The Research On Gas Extraction By Drilling Borehole In Coal Seam Of Low Permeability

The drainage technology of hole drilled along seam is main measure of regional gas control. It’s more adapted to single coal seam of low permeability with low construction costs and long effective drainage section, large control range and drainage volume of one hole. But arrangement hole is blind, sealing is more difficult for influence of roadway relief, plugging collapsed boreholes is serious in soft coal. So the paper carried out research of optimization drainage technology of hole drilled along seam.Gas drainage coupled model in hole drilled along seam was established based on mass conservation equations, Darcy’s law and geometry and balance equation of coal and rock, with dynamic changes of porosity and permeability. The paper analyzed factors of effective gas drainage radius and stress distribution around boreholes, plastic zone distribution around boreholes with influence of roadway relief by COMSOL Multiphysics software. In order to obtain the quantitative characterization and systematically evaluation of borehole destruction level, the concept of borehole destructiveness is proposed in this paper, and its expression is obtained by theoretical analysis as well. The destructiveness caused by small angle drilling is low when the geostress influence factor is small. However, as the geostress influence factor increases,the destructiveness increases as well, and there is a appropriate drilling angle to effectively reduce the destructiveness. When the geostress influence factors is large, the direction of smallest borehole destruction is always tending to parallel to the direction of largest crustal stress.The 3# coal seam in Huoerxinhe coalmine mined is low permeability and high gassy. To optimize drainage technology conducted tests of effective gas drainage radius.The results show: ①The curves of effective gas drainage radius with drainage time were same, which determined by theoretical calculations based on gas drainage coupled model and testing methods with residual gas content, and effective gas drainage radiuswas 1.25 m. ②Initiative support sealing highest and lowest daily average drainage concentration were 32.25% and 14.69%, the highest and lowest concentration of ordinary capsular bag sealing were 10.76% and 2.93%, 3.58% and 0.48% of polyurethane sealing, and effect of initiative support sealing was better than the other two methods.