Experimental Study on Coal Structure Variation and CH4 Adsorption Properties under the Effect of Cocamidopropyl Betaine

Surfactants can be used to improve the coal surface wettability and reduce the CH4 adsorption capacity of coals, so they are usually added to fracturing fluids to promote coalbed methane (CBM) production. In this work, the impacts of a surfactant cocamidopropyl betaine (CAB) on coal structures and CH4 adsorption characteristics were investigated. The CAB solutions with five CAB concentrations (0, 0.002, 0.006, 0.008, and 0.01 mol/L) were prepared and used to treat coal samples. The treated coal samples were dried and tested for infrared spectroscopy experiments, liquid nitrogen adsorption experiments, and CH4 adsorption experiments. The results indicate that the hydrophilicity of the coal samples was enhanced after being treated with CAB solutions; the number of hydroxyl functional groups increased; and some aliphatic, aromatic, and oxygen-containing structures disappeared, which promoted the replacement of CH4 by H2O and reduced the amount of adsorbed CH4. Meanwhile, the pore volume of the coal samples increased and the CBM migration rate was improved. Among the five CAB concentrations, 0.008 mol/L CAB was determined to be an optimal concentration to increase the CBM extraction efficiency. Compared to the untreated coal, the amount of adsorbed CH4 decreased by 13.18% and the CBM migration channel volume increased by 16.74% at 0.008 mol/L CAB. The results reported in this work will offer new insights into the improvement of CBM extraction efficiency using CAB.