Experimental study on the effect of coal rank on the efficiency of N₂ injection on CH₄ displacement

This study investigated how coal rank affects the displacement of methane (CH₄) by nitrogen (N₂) injection under different injection pressures. Experiments were conducted using a laboratory gas-coal coupling testing system, along with surface area and pore size distribution analyses, to quantify displacement performance and explain the role of pore structure. The results show that displacement performance varies systematically with increasing N₂ injection pressure across coal ranks. Low-rank coal, which contains well-developed, partly connected slit-shaped pores, showed the best performance, with displacement efficiency ranging from 75.91% to 92.48% and the displacement ratio decreasing from 0.47 to 0.03. Medium-rank coal had more diverse pore types but weaker pore connectivity, resulting in a displacement efficiency of 63.17% to 81.37% and a displacement ratio decreasing from 0.17 to 0.03. High-rank coal had high open pores but a denser structure and poorer connectivity, giving the lowest displacement efficiency, from 39.97% to 69.52%, and a displacement ratio decreasing from 0.09 to 0.01. During displacement, all coal samples experienced shrinkage, which decreased rapidly at first and then gradually stabilized. Coal shrinkage was negatively correlated with N₂ injection pressure. Low-rank coal showed the greatest shrinkage because of its looser pore structure, conversely, high-rank coal showed the least. Overall, low-rank coal is more suitable for N₂ injection displacement because its well-developed and better-connected pores improve methane recovery while reducing operational costs.