Porosity Optimization Model to Characterize Gas Seepage Behavior in Longwall Mining Gobs: An Application Case and Simulation Analysis

Abstract

The degree of fragmentation of coal rock determines the porosity distribution of goaf porous media, which can be expressed using empirical formulas. However, this method fails in regions with nonuniform caving, dynamic disturbances, or deep high-stress conditions. Therefore, this study developed a porosity optimization model comprising A and B values. The A value represents the upper limit threshold of the porosity and is related to the total air volume that leaked into the gob, and the B value is related to the distribution pattern of the collapsed coal rocks and determines the migration path of airflow. Based on this, the permeability coefficient of Yangchangwan Coal Mine was determined through an inversion calculation, and the evolution laws of multi-physical fields under different seepage environments were explored. The results indicate that the air leakage velocity variation is approximately S-shaped in the working face inclination direction. The temperature rise in gobs was primarily influenced by the advance rate and residual coal thickness. Thus, the mining rate should be increased as much as possible, and leak prevention measures should be employed to prevent spontaneous combustion fires. This study provides a more accurate and practical method for evaluating the porosity distribution in fracture zones.