Anomalous Variation Law in the Effects of Different Surfactants on Coal Wettability under a Gas Atmosphere

Surfactants are widely used to reduce the surface tension of a solution, thereby enhancing the coal–water wettability. However, natural coal reservoirs contain significant amounts of gas under pressure. The wettability of gas-containing coal is influenced not only by gas adsorption but also by environmental pressure. Consequently, surfactants selected under atmospheric pressure may not be suitable for gas-containing coal. To investigate the wetting characteristics of surfactants on gas-containing coal, surface tension, sedimentation, and contact angle experiments were conducted. Additionally, molecular dynamics simulations were employed to examine the electrostatic potential, relative concentration, and mean square displacement. The results showed that the wettability of coal by aqueous solutions significantly improved with increasing surfactant concentration under atmospheric pressure. However, for gas-containing coal, as the gas pressure increases, the coal–water contact angles increase notably, reflecting a decrease in wettability. It is noteworthy that, under atmospheric pressure, the contact angles followed the ranking: Rapid penetrant T < SDBS < JFC < OP-10. Under 2 MPa gas pressure, however, the ranking changed to SDBS < Rapid penetrant T < OP-10 < JFC. The anomalous changes indicate that in a gas environment, the number of Rapid penetrant T and JFC molecules adsorbed on the coal surface is reduced, leading to a significant decline in their wettability. In contrast, SDBS and OP-10, due to the strong directional properties of their alkyl chains, continuously aggregate and bend toward the coal surface through electrostatic interactions. However, the stacking of molecular chains partially masks the hydrophilic sites, slightly reducing their wettability. This phenomenon led to abnormal surfactant wetting in gas-containing coals. This study clarifies the mechanism of surfactant action in the wetting process of gas-containing coal, providing a theoretical basis for improving coal–water wettability in a gas environment.