Gas-liquid competitive adsorption characteristics and coal wetting mechanism under different pre-adsorbed gas conditions

Coal seam water injection was widely used to prevent mine gas and dust disasters. It is undeniable that the gas adsorbed on coal surface can influence the coal-water wettability. In order to investigate how the gas environment affects the coal-water contact characteristics, the coal-water contact angle was tested under different gas environments, including CH4, N-2 and He. Then the microscopic wetting mechanism were revealed by combing with the molecular dynamic simulation. Results showed that, the coal-water contact angle gradually increases in the range of 0 similar to 2 MPa. The contact angle shows the most significant growth trend in CH4 environment, increasing from 72.11 degrees to 106.9 degrees, and the wettability transits from hydrophilic wetting (<90 degrees) to neutral wetting (=90 degrees), eventually it becomes hydrophobic wetting (>90 degrees). At the same gas pressure, the coal-water contact angle in the CH4 atmosphere is the largest, followed by N-2 atmosphere and He atmosphere. Molecular dynamics simulation results showed that the adsorption capacity of water molecules decreases as the pre-adsorption gas pressure grows. The adsorption capacity of water molecules decreased from 1.84 mmol/g, 2.04 mmol/g and 2.23 mmol/g to 1.46 mmol/g, 1.51 mmol/g and 1.69 mmol/g under CH4, N-2 and He atmosphere, respectively. Furthermore, the surface free energy and adsorption energy of water and gas were analyzed, finding that the surface free energy drops down with the rising pressure and the gas adsorption capacity, leading to the poor wettability. The adsorptive gas shows a more remarkable effect in weakening coal-water wettability due to the gas adsorption layer and gas-water competitive adsorption.