Macro- and Microscale Interaction Properties between Surfactant Solution and Gas-Bearing Coal

Coal seam water injection was widely applied for prewetting dust suppression and gas control. In high-gas coal seams, gas significantly interfered with the wetting of coal by H₂O, resulting in poor wettability of gas-bearing coal. Dodecyl dimethyl betaine (BS-12), known for its excellent wetting characteristics and biodegradability, was frequently used in coal wettability enhancement studies. The wetting phenomenon of H₂O on coal was actually a macroscopic and microscopic process, and its characteristics at different scales were not yet clear. Macroscopic results indicated that as the gas pressure was decreased and the concentration was increased, the contact angle value between the BS-12 solution and the gas-bearing coal gradually reduced. Microscopic analysis indicated that raising the BS-12 concentration proved to be beneficial to the displacement of adsorbed methane. The number and average bond angle of the BS-12 molecules adsorbed onto the surface of coal decreased with increasing gas pressure but increased with BS-12 concentration. Monte Carlo adsorption simulations showed that with rising BS-12 concentration and decreasing gas pressure, the number of H₂O adsorbed molecules increased. The greater the number of gas molecules adsorbed by coal, the smaller the electrostatic potential difference between the coal-gas system and H₂O molecules, leading to reduced wettability. Conversely, the higher the number of BS-12 molecules, the larger the electrostatic potential difference between the BS-12-water system and the coal-gas system, thus improving the wettability of the gas-bearing coal. These findings offer some theoretical guidance for the use of surfactants in coal-bed methane treatment.