Mechanisms of Al2O3 nanofluids affecting coal wettability and gas adsorption-desorption under different pH conditions: Molecular simulation and experimental study

Abstract

Coal dust pollution during mining is a major cause of occupational diseases. Although hydraulic fracturing is widely applied for coalbed methane extraction, poor coal wettability, limited dust suppression, and low gas desorption restrict its efficiency. This study prepares Al₂O₃ nanofluids with varying pH values and concentrations and investigates their dispersion and effects on coal surface modification, wettability, oxygen-containing functional groups, and gas adsorption-desorption behavior. Molecular dynamics simulations, contact angle measurements, zeta potential analysis, mercury intrusion porosimetry, methane adsorption, ESEM, and FTIR are employed. In the Coal/Al₂O₃/H₂O system, the adsorption layer thickness increases to 32.96 Å, a 44.43 % rise compared with Coal/H₂O. Acidic nanofluids maintain good dispersion for 48 h, whereas alkaline solutions precipitate easily. Increasing Al₂O₃ concentration significantly enhances coal wettability, with the contact angle decreasing by 65.7 % at pH 3 and 4.00 wt%. Both acidic and alkaline treatments increase surface oxygen-containing functional groups, negatively correlating with the coal-water contact angle, while the contact angle positively correlates with gas adsorption. Acidic corrosion alters coal microstructure and promotes macropore expansion. The synergistic "dispersion–wettability–corrosion" mechanism effectively enhances coal wettability and gas desorption. This study provides theoretical and practical guidance for improving wettability and efficient gas extraction in high-gas coal seams.