Dynamic evolution characteristics and microscopic mechanisms of contact between gas-bearing coal and water

The effectiveness of coal seam water injection in controlling gas and dust is closely related to the wetting effect of water on the coal. A dynamic wetting process is formed after the contact between gas-bearing coal and water, which is crucial for revealing the wetting mechanism of coal seam water injection and the application of coal seam water injection. However, the contact dynamic evolution characteristics and microscopic mechanism between gas-bearing coal and water are still unclear. Based on this, this paper uses the self-developed surface tension analysis software between water and gas, the contact angle experimental equipment of gas-bearing coal, and the water droplet contour extraction software to study the contact evolution characteristics between gas-bearing coal and water. The microscopic mechanism of contact evolution between gas-bearing coal and water is revealed based on molecular dynamics simulation. The results show that the relationship between surface tension and gas pressure satisfies the negative exponential function. The contact angle is a function of gas pressure and time, which is negatively correlated with time and positively correlated with gas pressure. Under the same gas pressure condition, with the increase of wetting time, the interface energy of coal and water decreases gradually, the surface energy of coal increases gradually, and the adhesion work increases gradually. With the increase of gas pressure, the coal-water interface energy becomes more extensive, the surface energy of coal becomes smaller, the adhesion work becomes smaller, and the difficulty of water spreading on the surface of coal increases. With the increase of wetting time, the height of the drop of the highest point of the water droplet contour increases gradually, and the contact surface width between the water droplet and the gas-bearing coal increases gradually. The gas pressure is negatively correlated with the height of the drop at the highest point of the water droplet profile and the increase of the contact surface width between the water droplet and the gas-bearing coal, which is consistent with the contact evolution law between gas-bearing coal and water obtained from molecular dynamics simulation. The concentration of water molecules appears at a peak point above the interface between coal and water. At a certain distance above the peak point, the concentration distribution of water molecules increases with the decrease of gas pressure and increases with the increase of gas pressure. For high gas coal seam, the coal seam permeability improvement measures and water injection measures can be used in combination, and the permeability improvement and drainage gas measures can be taken to reduce the gas pressure of coal seam before water injection, so as to increase the wetting range and wetting degree. The research results provide some theoretical support for revealing the dynamic wetting mechanism of water in gas-bearing coal and the application of coal seam water injection.