Drilling Mechanism Of Combined Bit In Coal Seam Drilling Pressure Relief And Its Application

Rockbursts are relatively common dynamic phenomena in deep coal mines,and one of the important rockburst preventions is to implement water-injection drilling or pressure-relief drilling in burst danger areas.However,in the process of drilling the working face of the coal seam buried deep,due to the stress of surrounding rocks,drilling dynamic phenomena such as pipe sticking,bailing-up sticking or rod breaking often occur in the holes,which seriously affect the implementation of relevant rockburst preventions.Therefore,how to improve the drilling efficiency in deep coal seams has become the key problem to be solved urgently.Because of its strong adaptability in deep coal seam drilling,drilling bit assembly is often used in deep hole drilling of large diameter drilling pressure relief,water injection pressure relief,blasting pressure relief and other technologies in coal seams with local burst tendencies,but the research of the influence of the structural characteristics of the bit assembly on the performance of the drilled coal body is obviously lagging behind the engineering application.The research on the mechanism of bit drilling in the past mainly took the pressing of bit teeth into the elastic half space body as the force model,and considered the stress distribution of the coal body under the static action of the bit on this basis,and then studied the mechanism of coal fragmentation by combining different strength criteria.This theoretical model can be applied to the type of geological bits with the same stress on each cutter tooth,but for bit assembly with teeth of different size parameters,this model is no longer reasonable because each cutter tooth does not break coal simply by pressing in cutting,but drill by the mechanism of the interaction between the cutter teeth in different directions and the coal body under the stress of surrounding rocks.In the study of the drilling mechanism of the bit assembly used for drilling the working face of the coal seam,the structural characteristics of the bit assembly should be taken as the basis,and the drilling mechanism should be analyzed according to the adaptability under different geological conditions.Therefore,by analyzing the stress state of the bit assembly in the process of drilling,and deducing the contact stress between the bit assembly and the hole walls of the coal body with the theories of contact mechanics,the stress model of the bit assembly has been established.Based on the analysis of the contact between the bit of different geometric parameters and the coal body,a physical model has been established to analyze the factors influencing the drilling efficiency and its change rules,analyzed the development of cracks around the hole walls when drilling the coal body under the stress of the surrounding rocks according to the corresponding rock strength criterion,and established an evaluation system of bit assembly drilling performance.Moreover,the change rules of the drilling performance of bit assembly under different geological conditions has been analyzed through the micro experiment of geometric-parameter-optimized bit assembly drilling the coal body on the basis of the mechanics parameters of the coal body physics.In the end,the drilling effect of the bit assembly is tested in the field,and the drilling adaptability is compared with the research results.The details are as follows:(1)Based on the analysis of the structural characteristics of the bit assembly and taking the kinematic characteristics of "drilling+progressive reaming" of the bit assembly as the research objects,and the mechanical model of bit assembly drilling has been established using contact mechanics theories.Through theoretical calculation,the analytical expressions of torque and feed force of different regions of drill bits have been obtained,and the main factors influencing the bit assembly drilling have been revealed from the essence of the broken coal body,so as to provide theoretical ground for the evaluation system of bit assembly drilling performance.(2)ABAQUS software is used to establish the discrete element simulation model of the drilling process of the bit assembly,the simulation model is used to study the drilling performance influenced by the geometric parameters and the motion parameters of the bit assembly,the influence of the working angle,the distance between the cutters,the arrangement of the cutters,the rotation speed and the feed speed on the stress state of the bit assembly is analyzed,and the influence rule of the structural parameters of bit assembly on its drilling performance has been generalized.(3)A biaxial loading bit assembly drilling experiment platform has also been set up to study,through experiments,the drilling performance of optimized bit assembly under different stress of surrounding rocks and different magnitudes of coal bodies.In the meantime,this paper provides a detailed explanation of the effects of coal mass strength,confining pressure and drilling parameters on the drilling performance of combined drill bits from the aspects of feed force,torque,SEC value for drilling,average cuttings,etc.(4)Taking the impact of surrounding rock stress on coal seam during the drilling process on the working face as a starting point,the paper creates a numerical simulation model of drilling based on the constitutive relationship of HJC material to analyze the influence mechanism of combined drill bit load,drilling torque and weight of drill cuttings under different sizes and distributions of confining pressure,while exploring the mechanism of combined drill bits under confining pressure.(5)In view of the power phenomenon during the process of pressure relief drilling in the deep coal seams,the working face with typical impact tendency—Xinjulong Coal Mine 2302S working face was selected as the site for the drilling test.The optimized combination of drill bits with proper drill rods and rigs was applied in the process.After verification of the feasibility of the improved solution,the water injection pressure relief hole and large-diameter pressure relief hole were drilled separately,and the performance of combined drill bits was verified on torque,weight of drill cuttings,as well as drilling depth after equipment improvement.Figure[86]table[32]reference[161]...