Study On New Sealing Technology And Reasonable Parameters For Gas Drainage Borehole In Soft Coal Seam

The drilling hole of gas drainage is the main means of prevention and utilization of coal seam gas in China,the poor sealing performance of boreholes affects the effect of gas extraction,and how to improve the sealing quality of boreholes is an urgent engineering problem to be solved.Hence,how to improving borehole sealing is an urgent engineering problem to be solved.Especially,the soft coal seam gas drainage borehole is very difficult to seal,Under the influence of secondary stress,the coal body around the borehole is severely fractured and fissure are developed.The internal interconnected fissures formed leakage channel.Under the negative pressure of borehole drainage,the air of roadway enters the hole along the leaking passage,which reduces the extraction effect.Therefore,it’s a great significance for gas control and benefit to production of coal mine to optimize borehole sealing technology and enhance the performance of borehole sealing through the study of borehole leakage and sealing model.This paper focuses on the study of sealing-isolating integration technology and reasonable parameters of borehole in soft coal seam gas drainage,so as to improve the effection of gas drainage.The research work of this paper as following four parts:(1)Based on rock mechanics and related theories,the stress distribution and mechanical characteristics evolution of coal around borehole excavation are studied.To simplify the calculation conditions of borehole and based on the mohr-coulomb strength criterion,The stress solutions and influence radius formulas of each stress zone in the loose zone of coal and rock mass around roadway and borehole are obtained.The fracture distribution and permeability characteristics in area of roadway are analyzed.Based on the deformation equation of coal around borehole and the equation of porosity,the permeability change equation is deduced.(2)According to the geological conditions of No.8 coal seam in Wulunshan Coal Mine,the FLAC 3D software is applied to simulate the damage and deformation of the surrounding coal body after the tunnel and the borehole be excavated.The general distribution range of stress zones around roadways and boreholes is obtained,which provides guidance for determining reasonable parameters of the sealing-isolating integration technology.(3)This paper mainly expounds the influencing factors of gas leakage in boreholes from three aspects: coal failure type and structural characteristics,borehole instability characteristics,sealing materials and grouting pressure.According to the damage situation of coal body around roadway and borehole and combination with the cause of gas leakage,the gas leakage form of borehole can be divided into three categories: gas leakage in roadway fissure zone,gas leakage in borehole fissure zone and material leakage in sealing section.Based on Darcy’s law of fluid mechanics,the models of material leakage and gas leakage in drilling fracture zone and sealing section are established.The diffusion pattern of seepage grouting in boreholes can be divided into spherical diffusion and cylindrical diffusion.Combining with the diffusion theory of seepage grouting,the sealing model of grouting is established.(4)In view of the existing problems and shortcomings in the sealing technology of borehole along coal seam,combined with the ultra-high pressure hydraulic slotting technology,a new sealing-islolating integration technology is proposed and reasonable sealing technical parameters are determine.Through the comparison and analysis of industrial test results,the average initial concentration of boreholes with sealingisolation integrated technology is increased by 1.5-2 times and the gas drainage concentration is increased by 2-3.5 times,which greatly improves the performance of boreholes sealing and the effection of gas drainage.