Gas migration channels and efficient gas extraction levels of soft-hard alternate coal seam composite roof

Efficient gas extraction is important for reducing coal mine gas accidents and protecting the environment. A prerequisite for achieving efficient gas drainage is to clarify the evolution rules of mining fractures and the development characteristics of gas migration channels. In this study, aiming at the rock structure of coal seam composite roof, the evolution rules of gas migration channel in the mining process of the composite roof are analyzed, the multifractal characteristics of mining fracture development are discussed, and the most effective construction layer of high-level boreholes is quantitatively characterized. It has been found that the development process of gas migration channels can be divided into four distinct stages. During the mining of the working face, the Delta f that characterizes the uneven distribution of mining fractures in the multifractal spectrum shows a trend of increasing first and then decreasing and then stabilizing, and the Delta f(alpha) that characterizes the size difference of fractures in the multifractal spectrum shows a trend of decreasing as a whole, while the quotient of Delta f and Delta f(alpha) on the open-off cut side, and the working face side shows an increasing trend. Furthermore, the range of gas transition flow channel area in the composite roof is 9-17 times of mining height, the permeability in this range is concentrated in 6.92E-09 similar to 4.43E-07 m(2). Notably, the hard rock strata within the range of 9-13 times of mining height are the best gas drainage levels. These results provide theoretical and technical support for gas drainage in coal mines.