Wettability Alteration of Coals with Different Coal Ranks Induced by Fluorocarbon Surfactant FS-61

Abstract

Abstract Coal wettability strongly influences coalbed methane (CBM) recovery and water-block damage in coal reservoirs. Wettability reversal from hydrophilic to hydrophobic is therefore an effective approach for improving CBM production. In this study, low-rank long-flame coal, medium-rank coking coal, and high-rank anthracite were selected to investigate the rank-dependent wettability reversal induced by the fluorocarbon surfactant FS-61. Contact angle, surface tension, SEM-EDS, and Fourier transform infrared spectroscopy (FTIR) were combined with low-field nuclear magnetic resonance (NMR) and low-temperature nitrogen adsorption to characterize changes in surface wettability, morphology, functional groups, and pore structure. The critical micelle concentration of FS-61 was determined to be 0.3 wt %, and the surfactant markedly reduced the surface tension of the aqueous solution. FS-61 treatment decreased hydrophilic hydroxyl groups and introduced hydrophobic C–F moieties on coal surfaces, resulting in enhanced hydrophobicity. The wettability reversal efficiency decreased with increasing coal rank, with the water contact angle of long-flame coal increasing from 51.70° to above 90%, indicating the strongest modification effect. In contrast, anthracite exhibited a weaker response, which was attributed to its relatively compact pore structure and intrinsically higher hydrophobicity. Nitrogen adsorption showed that FS-61 increased the adsorption capacity of long-flame coal by 248.85% while decreasing that of anthracite by 27.54%, demonstrating a pronounced rank dependence of pore-structure modification. EDS mapping further revealed that fluorine adsorption followed the order of long-flame coal > coking coal > anthracite. FTIR results indicated that FS-61 mainly modified surface functional groups without substantially altering the coal macromolecular framework. Low-field NMR demonstrated enhanced liquid flowback from coal pores after treatment, indicating effective mitigation of water-block damage. These findings reveal the strong dependence of FS-61-induced wettability reversal on coal rank and provide a basis for targeted wettability modification and enhanced CBM recovery.