Ultra-deep reservoirs gel fracturing fluid with stepwise reinforcement network from supramolecular force to chemical crosslinking

Hydraulic fracturing stimulation is a crucial treatment for the development of deep/ultra-deep reservoirs. The success of this treatment is highly dependent on the property of the fracturing fluid, particularly its performance and ability to withstand high pressure without degradation. The high temperature and long wellbore characteristics of deep/ultra-deep reservoirs demand fracturing fluid with high requirements for shear resistance and thermal stability. These attributes are crucial for the effective transportation of proppants over long distances and for minimizing fluid loss. Enhancing the chemical crosslinking intensity can improve the thermal stability of the gel fracturing fluid. However, premature crosslinking can result in frictional pressure loss and mechanical degradation of the fracturing fluid along the long wellbore. In this study, polyacrylamide-based quaternary copolymers with a multi-functional imidazole ring are synthesized. The strong supramolecular force existing between the imidazole ring and carboxylic acid allows for sufficient time intervals before initiating the crosslinking reaction through steric effects, thereby alleviating mechanical degradation and reducing excessive frictional pressure loss. Furthermore, the anti-shear ability of the fracturing fluid is strengthened by a three-dimensional physical network structure. Additionally, thermal/shear stability tests demonstrate that the working temperature of the synthetic polymer (SP) gel fracturing fluid can reach up to 160 °C. Moreover, the fracturing fluid degrades without leaving visible residues under static conditions, which is beneficial to maintaining the integrity of the reservoir for post-fracturing treatment.