Enhanced Filtration Performance of Cellulose Nanofiber/Bentonite Water-Based Drilling Fluids with Temperature- and Salt-Resistant Polymers for Extreme Conditions

Summary Drilling fluids play a crucial role in transporting cuttings to the surface, maintaining wellbore stability, and controlling formation pressures. However, high temperatures, high pressures, and high salinities frequently impair their effectiveness in deep and intricate formations. In recent years, cellulose nanofibers (CNFs) have gained attention due to their high surface area, aspect ratio, and fiber-like structure, making them promising candidates for rheology and filtration modification in bentonite water-based drilling fluids (BT-WBDFs). Nevertheless, their filtration performance under high-salinity and high-temperature conditions has been found insufficient. For this study, we used acrylamide (AM), 2-acrylamide-2-methylpropane sulfonic acid (AMPS), and dimethyl diallyl ammonium chloride (DMDAAC) to synthesize a series of polymers, including poly(AM-AMPS) (AA), poly(AM-DMDAAC) (AD), and poly(AM-AMPS-DMDAAC) (AAD), with excellent temperature and salt resistance. These polymers were evaluated for their ability to enhance the filtration performance of CNF/BT-WBDFs under extreme conditions. Experimental results revealed that terpolymer AAD demonstrated the best salt and thermal resistance under conditions of 26 wt% NaCl concentration and aging at 200°C for 16 hours. Further optimization of the monomer ratio for AAD showed that when the AM/AMPS/DMDAAC molar ratio was adjusted to 3:2:1, the API fluid loss and high-pressure, high-temperature (HPHT) fluid loss were minimized to 7.3 mL and 70 mL under the same extreme conditions. Mechanistic analysis suggested that maintaining the uniform dispersion of BT and CNFs is critical for reducing fluid loss, with AAD exhibiting the strongest ability to preserve colloidal stability. These findings pave the way for developing sustainable drilling fluids with improved thermal stability and filtration control tailored for challenging drilling environments.