Phosphorylated Nanocellulose/Surfactant-Stabilized Middle-Phase Microemulsions for Enhanced Oil Recovery

Middle-phase microemulsion flooding provides stable oil-displacing fluids which are highly desirable for enhanced oil recovery (EOR). Although nanocellulose-surfactant systems have been widely explored for emulsion-based EOR, their use for stabilizing middle-phase microemulsions has been rarely reported. In this work, a novel middle-phase microemulsion flooding system was formulated by combining phosphorylated cellulose nanocrystals (P-CNCs) with a binary surfactant mix including anionic sodium dodecylbenzenesulfonate (SDBS) and amphoteric dodecyl/myristyl dimethyl hydroxypropyl sulfobetaine (HSB1214). The formulation was evaluated using n-decane as a model oil, NaCl as a model brine, and sandstone cores for core flooding tests. The negatively charged P-CNC adsorbs strongly onto the mixed surfactant micelles via electrostatic interactions and hydrogen bonding, creating a robust interfacial layer. The resulting dispersion remains single-phase and stable in 4.0 wt % NaCl for over 75 days, reflecting the intrinsic macroscopic storage stability of microemulsions. Phase behavior observation, solubilization parameters (SP_o, SP_w), and dynamic/static interfacial tension (IFT) measurements collectively confirmed the optimal Winsor III middle-phase microemulsion formulation: 0.05 wt % P-CNC, 0.5 wt % SDBS/HSB1214, and 4.0 wt % NaCl. This formulation achieved a dynamic oil-water IFT of 2.4 × 10^-4 mN/m, with static equilibrium IFT on the order of 10^-3 mN/m. In microscopic EOR and core flooding test (core permeability 217.68 × 10^-3 μm², porosity 24.45%), the optimized P-CNC-surfactant system effectively removed residual oil films and achieved an overall oil recovery of 88.85% (a 25.13% increment over conventional water flooding). Notably, the incorporation of P-CNC suppresses the SDBS adsorption loss rate from 81.60% to 47.06%, corresponding to a 42.34% reduction in surfactant loss, while the P-CNC retention in sandstone cores is only 17.61 μg/g, indicative of excellent pore-scale transport compatibility. Overall, the P-CNC-surfactant flooding system offers a stable, high-performance chemical EOR agent capable of ultralow IFT and robust operation under NaCl model brine conditions.