| Roadways gas extraction that are above the roof play key roles in outburst prevention process.On one hand gas pre-drainage boreholes can be drilled from there and on the other hand,the boreholes can be used for goaf gas drainage.These two benefits are obvious.However,due to the lack of convenient and efficient pressure relief measures,gas control in roof roadway is mainly carried out through construction of dense boreholes.In order to solve the problems that are encountered during the gas drainage process,in this paper,we are based on the Pingdingshan coal area.Through field observation,laboratory analysis and model development,the high efficiency gas drainage means and coal stress relief mechanism are obtained.According to the technical difficulties of downhole coal cavitation,the advantages of coal breaking,coal breaking process and force characteristics of the new mechanical cavitation technology in submerged environment were demonstrated.Based on the similarity simulation experiment and numerical simulation analysis of mechanical cavitation,the coal breaking characteristics of the downhole mechanical cavitation tool and the damage and anti-reflection characteristics of the cavitation coal are obtained.At last,a system is developed based on the field trials of downward borehole coal breakage and it is verified through field production.The main findings are as follows:(1)Statistical analysis found that the gas geology of the east and west mines in Pingdingshan mining area is quite different.In the eastern mine,the maximum principal stress is 49 MPa,the maximum gas pressure is 3.5 MPa,and the maximum gas content is 27 m~3/t.Compared with the western mine,the ground stress is about 27 MPa higher,the gas pressure is about 0.8?2.0 MPa higher,and the gas content is about 5?10 m~3/t higher.Under the same buried depth in the same area,the gas pressure and gas content of coal of group Ji are about 0.7 MPa and 6 m~3/t higher than those of group Wu,respectively.The outburst risk is higher in the east than in the west and higher in Ji group of coal than that in Wu.Combining with the gas control mode of typical outburst mines,it is found that roof roadway is used for gas control in coal of Group Wu and Group Ji in western mine,where gas pressure and gas content are relatively low.In the eastern coal mines,bottom-drawing roadways are often used to control gas.That shows that the roof roadway has certain advantages in coal seams that focus on stress relief while taking into account gas extraction.The results show that the difference of coal quality and pore fracture structure between two groups of coal samples with different buried depth is not obvious.Therefore,the different stress environment is the main reason for the difference in gas drainage efficiency.On this basis,a permeability evolution model considering the stress response is established,and combining with the measured permeability variation with the buried depth,it is proved that unloading is the fundamental way to realize efficient gas extraction in high-stress and low-permeability coal seams.(2)When the initial confining pressure is 5 MPa,10 MPa,and 15 MPa,the peak strength of coal going through relieving confining pressure(25 N/s)and adding axial pressure is 41.4%,29.0%,and 34.3%of that of the force path of constant confining pressure and adding axial pressure.And correspondingly,the permeability increases from 119.1 times,75.2 times and 86.8 times to 308.4 times,272.6 times and 183 times after coal sample destruction.The results show that under the condition of discharging confining pressure,the coal is more prone to damage and produces higher anti-reflection effect.However,the peak strength of coal going through relieving confining pressure(50 N/s)and adding axial pressure is 77.7%,77.6%and 62.2%of that under the condition of confining pressure(25 N/s)and adding axial pressure.And the permeability increases from 308.4 times,272.6 times and 183 times to 340.6 times,314.9 times and 342.9 times after coal sample destruction.That means the faster the unloading rate,the more significant the permeability increase.The results of coal permeability measurement with helium show that when the hydrostatic pressure drops from 30 MPa to 2 MPa,the coal permeability increases by 51 times,indicating that unloading can effectively increase the coal permeability.(3)The analysis results of coal breaking depth and coal breaking volume under the downhole environment with traditional hydraulic cavitation technology and new mechanical cavitation technology show that:The efflux propagation velocity decreases significantly in submerged environment,and with the increase of water jet velocity,the coal breaking depth increases,but the effect is not significant.However,the process of mechanical coal breaking is not affected by the submerged environment.At the same time,the coal breaking depth by mechanical cavitation tool is 5.8 times,4.9 times and4.2 times higher than that of hydraulic cavitation at different speeds(170 m/s,190 m/s and 210 m/s).Under the same advancing distance condition,the coal volume of mechanical cavitation tool is 9.7,7.8 and 6.3 times higher than that of hydraulic at different speeds(170 m/s,190 m/s and 210 m/s).The difference of efficiency between the two technologies fully proves that mechanical coal breaking cavitation technology is obviously superior to water jet coal breaking.(4)Based on the structure of the mechanical cavitation cutter,the stress characteristics of the cutter during coal breaking process were obtained by numerical simulation.Combined with similarity simulation experiment of mechanical cavity technology,the splaying process of cavitation cutter can be divided into two stages.The cavitation cutter were splayed with an expansion angle of 6.1°and 46.3°in the first stage and the second stage,respectively,then were fully splayed rapidly in the second stage.Additionally,it is considered that the advance speed during mechanical cavitating should not exceed 20%of the normal drilling speed.The numerical model of coal unloading damage is established combined with similar simulation results.The numerical simulation results shown that the radius of radial stress relief zone increased by 4.8 times from 1.3 m to 6.2 m.The maximum radius of plastic failure zone increased by 12.5 times from 0.3 m to 3.75 m.The range with a ten-fold increase in permeability increased by 6.3 times from 0.95 m to 6 m,and the effective extraction radius within30?180 d increased by 1.94?2.14 times.(5)The parameters of mechanical cavitation technology(propulsion pressure is 8MPa,rotation speed is 90 r/min,propulsion speed is 0.2 m/s)and slag removal(pumping flow range 550?600 L/min)were determined through field tests.In the field investigation,coal flush weight of the mechanical cavitating borehole section was about262 kg/m,which is greater than the value of calculation of 255 kg/m,and the diameter of cavity is larger than design value of 500 mm.The coal permeability increased by23.9 times from 0.0018 m D to 0.0431 m D after cavitation after mechanical cavitating and gas emission per hundred meters of incipient borehole increased by 5.8 times from0.36(m~3/min·hm)to 2.1 m~3/(min·hm).In addition,the number of mechanical cavitation boreholes reduced by 70%compared with ordinary boreholes and the gas drainage time of reaching expected level was decreased from 90 d to 70 d.After gas drainage,the tunnel excavation rate was increased from 4.2 m/d to 4.6 m/d and the maximum drilling cutting weight was reduced from 4.5 kg/m to 3.9 kg/m,indicating that the risk of coal and gas outburst was significantly reduced during tunneling.This work contains 126 figures,27 tables,and 184 references. |
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