Research On The Gas Migration Law Of Pressure Relief In Close-range Coal Seam Mining And Precise Extraction By Directional Long Drillin

As an important energy base in China,Guizhou Province is known as the"Jiangnan Coal Sea,"but most of the coal mines in the province have close distance,high gas content and complex geological conditions.After the coal seam is mined,the pressure relief gas flows into the working face through the mining fissure and gathers in the upper corner of the return airway with the airflow,and the gas accidents occur frequently.Based on this,this paper takes Anshun Coal Mine as the engineering background,and uses a variety of research methods such as mathematical evaluation method,theoretical analysis,physical similarity simulation test,finite element and discrete element numerical simulation to comprehensively study the movement and deformation,failure characteristics,stress distribution,plastic zone evolution and crack propagation of the overlying coal and rock mass after the mining of the lower coal seam in the close distance coal seam.On this basis,the permeability evolution law of the upper adjacent layer and the migration and enrichment law of the pressure relief gas in the fracture field are studied.The directional long borehole segmentation extraction technology of mining pressure relief gas in close distance coal seam is proposed,and the main factors affecting the extraction effect are simulated and analyzed.The extraction scheme of directional long borehole is designed in detail.The research results can provide solutions for coal and gas co-mining in mines with similar occurrence conditions in Guizhou Province.The main research results are as follows:1.The gas disaster accidents in Anshun Coal Mine and adjacent mines were investigated.The IF-AHP-improved entropy weight method is used to comprehensively evaluate the current gas control system of Anshun Coal Mine,and the safety evaluation system of coal mine gas prevention and control system is established.After the process of’qualitative-quantitative-qualitative’,based on the evaluation results,the existing problems of the mine are put forward.2.The theory of three zones of overlying strata in goaf is summarized and calculated by empirical formula.The factors affecting the range of three zones are explored.It is concluded that the height of caving zone of overlying strata in goaf after mining in Anshun coal mine is 2.13～5.13 m,and the height of fracture zone is10.1 m～18.1 m.Based on the mechanical model,the pressure distribution and displacement of M8 coal seam are analyzed by using the function expression derived from the displacement function method of elastic theory.The stress and displacement function expressions are solved by Mathcad software,and the stress distribution and displacement change line diagram of M8 coal seam are drawn.The basic equations of the mathematical model of gas flow are summarized systematically.The classical mining overburden fracture field distribution theory and the pressure relief gas migration mechanism of 9305 working face in Anshun Coal Mine are analyzed.3.According to the actual production geological conditions of Anshun Coal Mine,the geometric similarity constants,motion similarity constants,bulk density similarity constants,stress similarity constants and material ratio parameters of similar simulation experiments were determined,and the deformation and failure characteristics and fracture evolution law of overlying strata in close coal seam mining were studied.By introducing the images collected during the similar simulation experiment into the digital speckle software for calculation,the displacement field and strain field evolution law of overlying strata in close coal seam mining were revealed,and the potential correlation between the maximum principal strain concentration area and the gas enrichment area was discussed.4.Using FLAC^3Dand UDEC numerical simulation software,the stress evolution law,plastic zone distribution law,displacement field evolution law and fracture distribution law of overlying coal and rock mass are obtained.Finally,the distribution of overburden fracture network is quantitatively analyzed by fractal theory.The influence of stress on permeability distribution is analyzed in detail,and the effective stress of upper coal seam is transformed into coal seam permeability by using Fish language.The evolution law of permeability coefficient of upper coal seam at different advancing distances of lower coal seam is analyzed in detail.5.By introducing the overburden fracture field calculated by UDEC into COMSOL,the law of pressure relief gas migration in close distance coal seam is studied.It is found that the mining fracture plays a guiding role in the migration of pressure relief gas.With the advancement of the working face,the guiding effect of the fracture field on the gas migration becomes more and more obvious,and provides an enrichment space for the gas migration.However,the enrichment space is in a dynamic state before the initial mining of the working face.With the continuous advancement,the scope of the enrichment area gradually tends to be stable.The targeted arrangement of the extraction system in the enrichment area can minimize the gas concentration in the goaf and the upper corner of the working face.6.Through the above similar simulation,numerical simulation and analysis theory,the mining of the working face is divided into three stages:the initial mining period of the working face,the upward development period of the fracture development height,and the stable period of the fracture development height.Combined with the characteristics of directional drilling trajectory control,wide coverage and mine pressure characteristics,the directional long drilling section extraction technology is proposed.The numerical simulation software FLUENT was used to simulate the gas concentration distribution in the goaf at three stages,and the factors affecting the extraction effect were simulated and analyzed.Finally,the precise extraction scheme of directional field drilling was designed and optimized in detail.