Numerical simulation of influence of Langmuir adsorption constant on gas drainage radius of drilling in coal seam

To determine reasonable distance of gas pre-drainage drillings in coal seams, a solid–gas coupling model that takes gas adsorption effect into account was constructed. In view of different adsorption constants, the paper conducted the numerical simulation of pre-drainage gas in drillings along coal seam, studied the relationship of adsorption constants and permeability, gas pressure, and effective drainage radius of coal seams, and applied the approach to the layout of pre-drainage gas drillings in coal seams. The results show that the permeability of coal seams is on the gradual increase with time, which is divided into three sections according to the increase rate: the drainage time 0–30 d is the sharp increase section; 30–220 d is the gradual increase section; and the time above 200 d is the stable section. The permeability of coal seams is in negative linear and positive exponent relation with volume adsorption constant VL and pressure adsorption constant PL, respectively. The effective drainage radius is in negative linear relation with VL and in positive exponent relation with PL. Compared with the former design scheme, the engineering quantity of drilling could be reduced by 25%.