| The technology of gas extraction borehole in coal seam is currently regarded as one of the most important methods for gas controlling and preventing during the process of underground mining.The deformation and instability of borehole constantly affect and hamper the gas drainage efficiency.Sealing segment of borehole,being central part to connect drainage system of side of gates and the pipe system in coal seam,that is usually blocked and sealed with something like cement and polymer and the like.Meanwhile,it's generally believed that sealing material and corresponding sealing technology both have significant influence on ensuring borehole stability and achieving high standard of gas extraction concentration.Considering issues mentioned above,the mechanics of deformation and instability of boreholes and active principles of sealing material were firstly analyzed in this paper.Meantime,the Lab experiments were carried out to research on the physical characteristics of compound material,like strength,expansion and gas permeability.Finally,the composite sealing material was applied in field test and the actual effect of this was compared with that of other sealing materials.Comprehensively theoretical analysis and experiment and field test results,the following conclusions can be got:(1)The stress distribution expression being established based on elastic mechanics,seepage mechanics and effective stress principle,and general Hoek-Brown strength failure criteria that was deducted for examining stress state and stability of surrounding coal mass of borehole,and given that characteristic of the non-linear porous pressure distribution around the rock mass.Meanwhile,the several key factors that have influence on the borehole stability and size of plastic area was analyzed.Besides,the effect of protecting borehole wall using the sealing material and the mechanics of slurry migration and sealing were studied.The theoretical analysis shows that sealing material should have proper strength,favorable expansion and desirable diffusion-seepage ability and perfect compact structure.(2)By applying the RMT-150 C mechanical testing machine,the compressive strength of the compound sealing material mixed with different percentage of sodium bentonite and aluminum powder was tested and analyzed under the different curing time.The results show that the strength of compound material adding with sodium bentonite and aluminum powder tends to decrease in some degree.However,the volume expansion rate of this compound material that adding with aluminum powder and sodium bentonite respectively also can be effectively improved.(3)Applying the KD dynamic damage evaluation system of permeability of coal seam,the permeability of compound material that added with different amount of sodium bentonite and aluminum powder was tested.Along with variable mixing amount of sodium bentonite and aluminum powder,the permeability of compound cement material demonstrates dual changing characteristics.(4)By using this compound sealing material,the field test result shows that the gas extraction concentration can be significantly enhanced.The average gas concentration is 40% or so within the 45 days,which is more than twice as high as the conventional sealing method.It's important that compound sealing material of this sort can be applied to engineering practice,given those practical results. |
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