Multi-Cyclic Pentamer Copolymers as High-Temperature and Salt/Calcium-Resistant Fluid Loss Additives in Water-Based Drilling Fluids

To meet the stringent demands posed by high-temperature, high-salinity conditions in deep and ultradeep formations, this study developed a novel polymeric fluid loss reducer with excellent thermal stability and resistance to salt and calcium contamination. The polymer was synthesized via free-radical solution polymerization using five functional monomers: acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, N-vinylpyrrolidone, N,N-dimethylacrylamide, and N-acryloylmorpholine. Its chemical structure and thermal properties were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance (1H NMR), and thermogravimetric analysis. The performance of the polymer was systematically evaluated under high-temperature conditions and in high-salinity (20 wt %NaCl) and high-calcium (1.5 wt %CaCl2) environments. Results showed that the polymer achieved an API filtrate volume as low as 10.4 mL at 220 °C (2.0 wt % dosage), while the HTHP fluid loss was controlled within 24.8 mL at the same temperature. The rheological parameters remained stable, with a yield point of approximately 8 Pa after hot rolling at 220 °C, and TGA revealed an onset decomposition temperature above 320 °C. In addition, the fluid-loss reducer exhibited strong tolerance to salt and calcium contamination, effectively suppressing filtrate invasion and preserving the integrity of the filter cake structure. These outstanding properties are attributed to the synergistic effects of multiple functional groups, which endow the polymer with strong thermal resistance, hydration capability, and ion-shielding performance. Overall, this material holds significant promise for applications in complex and harsh drilling environments.