Dynamic evolution mechanism of water-bearing coal permeability and water film under stress

To explore coal permeability evolution mechanism under the comprehensive action of multiple factors including stress - adsorption - water and slippage effect, considering the coal deformation induced by stress - adsorption, the expression of water film thickness was corrected to quantitatively characterize the effective pore size, and based on this, the intensity of gas slippage effect of water-bearing coal was further quantified and the permeability model was established under the comprehensive action of multiple factors. Combined with experimental research to verify the reliability of permeability model, and then the evolution mechanism of coal permeability, water film and slippage factors under the comprehensive action of multiple factors was further revealed. The results show that under different water saturation conditions, the permeability decreases sharply first and then tends to flat with the increase of effective stress; under the same effective stress condition, the permeability decreases with the increase of water saturation. The water film thickness changes dynamically under the action of stress - adsorption – water, the water film thickness has a negative correlation with stress and adsorption, but a positive correlation with water saturation; the slippage factor increases gradually with the increase of water saturation, but the increase trend is gentle under low stress condition, and more sharply under high stress condition. In addition, based on the disjoining pressure of gas-liquid-solid surface, the expressions of dynamic water film in square and equilateral triangle under the effect of stress-adsorption were deduced, and the evolution mechanisms of gas permeability, water film and slip coefficient of pores with different geometric shapes are compared and analyzed. Due to the presence of corner holes, the order of water film thickness in pores of different geometric forms is circle > square > equilateral triangle from large to small, the order of permeability is opposite; the slippage factor in circular is larger than that in angular pore, while the slippage factor in square and equilateral triangle pore has little difference.