Study on Surface Charge Regulation of Nano-SiO2 and its Oil Displacement Characteristics in Low Permeability Reservoirs

Nanofluid flooding technology has shown great application potential in enhancing oil recovery in low-permeability reservoirs. To better leverage the size advantage of nano-SiO 2 and improve its dispersion stability, this paper describes the preparation of three types of nano-SiO 2 modified with negatively charged (NPs-YI), positively charged nano-SiO 2 (NPs-YA), and zwitterionic nano-SiO 2 (NPs-LX) by grafting three different electrically charged groups onto the surface of nano-SiO 2 using silane coupling agents. Fourier transform infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis, and particle size analysis show that the surface modification of nano-SiO 2 is successful. The median particle size of NPs-YI is 35   nm, and that of NPs-YA is 55   nm. The modified nano-SiO 2 exhibits good dispersion performance. The effects of temperature, salt concentration, and pH on the dispersion stability of the modified nanofluids were investigated. Results showed that NPs-YI and NPs-YA exhibited minimal changes in median particle size, zeta potential, and transmittance within a 90℃ range. Even under alkaline conditions and within a salt concentration range of 30000   mg/L, the modified nanofluids maintained strong dispersion stability. Static sedimentation experiments demonstrated that under simulated formation water conditions of 30000   mg/L, NPs-YI and NPs-YA remained stable for over 14 days, NPs-LX for over 7 days, while unmodified nano-SiO 2 completely settled within one day. The addition of the modified nanofluids altered rock wettability; after treatment with NPs-YA, the contact angle on the rock surface decreased from 159.17° to 40.24°, indicating a water-wet rock surface. Furthermore, the oil-water interfacial tension decreased to a minimum of 8.19 mN/m after nanofluid treatment. Core displacement and micro-displacement experiments showed that the oil recovery rate of 1000   mg/L NPs-YI crude oil reached a maximum of 43.8%, with a permeability retention rate of 90.8% and minimal core damage. Modified nanofluids primarily enhance oil recovery by reducing oil-water interfacial tension, altering rock wettability, and generating structural separation pressure. The application potential of the three nano-SiO 2 fluids with different surface charge types proposed in this paper in low permeability reservoirs has been verified.