A mild oxidation cleavage-and-polymerization strategy towards sulfonate-modified carbon dots for enhanced oil recovery

As a new type of oil displacement agent, carbon dots (CDs) with ultrasmall size show great potential in developing unconventional reservoirs. However, the complex synthesis process, high energy consumption, and difficulty in surface modification restrict their practical applications. Herein, we propose a facile and mild oxidation cleavage-polymerization strategy for engineering sulfonate-modified carbon dots (sm-CDs) based on a low-temperature hydrothermal technology for enhanced oil recovery (EOR). This methodology employs humic acid as the carbonaceous precursor and sodium sulfamate as the sulfonation agent, utilizing hydrogen peroxide-mediated oxidative initiation under mild hydrothermal conditions (90 °C). The synthetic method achieves concurrent oxidative cleavage of humic acid macromolecules and subsequent polycondensation with sulfonation moieties, enabling one-step in situ functionalization of CDs with modified surface sulfonate groups. The sm-CDs demonstrate outstanding performance in dispersion stability under extreme reservoir conditions (high-temperature: 90 °C and high salinity: 5 × 10⁴ mg/L NaCl), and the interfacial activity. Core flooding tests show that 0.2 wt% sm-CDs nanofluid significantly reduces the injection pressure by 62.15 % and increases the oil recovery from 48.14 % to 63.92 % compared with water flooding. Subsequent investigation indicates the tailored sm-CDs adsorb on core surface, which improves the heterogeneity of wettability and decreases the average oil-rock adhesion forces from 1.621 to 0.091 nN, thereby effectively enhancing the oil displacement. This study provides a valuable reference for the development and application of efficient CDs nanomaterials in the oilfield.