Evolution laws of mining-induced fractures in overlying strata of inclined extra-thick coal seam and the identification of gas migration and enrichment regions

Accurately identifying gas migration and enrichment regions (GM&ERs) is crucial for facilitating precise gas extraction and ensuring safe mining. Identifying GM&ERs relies on comprehending the temporal and spatial patterns of mining-induced fractures in overlying strata. To identify the GM&ERs of inclined extra-thick coal seams (IETCs), the 3D numerical simulation was conducted based on the Liuhuanggou coal mine as the research prototype, aiming to investigate the overburden movement process. The research revealed the evolution of broken and separated fractures and mining-induced fracture angles while further examining the fractal characteristics of fractures. The results indicated that broken and separated fractures were predominantly concentrated in the upper section of the goaf, influenced by gravitational and sliding forces. The fractal dimension of mining-induced fractures (DM-IFs) experienced three distinct phases during the mining of IETC: a rapid increase, a fluctuating increase, and a gradual decrease. The complexity of fracture development in the upper section of the goaf was conducive for gas migration and enrichment. A novel method for identifying the GM&ERs, which considers both broken and separated fractures as well as the DM-IFs, was proposed. Based on the proposed method, the low-level gas enrichment region in the 9–15(06) working face was located above the floor at depths ranging from 17.48 to 31.97 m and extended within a 30 m range beneath the air return roadway. A layout plan for the high-level drilling site was designed, and gas extraction proved to be highly effective. The research provides a valuable reference for the precise gas extraction in the goaf of IETCs.