Mechanism of cross-fusion in gas transportation-storage area in fully mechanized mining face of high gas thick coal seam under different advancing speeds

The fracture structure is complex and irregular, which has a decisive influence on the migration and storage of pressure-relief gas in overlying strata. During the advancing process of the fully mechanized mining face in the high-gas thick-coal seam, in order to determine the impact of dynamic change of intersection and fusion of the gas transport and storage area on the gas occurrence, and the gas transport and storage law in the overlying strata under different advancing speeds, the two-dimensional physical similarity simulation was conducted to analyze the fracture distribution characteristics of the overlying strata and the geometric change law of the gas transport-storage area, and explore the change characteristics of the gas transport-storage area with the periodic weighting under different advancing speeds. Additionally, taking the advancing speed of the working face as a key parameter, the quantitative characterization model of the propulsion speed effect of the gas transport-storage area was established, and the symmetrical periodic construction mechanism of the gas transport-storage area under the influence of the advancing speed was revealed. The results show that the inner and outer boundaries and the boundary between the gas transport area and storage area are located in the mutation areas of the fracture openness and through degree respectively. With the increase of the advancing speed, the boundary of the transport-storage area is reduced from the middle of the goaf to both sides. As the working face advances, the gas storage area gradually transits into a migration area, and the two areas cross and fuse with each other. During the whole process, the transport-storage area is in the cycle of “construction-destruction-construction”, and finally form a complete symmetrical elliptical parabolic banded overburden fracture field. The change of fracture entropy increases first and then decreases with the advance of the working face, and the change of fracture rate increases and decreases twice with the advance of the working face respectively, indicating that the gas migration area and the storage area show the dynamic change of “initial formation-cross fusion-regional separation and expansion” with the advance of the working face. In addition, based on the theory of mining-induced fracture elliptic paraboloid zone, the quantitative characterization model of propulsion speed effect of gas transport-storage zone was constructed, and the boundary and state determination process of gas transport-storage zone was established. The cross-fusion evolution mechanism and symmetric periodic construction mechanism of gas transport-storage area under the influence of advancing speed were revealed, which provide a guidance for studying the distribution of pressure-relief gas in goaf and realizing the directional and accurate extraction of gas enrichment zone.