Influence of combination forms of intact sub-layer and tectonically deformed sub-layer of coal on the gas drainage performance of boreholes: a numerical study

High concentration and large flow flux of gas drainage from underground coal seams is the precondition of reducing emission and large-scale use of gas. However, the layered occurrence of coal seams with tectonically deformed sub-layers and intact sub-layers makes it difficult to effectively drain gas through commonly designed boreholes. In this study, the gas drainage performance in coal seams with different combinations of tectonically deformed sub-layers and intact sub-layers was numerically analyzed. The analysis results show that the gas drainage curve changes from a single-stage line to a dual-stage curve as the permeability ratios of Zone II (k(II)) and Zone I (k(I)) increase, raising the difficulty in gas drainage. Furthermore, a dual-system pressure decay model based on the first-order kinetic model was developed to describe the dual-stage characteristics of pressure decay curves with different permeability ratios. In the end, the simulation results were verified with reference to in-situ drainage data from literature. The research results are helpful for mines, especially those with layered coal seams comprising tectonically deformed sub-layers and intact sub-layers, to choose appropriate gas drainage methods and develop the original drainage designs for achieving better gas drainage performance.