Experimental study of drainage radius considering borehole interaction based on 3D monitoring of gas pressure in coal

Abstract

In practical engineering, borehole drainage is the most effective and widely used method of extracting coalbed methane (CBM). The drainage radius of a borehole is a key parameter in determining the design and layout of drainage boreholes, and the interaction among boreholes in multiple-borehole system significantly influences the drainage radius of each individual borehole. Therefore, the physical simulation experiments of gas drainage with single, double, and multiple boreholes were carried out using a self-developed device. This paper discusses the drainage radius considering borehole interaction based on 3D monitoring of gas pressure in coal, which can provide reference data for numerical simulation and offer practical guidance for engineering. The results show that multiple boreholes lead to a faster drop in gas pressure along the monitoring line and require less time to reach the safety line, with two distinct propagation patterns observed in the effective drainage zones. Besides, a calculation model was proposed for obtaining the drainage radius of a borehole. The drainage radius can be fitted using a power function with respect to drainage time, with larger values giving an increase in the drainage radius. Meanwhile, a benchmark borehole was selected from which the gas pressure decrease coefficients at different monitoring points are calculated; these coefficients reflect the degree of borehole interaction. The relationship between the pressure decrease coefficient and drainage time is approximated by a negative exponential function, with multiple boreholes exhibiting more intense interaction. Finally, it is found that the influence of other boreholes on monitoring points near the reference borehole is relatively small and subject to a time lag.