Spatiotemporal evolution model of protected coal seam permeability before and after first fracture of overlying key strata: a case study of Mengjin coal mine

The fracture movement of the overlying key strata (OKS) significantly impacts the permeability evolution of protected coal seam (PDCS), with distinct variation patterns observed before and after the first fracture. However, a spatiotemporal evolution model has not yet been established. This paper proposes a method for constructing a spatiotemporal evolution model of PDCS permeability. A model was developed, applied, and validated based on the conditions of Mengjin coal mine to elucidate the effects of OKS fracture movement on PDCS permeability evolution. The results show that at the test site, only KS₁ fractures. Before the first fracture, the PDCS permeability increased slightly, with a maximum of 6.9 times the initial permeability. Within 6 m of the fracture, the permeability increased sharply by 148–169 times, peaking at 190–3,372 times before the first square dimension. Subsequently, the compaction effect of the goaf became evident, causing the permeability to decrease by 92–1,198 times, while the permeability valley region increased steadily after the second square dimension. The peak permeability period occurred 18–213 m behind the working face. Overall, OKS fracture movement governs the permeability evolution by controlling the stress field and the deformation of the coal and rock structures.