Evaluating the role of monovalent and divalent salts on interfacial tension reduction using nitrogen doped carbon dots for enhanced oil recovery

Abstract

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 Ca2+ and Mg2+ 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, specifically ethylenediamine, urea, and thiourea with citric acid to synthesize E_CDs, U_CDs, and T_CDs, respectively. Characterization and IFT tests reveal that T_CDs have the highest polarity, with Ca2+ ions most destabilizing CDs and Mg2+ ions most effectively reducing IFT when CDs are stable. NaCl significantly reduces IFT in low-polar E_CDs (18 mN/m IFT reduction), while Ca2+ and Mg2+ increase IFT due to instability. In high-polar T_CDs, NaCl’s IFT reduction ability decreases (0.67 mN/m IFT reduction), but Ca2+ and Mg2+ more effectively reduce IFT by occupying interface sites (2.53 and 3.37 mN/m IFT reduction, respectively). Moderate-polar U_CDs (urea-derived CDs) show varied IFT reduction based on salt type and concentration. Increased citric acid as a reactant enhances the polarity of T_CDs and improves their IFT reduction capability for MgCl2 (from 3.37 IFT reduction to 8.88 mN/m IFT reduction) and CaCl2 (from 2.53 to 5.3 mN/m), while reducing the impact of NaCl on E_CDs. These findings highlight the complex interactions between nitrogen-doped CDs and salts, providing valuable insights for optimizing EOR operations in dense rock reservoirs. Nitrogen-doping and specific salt interactions can significantly influence the development of effective EOR strategies, maximizing recovery while minimizing operational costs and issues such as nanoparticle instability and reservoir pore plugging. By optimizing the interactions between CDs’ functional groups and salt ions, the industry can improve nanofluid stability and performance under various reservoir conditions, leading to more effective EOR processes.