Evaluating the Role of Monovalent and Divalent Salts on Interfacial Tension Reduction Using Nitrogen-Doped Carbon Dots for Enhanced Oil Recovery

Nanofluids based on carbon dots (CDs) effectively reduce interfacial tension (IFT) in enhanced oil recovery (EOR), but their stability against salt ions can be compromised by structural defects. Divalent ions like Ca ²⁺ and Mg ²⁺ can destabilize CDs, causing precipitation, while salts can also enhance IFT reduction. This study evaluates the impact of salt concentration and CD type on IFT between crude oil and water, examining various synthesis parameters and nitrogen-doping reactants. Characterization and IFT tests reveal that T_CDs have the highest polarity, with Ca ²⁺ ions most destabilizing CDs and Mg ²⁺ ions most effectively reducing IFT when CDs are stable. NaCl significantly reduces IFT in low-polar E_CDs (18.3 mN/m), while Ca ²⁺ and Mg ²⁺ increase IFT due to instability. In high-polar T_CDs, NaCl’s IFT reduction ability decreases (0.67 mN/m), but Ca ²⁺ and Mg ²⁺ more effectively reduce IFT by occupying interface sites (2.53 and 3.37 mN/m, respectively). Moderate-polar U_CDs show varied IFT reduction based on salt type and concentration. Longer reaction times and increased citric acid as a reactant enhance CD polarity and IFT reduction for MgCl ₂ (8.88 mN/m) and CaCl ₂ (5.3 mN/m) while reducing NaCl’s impact. These findings highlight the complex interactions between nitrogen-doped CDs and salts, providing valuable insights for optimizing EOR operations in dense rock reservoirs.