Seepage Characteristics of Coal under Complex Mining Stress Environment Conditions

In this study, four types of coal samples (complete, parallel fracture, vertical fracture, and inclined fracture) were prepared, and four mining stress paths were designed. Cyclic loading–unloading seepage tests were conducted on coal samples to study the evolution of the seepage characteristics. Evolution characteristics of permeability and porosity were investigated. The cycle factor was introduced to research the effect of the cycle number on permeability. Stress sensitivity coefficients were obtained by fitting the permeability curves. The relationship between permeability and porosity was analyzed. Results indicate that the permeability enhancement effect of the coal seam is positively correlated with the number of repeated mining and pressure relief range, which means that permeability increases with the increase in the cycle number and σ3 – σ1, and gas extraction is most effective in high-pressure relief areas. Furthermore, evolution characteristics of porosity and permeability are similar, and their fitting curve relationship is a cubic law. Permeability changes significantly when stress reaches a critical point, which is most evident in fractured coal samples. In addition, as the cycle number increases, the compaction effect is reduced and the expansion effect is improved, and the two effects are improved in the following order: vertical fracture > inclined fracture > parallel fracture > complete. As the cycle number increases, the stress sensitivity coefficient decreases in the loading stage, while it increases in the unloading and recovery stages. Meanwhile, the stress sensitivity of coal samples is in the following sequence: inclined fracture > parallel fracture > vertical fracture > complete. Results are expected to provide a theoretical guidance for strengthening gas extraction.