Optimization of the Layer Location of Extraction Wells Based on the Unpressurized Gas Transport Law.

Protective layer mining is one of the most effective measures to control outbursts of coal seam gas in coal mines. Accurately grasping the overlying rock movement and pressure-relief gas migration patterns under protective layer mining conditions is a prerequisite for efficient surface coalbed methane extraction; it is the basis for green emission reduction in coal mines. A physical model was established using the Ji-33200 working face of Pingmei Shi Mine as the research object, and a method combining theoretical calculation and numerical simulation was used to obtain the overlying rock movement. In situ stress distribution characteristics of the stope after the upper protective layer was mined to explore the upper protective layer migration rules of pressure-relief gas after mining. On this basis, the location and layer of surface coalbed methane production wells was determined. The research results show that the coal and rock formations on the floor of the goaf experienced a deformation process of compression → expansion → rebalance during the mining process; the stress changes of the overlying and underlying coal strata in the goaf have experienced a process of increasing → decreasing → rebalance; and gas migrates upward through the fissure zone in the coal layer and slowly diffuses in other microfissure areas. When the pressure reaches a certain value, it is enriched in the crack development area and the upper part of the fissure zone; combined with the relevant geological conditions of the study area, it was determined that the upper part of the roof of the Ji coal seam is a gas-rich area. By comparing three mining vertical wells at different positions in the horizontal direction, it was found that the extraction effect was significant in the "O″ ring, near the excavation face.