Modeling of gas migration in water-intrusion coal seam and its inducing factors

Formation water intrusion into the coal seam through cross drainage boreholes seriously affects the gas extraction efficiency. Particularly in the downward boreholes, the formation water cannot be discharged through the drain. Using the conventional seepage model to estimate the effect of gas drainage will cause that the coal seam residual gas pressure is underestimated. In this paper, we construct the gas migration models in a water-intrusion coal seam, which include the gas seepage in original coal seam and the gas diffusion in water-saturated coal seam. By comparing with the field data, the mathematical models are authenticated. Then, the influences of diffusion coefficient, permeability, Henry's constant and the radius of water-saturated coal seam on the residual gas pressure and gas production are analyzed by numerical calculations. The results show that the gas pressure decreases with the increase of the diffusion coefficient, while increases with the increase of Henry's constant and the radius of the water-saturated coal seam. The impact of permeability is more complex. In the high permeability coal seam, the overall pressure relief effect is better. However, in the low permeability coal seam, the pressure relief effect around the borehole is better. Finally, the influence mechanism of each factor is analyzed to revel the gas production in the water-intrusion coal seam. The results of the work are of great significance to enrich the theory of gas migration and the prevention and control of mine gas disaster.