It is very important from the point of view of gas control and production safety to efficiently extract gas from stress-relief fractures in the initial phase of the longwall working face. In the initial phase of longwall mining, the overburden deformation significantly affects the instability of gas emission and the mining speed positively correlates to the volume of gas emission, so appropriate mining speed can help to increase the efficiency of gas extraction. The monitoring data on surrounding rock deformation show that the horizontal separation fractures first appear from the outer to inner layers and continue to deform along with the mining working face progressing, then the vertical fractures gradually evolve from the lower to the upper layers and finally a stable fracture zone comes into being, which provides space and pathways for both gas ‘stress-relief and retention’ and its abnormal emission during the mining initial phase. Moreover, this paper proposes a new spatiotemporal division model called overburden ‘three belts and five zones’ in the mining initial phase, i.e. the vertical ‘three belts’ including the caving belt, the dynamic fracture evolution belt and the curved subsidence belt; and the horizontal ‘five zones’ consisting of the key gas drainage zone for fracture development, ‘stress-relief and gas-retention’, overburden pressure-relief zone, gas seepage zone in goaf, coal seam pressure-relief zone in front of the working face and the difficult gas drainage zone in coal wall stress concentration area. The research results of this paper can provide theoretical and technical support for optimizing establishment of gas drainage parameters.

Gas drainage is an important technology to prevent coal and gas outburst, and the drained gas is a kind of clean energy. The gas pressure can characterize gas drainage effectiveness. In this paper, we investigated the effectiveness of gas drainage by gas pressure. Determined by the space shape of the gas flow field, the gas flow state surrounding the drainage boreholes is radial flow. According to the basic equations of radial flow, discrete equations were achieved by the implicit difference scheme, and then we obtained the gas pressure surrounding the drainage boreholes. Results showed that the midpoint between two holes presents the highest gas pressure, and gas pressure declined from the midpoint of two boreholes to both sides. The midpoint gas pressure of the two holes reflects gas drainage effectiveness in a certain degree. Gas pressure declined with segmented characteristics in the first period decline curve in the form of a cubic curve, and the second period decline curve in the form of a straight line. When the drainage pressure reaches a certain value, the decline rate of gas pressure had little relationship with the drainage negative pressure, mainly influenced by the permeability coefficient. To improve the drainage effectiveness, anti-reflection measures are feasible, instead of increasing the drainage negative pressure. Moreover, the conclusion was verified by field data.