Research on the Construction of a Radial Seepage Model for Porous Media and Simulation of the Response Mechanism in Different Zones

To investigate the radial gas seepage characteristics and response mechanisms of the coal surrounding boreholes, this study selects a working face of the Zhaogu No. 2 mine in the Jiaozuo mining area as the research object, develops a radial gas permeability evolution model of the coal body around the boreholes, and employs COMSOL numerical simulations to systematically analyze the zonal response mechanisms of gas seepage. The results indicate that following borehole excavation, the stress around the hole exhibits a characteristic "butterfly-shaped" distribution, forming in sequence along the radial direction a pressure relief zone, a stress concentration zone, and an original rock stress zone. The gas pressure in the coal surrounding the borehole gradually increases with the radial distance, and the streamline distribution on the lateral sides of the borehole is denser than that observed at the top and bottom. Further analysis reveals that under different time steps the gas seepage velocity and pressure gradient in the coal body surrounding the borehole both exhibit an attenuating trend with increasing radial distance. Among these zones, changes in the pressure relief and stress concentration regions are the most pronounced, whereas the original rock stress zone shows no significant variation. The stress concentration zone primarily exhibits a Darcy-type linear seepage across different time steps. Although the pressure relief zone maintains a Darcy-type flow state in the initial stage of seepage, with increasing pressure gradient, its seepage behavior transitions from Darcy-type linear seepage to Forchheimer-type nonlinear seepage. However, as seepage progresses due to the low permeability of the stress concentration zone and the influence of the Klinkenberg effect, the seepage state in the pressure relief zone gradually reverts to Darcy-type linear seepage.