Study On Seepage Characteristics Of Coalbed Methane In Abandoned Mine Goaf Under Surface Vertical Well Conditions

The extraction of methane in abandoned goaf can not only produce considerable economic benefits,improve the energy structure of our country,but also solve the hidden danger caused by gas accumulation,and provide safety guarantee for the recovery of residual coal.In this paper,Yuecheng and Jinsheng coal mine are regarded as the research object.The method of theoretical analysis,UDEC and COMSOL numerical simulation are applied to study the spatial distribution characteristics of the abandoned goafs.The discarded goaf is classified on the basis of this,and then the different wells are located on different wells.The characteristics of the methane seepage in the goaf under the condition of surface vertical well are studied,location of well is optimize.the main research results are as follows:(1)According to the geological data of Yuecheng and Jinsheng mine,the UDEC numerical simulation test is carried out,and the goaf is divided into three types according to the strike length of goaf L.When L? 45 m,the roof is not broken down in the goaf,and when 45 m < L ?100m,the roof collapse and not compacted goaf is formed.When L > 100 m,roof collapse and compaction goaf will be formed.(2)A three-dimensional seepage flow model for methane in abandoned goaf is established and solved by COMSOL Multiphysics.Three kinds of abandoned goaf under the typical well pumping conditions are studied in the three-dimensional,two-dimensional and one-dimensional coal bed migration and migration rules,and the high-speed flow area of coal bed gas and the "blind area" are divided.The three kinds of blind areas of the goaf are respectively the top of the goaf,the broken rock part and the bottom of the goaf,1.67 times the height of the mining area.The coalbed methane in the blind area will continue to flow along with the extraction process,and will slowly diffuse into the high speed flow zone under the action of the concentration gradient.(3)According to the velocity distribution of coalbed methane and the range of high speed flow area,combined with the influence of time factors,the best well location of different types of abandoned goaf is determined.The best well location is in the middle of the goaf for the top slab mined out area.For the roof collapse and not compacted goaf,the best well location is in the middle part of the goaf;for the roof collapse and the compacted goaf,the best well location is in the four angle of the "O" ring in the goaf.(4)Statistics the size of the goaf and the surface vertical well distribution in Yuecheng and Jinsheng mine,and the effect of two key factors of the area of the goaf and the surface vertical well on the gas production capacity respectively is studied by variable-controlling method.The results show that when the well position is constant,the instantaneous flow rate increases linearly with the increase of the abandoned goaf,and the gas production of surface vertical well is in good agreement with the numerical simulation results.