The Mechanisms Of Impacting And Drilling In Coal By Combined Jets And Its Technology Application

The primary technique of coalbed methane(CBM)drainage in Chinese coal mine is to drill drainage boreholes into coal seams.The cross-layer vertical drilling technology is facing some problems of the short hole length in coal seam but long length in rock and the limited extraction range.The horizontal boreholes are usually drilled from the roadways in coal seams,and it means that the roadways must be tunneled before the gas extraction.However,the mining technology without roadway in coal seams is advantageous to safe production,so a kind of ultra-short radius steering drilling technique that can drill horizontal boreholes in coal seam from the cross-layer vertical borehole is necessary.When using the traditional mechanical steering drilling method,the steering curvature is so large that the steering process must start in rock and the drilling length in rock is large.The water jet drilling technique,that uses high-pressure hose as power source and a small drilling bit of simple structure that can turn easily,provides a new way to solve this problem.The previous water jet drilling techniques usually adopt combined jet,rotary jet,or mixed jet,and combined jet has more advantages in terms of ultra-short radius steering drilling due to the drilling bit of simple structure.The rock-breaking efficiency of combined jet depends on the arrangement of jet parameters,and the rock-breaking and drilling mechanisms are unknown,which hinders the development of combined jet technique.Based on the theoretical analysis and numerical simulation combining to the coal drilling experiments and CT test,the impact breaking mechanism of combined jet on coal,the self-propelling drilling mechanism of combined jet bit,and the ultra-short radius steering drilling technique were studied,and the main achievements are as follows:(1)Based on the jet impact properties and the mechanical characteristics of coal,the process of water jet breaking coal and its mechanism were studied.Coal damages mainly occured in the water-hammer pressure stage.Under the action of jet impact,coal responses contained two aspects.One is the water-hammer pressure breaking coal directly.Through analyses on water-hammer pressure and action time,the threshold condition of coal breaking under the jet impact was obtained,and the failure criterion of coal fracture and yield was established.Combining with the coal-breaking experiments,the volume calculation model of jet breaking coal was obtained.The other is that the stress wave was generated in coal under the effect of water jet impact,and it can promote the propagation of microcracks inside coal and thus form crack networks composed by circumferential cracks and radial cracks.(2)Revealing the mechanism of combined jet impacting and breaking coal and rock.The impact characteristics of single jet stream were studied.Combining with the interaction theory of shock wave,the stress wave interference phenomena when combined jet impacted the target body were elaborated.Based on the coal-breaking experiments,the mechanism of combined jet impacting and breaking coal was revealed.Using CT scan method,the relationships between the length and angle of conical cracks and jet velocity and nozzle diameter were obtained,and a jet combination and rock-breaking method that can make full use of crack propagation was proposed,and the calculation model of combined jet spacing was established.Meanwhile,the pressure distribution on the target surface impacted by combined jet was researched by numerical simulation,and the results revealed the rock-breaking mechanism of combined jet under dynamic target distances.(3)The self-propelling drilling mechanism of combined jet and the stability of hole wall were analyzed.Based on the rock-breaking mechanism of combined jet,the combination self-propelling bit was designed.By conducting self-propelling drilling experiments,the hole drilling principle was studied,and the structures of self-propelling bit were optimized.The hole forms were quantitatively analyzed based on the hole wall roughness.By conducting the true-triaxial hole wall destructive tests,combining with the stress distribution around the horizontal drilling in coal seam,the hydraulic hole stability under the real stress state was analyzed.Results show that the hole wall roughness have a significant influence on the stability of hole wall in coal seam.The increase of hole wall roughness causes hole wall stability reducing.Hole wall roughness was associated with the number of nozzle,and the increase of the backward nozzle number can reduce hole wall roughness and improve the stability of hole wall.According to the linear relationship between a value and hole wall roughness,the MG-C failure criteria was amended,and the evaluation model of hole wall stability by hydraulic drilling was established.(4)The technique of ultra-short radius steering drilling was designed,and the calculation model of hydraulic system parameters was established.Based on researches on the rock-breaking and drilling mechanisms of combined jet,the method of ultra-short radius steering drilling in coal seam was put forward,and its technical principle and system composition were introduced.Through the stress analysis of the self-propelling bit,combining with the system hydraulic loss calculation,the limit depth model of ultra-short radius steering drilling was built,and the influences of various construction parameters on drilling depth were analyzed.The results show that pump pressure is the main factor influencing limit depth,and limit depth increases with the increase of pump pressure.With the increase of nozzle diameter,limit depth first increases and then decreases.With the increase of high-pressure hose length,it also first increases and then decreases.The drilling equipment was applied in Chongqing Fengchun coal mine to conduct hydraulic drilling field experiments.The results showed that the average drilling depth by combined jet was 10.2 m,and the average drilling rate reached 1.09 m/min.No sticking phenomenon appeared during all drilling processes,which confirmed the feasibility of ultra-short radius steering drilling.The research results in this paper have made some contributions to the perfection of water jet rock-breaking theory,provide a new research idea of pure water jet technology,and contribute to the innovation application of water jet technology in mining engineering.