Research And Application Of Cinder Transportation Laws For Air Directional Drilling In Broken Soft Coal Seams

Air directional drilling technology plays an increasingly important role in gas extraction of broken soft coal seam in coal mine because of its small disturbance to coal seam.However,air directional drilling technology still has limitations such as poor slag discharge in lower inclined holes and lack of theoretical guidance for parameter selection.In view of above problems,this paper uses the theory of fluid mechanics and air conveying,through theoretical analysis,numerical simulation,field test,etc.,to derive the quantitative calculation formula of minimum wind pressure and critical wind speed,and uses Fluent software to simulate the coal slag migration law under different working conditions.Finally,the rationality of the formula is verified in Wangpo Coal Mine.(1)The role of physical properties of nitrogen in the process of cinder transport is explained.Among them,the flame retardancy of nitrogen provides a guarantee for safe and efficient drilling;the suspension,rheology and compressibility provide the basic conditions for cinder transport.The cinder particle density sedimentation test and particle size sieving test were conducted to obtain the test values of the above parameters.(2)The suspension motion state of single particle and particle swarm is studied respectively,and the function relationship between suspension velocity and migration distance is deduced.The pressure loss of gas-solid two-phase flow in annular space is studied.The theoretical calculation formulas of two key parameters of air directional drilling,air pressure and air volume,are deduced,and the influencing factors are classified and analyzed.The hole depth is positively correlated with the wind pressure and positively correlated with the power law of air volume.The particle size of coal cinder is positively correlated with air pressure and positively correlated with air volume.The borehole inclination angle is negatively correlated with the sinusoidal power law of wind pressure and wind volume.The density of coal slag is positively correlated with wind pressure and wind volume.The solid-gas ratio is linearly positively correlated with wind pressure and positively correlated with wind volume power law.(3)Through numerical simulation,the cinder transport law was observed under different hole depth,cinder particle size and solid to gas ratio conditions.The results show that the cinder particles are mainly transported in a complex spiral pattern in the annular space gap,while there are collisions with the hole wall and drill pipe in the radial direction.The fluidization process of cinder at the bottom of the hole under the action of airflow is studied,and the suspension state is completely reached when the time t=6s under the given initial conditions.(4)The air pressure and air volume formulae were verified by combining the actual drilling data in the field,and the error between the theoretical air pressure and air volume and the actual data was within 10%,and the formulae had certain guiding properties.Further analysis of the influence weights of particle size,density,hole depth and inclination angle changes on wind pressure and air volume showed that the particle size had the greatest influence on the values of wind pressure and air volume.The research results of this paper provide guidance for improving the efficiency of air directional drilling and optimizing drilling parameters,and have theoretical guidance and engineering reference significance for the research and application of dynamic directional drilling technology for broken soft coalbed methane.