Study On Slags Entering And Discharging Rules Of Hole-shielding And Pressure-relief Drilling Tool In Outburst Coal Seam

Drilling in outburst coal seam is a recognized technical problem in the industry.In borehole drilling process,borehole collapse and gas outburst phenomenon are frequent.It makes drilling process and slags discharging difficult,resulting in shallow borehole depth,low efficiency and more safety accidents,which seriously affects the efficiency of gas extraction and restricts the safe and efficient production of coal mine.In view of the slags discharging problem in outburst coal seam borehole drilling,this dissertation puts forward a hole-shielding and pressure-relief(SHRP)drilling technology with reliable internal slag discharging channel.It is mainly through the three-channel SHRP drill pipe appearance shielding borehole to prevent borehole wall from collapsing excessively,and drilling slags enter into internal slag discharging channel in center of drill pipe through the sieve holes on drill pipe appearance and discharge outward under the action of air-flow,so as to reduce the force on drill pipe and avoid drill pipe stuck.For this new drilling technology,the primary core problem is to clarify how the slags outside of drill pipe enters into sieve holes and discharges from inner hole of drill pipe.Therefor,the SHRP drilling tools were trial-produced,slags entering and slags discharging physical simulation test devices were self-developed,and slags entering rules and slags discharging rules of SHRP drilling tool have been explored by theoretical analysis,physical simulation tests and numerical simulation calculation.And the strength analysis and parameter optimization of drill pipe were also conducted.Research results lay a theoretical foundation for application and improvement of SHRP drilling technology in outburst coal seam.The main research results obtained are shown as follows:(1)The causes of borehole wall collapse and borehole deviation and its harms to drilling construction in outburst coal seam are analyzed systematically,and based on this,SHRP drilling technology is put forward accordingly.The processes of slags entering into sieve holes and slags discharging of SHRP drilling tool are analyzed by building mechanical models.It is preliminarily concluded that the upper middle areas of horizontal borehole is the slags entering region(SER).Only when sieve holes passes through the SER with drill pipe rotation,drilling slags outside of drill pipe can enter into sieve holes.Moreover,the change of drill pipe rotation speed and external squeeze force will change the range of SER.Wind velocity,amount and particle size of slags entering into sieve holes are the key factors affecting the migration of drilling slags in inner hole of SHRP drilling tool,so controlling the amount and particle size of slags entering into sieve holes is helpful to improve slags discharging effect of drilling tool.(2)Through physical simulation test and gas-solid coupling numerical simulation of slags entering,the factors affecting slags entering into sieve holes and its influence rules of SHRP drilling tool have been studied.The studies find that squeeze force and gravity are the main forces of slags outside of drill pipe entering into sieve holes.The increase of squeeze force on drilling slags outside of drill pipe can increase the amount and particle size of slags entering into sieve holes.Moreover,setting grooves on the surface of drill pipe,increasing sieve hole diameter and reducing sieve hole depth can observably improve the effect of slags entering into sieve holes.Besides,SHRP drill pipe has a critical rotation speed with the best effect on slags entering into sieve holes.When drill pipe rotation speed is greater than the critical rotation speed,the effect of slags entering into sieve holes decreases with drill pipe rotation speed increasing.The particle size of slags entering into sieve holes also decreases with drill pipe rotation speed increasing,so the amount and particle size of slags entering into sieve holes can be controlled by adjusting SHRP drill pipe rotation speed.The problem of sieve hole blockage can be effectively solved by increasing the diameter-depth ratio of sieve hole.(3)Through physical simulation test and gas-solid coupling numerical simulation of slags discharging,the factors affecting slags discharging and its influence rules of SHRP drilling tool have been studied.The studies find that when wind velocity is low,the large particle size drilling slags will be deposited at the bottom of drill pipe inner hole,which reduces the cross-section of internal slag discharging channel.Increasing wind velocity can reduce the deposition of drilling slags in inner hole of drill pipe,and increase the migration velocity and discharging amount of drilling slags.The increase of drilling slags amount and deposition of large particle size slags in inner hole of drill pipe will cause the reduction of overall migration velocity of drilling slags,which is not conducive to slags discharging.Drill pipe rotation speed has little influence on the migration discharging of slag particles in inner hole of drill pipe,but slags discharging can be indirectly affected by adjusting drill pipe rotation speed to control the effect of slags entering into sieve holes.Slags entering into sieve holes makes drilling slags of different velocities exist in the inner hole of drill pipe,which leads to drilling slags colliding easily.It is not conducive to the continuous acceleration or stable migration of drilling slags and reduces the overall migration velocity of drilling slags in inner hole of drill pipe.SHRP drilling tool helps to overcome the harm of drilling cave on slags discharging,which is mainly because that drilling slags in inner hole of drill pipe can overcome the adverse effects of drilling cave split-flow deceleration and pass through drilling cave area along the equal cross-section and linear slag discharging channel under the joint actions of its own motion inertia and air-flow.(4)Through numerical simulation and theoretical calculation,the strength of SHRP drill pipe was analyzed.It is found that that sieve hole diameter,sieve hole spacing and axial groove depth are the key parameters affecting the strength of drill pipe,and drill pipe is most stressed at the intersection of sieve hole,axial groove and spiral groove.Based on the slags entering and discharging rules and strength analysis results of SHRP drilling tool,the structure parameters of drill pipe were optimized,and the dynamic regulation processes of drilling resistance and slags discharging condition of borehole by adjusting SHRP drill pipe rotation speed in borehole construction process were discussed.SHRP drilling tool has significant advantages in reliability of slags discharging channel,slags controllability and overcoming the harm of drilling cave,which are important for improving the borehole depth in outburst coal seam.The dissertation includes 146 figures,18 tables and cites 172 references.