Study on Fracture Evolution and Air Leakage Simulation in Composite Goaf of Close-Distance Coal Seams and Prevention/Control of Induced Coal Spontaneous Combustion: A Case Study

Coal spontaneous combustion (CSC) occurring within the composite goaf of close-distance coal seams remains one of the most challenging issues in mine fire prevention and suppression efforts. To address the high-temperature hazards within the close-distance composite goaf areas at a mining site operated by Pingmei Group, a combined approach of numerical simulation, field testing, and theoretical analysis was employed. This study investigated the fracture development characteristics, air leakage patterns, and measures for preventing CSC in the region. Research indicates that advancing the working face of the underlying coal seam beyond the stopping line of the overlying coal seam creates numerous air leakage pathways through disturbance-induced inter-seam fracture structures. This process also caused the collapse of the overlying unmined seam, leading to significant residual coal distributed three-dimensionally between the overlying coal seam and underlying coal seam. The composite goaf’s airflow typically shifts from the underlying coal seam to the underlying coal seam, influenced by the variance in pressure inside it. Residual coal in the composite goaf results in a differential oxygen concentration distribution. Using the mine’s geological conditions and composite goaf air leakage patterns, the precise locations of high-temperature CSC hazard areas were determined. Effective mitigation measures, including cement-based material filling and sealing, inerting and cooling with liquid nitrogen and carbon dioxide, and gel foam coverage for oxygen isolation, were successfully implemented to address the high-temperature hazards of composite goafs. These findings provide valuable technical references for preventing and combating CSC in close-distance composite goaf areas.