Study On Gas Distribution And Migration Law In Goaf Of Close-distance Coal Seam Mining

In the mining process of close coal seam,affected by mining,the rock mass cracks in the overlying goaf are re developed,resulting in the formation of vertical cracks in the upper and lower goaf,and even connecting the surface,which enhances the oxidation degree of the residual coal in the overlying goaf and the lower goaf.The toxic and harmful gases such as carbon monoxide and carbon dioxide generated by the oxidation of the residual coal enter the working face of the lower coal seam through the overlying rock cracks in the goaf under the action of wind pressure difference,It seriously affects the safety production of lower coal seam working face.Taking No.8 coal seam in shaping mine as the research object and 1818 working face as the research platform,this paper obtains the gas composition and concentration distribution of the upper and lower goaf through on-site construction drilling and beam pipe arrangement.At the same time,the gas volume fraction between the return air corner and the support of the working face is measured by the air bag sampling method,and the three-dimensional gas distribution model of the composite goaf under different surface air leakage is established by the numerical simulation method.According to the static and dynamic distribution law of the short-distance goaf obtained from the field data,combined with the numerical simulation results of the three-dimensional distribution of gas in the composite goaf,the temporal and spatial distribution and migration law of gas in the short-distance coal seam goaf in the mining process are studied.The main conclusions are as follows:Through the methods of construction drilling and pipe bundle monitoring,the distribution characteristics of gases in the upper and lower goaf and the working face of the lower coal seam are studied.The dynamic and static distribution laws of the gases in the upper goaf,i.e.the goaf of the upper coal seam 8,under the condition that the lower coal seam 8 is not mined and in the mining process,as well as the gas distribution laws in the working face of the lower coal seam1818,the return air corner and the goaf are obtained,and the distribution areas of spontaneous combustion"three zones"in the goaf are divided.According to the key layer of the overlying structure of the goaf and the theoretical research of"O"ring,combined with the physical parameters of the overlying strata in the coal seam goaf,the estimation formula of the height of the fracture zone and caving zone in the short distance coal seam goaf is obtained.Combined with the on-site ground pressure and rock displacement data of the mine,the three-dimensional calculation formulas of the broken swelling coefficient and porosity of the short-distance composite goaf are obtained,and the basic differential equations of the short-distance coal seam composite goaf including energy conservation equation,continuity equation,momentum conservation equation,material conservation and other flow fields are established.By simulating the gas distribution law of the short-distance goaf under different surface air leakage conditions through fluent,the influence degree of the surface air leakage on the gas distribution of the goaf is obtained.The research shows that compared with no surface air leakage,the surface air leakage has less influence on the heat dissipation zone of the goaf on the air inlet side,but extends the oxidation zone by 15m.Similarly,the surface air leakage has a serious impact on the front area of the goaf at the return air side.The heat dissipation zone of the goaf at the return air side extends by 5m and the oxidation zone extends by 25m.It is estimated that the area of the heat dissipation zone is expanded to about 1.25 times and the area of the oxidation zone is expanded to about 1.30 times.According to the simulation results,most of CH₄ and CO₂ in the goaf on the return side migrate from the goaf on the inlet side and the middle of the goaf under the influence of the air leakage flow on the inlet side of the working face.For CO₂,the migration from the overlying goaf to the goaf of the coal seam accounts for about 15%of the total volume fraction,while the migration of CH₄ accounts for about 19.5%,Therefore,the proportion of CO₂ migrating to the return air side in other areas of the goaf reaches 85%and CH₄ reaches 80.5%.Based on the field test research and numerical simulation results of gas distribution in the goaf of close coal seam,the gas distribution law of the overlying goaf before and after mining is obtained.At the same time,combined with the gas distribution and volume fraction change law of the working face and return air corner,Finally,four migration mechanisms of gas in the goaf of close coal seam in shaping mine are obtained:first,there is migration between the gases in the goaf of upper and lower coal seams;Secondly,the air inlet side of the lower coal seam faces the goaf of the lower coal seam and gas migration occurs;Third,gas migration from the goaf of the lower coal seam to the working face;Fourth,the gas generated by the secondary oxidation of floating coal in the overlying goaf and the oxidation of residual coal in the goaf of the lower coal seam gushes out from the return air corner of the working face of the lower coal seam under the action of wind pressure,which may cause the accumulation of toxic and harmful gas in the return air corner.