Characteristics of overburden fracture spatial distribution in abandoned mines goaf group

Developing and utilizing the remaining methane resources in abandoned mines could reduce resource waste, protect the environment, and promote the sustainable development of the coal industry. However, determining the distribution characteristics of the fracture space in abandoned mines was the prerequisite and basis for the effective identification of methane-enriched areas and the precise development and utilization of resources. At present, most abandoned mines had goaf group. Compared with the mining of a single working face, the distribution patterns of their overburden fracture fields might be different. Based on this, taking the east area of Panji No.1 Coal Mine in Huainan as the research background, the three-dimension distinct element code discrete element software was used to construct a three-dimensional numerical model of the stope. A calculation program for the fractal dimension of overburden fractures in goaf group was proposed, and the fractal characteristics of the overburden fracture field were analyzed. The coal-rock strata in the dip direction of the goaf group were divided into three zones: the concentrated fracture-development zone (I), the recompaction zone (II), and the top-separation zone (III). The distribution range of the dominant channels for methane migration was determined. Meanwhile, based on the bulking characteristics of the caved rocks in the caved zone and the subsidence characteristics of the overburden in the fractured zone, the fracture porosity distribution function was obtained, and the three-dimensional distribution law of the overburden fracture porosity in goaf group was expounded. The research results indicated that: for each goaf within the goaf group, the fracture porosity of the caved zone decreased continuously under the influence of repeated mining. Its distribution presented a “shovel” shape, with a higher fracture porosity around the goaf and a lower one in the middle of the goaf. The peak value of the fracture porosity reached 32.5% and was located at the upper and lower corners of each goaf. The fracture porosity in the fractured zone continuously increased under the influence of repeated mining. The distribution of fracture porosity in the middle presented a “double-hump” shape. The fracture porosity was large in the outer convex areas and small in the inner concave areas, with a peak fracture porosity reaching 14.3%. The distribution of fracture porosity at the top presented a “single-peak” shape. The fracture porosity was small around the goaf and large in the middle of the goaf, with a peak fracture porosity of 16.7%. The fractal dimension values of overburden fractures and the distribution of fracture porosity on the side of the section coal pillar increased significantly compared with those on the side of the boundary coal pillar. The fractures in the caved zones on both sides of the section coal pillar penetrated each other, forming a fracture-intensive area, which served as the dominant channel for methane migration in the goaf group of abandoned mines. The research results provided a basis for the precise identification of methane-enriched areas in the east area of Panji No.1 Coal Mine and laid a theoretical foundation for the evaluation of methane resources in abandoned mines.