| Mostly in the form of coal seam groups,the geological conditions of coal seams in Guizhou region of China are very complicated,coal seams or composite coal seams coexisting with structural coal and primary coal have been formed,the permeability coefficient of which is very small,causing the gas difficult to extract and a high degree of outstanding danger.Practice has proved that the high-pressure water jet punching technology can effectively reduce the pressure and increase the permeability of coal seams,and significantly improve the gas drainage effect.By drilling through the layer to the target coal seam,the high-pressure rotating water jet out a large number of soft coal bodies and gas,forming a larger diameter cavity in the coal seam,this method is so called the high-pressure water jet punching technology.It provides the expansion and deformation of the coal body space,on the one hand,increases the permeability of coals around the cavern on the other hand,thus improving the gas drainage efficiency,thereby eliminating the danger of coal and gas outburst.The coal body tend to become soft and fragile under the action of special geological structures,leading to changes in physical and microstructure of the coal body,and the pore and fissure system is more developed.Respectively the micropore and small pore volume of soft coal are 1.37 times and 1.72 times that of hard coal,the specific surface area ratio is 1.43 times and 1.59 times of that hard coal,respectively.Increasing the pore volume of the coal body 's micropores and small pores results in an increase in the capacity of adsorption and desorption.The initial desorption speed of soft coal is obviously higher comparing to hard coal,the desorption amount of soft coal in the 5 minutes before the desorption experiment has reached 50% of the total desorption amount,while hard coal needs 25 minutes to reach half of the total desorption amount.The average gas release initial velocity ?p is 37.5 mmHg,which is 2.03 times that of hard coal.The average firmness coefficient of soft coal in composite coal seam is 0.5,and the average firmness coefficient of hard coal is 1.9 which shows a significant difference.The numerical simulation method was used to reveal the pressure relief and permeability enhancement mechanism after hydraulic punching from the plastic zone volume,stress distribution and gas pressure of the coal around the single-hole and double-hole caverns.The plastic zone and the secondary stress distribution also appear after hydraulic punching,the horizontal stress of the coal body between the caves decreases and the vertical stress increases,the stress concentration phenomenon occurs,and then it is shifted to the outside by the crushing and crushing action of the ground stress.From the perspective of the same radius,different cavity spacing and the same cavity spacing,different radius,the range of the pressure relief zone and the gas extraction effect are comprehensively considered.It is determined that when the punching radius is 0.5m,the optimal spacing is 8.48 m.Field tests were carried out on the 4 # outburst seam of Xintian Coal Mine using hydraulic punching technology.The results show that after high-pressure hydraulic punching,the hydraulic punching time for high-concentration gas extraction increased by 5 times,and the average 100-meter pure quantity in 180 days increased about 3 times.The residual gas content and residual gas pressure of the 4 # coal seam within the control range of 785 m to 940 m of the 1402 cut-out both decreased significantly,and the gas drainage effect was significantly improved,that is,the prominent danger of the coal seam was effectively eliminated within the judgment range.The paper has 43 pictures,14 tables,and 90 references. |
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