Enhancing temperature resistance of polymer gel fracturing fluids: The role of alcohol

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The temperature resistance of polymer gel fracturing fluids is the key to ensure its application in deep oil and gas reservoirs. Currently, the methods to improve the temperature resistance of polymer gel fracturing fluids are mainly based on the additives commonly used in fracturing fluids, but rarely studied on improving the strength of crosslinked systems by strengthening its non-covalent interactions. Herein, we separately introduce ethanol and glycol into the partially hydrolyzed polyacrylamide (HPAM) crosslinked zirconium polymer gels to form the fracturing fluid system with higher temperature-resistant ability. The concentrations of the HPAM and crosslinking agent are all 0.6 wt%. After shearing at 120 °C and 170 s−1 for 1 h, the addition of ethanol and glycol can increase the retention viscosity of the polymer gels from 43 mPa s to 148 mPa s and 204 mPa s, respectively. Microscopic characterization demonstrates that the addition of ethanol and glycol makes the polymer gels more dense “honeycomb” and “dendritic” structures, which may be the main reasons for the temperature resistance enhancement of the system. Additionally, the molecular dynamics simulations show that the supramolecular interaction in the polymer gel system is greatly enhanced by the formation of hydrogen bonds when the alcohol is added at low concentrations. Meanwhile, the zirconium ions play a bridging role in connecting oxygen atoms in the carboxyl group in HPAM and oxygen in the alcohol hydroxyl group. The hydrogen bonds and O-Zr-O bridges can strengthen the non-covalent interactions of the system, inhibiting the destruction of polymer gel structure at high temperatures and enhancing its temperature resistance. However, when the alcohol concentration of the system is too high, the crosslinking strength of the polymer gels is weakened to different degrees due to the competitive effect of the alcohol on the crosslinking agent. Both experimental results and theoretical simulation further deepen the understanding of alcohol to improve the temperature resistance of polymer gels, and can promote the subsequent application of alcohol to polymer gel fracturing fluids for property enhancement.