| Coal seam methane is a sort of efficient clean energy, but as the coalbed gas which is one of the main gas disasters threatening coal mine safety in production.Extracting methane in coal seam effectively is one of the most effective methods to reduce the mine gas accident, While most coal seam belongs to low permeability coal seam in China,in order to ensure safe production of coal mine and improve coal production efficiency,solving low permeability gas extraction is a key problem. Slotting and enhancing permeability of coal by high pressure rotational jetting and its appliation in low permeability coal seam were studied in this text to enhance drainage efficiency.Mechanism of rock breaki were studied in this text ng by shear, extruding, erosion and grind was investigated in this paper.Free rotating water jet flow model under the condition of submerging borehole was established through theoretical analysis, rotating water jet velocity analytical solution in borehole was derived based on self-similar movement theory.Mechanism of slotting and enhancing permeability by high pressure rotational jetting was researched by numerical simulation,after coal was cut double symmetrical slot by high pressure rotational jetting,coal distortion was caused around slot,stress field was redistributed,damage failure due to yield stress was happened on the periphery of slot. fracture was increased and permeability was enhanced;permeability of distant area coal was increased with the decrease of stress owing to pressure relief, ventilation property of coal seam was improved. Effect of slotting and enhancing permeability by high pressure rotational jetting is better than ordinary excavation cut. Mathematical model of gas extraction migration by slotting was established.Numerical simulation about rotational jetting rule under the condition of submerged and non-submerged nozzle parameters was done by FLUENT, Simulation results showed that jet diffusion angle and jet velocity was the decreased along with the increase of nozzle exit length. High pressure rotational jetting nozzle was designed based on the kinetic parameters of rotational jetting, The optimal guidance angle for impeller was 45 o and the best export length was L=1.8D. Dynamic pressure of rotational jetting annular impact area can reach at least 12MPa in the distance 70D from nozzle exit that can meet requirements of rock breaking.High pressure rotational jetting cutting equipment was developed.It was consisted of high-pressure hydraulic pump,control system,executive system and supporting system,that formed a set of downhole mobile and convenient, efficient and practical hydraulic pressure cutting system,the highest working pressure of this equipment is 47Mpa and the highest water flow is 120L/min.Down-hole coal seam rotational jetting slotting and gas extraction were tested on the right second surface of 3B# coal seam in the western right working area of Chengshan coal mine which belongs to Jixi mining industry (group) limited liability company. The test results show that:(1)The increase of slot depth was nonlinear with the increase of jet pressure, but the trend became slow gradually and the slot depth tend to be a ultimate value,the increase of slot depth was diminishing with the increase of slotting speed, Eficiency of cutting coal by rotational jetting is 1.37~1.47 times as direct jetting on the same pressure and slotting speed.(2)The parameters of rotational jetting slotting are shown as follow, boreholes diameter are 94mm and depth is 60m, boreholes pitch are 10m, working pump pressure is 40Mpa, diameter of nozzle is 2mm and translational speed of spray-head is 0.1～0.2m/min when using directional slotting by spray-head with two nozzles and jet direction of nozzle is perpendicular to bit's axis. slotting total depth reachs 1.1 m and ite width reachs 120mm.(3)Process flow of slotting by rotational jetting was designed and a set of workable safety protective measures were formulated to ensure implementation of rotational jetting.(4)Naturally initial drilling methane emission of was 19.452×10-3m3/min·hm before slotting in 3B# coal seam,which can reached 54.635×10-3m3/min·hm after slotting. Ultimate methane emission are 186.03m3 of hundreds meters coal seam drill,which can reach 565.62m3 after slotting, naturally initial methane emission of hundreds meters coal seam drill was 3.04 times after slotting as before slotting.(5)Everyday methane extraction volume of hundreds meters slotting borehole and routine borehole were 148.028m3/d.hm and 66.037m3/d.hm respectively,the speed of extracting methane of meters slotting borehole was 2.24 times as routine borehole. In the two months, methane extraction ratio of slotting borehole can reach 24.42%,which of routine borehole was only 10.48%. Methane extraction ratio of slotting borehole was 2.33 times as routine borehole.
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