Research On Key Technology Of Foam Drilling In Underground Broken Soft Coal Seam

Gas extraction through borehole drilling in coal seam is an effective means to prevent gas accidents,and also a safe,economic and effective measure to control gas overruns and improve mining efficiency.In China’s recoverable coal seams,broken and soft coal seam accounts for a large proportion.Due to high gas content,high gas pressure,and poor stability of coal seam,inseam borehole depth is always short and the probability of pore formation is low because of low drill cuttings removal efficiency and collapse of borehole.This situation is seriously affecting the gas extraction effect and high-efficiency mining.At present,mid-pressure air drilling equipment and technique are used for most of the drilling of broken soft coal seams in China,which have the following problems,(1)Compressed air has a poor cooling effect as a circulating medium.(2)Borehole collapse may cause hidden danger of fire in the hole after the generation of heat by rotation friction of the drill string.(3)In case of water-bearing coal seams,the sludge adheres to the outside of the drill rod,which easily causes drilling accidents.Combining advantages of foam drilling such as strong drill cuttings removal capacity,good purification effect in the hole,good cooling effect and suitable for drilling in water-bearing strata of foam drilling,the following researches have been carried out in this paper with the key technology of foam drilling for broken and soft coal seam in coal mine as the research object on the basis of the major national science and technology project"Efficient Extraction Technology and Equipment for Coalbed Methane Underground Coal Mines"(No.2016ZX05045-003).(1)Based on the theory of multiphase flow,the flow law of foam fluid inside the drill rods,drill bits,and annulus clearance,etc.And the force and motion of pulverized coal particles in annular clearance are analyzed and researched,providing a theoretical basis for the study of drilling process technical parameters.(2)It is proposed that the use of short-wing auger drill rod to increase the turbulence of the foam flow field to improve the efficiency of drill cutting removal efficiency.Combining the force and motion analysis of pulverized coal particles,the dust stirring,and auxiliary dust-exhaust capacities of the short-wing auger drill rod have been researched,to obtain the calculation method of the dust’s super-boundary rotation speed and the spiral groove lead angle.In addition,the main parameters that affect the drill cuttings removal capacity of spiral groove has also been analyzed.(3)Based on the drilling requirements of broken soft coal seams,the research on anti-collapse emulsion foam formulation has been carried out.It is proposed to mix anti-collapse emulsion and foaming agent for directional drilling to develop an emulsion foam rinse fluid system with strong anti-collapse capacity.The foam volume,density,apparent viscosity,dynamic shear force,fluidity index,consistency coefficient and other parameters of different anti-collapse foam emulsion formulations have been studied by Waring-Blender stirring method and orthogonal test method.Optimal formula of anti-collapse foam emulsion for broken soft coal seams is obtained,which is water+0.2-0.3%anti-collapse emulsion+0.5%foaming agent K12+0.4-0.8%clay inhibitor NH-1.Besides,scanning electron microscopy(SEM)analysis was performed on the coal samples of the broken soft coal seam,and the collapse prevention mechanism of the anti-collapse emulsion foam on the broken soft coal seam was studied.(4)The drilling annular flow field has been analyzed.By using the finite element method,numerical simulation of the dust carrying capacity of external flat drill rod and short-wing auger drill rod under the action of foam through fluid mechanics simulation software are carried out respectively.Further,the contrastive research on the law of foam fluid pressure and flow velocity in the annulus when using short-wing auger drill rod and regular flat drill rod are also carried out,to obtain the correction coefficient of pressure loss in the annulus foam flow of the short-wing aguer drill rod.A theoretical calculation method for the volume flow rate of foam drilling gas in coal mine is proposed through the analysis and research on the process parameters of foam drilling pressure loss,compressed air and foam liquid injection volume in broken soft coal seam.(5)Through comprehensive research on foam infusion technology and defoaming technology in coal mines,foam generators,drilling defoaming devices and other equipment have been developed,and a complete set of foam drilling equipment in coal mines has been integrated.(6)On the basis of theoretical analysis,simulation and laboratory research,a series of experimental studies such as foaming experiments,defoaming tests,anti-collapse tests and on-site drilling have been carried out.Drilling comparison tests were carried out on the site of broken soft coal seam.Compared with the medium-pressure air drilling technique,application of foam drilling method and short wing auger drill pipe achieved the reduce of drilling rig rotary pressure up to 48%.Field experiment proves that the foam drilling process is suitable for to conduct 200 meter deep borehole drilling in broken and soft coal seams.