Microscopic effects of nanofluid and surfactant compound ratio on the structural modification and strength weakening of wetted coal

Nanofluid-surfactant compound liquid exhibits significant application potential in improving the mechanical properties of wetted coal and enhancing the effectiveness of coal seam water injection. Determining its optimal ratio is the key to enhancing its performance. Through stability analysis, contact angle test, nanoindentation mechanical experiments, and scanning electron microscopy tests, the influences of different ratios of compound liquid on the micromechanical parameters of the coal are investigated. The results indicate that the liquid stability is significantly improved by adding cationic surfactant. In addition, the contact angle of coal remarkably reduces after the modification of the nanofluid-surfactant compound liquid. Maximum indentation depth of the coal first increases and then decreases as the compound ratio rises. When the compound ratio reaches 2, the mechanical parameters such as hardness, fracture toughness, and elastic modulus of the coal decrease significantly, and the proportion of elastic energy is minimized. Appropriately increasing the ratio of SiO2 nanofluid can boost the effectiveness of coal modification. This facilitates greater water molecule penetration into the coal, the binding forces between mineral particles are weakened, and the mechanical properties are ultimately degraded. The research findings are expected to provide a basis for optimizing the wetting agent ratio in coal seam water injection.