Active Nanofluids for Enhanced Shale Oil Recovery: Synergistic Imbibition and Multiscale Pore Access

Shale oil development faces challenges such as rapid production decline and high-cost formation damage caused by refracturing. To address these issues, this study proposes an active silica-based nanofluid with a total concentration of 0.12 wt % (0.045 wt % nanoparticles and 0.075 wt % AES surfactant). Dynamic light scattering (DLS) analysis demonstrates that the nanofluid exhibits excellent stability under high-temperature and high-salinity conditions (90 °C, salinity of 1.0 × 105 mg/L), maintaining an average hydrodynamic diameter of approximately 22 nm, which is significantly smaller than that of the unmodified system (30 nm). Interfacial tension (IFT) measurements revealed a stable IFT of 0.358 mN/m at a concentration of 0.3 wt %, while dynamic contact angle tests confirmed its strong wettability alteration capability. Spontaneous imbibition experiments revealed enhanced oil recovery, with increases of 23.8 and 10.62% compared to simulated formation water (SF water) and AES alone, respectively. Additionally, results indicated that higher core permeability facilitated easier oil–water extraction, while oil saturation primarily influenced oil–water displacement through the starting pressure gradient. A multiscale pore classification model was established by integrating nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and nitrogen adsorption analysis, enabling the quantification of oil contributions from different pore size ranges: micropores (<0.07 μm) contributed 50.94%, macropores (>3.67 μm) 28.92%, and mesopores (0.07–3.67 μm) 20.15%. These results demonstrate its crucial role in overall oil recovery during shale imbibition. Finally, the mechanism behind enhanced recovery was elucidated through synergistic wettability alteration and capillary displacement equations. This work presents a cost-effective nanofluid formulation and a multiscale pore analysis methodology, providing practical approaches and theoretical insights for enhancing shale oil recovery.