Objective and Methods The spatial distribution and long-term evolution of effective migration pathways determine the migration pathways of residual coalbed methane (CBM) in abandoned mines. Furthermore, they provide an important basis for selecting the optimal target areas for CBM extraction drilling in abandoned mines and maximizing extraction efficiency. This study investigated the first mining area at the first horizontal elevation of the Songzao Coal Mine in Chongqing. Using a comprehensive approach that integrated theoretical analysis, numerical simulations, and physical simulation experiments using similar materials, this study systematically examined the distribution patterns of effective CBM migration pathways in abandoned mines under the mining of multiple inclined coal seams. Furthermore, it proposed a scheme for selecting the optimal target areas for CBM extraction.
Results and conclusions Theoretical calculations, numerical simulations, and physical simulation experiments using similar materials revealed that the fracture zones after the repeated mining of coal seams K2b, K1, and K3b exhibited maximum heights of 72.89 m, 76.4 m, and 77.3 m, respectively, suggesting a high consistency. The stress fields in the overburden underwent three stages of evolution: formation of arch-shaped pressure relief zones, asymmetric stress transfer, and dynamic stress equilibrium. This process culminated in the formation of a three-dimensional asymmetric structure consisting of a pressure relief dome of the roof and lateral stress shells. The fracture development in the overburden followed a coevolutionary pattern characterized by mining predominance, spatial differentiation, and temporal accumulation sequentially. Specifically, in the initial stage, the mining-induced rapid migration of the overburden led to the formation of a basic fracture network. In the middle stage, fractures experienced superimposition and propagation under the influence of repeated mining. In the late stage, three-dimensional O-shaped fracture traps were finally formed after over 10 years of consolidation and recovery. As a result, the fracture network entered a relatively stable stage, providing effective pathways for CBM migration and forming a CBM enrichment area under the sealing of the overlying thick and hard limestones. After long-term compaction, the fracture heights decreased to 67.97 m and the fractures presented a differentiated framework featuring a symmetric trapezoidal shape along strikes and an inclined trapezoidal shape in the dip directions. In combination with the CBM migration patterns, it is recommended that the optimal target area for CBM extraction should be the upper corner within burial depths ranging from 6.41 m to 36.70 m above the goaf floor of coal seam K3b.
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