Effects of Al2O3 nanofluids and pH conditions on wettability and micromechanical properties of coal

Conventional hydraulic fracturing is widely used for coalbed methane extraction. However, in deep coal seams, poor wettability and high stress concentration often lead to rapid elastic energy release, resulting in an elevated risk of outbursts. This study systematically investigates the regulatory mechanisms of Al 2 O 3 nanofluids on coal wettability and micromechanical properties under different pH conditions by integrating molecular dynamics simulations with laboratory experiments. The results indicate that acidic solutions corrode and partially dissolve calcite, hematite, and kaolinite, while exerting limited influence on quartz; in contrast, alkaline solutions dissolve quartz and generate Na 2 SiO 3 precipitates. Al 2 O 3 nanofluids significantly reduce the coal–water contact angle. Under pH=3 conditions, the contact angle decreased to 47.5°, representing a 38.31% reduction compared with raw coal, and the adsorption layer thickness increased from 16.77 Å in pure water to 28.79 Å, transforming the coal from hydrophobic to hydrophilic. Different pH conditions also caused significant variations in coal’s micromechanical properties. Under acidic conditions (pH=3), the elastic modulus and hardness decreased by 50.11% and 34.69%, respectively, while peak displacement and contact displacement increased by 26.32% and 33.46%. Under alkaline conditions (pH=11), the mechanical properties were intermediate between those of raw coal and the acidic group, but were more prone to delayed creep during load holding. This study elucidates the composite modification mechanism of pH conditions and Al 2 O 3 nanofluids, providing theoretical support and new insights for enhancing fluid injection efficiency and mitigating dynamic disasters in deep, high-gas coal seams.