Evaluation and modeling of gas permeability changes in anthracite coals

The gas permeability of anthracite coal is altered as a result of the effective stress increase, the coal matrix shrinkage and the gas slippage that occur during the gas pressure depletion process. This paper describes an investigation of changes in the adsorbing-gas (CO2) permeability of three anthracite coal cores (samples A, B and C). The changes in permeability under a constant confining stress condition (4.3 MPa) were found to be distinct for the three cores, and these observations are considered as the superimposed results of the three effects based on the following findings. During the gas pressure depletion: (a) the permeability is negatively proportional to the effective stress, and the slopes of the straight lines are near unity when the mean gas pressures are greater than 0.2–0.4 MPa; (b) the permeability increment induced by the gas slippage levels off to an approximately constant value (core A, 0.5 μD; core B, 0.6 μD; core C, 0.02 μD) when the mean gas pressure is greater than 0.8 MPa and subsequently becomes more significant at pressures less than 0.8 MPa; (c) the matrix shrinkage-induced permeability increment increases linearly for cores A and B but increases logarithmically for core C; and (d) the effect on the permeability of the effective stress predominates for core A, but for core C, the effect of the matrix shrinkage is most significant. Furthermore, an empirical model that predicts the adsorbing-gas permeability change was proposed as a function of the permeability increments caused by the effective stress, the matrix shrinkage and the gas slippage at each gas pressure under a constant confining stress condition, and can be mathematically expressed as: k gt = k oi + Δ k sli ( p t ) + Δ k shr ( p t ) + Δ k eff ( p t ) . The results demonstrate that good agreement was achieved between the empirical models and the experimental data.