5 / 2023-04-19 17:37:07

Characteristics of low gas permeability area in overlying strata under different dip angles of coal seam

Up-dip,Crack propagations,Dip effect model,Elliptic paraboloid zone,High-level boreholes

In order to study the fracture development law of overlying strata under different coal seam angles, the characteristics of low permeability zone of overlying strata are analyzed. Then reasonable layout of the goaf drilling field, to ensure that the working face and upper corner of the gas content does not exceed the limit, to ensure the safety of the site staff. Based on the experimental background of a high gas coal mine in Heshun County, Shanxi Province, the influence of coal seam dip angle on the evolution law of low gas permeability zone of inclined the up-dip fully mechanized mining face is discussed through physical similarity simulation test. Based on theoretical analysis, the crack propagations law of fractures and mechanical properties of overlying strata in low gas permeable zone under different coal seam dip angles are studied. The results show that with the continuous mining of coal seam, the amount of separated layer presents an "M" shape as a whole. The fracture field can be divided into high permeability zone and low permeability zone according to the abrupt change of separation volume. The separation rate of low permeability zone is much lower than that of high permeability zone. According to the simulation results of similar materials, it is found that in the process of coal mining, the mechanical balance of overlying strata is broken, and the stress concentration area appears at the same time of floor stress redistribution. The fracture field of overlying rock forms three regions. The fracture height of 30° is greater than that of 0° and 15°, that is, with the increase of coal seam dip Angle, the average weighted posture gradually decreases. With the increase of coal seam dip Angle, the width and height of low breathable zone increase gradually. And with the increase of the caving Angle on one side of the working face, the caving Angle on one side of the trough face decreases. The two boundaries of the low permeability zone show an elliptical parabolic shape. The width and height of low breathable zone are affected by the mining distance and dip Angle of coal seam. Therefore, the dip effect model of the influence of different angles on the low permeability zone is established based on the theory and experimental results of mining-induced fracture elliptic paraboloid zone. Finally, according to the results of physical similarity simulation and theoretical analysis, high-level boreholes is designed on the test face. According to the field monitoring data, it is considered that the gas density of upper corner coal seam and mining roadway are well controlled. The maximum gas concentration in the upper corner is 0.35%, and that in the return air lane is 0.36%, both of which are less than 1%. According to the results of field test, reasonable arrangement of gas extraction boreholes can ensure the safety of coal mine production. In addition, the study on the characteristics of low permeability zone under different coal seam angles provides a reliable theoretical basis for accurately locating the high concentration gas zone in the overburden fracture zone.