Preparation and Evaluation of High-Temperature-Resistant Copolymer Gels for Enhanced Oil Recovery: A Study on Gelation Properties and Thermal Stability

In the late stage of oilfield water flooding, the rapid increase in water cut of produced fluids significantly reduces oil well productivity. To tackle the challenge of excessive water production in ultra-high-temperature (150 °C) reservoirs, this study introduces a copolymer (acrylamide/vinylpyrrolidone copolymer, acrylamide/2-acrylanmido-2-methylpropanesulfonic acid copolymer)-based gel system. The gelation performance of copolymers with varying compositions and molecular weights was systematically investigated at 150 °C using gelation visualization codes, mechanical strength tests, microstructural analysis, thermogravimetric analysis (TGA), and nuclear magnetic resonance (NMR) spectroscopy. These approaches provide insights into the thermal and mechanical behavior of the gel under high-temperature conditions. Experimental results show that under optimized conditions—specifically with a vinylpyrrolidone (NVP) content of 30–40% in the copolymer and a copolymer concentration of 1.0 wt%—the gel system exhibited the best performance: a gelation time of 9.5–11 h, storage modulus (G′) of 14.7–16.0 Pa, and stability exceeding 6 months at 150 °C. Moreover, increasing the molecular weight from 1.78 × 106 to 3.82 × 106 shortened the gelation time from 18.5 h to 14 h and raised the gel strength code from F to G. Although higher molecular weight led to a finer microstructure lattice and somewhat lower chemical structure stability, it also reduced the gel’s water-binding capacity compared to lower-molecular-weight analogues. The copolymer gel system developed in this work offers a promising technical solution for improving water flooding efficiency in ultra-high-temperature reservoirs.