Study on the effect of CO₂ displacement and CH₄ injection pressure and its optimal threshold in weakly caking coal of the Huangling Mining Area

Low-permeability coal seams often have low gas drainage efficiency, and the appropriate injection pressure for CO₂-enhanced methane recovery remains difficult to determine. This study investigates the injection pressure characteristics and optimal pressure range for CO₂ displacement of CH₄ in weak coking coal from the Huangling Mining Area using theoretical analysis and laboratory experiments. The injection pressure was found to have two main effects: it changes the adsorption state of CO₂ and CH₄ by altering adsorption potential, and it promotes free-gas migration through pressure gradients. Based on Langmuir adsorption theory, the adsorption capacities and displacement ratios of CO₂ and CH₄ were calculated under different pressures. CO₂ displacement experiments were then conducted at varying injection pressures using a self-developed experimental system. The results show that CO₂ displacement in weak coking coal is highly sensitive to injection pressure, and the displacement efficiency increases in three stages as pressure rises. After normalization, the curves for displacement efficiency and displacement ratio intersect at 3.75 MPa, indicating a critical point. This point marks the beginning of the high-efficiency displacement range and represents a balance between methane recovery and CO₂ consumption. Considering the adsorption transition point, displacement efficiency, and engineering economics, the reasonable injection pressure range for weak coking coal in the Huangling Mining Area is determined to be 3.8-7.0 MPa. These findings provide practical guidance and data support for designing CO₂-enhanced methane recovery parameters in low-permeability coal seams.