Investigation of carbon-based nanofluid imbibition processes in low-permeability reservoirs using nuclear magnetic resonance

Spontaneous imbibition has been demonstrated to be an important approach to enhance the oil recovery of unconventional reservoirs after hydraulic fracturing. Adding chemical agents to modulate the properties of the oil-water and oil-rock interfaces is a common method to enhance imbibition. Carbon-based nanofluid showed excellent ability to reduce interfacial tension and alter wettability due to their distinct surface properties and ultra-small particle size. However, there are few systemic studies on the imbibition process of carbon-based nanofluid and the unclear mechanism of the enhanced oil recovery (EOR) restricts their oilfleld application. In this study, the carbon-based nanofluid was prepared by dispersing carbon nitride quantum dots (CNQDs) of 2.34 +/- 0.34 nm synthesized by citric acid and urea in deionized water. The spontaneous imbibition process of nanofluids was monitored by nuclear magnetic resonance (NMR) technology. The T 2 relaxation distribution results suggested that the oil recovery of CNQD S nanofluid by spontaneous imbibition was 24.6%, which was 5% higher than that of nano-SiO 2 nanofluid. Moreover, the variations of oil saturation proflles analyzed based on NMR data indicated that the maximum imbibition distance of the carbon-based nanofluid was about 20.34 mm which is also larger than that of the nano-SiO 2 nanofluid of 16.02 mm. The oil saturation variations of saturated oil cores imbibed in carbon-based nanofluid was also signiflcantly improved compared with nano-SiO 2 nanofluid. This flnding was attributed to the fact that compared with nano-SiO 2 nanofluid, the CNQDs nanofluid can not only promote the fluid to enter deeper matrix, but also effectively reduce the residual oil saturation in the area that the nanofluid accesses to. The interfacial tension and contact angle measurements were conducted to reveal the underlying EOR mechanisms of CNQDs nanofluid. This study provides vital insights into the prospective application of carbon -based nanofluid in low -permeability reservoirs.