| In Western China, there are some mining areas with complex geological structureand significant hazards of gas, coal and gas outburst. The outburst hazard not onlythreatens safety of mine gravely, but also makes it difficult to explore coal resourcesin some regions. Aiming at the characteristics of the complex structure, poor coalseam permeability and multi-seam mining in the mining area, with the purpose ofeliminating the danger of outburst and decreasing the gas content of coal seam, thispaper makes a systematic research on the mining of the lower distant protective layer,the technology of pressure-relieved gas drainage using surface boreholes andeliminating the potential dangers of coal and gas outburst.By constructed the model of similar materials experiments of stope andnumerical simulation, we have studied the depressurization effect of protected coalseam under the mining condition of lower distant protective layer:(1)Duringextraction of the lower distant layer, the stress of protected layer could be reducedsufficiently, and gas permeability coefficient could be increased substantially,providing favorable condition for gas drainage;(2)As the double protective seammining,40～50m area of intake entry and return entry of mining face in protectedcoal seams is dilated area, which is the proper location of surface boreholes;(3)Repeat depressurization mode is found that the gob of the extracted seam below thegassy one provides a "Buffering Effect" for the overlying coal seam which makes thedepressurization of the overlying seam weaken.The results from mechanical model for analysis of the stability of surfaceborehole are:(1)The greater the difference between the strength of adjacent strata is,the bigger the squeezing and shearing stress in the casing become;(2)Under thecondition of leaving the space between the casing and borehole wall, the max shearingstress of casing decreased by50.8%evenly, and the probability of failure of casingwas reduced significantly;(3)The longer the production casing is, the bigger thedeflection value is and the greater anti-bending ability the casing gets. Based on theabove results, we designed a structure of surface borehole against shearing andrupturing. There leaves the "buffer spacing" between the cementing casing orborehole wall and the production casing which is whole-tube and screen section washandled by "casing enhancement technology", effectively improving the stability ofsurface borehole. The law of pressure relief methane drainage using surface borehole had beendemonstrated:(1)Illustrated the distance from working face to borehole (showing themethane permeability coefficients of overlying coal seam and fractured zone), is thekey influencing factor of gas production rate of borehole;(2)Revealed the changingmechanism of "fast increase-slowly decrease" of the gas production rate of boreholeduring the protective coal seam mining, and determined the best layout parameters ofsurface boreholes;(3)Built a mathematical model for determining the gas flow rate ofoverlying coal seam and gob, which can provide a basis of assessment for gasdrainage of overlying coal seam;(4)Proposed the method of improving gas drainageform overlying pressure released seams with increasing the lower borehole resistance.The research results mentioned above have been carried out in WuLan CoalMine of ShenHua Group, which show that the effect of eliminating outburst byextracting pressure relieved gas with surface boreholes is remarkable. The pure gasdrainage quantity of boreholes in the testing area has totally reached1512.96×10~4m~3.The max spacing of borehole near the intake entry and the return entry are150mand169m respectively. The remaining gas quantities of the protected coal seams areeliminated to3.63m~3/t and3.14m~3/t, and the gas drainage rates are65.8%and68.0%,respectively, completely eliminating the outburst danger of the coal. |
PDF Full Text Request