Quantum Dot-Hybridized Temperature- and Salt-Resistant Polyacrylamide for Enhanced Oil Recovery in High-Temperature and High-Salinity Reservoirs

Conventional partially hydrolyzed polyacrylamide (HPAM) suffers severe chain coiling, viscosity attenuation and precipitation under high-temperature and high-salinity reservoir brines, restricting its tertiary oil recovery efficiency. Herein, a novel carbon quantum dot hybrid terpolymer (QDHSTP) was synthesized via free-radical copolymerization, where silane-modified nitrogen-doped carbon quantum dots (FNCQDs) were covalently bonded to acrylamide (AM)/2-acrylamido-2-methylpropane sulfonic acid (AMPS)/diallyldimethylammonium chloride (DMDAAC) backbones. FTIR, 1H NMR and thermogravimetric analysis (TGA) verified successful grafting of FNCQDs, while SEM revealed a continuous three-dimensional entangled network constructed by polymer chains. Steady and oscillatory rheology systematically characterized the solution viscoelasticity: QDHSTP solutions followed the power-law shear-thinning model, with flow behavior index n decreasing from 0.698 to 0.676 and consistency factor k rising from 80.91 to 131.13 mPa·sn as concentration increased from 2000 to 3000 mg/L. All samples behaved as viscosity-dominated viscoelastic fluids, with elastic modulus exhibiting stronger frequency dependence. Benefiting from embedded FNCQDs, QDHSTP retained 79.74% and 76.06% of initial viscosity in 1.0 × 104 mg/L NaCl and CaCl2 brine, respectively, markedly better than that of the polymer without incorporated FNCQDs respectively, and maintained thickening capacity at 90 °C. Artificial sandstone core flooding demonstrated an incremental oil recovery of 29.8% over baseline waterflooding, attributed to mobility control and elastic residual oil stripping. This covalent nanohybrid strategy provides a facile route to construct thermo-salt tolerant polyacrylamides, offering a promising candidate polymer for harsh oil reservoir chemical flooding.