Filtration loss mechanisms of polyacrylamide-based copolymers in high-temperature drilling fluid

Abstract Deep and ultra-deep oil and gas exploration imposes stringent requirements on water-based drilling fluids. Conventional fluid loss additives often fail to deliver stable performance in these extreme environments, largely due to inadequate functional group composition and thermal stability. This work introduces an innovative strategy by regulating the structure of quaternary AM/AMPS/NVP/DMDAAC copolymers and clarifying their high-temperature filtration loss mechanisms. Five polymers with different compositions were synthesized, and their structural features, thermal stability, filtration loss performance, and mechanisms were systematically evaluated. The results showed that the monomer composition significantly influenced the polymer’s structural characteristics and thermal stability. Increasing the N-vinylpyrrolidone (NVP) content to 25–40 % enhanced intermolecular interactions and thermal stability at high temperatures, with a filtration volume of 25.7 mL in freshwater-based mud after aging at 240 °C. It was found that Poly D formed a dense filter cake through strong hydrogen bonding with high adsorption. Moreover, high dimethyldiallylammonium chloride (DMDAAC) content triggered charge shielding effect and disrupted bentonite dispersion. And NVP reduces permeability by modulating the equilibrium of filter cake affinity. This study reveals the structure-property relationship of copolymers under high-temperature and high-salt conditions, which provides new perspectives and experimental bases for designing and synthesizing high-temperature water-based fluid loss control agent for drilling fluids.