Abstract: In order to study the permeability evolution law and distribution characteristics of goaf during protective seam mining and its pressure-relief mining effect, a calculation model to calculate gas flow rate of drilling well extracting gas from goaf and pressure-relieved seams has been simplified. This model has made full use of gas extraction monitoring data of the working face 1242(1) of a coal mine in Huainan city. Combined with Darcy’s law, the equivalent permeability of goaf has been obtained. Meanwhile, by fitting gas extraction volumes from protected seam, the decline coefficient of gas extraction has been obtained. A decrease multiples of decline coefficient has been introduced to reflect the pressure-relief performance of protective seam mining. The results have shown that: 1) The permeability evolution of goaf has experienced three stages including sharp rise stage, the decline stage, and the stable stage. Longwall face advancing distances of the first two stages are 19.24 m and 186.2 m respectively. The permeability in the middle of the compacted goaf is much smaller than the permeability on the edge of goaf, and presents O shape trend after longwall face mining. 2) Gas extraction volume from protected seam has also experienced three stages including sharp rise stage, the decline stage and the stable stage. The time of the first two stages are 10 d and 125 d. 3) Decrease multiples of decline coefficient of 1^# drilling well is as high as 7.7, which has indicated that the protective seam mining has great pressure-relief effect.

Key words: surface well drilling, pressure-relief mining, permeability calculation model, goaf, decline coefficient