Mechanical Response and Damage Evolution of Coal Boreholes Stabilized with Drilling Fluids Containing Nanoparticles

Adding nanoparticles (NPs) to drilling fluid can effectively suppress the weakening effect of water on borehole stability. In this study, isostatic triaxial mechanical tests were performed on the thick-walled hollow cylinder (TWHC) coal specimens that had been immersed in the drilling fluids containing different NPs. The results show that the critical initiation pressure of coal specimens treated by the Water-based drilling fluid (WBD) containing 30 nm Fe 2 O 3 NPs and 50 nm SiO 2 NPs increases by 22.2% and 21.6% respectively. The elastic energy of the Fe 2 O 3 -treated group exceeds that of the SiO 2 -treated group, reaching approximately 50 kJ/m 3, accompanied by a smaller dissipation ratio. Computed tomography (CT) scans and acoustic emission (AE) monitoring reveal that the addition of NPs shifts the failure mode of coal specimens from brittle fracture to a complex mode dominated by shear. NPs effectively suppress macro-crack propagation through physical blocking and surface effects, reducing crack length and quantity. However, under high confining pressure, the likelihood of localized failure in the coal specimens increases. Mechanical analyses of borehole stress states using the Mogi-Coulomb criterion show that the predicted failure values are consistently lower than the measured values, indicating that the Mogi-Coulomb criterion is not fully applicable to NPs-treated TWHC coal specimens. This discrepancy is attributed to the NPs, which form a hydrophobic rough film on the coal surface and plug pores and microfractures of the coal matrix, thereby reducing drilling fluid filtrate invasion and enhancing borehole stability.