Graphene Quantum Dots for the Mobilization and Solubilization of Nonaqueous Phase Liquids in Natural Porous Media

This study proposes an approach to synthesize graphene quantum dots (QDs) with remarkable physicochemical properties that allow them to effectively mobilize and solubilize nonaqueous phase liquids (NAPLs) such as crude oils in porous media. QDs are first extracted by mild oxidation of coal and then partially functionalized by alkylamines using a starch template to generate engineered QDs (EQDs). When mixed in equal amounts, the nanofluid can synergistically interact at oil/water interfaces and lower the interfacial tension (IFT) from 19.6 to 0.9 mN/m. The two quantum dots exhibit different behavior on minerals. EQDs do not adsorb because of steric hindrance caused by their aliphatic tails, whereas QDs adsorb moderately on sandstones through hydrogen bonding, leading to wettability alteration from oil-wet to weakly water-wet state. The nanofluid provides mixed-wet conditions that, together with IFT reduction, result in 21 vol % of incremental NAPL recovery compared to waterflooding. The performance of this mixture is curbed in carbonates with only 9.6 vol % of additional recovery because of strong QD adsorption and pore plugging. The ability of this nanofluid to alter wettability was further confirmed in a miniature silicate-rich rock using X-ray microtomography. The in situ contact angle distributions on quartz and carbonate shifted to a more water-wet state and covered a wide range of values, denoting a nonuniform wettability reversal. Thus, the application of QD-based nanofluids is more suitable in silicate-rich formations where chemical retention by the rock is minimum. This study paves the way for the development of more EQDs by tuning their structure via chemical functionalization of their oxygen-rich active sites.