Influence of Nanoparticle–Surfactant Compound Solution on Coal Wettability

Poor coal wettability limits the effectiveness of water injection in coal seams. Nanofluids and surfactants can improve the coal wettability. In this paper, the impact of four types of composite fluids containing surfactants and SiO2 nanoparticles on coal wettability was investigated. By analyzing the variation in contact angle and moisture content as well as functional groups before and after modification, it is found that the compound solution can effectively enhance coal surface wettability and increase the moisture content of coal samples. Compared to deionized water and SiO2 nanofluids, the contact angle of coal modified by the compound solution does not rebound quickly with the loss of moisture. Moreover, the average strengthening rate and water absorption mass strengthening rate were utilized to quantify the modification effects of the compound solution. It can be seen that the combination of cationic surfactant CTAB with SiO2 nanoparticles provides the most effective modification. FTIR results imply that the modification results in increased activity of free hydroxyl groups in the coal, enhancing its hydrophilicity. The synergistical interaction of nanoparticles and surfactants can occupy vacant adsorption sites on the coal surface. Additionally, surfactants adhere to the nanoparticle surface due to van der Waals forces, while hydrophilic groups attract water molecules to the coal. The research achievements provide a theoretical foundation for the potential application of nanoparticle–surfactant compound solution in coal seam water injection.