Effect of Different Placement Sequences of Water on the Methane Adsorption Properties of Coal

After the coal seam is injected with water, the moisture content in the coal body increases, which affects the output capacity of coalbed methane (CBM). In order to improve the effect of CBM mining, the classical anthracite molecular model has been selected. To analyze the influence of different placement orders of water and methane on the characteristics of coal-adsorbing methane from the micro point of view, a molecular simulation method is used for comprehensive consideration in the study. The results show that H₂O does not change the mechanism of CH₄ adsorption by anthracite, but it inhibits the adsorption of methane by anthracite. When water enters the system afterward, there arises an equilibrium pressure point where water plays the most significant role in inhibiting methane adsorption by anthracite coals, which increases with increasing moisture content. When water enters the system first, no equilibrium pressure point occurs. The excess adsorption of methane by anthracite when water enters second is higher. The reason is that H₂O can replace CH₄ at the higher energy adsorption sites of the anthracite structure, while CH₄ can only be adsorbed at the lower energy sites, and some of CH₄ is not adsorbed. For the coal samples with a low-moisture content system, the equivalent heat of adsorption of CH₄ increases first rapidly and then slowly with the increase of pressure. However, it decreases with pressure in the high-moisture content system. The variation of the equivalent heat of adsorption further explains the variation of the magnitude of methane adsorption under different conditions.