Heat-Fluid-Solid-Coupled Model for Flue Gas Displacement of CH₄ in Coal Seams and Its Applications

The objective of this study was to evaluate the effect of injecting flue gas (CO₂, N₂, and O₂) originating from coal-fired power plants into a coal seam on CH₄ extraction and CO₂ geological storage. To this end, a multifield thermal-fluid-solid-coupled mathematical model of flue gas injection extraction was established. The results showed that with the increase in time increase, the volume concentration of CH₄ decreased, but the CO₂, N₂, and O₂ increased. Compared with single extraction, the gas injection extraction brought about a significant reduction in the pressure and content of CH₄, an increase in the CH₄ extraction rate, and an increase in the effective radius of CH₄ extraction. In the single extraction, the temperature of the reservoir decreased, and its permeability increased. In the gas injection extraction, the temperature near the gas injection hole increased, whereas the temperature near the extraction hole decreased, and the permeability decreased overall. A method of measuring the effective radius of gas extraction by temperature is presented. The storage and extraction times of CO₂ exhibited a linear relationship, and the CO₂ escape rate increased gradually. The longer the gas injection extraction time, the greater the risk of coal and gas (CO₂) outbursts.