Mechanisms of wettability-enhanced gas displacement and nanomechanical strength weakening in coal modified by active nanofluids

To address the limitations of conventional hydraulic fracturing in deep coal seams, this study constructed active nanofluid systems by combining SiO 2 nanoparticles with three types of surfactants: the cationic surfactant dodecyltrimethylammonium bromide (DTAB), the anionic surfactant sodium dioctyl sulfosuccinate (AOT), and the nonionic surfactant alcohol ethoxylate (AEO-9). Experimental characterization and molecular simulations were integrated to systematically investigate their effects on the physicochemical structure, wettability, gas adsorption and desorption behavior, and nanomechanical strength of coal. The results showed that the addition of surfactants significantly improved the dispersion and attachment of SiO 2 nanoparticles on the coal surface, enabling the composite systems to form an interfacial adsorption configuration in which hydrophobic chains were oriented toward the coal surface, while hydrophilic groups faced the aqueous phase. After treatment with active nanofluids, the response of oxygen-containing functional groups, including C–O and C==O, was enhanced on the coal surface. The total absorption peak area of oxygen-containing functional groups was negatively correlated with the coal-water contact angle, whereas the coal-water contact angle was positively correlated with CH 4 adsorption capacity. As coal wettability increased, the elastic modulus and hardness generally decreased, while the maximum displacement, contact displacement, and creep displacement increased. Among the treated samples, the AOT + SiO 2 group exhibited the lowest elastic modulus and hardness, which decreased by 25.21% and 25.14%, respectively, compared with raw coal, indicating that this system produced the strongest wettability enhancement and the most pronounced weakening of nanomechanical strength. This study elucidates the synergistic modification mechanism of surfactant-SiO 2 composite nanofluids and provides theoretical insights and a potential approach for improving fluid injection efficiency and mitigating dynamic hazards in deep coal seams.