Experimental study on the effect of cationic/nonionic surfactants on the wettability of medium rank coal

Coal seam wettability is a critical factor affecting coalbed methane productivity. To investigate the mechanism of cationic and nonionic surfactants on the wettability of medium rank coal, this paper uses No. 8 coal seam from Baode Coal Mine as the research subject. The coal samples are treated by soaking in cationic surfactant 31526 and nonionic surfactant OP-10, respectively. Systematic tests are conducted to measure the changes in contact angle, mineral composition, and surface roughness of the coal samples before and after treatment. The experimental results show that after treatment with distilled water, surfactant solution 31526, and surfactant solution OP-10, the contact angles of the coal samples are 65.6°, 71.8°, and 41.4°, respectively. After treatment with 31526, the quartz content in the coal samples decreases, while the kaolinite content increases, along with an increase in surface roughness. The surfactant molecules occupies the adsorption sites on the coal, forming a structure with hydrophilic groups facing inward and hydrophobic groups facing outward, thereby increasing the contact angle and reducing wettability. In contrast, OP-10 treatment increases both the quartz and kaolinite content in the coal samples, enhances surface roughness, and exposes more hydrophilic functional groups, thereby strengthening surface hydrophilicity. As a result, the contact angle significantly decreases, and wettability improves. The findings of this study can provide a theoretical basis for the selection and optimization of surfactants in coalbed methane fracturing fluids.