Research And Application Of Permeability Enhancement Effect Of Coal Seam Under Different Layered Pressure Relief Conditions

Aiming at the difficulty of coal transportation and stress concentration in the edge caused by the pursuit of large coal output by conventional hydraulic punching technology,a layered pressure relief hydraulic punch technology is proposed.Based on the principle of pressure relief of protective layer,a soft layer is punched out,which enables the upper and lower coal body to be depressurized and penetrated.On this basis,through similar simulation experiments,multiphysicals theoretical analysis,numerical simulation and the method of field test verification are used to compare the deformation characteristics of coal rock loaded by boreholes and different deformation characteristics of different coal seam pressure relief and permeability enhancement results under the condition of changing the parameters of layered pressure relief jet.In order to obtain the deformation characteristics of loaded coal and rock under different layered unloading conditions,the similar simulation experiments of loaded coal and rock deformation characteristics of layered pressure relief punching were carried out.Based on the conclusion that the cross section of layered pressure relief punching borehole is an irregular ellipse with a circular end at the top,the cross section of the borehole is simplified as a rectangle,and then the deformation characteristics of loaded coal and rock were studied.On the basis of establishing the fluid structure coupling model of gas drainage,the deformation characteristic results of UDEC numerical simulation are taken as the initial physical model of COMSOL software to carry out the numerical simulation of gas drainage.The results show that:with the change of jet step,the coal and rock will appear bending deformation,separation,fracture and collapse,and the specific deformation characteristics of the coal and rock are closely related to the spatial characteristics of the boreholes.When the space height of the boreholes are small,the coal and rock will appear bending or fracture,but not collapse.When the space height of the boreholes are large,the coal and rock will appear bending or fracture.And UDEC simulation results can be basically consistent with similar experimental results;in the simulation of coal seam gas drainage by COMSOL software,when the jet thickness is fixed and the jet step distance is 5.6m,the effective drainage radius and effective drainage range reach the maximum after 180 days of drainage,and the effective drainage radius is 9.8m.When the jet step distance is fixed,in the four groups of jet thickness,when the jet thickness is the largest（the jet thickness is the largest）,the results show that the maximum extraction radius is 3.1m after 180 days of extraction,and there is a strong positive correlation between permeability and porosity,but a strong negative correlation between permeability and pressure.In the horizontal direction,the change range of coal porosity is 0.024-0.041,and the change range of permeability is 1×10^-17m²-4.8×10^-17m².In the vertical direction,with the increase of the distance from the boreholes,the change range of coal porosity is 0.056-0.041,and the change range of permeability is 13×10^-17m²-4.8×10^-17m².In the field experiment of Xin’an Coal Mine,the advantages of layered pressure relief punching compared with conventional punching are compared and verified.The overall average coal output of the test boreholes is about 0.73 of that of the conventional punching boreholes,and the effective drainage time of the average gas concentration above 30%is more than 7 times of that of the conventional punching boreholes.At the same time,with the increase of jet thickness,the average gas flow and gas concentration will increase,and the drainage effect is better.