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

Area of Science:

• Materials Science
• Petroleum Engineering
• Nanotechnology

Background:

• Shale reservoirs face challenges in CO2 flooding, including gas channeling, low sweep efficiency, and water sensitivity.
• Developing novel materials is crucial to enhance oil recovery in these complex formations.

Purpose of the Study:

• To synthesize amidine-functionalized graphene quantum dots (FN-GQDs) for improved CO2 flooding in shale reservoirs.
• To investigate the performance of FN-GQDs in addressing key challenges of CO2 EOR (Enhanced Oil Recovery).

Main Methods:

• Synthesis of FN-GQDs via amidation of citric acid-derived GQDs.
• Characterization using FTIR, UV-Vis, and conductometric titration.
• Molecular dynamics simulations to predict saturation.
• Evaluation of viscosity, interfacial tension, wettability alteration, and clay swelling inhibition.
• Core flooding experiments and NMR T2 spectra analysis.

Main Results:

• FN-GQDs exhibited a nonlinear viscosity increase up to 2.0 mPa·s upon CO2 introduction.
• Maintained low oil-water interfacial tension (0.12-0.25 mN/m) at elevated temperatures.
• Rapidly reversed rock wettability from oil-wet to water-wet (contact angle reduced from 141.7° to 38.9°).
• Achieved 92% clay swelling inhibition at 0.20 wt% FN-GQDs.
• Alternating CO2 and FN-GQDs injection resulted in a final oil recovery of 52.5%.

Conclusions:

• FN-GQDs demonstrate significant potential for CO2-enhanced oil recovery in shale reservoirs.
• Synergistic effects including viscosity enhancement, interfacial tension reduction, wettability alteration, and anti-swelling contribute to improved oil recovery.
• The developed FN-GQDs offer a promising solution for overcoming limitations in conventional CO2 flooding techniques.