Productivity Simulation Of Coalbed Methane Well In Goaf Of Xishan Tunlan Coal Mine And Its Controlling Factors

The extraction and utilization of coalbed methane in coal mine goaf has multiple important significance.Taking Tunlan mine as the research object,this thesis focuses on the 12501 working face in the south fifth panel of Tunlan mine,systematically analyzes the geological conditions of in-situ coalbed methane and the geological conditions of coalbed methane in goaf,and discusses the secondary accumulation characteristics of coalbed methane left in goaf.The characteristics of pore and fracture structure and permeability change of coal before and after mining failure are summarized.The multi-physics coupling finite element software COMSOLMultiphysics is used to establish the multi-physics coupling model of coalbed methane in the goaf of ground well extraction,and the dynamic change law of multi-field in the process of coalbed methane extraction in goaf is analyzed.The influencing factors of coal seam gas well productivity in goaf were studied,and the productivity simulation was carried out.The main findings are as follows:First: The physical characteristics of the disturbed reservoir in the goaf and the accumulation characteristics of the remaining coalbed methane are described.Based on the research results of the geological conditions of coalbed methane in the in-situ area and the physical properties of the disturbed coal rock in the goaf,the coal reservoir is destroyed by the pressure-bearing effect.The medium pores in the pores and fractures are subjected to stress compression to produce plastic deformation.The mesopores and macropores are significantly reduced,and the large-aperture fractures and micro-pores are significantly increased.After the pressure is released,the fracture network is formed.The adjacent fractures extend and penetrate each other,which increases the permeability of the coal reservoir.The gas in the goaf migrates to the top of the fracture zone after mining,and the coalbed methane enrichment area is formed in the gasconducting fracture zone.Secondly,it reveals the geological engineering control factors of high yield of CBM wells in goaf.According to the characteristics of gas production curve in the study area,the goaf drainage curve is divided into desorption-free type,free-desorption type and free type.The methane concentration of gas production in goaf wells continues to decrease,between 98 % and 35 %,with an average of 56.60 %.The productivity characteristics of coalbed methane in the goaf show that the initial gas production is high and the attenuation rate is fast.The whole drainage process is accompanied by the supplement of desorption gas to free gas.The study believes that the factors affecting the productivity in the study area are divided into : in-situ geological factors(including structural morphology),goaf disturbance factors(including changes in goaf porosity and permeability),and engineering factors(including extraction negative pressure and wellhead position selection).Thirdly,based on the actual size and formation conditions of 12501 working face in Tunlan Coal Mine,the model is established.The COMSOL-Multiphysics multi-field coupling finite element software is used to study the multi-field dynamic change law of goaf.On this basis,the productivity simulation of CBM wells in goaf is carried out.The numerical model of coalbed methane extraction in goaf by ground well can successfully simulate and calculate the temporal and spatial variation of stress field and seepage field in the process of extraction.The whole process of coalbed methane extraction in goaf can be simulated by COMSOL.The extraction of coalbed methane is mainly controlled by the initial porosity and permeability of the coal seam and the layout of the well location.The elastic modulus and the negative pressure of the extraction have little effect on the extraction volume itself.The gas production effect of coalbed methane is the best when the well is arranged in the strike edge of the gasconducting fracture zone in the goaf,followed by the tendency edge of the gasconducting fracture zone,and then the recompaction zone,and the gas production effect in the pressure-bearing zone is the worst.The productivity of coalbed methane wells in goaf is simulated.It is considered that the gas production obtained by numerical simulation of the model can objectively describe the gas production process of coalbed methane wells in goaf,and the variation law of the gas production curve obtained is in line with reality.