Abstract:
Low-permeability coal seams often have low gas drainage efficiency, and the appropriate injection pressure for CO
2-enhanced methane recovery remains difficult to determine. This study investigates the injection pressure characteristics and optimal pressure range for CO
2 displacement of CH
4 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 CO
2 and CH
4 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 CO
2 and CH
4 were calculated under different pressures. CO
2 displacement experiments were then conducted at varying injection pressures using a self-developed experimental system. The results show that CO
2 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 CO
2 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 CO
2-enhanced methane recovery parameters in low-permeability coal seams.