Study On Gas Flow Of Borehole Drainage In Anisotropic Coal Seam

Natural coal seam is a kind of bedding structure with complex pore and fracture space,and its permeability and mechanical properties show obvious differences in different bedding directions,which will inevitably affect the gas flow in coal seam.Therefore,it is of great significance to explore the gas flow rules in anisotropic coal seam for reducing coal mine safety accidents and improving gas extraction efficiency.In this paper,the cylindrical anthracite samples with axes parallel and perpendicular to the bedding plane were made respectively,and the triaxial seepage compression tests were carried out to compare and analyze the differences of mechanical properties and permeability characteristics between the two kinds of coal samples.Based on this and combined with previous research results,the variation laws of gas pressure,effective drainage radius and outburst elimination range of single borehole and multi borehole drainage in anisotropic coal seam were analyzed by numerical simulation.Through the analysis of the test and numerical simulation results,the following conclusions were drawn:(1)The triaxial compression test shows that the mechanical properties of coal samples with bedding have obvious anisotropic characteristics.The elastic modulus of coal samples with parallel bedding is larger than that of coal samples with vertical bedding,but the peak strength is opposite.The increase of confining pressure has obvious inhibition effect on the axial deformation of coal samples with vertical bedding,while it has more significant inhibition effect on the radial deformation of coal samples with parallel bedding.As a result,the Poisson's ratio of coal samples with vertical bedding increases with the increase of confining pressure,while that of coal samples with parallel bedding decreases with the increase of confining pressure.In addition,the internal friction angle of coal samples with vertical bedding is larger than that of coal samples with parallel bedding,but the cohesion of the former is smaller than that of the latter.(2)The seepage test shows that the permeability of bedding coal sample also has obvious anisotropic characteristics.The results show that there is an obvious Klinkenberg effect in the gas seepage of two kinds of coal samples;the absolute permeability of coal samples with parallel bedding is greater than that of coal samples with vertical bedding,but the Klinkenberg factor is less than that of coal samples with vertical bedding.(3)The simulation results of single borehole drainage show that the greater the permeability anisotropy degree,the slower the gas pressure drop,and the worse the effect of extending the drainage time to expand the effective drainage radius or outburst elimination range and increasing the borehole length to expand the outburst elimination range.When the borehole does not penetrate the coal seam,the difference of effective extraction radius in different borehole depths decreases with the increase of permeability anisotropy degree.In addition,the degree of anisotropy of Klinkenberg factor has little effect on coal seam gas drainage,while the anisotropy of elastic parameters almost has no effect on coal seam gas drainage.(4)The simulation results of multi-hole drainage show that the superposition effect can be enhanced by increasing the number of boreholes or decreasing the spacing of boreholes,but the effect gradually weakens with the increase of permeability anisotropy degree.The expansion of outburst elimination area has experienced three stages: slow growth,rapid growth and stable growth.The greater the degree of permeability anisotropy,the smaller the growth rate of the first two stages,and the longer the time to reach the stable growth stage,and the growth rate of the stable growth stage is mainly determined by the permeability parallel to the bedding direction.In addition,the larger the borehole spacing,the smaller the range of outburst elimination in the slow growth and rapid growth stages,and the larger the range of outburst elimination in the stable growth stage.