Abstract: To accurately predict the dynamic permeability of low-rank coal, a new model was established on the basis of coal cube model, with consideration of the slippage effect and effect of matrix porosity on the permeability. Sensitivity analysis of factors affecting absolute permeability and slippage coefficient was carried out, and the effects of methane and nitrogen on matrix shrinkage and slippage effect were discussed. The results show that the permeability prediction models based on match stick hypothesis are special cases when the matrix porosity is not considered in the new model. Compared with the new model, P-M and S-D models have more evident effect on matrix shrinkage, the new model considering matrix porosity and slippage effect is more realistic. Langmuir strain is the key to matrix shrinkage, whether the absolute permeability of coal can rebound or not is the result of joint action by cleat compression, matrix pore expansion, matrix elastic deformation and matrix shrinkage. Under the same conditions, the matrix shrinkage of methane is stronger than nitrogen, and the opposite on the slippage effect. The factors affecting slippage coefficient include internal and external cause, variation law of slippage coefficient with pore pressure is opposite to that of cleat width. The slippage effect and matrix shrinkage effect together enhance the gas permeability. The lower the pore pressure is, the more significant the effect of slippage effect and matrix shrinkage is.

Key words: low-rank coal, matrix porosity, matrix shrinkage, slippage effect, dynamic permeability