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

Abstract

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 N2–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 N2–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–(N2 + 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 N2 concentration of the gas mixture, the more CH4 molecules in the hydrate phase can be replaced. In the gas exchange process, the portion of N2 occupied in guest gas molecules of the hydrate became larger with increasing the N2 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 N2–CO2 gas mixture injection method.

Experimental Investigation of Critical Parameters Controlling CH4–CO2 Exchange in Sedimentary CH4 Hydrates

“…when using a mixture of 20 mol % CO 2 and 80 mol % N 2 in an experiment with no flow. Simulation results indicated that the CH 4 recovery was maximized when the CO 2 content of the injectant was below 40 mol % …”

“…However, the reported recovery rates of CH 4 gas are very scattered and reflect the variety of experimental procedures and conditions used . Several studies involving continuous CO 2 injection into CH 4 hydrate-bearing sediments exist, − but none of them systematically address the effect of residual water saturation. The CH 4 hydrate-bearing sediments in nature must be permeable such that the injected CO 2 may penetrate deep into the formation and contact the CH 4 hydrate.…”

“…Adding nitrogen (N 2 ) to the injected CO 2 has thus been recognized as an effective technique to ensure injectivity and was implemented during the short-term CH 4 –CO 2 exchange field trial on the Alaska North Slope in 2012 . Adding N 2 to the gas mixture increases the hydrate equilibrium pressure and promotes CH 4 gas recovery. , Other destabilizing chemicals such as hydrogen and methanol have also been tested in the laboratory as a mean to increase the recovery rate of CH 4 gas.…”

Mechanical Characteristics of Hydrate-Bearing Sediment: A Review

“…Different from the exploitation of conventional oil and natural gas resources, NGH exploitation is driven by the disruption of its thermodynamic equilibrium to release the natural gas; specifically, techniques including thermal stimulation, depressurization, inhibitor injection, and gas replacement are proposed. In addition, some innovative technologies involving hydrates have also been developed, such as hydrate-based gas storage, , heavy metal separation from sewage, and carbon dioxide (CO 2 ) capture. , …”

“…In addition, some innovative technologies involving hydrates have also been developed, such as hydrate-based gas storage, 9,10 heavy metal separation from sewage, 11 and carbon dioxide (CO 2 ) capture. 12,13 At present, because of low-temperature and high-pressure thermodynamics, NGHs are mainly concentrated in the deep sea floor along continental shelfs and inland permafrost zones, where the environment is poor and lacks infrastructure. 14,15 Microscopically, the NGH pore habit depends on the fluid environment, 16 effective stress, 17 and pore size-dependent capillary pressure.…”