Objective The mining height of coal seams along fully mechanized mining faces significantly influences the formation and evolution of fractures in the overburden, as well as the characteristics of gas migration and storage zones. Investigating the influential patterns under varying mining heights will provide guidance for the arrangement of gas extraction boreholes, thus enhancing gas extraction efficiency.
Methods This study investigated mining face 203 in the Tianchi Coal Mine in Heshun County, Shanxi Province. By integrating methods such as physical simulation experiments with similar materials, theoretical analysis, and engineering verification, this study systematically examined the developmental characteristics of fractures in the overburden and the dynamic evolutionary patterns of gas migration and storage areas under three different mining heights (i.e., 2 m, 4 m, and 6 m).
Results and Conclusions Based on the thresholds of abrupt changes in the fracture aperture (1 m) and penetration degree (0.68) of the overburden, the goal was divided into three zone: the overburden compaction zone (fracture aperture: ≤1 m, and connectivity ≤0.1), the gas migration zone (fracture aperture: >1 m, and penetration degree ≥0.68), and the gas storage zone (fracture aperture: >1 m, and penetration degree <0.68). Analysis of fracture development under different mining heights reveals that the vertical and horizontal heights of fractures, as well as the frequency of their abrupt changes, significantly increased with the mining height. The cross and fusion states of the gas migration and storage zones were characterized based on the degree of synchronicity of variations in the fracture rate and entropy. These zones experienced four stages of evolution under periodic weighting: integrated migration and storage zones and the preliminary formation, cross and fusion, and separation of migration and storage zones sequentially. A comparison indicates that the horizon and extent of the cross and fusion were positively correlated with the mining height. Based on the elliptic paraboloid zone theory, this study established mathematical models to describe the impacts of the mining height on the cross and fusion of the migration and storage zones of pressure relief gas. Furthermore, it proposed a process for identifying the boundaries of the migration and storage zones using the characteristic parameters of fractures, determining the middle-upper part of the gas migration zone as the optimal locations of gas extraction boreholes. Field tests indicated that gas drainage accounted for up to 52.3% on average when directional boreholes were arranged in the middle-upper part of the migration zone. This result verified he applicability of the proposed theoretical models. The results of this study will provide a theoretical basis for the efficient extraction of pressure relief gas.
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