Interfacial tension reduction using nitrogen graphene quantum dots with various precursors, molar ratios, and synthesis durations for enhanced oil recovery.

Nanofluids have the capacity to reduce interfacial tension (IFT) of crude oil and water for enhanced oil recovery (EOR) operations, but traditional nanoparticles are limited in tight reservoirs due to their inappropriate size for micro-nano pores and their tendency to aggregate. In this paper, Graphene Quantum Dots (GQDs) with simple and favorable properties are developed, and their performance and mechanism for reducing IFT are evaluated. The paper also aims to explore the effects of GQD precursor type, synthesis duration, and molar percentages of precursors on reducing IFT. For this purpose, citric acid was used as a carbon source, and ethylenediamine, urea, and thiourea were used as nitrogen sources to synthesize different GQDs. FTIR, XPS, HR-TEM, XRD, UV visible, and PL photoluminescence were used to identify the GQDs' characteristics. The highest IFT reduction value is achieved by using 1000 ppm ethylenediamine-derived GQDs, which reduces the IFT from 19.03 to 0.70 mN/m at 200,000 ppm NaCl concentrations without using any surfactants. The XPS analysis revealed that ethylenediamine-derived GQDs exhibit higher pyrrolic nitrogen content and a relative intensity ratio of sp3 C/sp2 C. It has been identified that the presence of more polar atomic bonds on the surfaces of GQDs decreases the ability of GQDs to reduce IFT. In this way, urea-derived and thiourea-derived GQDs exhibit less capability for IFT reductions from 19.03 to 12.33 and 18.04 mN/m at 1000 ppm GQDs and 200,000 ppm NaCl concentration, respectively. Furthermore, ethylenediamine-derived and urea-derived GQDs perform optimally at approximately 5 and 10 h, respectively, with a precursor molar ratio of 3 (mole ethylenediamine/urea per mole citric acid), while thiourea-derived GQDs show no significant IFT change with varying synthesis times or precursor molar ratio. The developed GQDs provide a promising solution for EOR operations in unconventional reservoirs by significantly decreasing the IFT between crude oil and nanofluids.