Fluorescent Carbon Quantum Dot–Based Nano-Surfactants for Multifunctional Interfacial Modulation and Improved Oil Recovery in Low-Permeability Reservoirs

Abstract

Carbon quantum dot-based cationic surfactants integrate nanomaterial transport advantages with interfacial activity, enabling simultaneous interfacial tension reduction, wettability alteration, and deep reservoir penetration, thus offering a fundamentally new strategy for enhanced oil recovery. Therefore, a novel nano-surfactant was derived from carbon quantum dots, prepared by hydrothermal method using pectin as the carbon source and N,N-dimethylpropanediamine as the nitrogen source, and characterized with FTIR spectra, XPS spectra to confirm the successful doping of nitrogen atoms into the carbon dot structure and quaternization by the introducing of carbon chains. And, through UV-visible spectroscopy and fluorescence measurement, the carbon quantum dots NCQDs and NCQDs-n surfactants solutions exhibited better optical properties and emitted blue fluorescence under UV light, indicating successful synthesis. Through measuring the surface tension by a surface tensiometer, the NCQDs-n surfactants possess good surface tension reduction abilities and the CMC of surfactants decreases with the increasing hydrophobic carbon chain which is similar to traditional surfactants. The interfacial tension of NCQDs-14 surfactant, determined by interfacial tensiometer, exhibits the best ability to reduce interfacial tensions, which can be reduced to 10 -3 mN/m, thus endowing with better emulsification ability. And the NCQDs-14 surfactant possesses the best wetting alteration ability, which is beneficial for converting capillary force from resistance to force, thereby promoting the driving fluid to permeate into the pore throats. Eventually, the NCQDs-14 surfactant possesses better oil removal ability (61.48%) and higher spontaneous imbibition recovery (21.52%), which can effectively reduce the injection pressure and improve oil recovery, indicating its favorable displacement performance and potential application in low-permeability reservoirs.