| Hydraulic punching is an effective measure for depressurizing and increasing permeability of coal seams.It has been widely used in gas prevention and control.However,the current conventional hydraulic punching pursues large coal output,which makes the coal out of the borehole not smooth,the transportation of coal slag difficult,and the stress concentration is easy to be caused at the edge of the punching hole.Based on this,inspired by the mining of the protective layer,this paper puts forward the technical idea of layer pressure relief by water jet in the coal seam:Selecting a weak layer in the coal seam,and using high-pressure water jet to jet out the coal in the form of porous and multi-point.The interconnected plum-shaped horizontal cavities are formed to connect the entire weak stratification,which promotes deformation of the upper and lower coal bodies,thereby increasing the permeability of the entire coal layer and improving the extraction effect.The related research on stratiform pressure relief and permeability enhancement effect of water jet in coal seam is carried out through the use of theoretical analysis,similar simulation,numerical simulation,and field investigation method,the following research has been carried out:In order to verify the feasibility of the technical idea that is stratiform pressure relief and permeability enhancement effect of water jet in coal seam and explore the failure characteristics of coal body between jet holes,we have carried out the similar simulation test of the failure characteristics of the coal body under the condition of loading.The experimental results show that when the jet hole spacing is less than or equal to 8cm,the coal body is severely broken and the coal body is decompressed under stress loading conditions.The displacement of coal seam roof and floor increases with the decrease of jet hole spacing.Establishing a theoretical model of fluid-solid coupling of gas-bearing coal based on the characteristics of stress field,deformation field and gas seepage field.Simulate and study the permeability improved effect of pressure relief in different water jet layers,and compare with conventional hydraulic punching with the help of COMSOL simulation software.The following conclusions are obtained:(1)The equivalent stress space distribution of layered water jets in the coal seam is "petal-shaped",and the6-petal flower shape formed by 6 jet branches is more reasonable.(2)Under the same coal output condition,the effective extraction volume of layered pressure relief of upper,middle and lower stratified water jet is 5.02 times,5.84 times and 5.02 times of the conventional hydraulic punching respectively at 90 days of extraction.(3)With the extension of the extraction time,the influence range of the extraction gradually increases.When the extraction time reaches 180 days,the effective extraction radius can reach 8.56 m by using the stratified water jet stratiform pressure relief with the coal output rate of 2%,which is 1.71 times that of conventional hydraulic punching of the same scale in the same period.In order to verify the feasibility of stratiform pressure relief and permeability enhancement effect of water jet in coal seam,we have carried out relevant tests in the middle floor tunnel of Xinyi Coal Mine 11090.The test found that the gas drainage effect of layered pressure relief of the upper layer water jet with smaller coal output is better than that of conventional hydraulic punching.The initial concentration of the two is about the same.With the extension of extraction time,conventional hydraulic punching usually decreases from high concentration to low concentration(less than10%)and remains stable within 10-20 days.However,the extraction concentration of water jet layered pressure relief will rise twice,and the maximum increased concentration can reach 60.7%,and the number of days to maintain the gas concentration above 30% is usually twice that of conventional drilling. |
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