Experimental Evaluation of a Green PECS–Carbon Nanodot CO ₂ Foam System for Mobility Control and Enhanced Recovery in Sandstone Reservoir Rocks

ABSTRACT CO 2 foam flooding is a promising chemical enhanced oil recovery method for improving gas mobility control and sweep efficiency in sandstone reservoirs, but foam instability remains a major limitation. In this study, a green hybrid CO 2 foam system was developed using powder‐extracted cottonseed surfactant (PECS) as a bio‐derived anionic surfactant and olive‐oil‐derived carbon nanodots (CNDs) as nanoscale stabilizing additives. CNDs were synthesized through a hydrothermal route and characterized by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR), ultraviolet–visible (UV–Vis) spectroscopy, and transmission electron microscopy (TEM). Static foam stability, bubble morphology, high pressure and high temperature (HPHT) interfacial tension (IFT), and tertiary core‐flooding tests were conducted to evaluate the PECS–CND system. Among the tested PECS concentrations, 1200 ppm showed the highest foam half‐life of 120 min and the lowest IFT, confirming its optimum performance near the critical micelle concentration. Adding 100 ppm CND increased the foam half‐life from 120 to 125 min and increased t 95 from 540 to 840 s, indicating delayed drainage and improved time‐dependent foam stability. Bubble morphology analysis showed that the PECS–CND foam retained a measurable and compact texture for a longer aging period than PECS alone. At 80°C and 1000 psi, CND addition reduced the IFT from 0.324 to 0.304 mN/m. Core flooding showed that PECS–CND CO 2 foam provided 22.6% original oil in place (OOIP) incremental recovery after waterflooding and increased the final recovery to approximately 81.5% OOIP. These findings demonstrate that combining a bio‐derived anionic surfactant with green CNDs can improve CO 2 foam persistence, support gas mobility control, and enhance residual oil mobilization in sandstone porous media. The results suggest that the PECS–CND system improves oil recovery through combined interfacial activity, lamella reinforcement, delayed drainage/coalescence, and CO 2 mobility control.