Zr, N-Co-Doped Carbon Quantum Dot Crosslinking Agents for Use in Fracturing Fluids

To further improve the performance of the fracturing fluid, the Zr, N-co-doped carbon quantum dot nano-crosslinking agent was synthesized by the hydrothermal method to solve the problems of easy agglomeration, instability, and gel breaking after crosslinking of the conventional polyacrylamide fracturing fluid crosslinking agent. The crosslinking agent has good stability and multiple crosslinking sites, as well as can be crosslinked with anionic polyacrylamide (HPAM) solution to form a high-strength hydrogel fracturing fluid. Moreover, it can be used as a nano-fluorescent tracer to test the flowback rate of fracturing fluid. The fracturing fluid composed of 0.3 wt % HPAM and 0.8 wt % crosslinking agent was named as Zr-CDG. At 180 °C, the viscosity of Zr-CDG remains above 180 mPa·s after shearing for 2 h, indicating that Zr-CDG has excellent temperature tolerance and shearing resistance. The residue is only 169 mg/L after gel breaking, which means low damage to the formation. Finally, the backflow process of fracturing fluid is simulated by core displacement. According to the good luminescence of the carbon quantum dot nano-crosslinking agent, the flowback rate of the fracturing fluid is calculated. It shows that the carbon quantum dot nano-crosslinking agent also has the role of fracturing fluid tracer. The results show that the properties of the prepared carbon quantum dot nano-crosslinking agent are more stable, and the performance of the crosslinked fracturing fluid system is greatly improved, which points out a direction for the application of carbon quantum dots.