Influence of tectonic fractures on CO₂ storage and enhanced CH₄ recovery

In the process of CO₂-ECBM, the tectonic fractures developed in the coal may become the preferential channels of gas-water seepage under the impact of injection pressure, and then influence the recovery of coalbed methane (CBM) and CO₂ storage. Based on the theoretical analysis, the displacing profile of CO₂-ECBM is established. A numerical model is constructed referring to the TS-634 field trial, and the influence of the tectonic fractures on CH₄ recovery and CO₂ storage is investigated. The CBM recovery and CO₂ storage in the TS-634 trial are evaluated, the mechanism of the rapid breakthrough of CO₂ is revealed, the enlightenment of the rapid breakthrough on the optimization of the injection-drainage process is dissected, and the proposals for the injection-drainage process in reservoirs with tectonic fractures are put forward. The results show that CO₂ enriched zone, CH₄-CO₂ mixed zone, CH₄ enriched zone, and water enriched zone can be formed among injection-production wells successively. With an injection, the zones affect the producers after their formation and evolution. The difficulty of water seepage determines the level of CH₄ output inhibition and then controls the CBM recovery and CO₂ storage. The producer along tectonic fracture can achieve an efficient recovery of CH₄, but a high permeability often leads to apparent CO₂ channeling which causes the loss of cumulative CH₄ production and CO₂ storage. During the trial period, the strong-breakthrough producer attains a remarkable increase in gas output, with the actual cumulative gas production increasing by 10.4% and bottomhole-pressure-equivalent cumulative gas output increasing by 92.3%. Tectonic fractures provide potential preferential channels for the CO₂ rapid breakthrough and become real preferential channels mainly induced by injection pressure. The existence of fracturing affected areas and liquid CO₂ phase transformation-induced expansion and energization accelerate the CO₂ breakthrough, but the coal matrix expansion-induced permeability decrease is expected to delay the breakthrough. To achieve a better displacement, enhancement, and sequestration, it is necessary to boost the bottomhole pressure in the early injection stage, fine-tune the pressure of the injector or increase the bottomhole pressure drop of producers in the inhibition stage, and maintain the pressure lower than the minimum principal stress of the reservoir in the whole stage. After the breakthrough, the exposure time between CO₂ and coal matrix can be increased by irregularly shutting down/restarting the producers or reducing the pressure of the injector, which is expected to further improve the CH₄ recovery.