New structure found in a filter cake with the addition of an amphoteric polymer fluid loss additive

ABSTRACT Water‐based drilling fluids always suffer from high‐salt and high‐temperature conditions, which cause safety andeconomic problems. Amphoteric copolymers are expected to solve this, although the underlying factors are still controversial. In this study, the AST (an amphoteric copolymer which was synthesized from 2‐acrylamide‐2‐methyl propane sulfonicacid, (3‐acrylamidopropyl)trimethyl ammonium chloride and acrylamide) was used as fluid loss additive, kaolinite (KT) was used as an evaluation condition, and a sandwich structure was found on the cross section of the filter cake. Four potential mainstream explanations were analyzed: the polymer properties, rheological behavior, dispersion of platelets, and structure of the filter cake. The sandwich structures in the cross section of filter cake were finally found and were confirmed by energy‐dispersive spectroscopy. The sandwich structure, which consisted of an amphoteric polymer and a clay platelet, affected the fluid loss dramatically. This method for studying the cross section of the filter cake on the basis of KT evaluation could be used to investigate the filtration process of other fluid loss additives. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47965.