Study on the coupling mechanism of coal damage and seepage around staged variable diameter gas extraction drillings

In order to reveal the damage evolution and seepage enhancement mechanism of coal around staged variable diameter gas extraction drillings under mining stress environment, taking the N2302 working face of Gucheng Coal Mine as the engineering background, a systematic study was conducted on the stress distribution, plastic zone evolution characteristics, porous mutual interference effects and their impact on permeability evolution of coal around drillings using a combination of theoretical analysis, numerical simulation and physical similarity experiments. Based on elastic-plastic mechanics and Mohr Coulomb yield criterion, an analytical model for the stress and plastic zone of coal around the drilling under different mining location conditions was established, revealing the inherent mechanism of the plastic zone evolution from approximately symmetrical to asymmetrical under mining support pressure. Based on FLAC3D numerical simulation, a porous staged drilling model was constructed, and the reconstruction characteristics of the stress field around borehole under porous conditions were analyzed. The “shielding effect” caused by the superposition of group hole pressure relief and the secondary stress concentration phenomenon at the pressure relief boundary were clarified, and the key control role of the aperture mutation zone in stress disturbance and damage evolution was revealed. On this basis, the spatial coupling relationship between the plastic zone around the pore and the permeability field was systematically analyzed. The results showed that the plastic failure zone is the main control structure for the formation of high permeability channels. Porous disturbances can promote the originally localized high permeability zone to gradually penetrate in the horizontal and vertical directions, forming a high permeability connected zone with obvious anisotropy. Furthermore, by conducting uniaxial compression experiments on coal bodies through staged drilling, combined with digital image correlation (DIC) technology and acoustic emission monitoring, multi-scale verification was carried out on the initiation, propagation, and penetration processes of fractures around the drilling. The experimental results showed good consistency with numerical simulations in terms of fracture evolution paths, damage concentration zones, and seepage enhancement trends. The research results can provide theoretical basis and technical support for optimizing the drilling structure and efficient gas extraction under complex mining conditions.