Investigation on gas drainage effect under different borehole layout via 3D monitoring of gas pressure

Abstract

Gas drainage using underground boreholes is an important method for the prevention of coal gas mine accidents. To study the influence of the increase in the number of boreholes on the gas drainage effect using a self-developed device for gas drainage, pressure sensors and branch boreholes were installed in a coal body. Four groups of physical simulation experiments of gas drainage were conducted. The results revealed that with an increase in the number of boreholes, the attenuation rate of the gas pressure is accelerated; the decrease in the gas pressure accelerates in the form of a natural logarithmic function. The effective drainage area, with characteristics of dynamic expansion and variations, has an approximately circular expansion with the borehole as the center. It gradually increases with the advancement of drainage. The relationship between the effective drainage area and the extraction time is in the form of a power function. With an increase in the number of boreholes, the effective drainage area increases more rapidly, and the regional outburst elimination occurs earlier. The effective stress and matrix shrinkage effect exhibit a coupled influence on the permeability, which causes the permeability evolution to exhibit periodic variations. At the early stage of drainage, the effective stress plays a major role, which reduces the permeability. At the middle and late stages of drainage, the matrix shrinkage effect plays a major role, which increases the permeability. With an increase in the number of boreholes, the attenuation rate and recovery amplitude of the permeability increase.