Wettability modification of long-flame coal by acid-based CO2 foam fracturing fluids: changes in mineral composition, microcrystalline structure, and surface chemistry

CO 2 foam fracturing shows promise for enhanced coalbed methane recovery. However, the relationship between coal wettability and the structural and surface-chemical changes following acid-based CO 2 foam treatment remains unclear. Long-flame coal was treated with regular CO 2 foam and with acetic acid-based, citric acid-based, and combined-acid CO 2 foam systems. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and dynamic contact-angle measurements were used to characterize the structural and wettability changes. The acetic acid-based system (A3) altered the relative mineral composition of the coal and induced partial disordering of its aromatic microcrystalline structure. FTIR and XPS showed higher relative contributions of C–O-related and hydrogen-bond-associated hydroxyl components and a lower contribution of C–C/C–H, although A3 did not have the highest total surface oxygen content. Across the five treatment groups, exploratory Pearson analysis showed that XPS C–O was negatively correlated with early contact angle, whereas d 002 was positively correlated. A3 showed the fastest wetting among the treatments: its contact angle was 47.84° at 0.5 s, compared with 106.56° for raw coal, and reached 0° by 3 s. Together, these results associate early wetting with concurrent changes in mineral composition, aromatic microcrystalline structure, and functional-group distribution, rather than total surface oxygen alone. They provide a physicochemical basis for understanding coal–water interfacial behavior relevant to fracturing-fluid retention and water blocking in coalbed methane reservoirs.