Impact of the Nano-SiO2 Particle Size on Oil Recovery Dynamics: Stability, Interfacial Tension, and Viscosity Reduction

Nanofluids, as a novel material for tertiary oil recovery, have shown promising potential in enhancing water flooding efficiency in low-permeability reservoirs and increasing oil recovery rates. However, the risk of particle agglomeration arises when the particle size is too small. Challenges such as maintaining dispersion stability, efficient long-distance transportation of nanofluids, and preventing particle retention and agglomeration have emerged as key obstacles hindering the widespread adoption and large-scale implementation of nanoflooding technology in enhancing oil recovery in various reservoir conditions. With issues such as poor nanoparticle dispersion stability, costly reprocessing, and limited reservoir fluid compatibility addressed, nano-SiO2 particles with average sizes of 45, 170, and 625 nm were synthesized using an enhanced particle size control technique based on the Stöber method. The impact of these different particle sizes on dispersion, emulsion stability, oil–water interfacial tension (IFT), and viscosity reduction before and after modification with fatty alcohol polyoxyethylene ether (AEO) were investigated. The findings demonstrate that modified nano-SiO2 exhibits enhanced stability and reduced oil–water IFT. At a primary AEO concentration of 0.25 wt %, micellar structure formation synergizes with SiO2 to achieve optimal IFT reduction. Smaller SiO2 particle sizes are more amenable to surface modification and exhibit superior adsorption capacity at the oil–water interface compared to larger particles, facilitating the interaction with asphaltene aggregates and, thereby, enhancing crude oil flowability. In a study, 170 nm nanosized SiO2 demonstrated a substantial viscosity reduction effect, with an apparent viscosity reduction rate of 95.3% at 0.0025 wt % SiO2 + 0.25 wt % AEO under test conditions of 45 °C and 5 s–1, indicating a significant viscosity reduction effect. This research introduces a novel concept for designing nanofluids to repel oil in low-permeability tight oil reservoirs, establishing an oil repulsion system. These findings not only advance the theoretical understanding of enhanced recovery in such reservoirs but also contribute to the efficient development of conventional and unconventional oil and gas fields, including deep shale gas and ultraheavy and thick oil reservoirs.