22% were recorded as an example. The validation of the simulation with experimental results strongly supported our study to find the efficient injection gas composition for methane recovery. Case studies of the numerical simulation were conducted with various CO2 concentrations of injection gas in the range of 24-72 mol %. Simulation results showed that the highest recovery factor was obtained for the 30-40 mol % CO2 gas injection cases. The higher the N-2 concentration of the gas mixture, the more CH4 molecules in the hydrate phase can be replaced. In the gas exchange process, the portion of N-2 occupied in guest gas molecules of the hydrate became larger with increasing the N-2 concentration of the gas mixture. From the discussion above, we concluded that the CO2 concentration of 30-40 mol % was the most effective for CH4 recovery by the N-2-CO2 gas mixture injection method.