Influence of Coal Stress Sensitivity on the Desorption Production Characteristics and Residual CH4 Distribution of Thin Multilayered Coal Seams

Abstract

Multilayer codevelopment technology can significantly enhance the efficiency of coalbed methane (CBM) production in multilayer thin coal seams. When facing multiple coal seams with low permeability and large differences in permeability between layers, investigating the impact of permeability and its stress sensitivity on the desorption and production patterns of methane (CH₄) is fundamental to the implementation of multilayer codevelopment techniques. Permeability stress sensitivity tests on coal cores, CH₄ desorption production experiments, and quantitative analysis of CH₄ distribution in coal were conducted on coal samples with varying permeabilities in order to identify an optimal pressure drawdown approach that mitigates the adverse effects of permeability differences on multilayer codevelopment. Experimental findings indicate that a significant shift in permeability ratios among coal seams is observed, from 1:18.9:5.4 to 1:43.1:10.8, which exacerbates interlayer differences, which occurs as reservoir pressure decreases to the abandonment. The linear pressure decline method is found to be effective in enhancing CH₄ recovery, particularly in low-permeability coal, with a high recovery rate of 71.6%. This method outperforms both stepwise and direct pressure decline methods, which achieve recovery rates of 67.5% and 54.7%, respectively. The study also reveals that high-permeability coal exhibits 4.8–9.5% higher CH₄ recovery rates than that of low-permeability coal with a linear decrease in pressure. The high-permeability coal also reaches the peak CH₄ production rate earlier and maintains it for a longer period. The higher adsorbed CH₄ recovery rates and more uniform distribution of residual adsorbed CH₄ in high-permeability coal suggest that CH₄ in micropores is more readily desorbed. The study underscores the importance of reasonable bottom hole flowing pressure control for optimizing multilayer codevelopment and provides a scientific basis for the effective development of CBM in the region.