Stimulus-responsive mechanism of salt-responsive polymer and its application in saturated saltwater drilling fluid

AM-AMPS-TAC polymers with different charge distribution are synthesized using acrylamide (AM), 2-acrylamido-2-methylpropanesulfonate (AMPS) and 3-acrylamidopropyl trimethylammonium (TAC) at different feed ratios by polymerization in solution. The salt-responsive behavior, reasons leading to salt-responsiveness, and effects of polymers molecular structure on salt-responsiveness are studied by laboratory experiments to find out the adaptability of the polymers. Rheology test under stepwise shear mode shows that the AM-AMPS-TAC polymers have salt responsiveness, and the closer the feeds of AMPS and ATC, the more significant the salt responsiveness will be. Conformation change of polymers molecular chain under salt stimulus is studied by turbidity test and micro-morphology analysis, and the responsive mechanism is further investigated by intrinsic viscosity test and copolymer composition analysis. Results indicate that the salt-responsive behavior of AM-AMPS-TAC polymers derives from the "curled to expanded" transition of chain conformation under salt stimulus, and this transition is led by the screening effect of salt which weakens polymers intramolecular ionic bond. Application in saturated saltwater drilling fluid shows that the AM90-AMPS5-TAC5 polymer has the best salt-tolerance and temperature-tolerance when used together with fluid loss controller PAC-Lv. The drilling fluid saturated with NaCl can maintain stable viscosity, good dispersion and low fluid loss for long time under 150 °C.