Mobility Control Mechanism of In Situ Viscosity-Enhancing Graphene Quantum Dots in Assisted CO2 Flooding

To address gas channeling, low sweep efficiency, and water sensitivity in CO2 flooding of shale reservoirs, amidine-functionalized graphene quantum dots (FN-GQDs) were synthesized via amidation of citric acid-derived GQDs. FTIR and UV-Vis confirmed successful grafting. Conductometric titration showed an optimal reaction time of 24 h with a grafting ratio of 58%, in good agreement with the 60% saturation predicted by molecular dynamics simulation. Upon CO2 introduction, protonation of amidine groups induced a nonlinear viscosity increase from 0.298 to 2.0 mPa·s at 0.02 wt% via electrostatic attraction and hydrogen bonding, forming a dynamic crosslinking network. FN-GQDs maintained low oil-water interfacial tension of 0.12–0.25 mN/m at 80–120 °C and rapidly reversed rock wettability from strongly oil-wet to water-wet, reducing the contact angle from 141.7° to 38.9° within 80 min. The positively charged surface inhibited clay swelling, achieving 92% at 0.20 wt%. Core flooding and NMR T2 spectra revealed that alternating CO2 and FN-GQDs injection at a 2:1 gas–water ratio achieved a final oil recovery of 52.5%, significantly higher than pure CO2 flooding. Through synergistic effects of interfacial tension reduction, wettability alteration, viscosity enhancement, and anti-swelling, FN-GQDs improve microscopic displacement efficiency and macroscopic sweep volume, showing great potential for CO2-enhanced oil recovery in shale reservoirs.