Effects of Recovery Ratio on the Fracture Evolution of the Overburden Pressure-Relief Gas Migration Channel for a Fully Mechanized Working Face

Combined with theoretical analysis and physical simulation experiments by using a fully mechanized working face at the Heshun Coal Mine in Shanxi Province, China, the fracture distribution laws of gas migration channel in overburden were investigated for different recovery ratios. The results showed that, with increase in recovery ratio, the height and average weighting step distance of the three zones decreased, whereas the maximum overburden subsidence volume, separation volume, and bulk coefficient increased. The through degree was adopted to describe the damage degree of fractures, and its boundary value was 0.6. The recovery ratio was positively and negatively correlated with the width and height of rock fractures, respectively, revealing that the fractal dimension increased with increases in distance advancement and recovery ratio, with the ranges of change being 1.29–1.52, 1.37–1.55, 1.43–1.62, and 1.51–1.67. The criteria for determining geometric position and the mathematical control model were established using as key parameters the through degree, subsidence volume, and separation volume extent. Through the optimization of the layout of the high-level drilling field, the gas extraction concentration increased from 28.36 to 52.08%, the gas extraction purity increased from 5.77 to 35.74 m 3 /min, and it is reasonable to control gas migration in the working face by gas extraction in a high-level drilling field.