| The prevention and control of coal and gas outburst in underground coal mines has always attracted the attention of scholars at home and abroad.At present,coal seam gas pre-drainage work is still the main measure of coal seam gas prevention and control,especially for outburst coal seams without protective layer mining conditions,but it is limited by the performance of ordinary drilling rigs.For example,before treatment,it is necessary to build a bottom(high)drainage roadway before drilling construction,and the length of drilling construction is short,and the number of drilling construction cycles is many,which seriously affects the treatment time and cost.With the development of directional long drilling and hydraulic punching technology,it has gradually been recognized by many enterprises.Directional long drilling has the technical advantages of long distance and precise positioning,and hydraulic punching has convenient and efficient antireflection effect.If the two technologies are used in combination,for the outburst coal seam without protective layer mining conditions,it can not only realize the gas control technology of long-distance pressure relief and permeability enhancement pre-drainage coal seam,but also reduce the number of construction cycles,shorten the gas drainage time,and achieve the purpose of efficient governance.Therefore,this paper takes the outburst coal seam of Xiangshan Mine as the research object,combined with the theory of coal seam gas occurrence and flow,rock mechanics,etc.,analyzes the geological characteristics of the mining area and the basic characteristics of the coal seam in the test area,demonstrates the necessity of the pressure relief and gas drainage technology of the directional long borehole in the coal seam,and puts forward the pressure relief and gas drainage technology of the directional long borehole in the coal seam.Combined with the numerical simulation software,the evolution law of the stress and plastic damage of the surrounding coal body before and after the punching is analyzed,and the numerical results of the pressure relief effect are obtained.Finally,through the field test,the effective extraction radius and coal output before and after punching are investigated and the numerical simulation results are verified.The drilling field is established by using the technology of punching and pressure relief gas extraction with directional long boreholes along the seam,and the effect of the technology on strengthening gas extraction is obtained through the systematic effect investigation.The main conclusions of this paper are as follows:(1)Through the analysis of gas occurrence law,drilling pressure relief theory,water jet and jet coal breaking mechanism,the law of limit stress state and coal energy dissipation process in stress transfer process is clarified.The submerged water jet structure and its whole process are mastered.Combined with the basic geological characteristics of the mining area and the basic characteristics of the coal seam,the reasons and necessity of selecting the gas control method in the stress concentration area of the low permeability coal seam to select the gas drainage technology of the directional long borehole punching and pressure relief are demonstrated.(2)The numerical simulation results of pressure relief and damage before and after punching of long directional boreholes along the bedding show that when the coal output is 0.4t/m,a hole with a diameter of about 0.6m can be generated,and the diameter of the borehole can be expanded to 696mm.Compared with the borehole with a diameter of 96mm,the maximum principal stress relief range will increase from 0.68m to 3.6m,with an increase of about 5.3 times,and the diameter of the plastic damage area will increase from 0.2m to 1.2m,with an increase of about 6times.The cavitation technology significantly affects the in-situ stress level of the surrounding coal,and the plastic failure range is greatly improved.The simulation comparison of different coal output shows that for every 0.3 t/m increase in coal output,the radius of the drilling hole is expanded by 0.1m,the radius of the drilling pressure relief area is expanded by about 1.5 times,and the diameter of the plastic damage area is increased by about 1.2 times.(3)In order to support the simulation results,the borehole extraction radius and coal output before and after hydraulic punching were investigated.Under the condition that the expected coal output target is 0.5 t/m,the actual coal output per meter is 0.5-0.6t,and the borehole radius can reach 0.38-0.41m,which achieves the expected goal.The actual pressure relief diameter is about 4.08m,which is different from the numerical simulation results.The main reason is that the in-situ stress will continue to produce creep and migration during the actual cave-making process.In the simulation,only the exposed area of the coal body and the failure pressure relief range of the balanced coal body are increased,but it does not affect the judgment of the influence range of pressure relief before and after punching and cave-making by simulation.Through comparison,the average daily extraction amount of punching boreholes is about 4.39 times that of ordinary boreholes,and the effective extraction radius of punching boreholes is between 1.96 times and 1.14 times higher than that of ordinary boreholes in 20 days to 180 days,and the extraction technical indicators have been greatly improved.Therefore,it fully illustrates the absolute advantage of punching hole drilling in extraction efficiency.(4)According to the field technical scheme,the test drilling field is constructed and the actual extraction capacity of the technology is verified.The statistical analysis data show that the average gas extraction concentration in the drilling field is about8.55%,the highest mixed extraction concentration is 21%,the average daily extraction purity is 1176.69m~3,the highest extraction purity is 6695m~3,and the total extraction purity in the drilling field is 176504m~3.In the comparison of the extraction concentration of the holes,it was found that the 1#and 2#holes were 1.64 times and1.56 times of the 4#holes,respectively.In the comparison of single holes,it was found that there was no significant difference between 5#and 7#single holes.The 3#hole-making hole was significantly better than the single hole without hole-making,and the extraction efficiency was increased by about 1.46 times.After the application of hydraulic punching hole-making technology,the treatment cost was saved by about15%-22.5%.After the treatment,the regional roadway was successfully drivage,the average driving speed was 5.7m/d,and the amount of drilling cuttings was reduced from the highest 5.4kg/m to 3.2kg/m.The statistical amount of drilling cuttings and the desorption index of drilling cuttings did not exceed the standard,and the gas concentration of the return air flow did not exceed the standard in the excavation.This work contains 44 figures,9 tables and 98 references. |
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