Carbon dots nanofluid: Reducing injection pressure in unconventional reservoir by regulating oil/water/rock interfacial properties

Effective development of unconventional resources is impeded by water flooding-related challenges, such as high pressure and under-injection, which hinder the replenishment of formation energy. Nanofluids have shown promise in reducing injection pressure, but traditional nanoparticles (NPs) face limitations in unconventional reservoirs due to their unsuitable size for micro-nano pores and aggregation tendencies. In this paper, novel carbon dots (CDs) with a simple, environmentally benign, and controllable synthetic approach have been developed, and their pressure-reduction performance and mechanism have been systematically explored. The particle size of the CDs could be tailored by a well-timely quenching reaction. CDs nanofluid demonstrated superior stability, dispersibility, and temperature/salinity resistance. In addition, CDs can regulate interfacial properties, including oil/rock and oil/water interfaces. It was found that CDs could be partially absorbed onto pore walls, reducing oil adhesion sites by 39%, altering wettability from oil-wet to neutral, and reducing rock surface roughness by 29.25%. Furthermore, CDs were rapidly adsorbed onto the remaining oil film in the pore space, reducing interfacial tension while solidifying the oil surfaces, increasing shear force around the oil surface by 1.7 times during subsequent water scouring, which makes the oil film break up more readily and remain smaller in size to migrate in the channeling. The modulation of the interface properties by CDs improved the stripping efficiency of the attached oil film and ameliorated the homogeneity and smoothness of flow channel, resulting in a significant reduction in injection pressure. Core displacement tests showed that a 0.1 wt% CDs nanofluid could achieve a pressure reduction rate of 35%. The developed CDs nanofluid represents a promising solution for reducing injection pressure and improving oil recovery in unconventional reservoirs.