Influence of effective stress on gas slippage effect of different rank coals

Gas slippage effect is one of the key factors which would influence gas seepage in coal. The gas permeability of various rank coals under effective stresses are analyzed, and the influence of effective stresses on gas slippage effect of various rank coals and its control mechanism are revealed by the experiment of gas seepage in coal. The results indicate that with the decrease of pore pressure, the gas permeability and the gas slippage effect in coal increases, which follows the variation laws of Klinkenberg slippage equation. The gas slippage effect in various rank coals is controlled by the effective stress, which shows that with the increase of effective stress, the coal permeability decreases and the gas slippage coefficient increases. The relationship between Klinkenberg permeability of coal and its effective stress obeys negative exponential function. With the increase of effective stress, the gas slippage coefficient in coal varies by the positive exponential function. On this basis, a novel evaluation method of gas slippage effect is proposed. The permeability improvement degree of coal is used to quantitatively evaluate the influence degree of gas slippage effect on the coal permeability. With the increase of pore pressure, the permeability improvement degree of different rank coals dropped by the power function. And with the increase of effective stress, the value of permeability improvement degree of coal increases.