Numerical study on the mechanism of air leakage in drainage boreholes: A fully coupled gas-air flow model considering elastic-plastic deformation of coal and its validation

Abstract Air leakage caused by mining-induced fractures around the borehole and roadway greatly affects the effect of underground gas drainage. Therefore, the study of the air leakage model considering three-dimensional stress and coal elastic-plastic deformation is of great significance to prevent air leakage. In this work, a model of air leakage outside borehole in mining-disturbed coal seam including a fully coupled gas-air flow and coal mechanics model and a mining-induced damage permeability model was developed and verified. The model was used to study the gas-air migration law and air leakage mechanism during gas drainage, and the influence of key parameters including initial permeability and sealing depth on gas drainage effect was analyzed. The results show: (1) The model can be used to characterize the air leakage of pre-drainage boreholes in mining-disturbed coal seams. (2) The gas and air pressure in the severe mining disturbance area rapidly decreased and increased in a short time. (3) Increasing the initial permeability and sealing depth will promote the gas flow in the borehole in the early stage, but the gas concentration will be reduced. The research results provide a scientific theoretical basis for improving the gas drainage effect and ensuring mining safety.