Carbon nanodots for enhanced oil recovery in carbonate reservoirs

Enhanced oil recovery (EOR) operations in oil-wet carbonate reservoirs face enormous challenges such as long-term chemical and thermal stability issues, high retention and adsorption, diffusion, and, accordingly, an inability to reach deep into the reservoir. This study investigates the EOR potential of Carbon nanodots (CNDs) in coreflood experiments using both outcrop and reservoir carbonate samples. CNDs have excellent colloidal stability and mobility in the harsh carbonate reservoir environment. We also compared its performance against an in-house Gemini surfactant (GS8) of excellent surfactancy properties. Experiments were performed at the "typical" Saudi Arabian reservoir conditions. We tested three flooding patterns using different rock permeabilities and additive concentrations in the SW. Based on the results, the added oil recovery with CNDs and GS8 ranged from 3%–23% and 6%–22%, respectively. At the same dilute concentration, CNDs are more effective than GS8 in increasing oil recovery. X-ray coreflood experiments provided a real-time 1-D saturation profile. It allowed gauging the efficacy of each additive in situ while tracking the oil and water saturation in and recovery from the carbonate samples. The saturation profiles were compared qualitatively against the oil recovery data from these experiments. Unsteady state, two-phase relative permeability curves of the X-ray coreflood experiments revealed the flow characteristics of oil and water during the injection of different fluids. Both CNDs and GS8 increased the oil's relative permeability, kro and reduced the water's relative permeability, krw. Overall, CNDs proved highly efficient, economical, and more scalable than a leading surfactant candidate. CNDs are based on a simple, single-pot synthesis process and have a proven-stability and mobility in the field. And in dilute concentrations (10–200 ppm), it alters the dynamic wettability of the carbonate reservoir favorably. Potentially, these can boost the oil recovery at depths (from the injector) that are unreachable by the conventional surfactants.