Study on influence of coal seam water content on effect of CO₂ isobaric diffusion displacing CH₄

In order to study the influence of moisture on the effect of CH₄ replacement by CO₂ injection in coal beds, a gas-containing multi-gas replacement experimental system developed independently was used to conduct CH₄ and CO₂ adsorption experiments on coal with different water content. The results show that: the presence of moisture in coal does not affect the pattern of CH₄ and CO₂ isothermal adsorption curves and the ranking of the magnitude of the capacity of coal adsorption of CH₄ and CO₂; under the same adsorption equilibrium pressure, the amount of CH₄ and CO₂ adsorbed gradually decreases with the increase of the water content of coal samples. Under the same adsorption equilibrium pressure, the adsorption amount of CH₄ and CO₂ gradually decreased with the increase of the water content of the coal samples, and the inhibition rate of CH₄ adsorption by water was larger than that of CO₂ adsorption by coal, which indicated that the inhibition degree of water on the adsorption of weakly adsorbable gases by coal is larger. In the experiments of CH₄ replacement by CO₂ injection in coal with different water contents under isobaric diffusion conditions, it was found that, under the same water content conditions, with the increase of adsorption equilibrium pressure, the replacement amount and replacement rate of CH₄ kept increasing, while the CO₂ injection ratio kept decreasing, and the replacement effect of CH₄ was better, while the utilization rate of CO₂ was lower. Under the same adsorption equilibrium pressure, with the increase of the water content of coal samples, the CO₂ storage quantity decreased continuously, and the comparison with the CH₄ replacement quantity found that, under the same conditions, the storage quantity of CO₂ in the coal body was always higher than the replacement quantity of CH₄ when the water content of coal samples was unchanged, and the adsorption equilibrium pressure was increased, the CO₂ storage quantity and CH₄ replacement quantity of coal samples were both enlarged, and the storage quantity of CO₂ in the coal body was also always higher than the replacement amount of CH₄. Under the same adsorption equilibrium pressure, with the increase of water content of coal samples, the replacement amount and rate of CH₄ and CO₂ injection ratio showed a decreasing trend, and the moisture inhibited the replacement of CH₄ in coal by CO₂, and the larger the water content of coal samples was, the worse the replacement effect was, and the higher the degree of inhibition was; the gradient of the decrease of the CO₂ injection ratio was small, and the degree of the decrease was not obvious, and the moisture had less influence on it. The research results further improve the theoretical analysis of the factors affecting the adsorption and desorption of coal on gas, which will provide a theoretical basis for the engineering application of gas injection and extraction of coalbed methane.