Research On The Deformation And Seepage Law Of Borehole Wall And Drilling Parameters Optimization

Methane extraction is an important technical means to solve the problem of coal mine gas outburst and gas exceeding the limit,and it is also a prerequisite for the development and utilization of coal bed gas resources.However,at present most of the mines in China are still facing the problem of low gas extraction concentration.In addition to the geological characteristics of low permeability of coal seam,the two key factors affecting the effect of borehole drainage are unreasonable setting of gas drainage parameters and failure of borehole sealing.Therefore,it is of great significance to study the extraction parameters and the sealing parameters of the drilling holes to improve the drilling extraction rate and to realize the rational exploitation and utilization of coal bed methane.In this paper,the research conditions of the boreho les along the seam of No.4 seam of ShaQu Mine and the He Xi Mine are studied.The theoretical analys is,finite element numerical simulation and the example calculation method are used to optimize the drilling parameters.And sealing parameters as the main objectives for research,obtained the following research results :(1)Based on the elastic-plastic mechanics theory,using the MohrCoulomb criterion and the dilatancy flow rule,the deformation of the borehole coal in the drainage borehole is regarded as a plane strain problem,and the fracture zone,plastic zone and elastic zone of the coal wall in the borehole wall are deduced.The stress-strain solution,combined with the effect of effective stress and matrix shrinkage on porosity and permeability,based on the definition of porosity and the Kozeny-carman equation,the dynamic change model of coal reservoir permeability during the extraction process was established.(2)Based on the model of dynamic change of permeability,combined with the seepage theory of porous media and relevant auxiliary equations,the differential equations of gas seepage under the dynamic change of permeability are established.The physical mathematical model can accurately simulate the effective drainage radius of drilling,Mining process optimization design provides the basis.Using COMSOL software,the effective drainage radius of No.4 seam drilling at different extraction time coincides with the effective drainage radius of downhole observation.(3)Based on the simulation results of the porosity and permeability of pore coal,the temporal and spatial variation of pore coal in borehole during drilling was analyzed.Coal permeability(porosity)along the radial direction,with the radius of the first shows a sharp decline in trend,after falling to a minimum and then slowly rises,eventually stabilize in the vicinity of the original permeability,and permeability of coal space.The distribution is closely related to the distribution of the tangential stress in the pore-wall coal.The permeability of the coal at the same location in the rupture zone and the plastic zone increases with the increase of the pumping time,while the permeability of the same coal body in the elastic zone follows.The increase of extraction time is gradually reduced because coal permeability is affected by the synergetic effect of matrix shrinkage effect(positive effect)and effective stress(negative effect),and the positive and negative effects on permeability are different in different regions.(4)Considering the influence of multi-borehole "pumping and stacking effect" on gas drainage,a multi-borehole extraction model was established and the effect of drilling gas drainage under different hole spacings was analyzed.The comprehensive safety and economy Considering many aspects such as sex and rationality,it was finally determined that the reasonable hole spacing of No.4 coal seam of Shaqu Mine was 4.13 ~ 4.95 m.(5)According to the stress distribution characteristics of surrounding rock,the spatio-temporal variation of permeability and the distribution law of seepage velocity,the coal body is divided into complete seepage zone,directional seepage zone,seepage shield zone and seepage boundary zone.The percolation characteristics of the above regions are analyzed based on simulation results.,which laid the foundation for the optimization research on the length parameter of sealing grouting hole.Based on the understanding of the traditional sealing hole length,considering the influence of dilatancy and fissures in the plastic zone on the sealability of the borehole and the effect of the coal body seepage zone on the drilling results,The conclusion is drawn that a reasonable length of the sealing hole should be greater than the radius of the plastic zone of the surrounding rock,while is less than the depth of the peak of the stress in the surrounding rock of the roadway.(6)A new grouting sealing and plugging mechanism was put forward.The radial seepage model of the sealing liquid was established,and the formula of the total grouting volume and the theoretical formula of grouting pressure and grouting time were derived.Engineering application of the results of the total sealing grouting borehole prediction,grouting pressure and time control have some reference and guidance value.