SiO2 nanofluids exhibit significant potential for preventing coal mine gas disasters through the technology of coal seam water injection by enhancing coal wettability and weakening its mechanical properties. Identifying the optimal nanofluid concentration is crucial for maximizing its modification effectiveness. Grounded in fluid dynamics, this study investigates the mechanisms of progressive failure and strength weakening in coal treated with SiO2 nanofluids at varying concentrations through triaxial mechanical testing, acoustic emission (AE) monitoring, computed tomography (CT) scanning, and scanning electron microscopy testing. Results indicate that, unlike the instantaneous failure of untreated coal, nanofluid-modified coal samples exhibit progressive failure. While overall strength is weakened, mechanical parameters, specifically elastic modulus and Poisson's ratio, exhibit the non-monotonic trend, decreasing initially and then increasing with nanofluid concentration. The plastic failure gradually intensifies, with elastic energy released progressively rather than instantaneously during failure. Furthermore, the AE signals shift from the post-peak concentrated release to a pre-peak sustained, low-amplitude distribution. CT analysis reveals that nanofluid modification increases crack quantity, crack volume ratio, and fractal dimension. Among all coal samples, the 0.5 wt. % SiO2 nanofluid modification results in the most significant crack propagation and distinct crushed fissures, correlating with the most significant progressive failure and strength weakening. However, higher concentrations cause excessive nanoparticles agglomeration, restricting further improvement. The observed non-monotonic trends are attributed to fluid dynamics mechanisms. The optimal concentration (0.5 wt. %) achieves a balance between enhanced capillary imbibition and interfacial lubrication facilitated by nanoparticle adsorption. Conversely, higher concentrations lead to dominant particle agglomeration, which restricts fluid penetration and thereby partially restores mechanical resistance. These findings not only provide theoretical guidance for optimizing nanofluid concentration in coal seam water injection but also offer a fundamental fluid physics framework for understanding how nanoparticle suspensions regulate the mechanical behavior of porous media.
Liang et al. (Sun,) studied this question.