Preparation and Performance Evaluation of a Core–Shell Nanosphere/Surfactant Composite System for Profile Control and Enhanced Oil Recovery in Low-Permeability Reservoirs

To address early water breakthrough and poor residual-oil mobilization in low-permeability reservoirs, a core–shell nanosphere/surfactant composite system was developed for profile control and enhanced oil recovery. The core–shell nanospheres were prepared by a semi-continuous seed-growth method, and a target particle-size window of 100–200 nm was selected based on pore-throat/particle matching. The representative sample, HK-0417, had an average particle size of about 120 nm and showed good dispersion stability in formation brine at 45 °C. After blending with the surfactant ALT-603, the system achieved an ultralow oil–water interfacial tension on the order of 10−3 mN/m and reduced the water contact angle of the oil-aged surface from 125° to 70°, indicating a shift toward near-neutral wettability. Core-flooding tests further showed that, under the same chemical dosage, slug injection (HK-0417 followed by ALT-603) demonstrated better performance than co-injection, with higher incremental oil recovery (15.49% vs. 13.17%) and higher plugging efficiency during subsequent water flooding (81.25% vs. 78.46%). The novelty of this work lies in integrating particle-size-window design, controllable preparation of core–shell nanospheres, and direct comparison of injection strategies within one system. The results provide practical guidance for formulation design and injection-mode selection for enhanced oil recovery in low-permeability reservoirs.