The one-step synthesis of B,N co-doped carbon dots as a fracturing crosslinker and fluorescent tracer in flowback fluid and their performance

In order to deal with the problem of the exploitation of low permeability reservoirs and to improve fracturing fluid performance, a carbon-dot-based nano-crosslinker was developed in which the same cross-linking point can be linked with multiple guar gum molecular chains; it also shows good stability. HRTEM and dynamic light scattering (DLS) studies were used to observe the morphology, crystal plane spacing (0.34 nm), and particle size distribution (1.20-1.42 nm) of the carbon dot crosslinker. Elemental mapping, Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to prove the successful doping with B and N. Then, rheological testing was carried out with carbon-dot-based fracturing fluid (CDG), and the results showed that the viscosity of CDG remained above 170 mPa s at 120 degrees C and a shear rate of 170 s(-1), performing better than organic boron fracturing fluid. Then, a Cole-Cole model was established based on the viscoelastic data from 25-120 degrees C; the results showed that the radius of curvature of the Cole-Cole plot increased with a decrease in temperature, and the system has strong compatibility. Displacement experiments were also used to characterize the fluorescence tracing properties. This work proves that the preparation of carbon dots to act as a crosslinker and fluorescent tracer was successful, and it also provides new insight for the application of carbon dots.