Effect of Stress Relief on Coal Seam Gas Migration: Experiment and Application Discussion

Stress relief-induced enhanced permeability is one of the crucial measures for promoting gas desorption flow and strengthening gas extraction. In order to examine the impact of stress relief and its magnitude on gas migration, this article explores the gas desorption flow during the stress relief process and elucidates the influence of stress relief degree on gas extraction. The results indicate that considering the analysis of the pore structure effect on gas seepage, the four coal samples' permeability is ranked as PDS > CSL > JZS > GHS. Throughout the stress relief process, the gas desorption rates of different coal samples under various stress paths exhibit varying degrees of increase. As an illustration, following 3600 s of stress alterations, the gas desorption rate of CSL1# experiences a notable increase, surging by 2.57 times; PDS2# shows 55.93 times increase after 4200 s, and JZS3# exhibits 3.13 times increase after 5400 s. A stress relief degree model is established to investigate the variation of horizontal stress and stress relief degrees under different borehole spacings, vertical stresses, cohesion, and internal friction angles for various borehole diameters (coal output). Optimal stress relief is achieved with a borehole diameter greater than 1.52 m with a borehole spacing set at 4 m. When the stress relief degree exceeds 30%, the corresponding borehole diameter ranges for different vertical stresses are 1.49-1.6 m. Similarly, for cohesion, the ranges are 1.25-1.68 m, and for internal friction angles, the ranges are 1.39-1.53 m. The research results can provide valuable insights for determining parameters in the on-site construction of stress relief boreholes.