Salt-responsive zwitterionic copolymer as tackifier in brine drilling fluids

Salt contamination has become the most critical challenge faced by water-based drilling fluids (WDFs) under extreme salinity and high temperatures, which deteriorates rheology and fluid loss properties, causing drilling accidents such as leakage, formation collapse and borehole instability. Herein, we report a salt-responsive zwitterionic polymer (PAMN) and its novel application as a tackifier in brine drilling fluid exploiting the anti-polyelectrolyte effect, in which the zwitterionic polymer molecular chain has undergone a transformation from a collapsed small spherical shape to an extended chain shape corresponding to the increasing ionic strength of the electrolyte. Compared with the saturated brine drilling fluid, the apparent viscosity and plastic viscosity of the saturated brine drilling fluid dispersed with PAMN increased by 542.9% and 925% respectively owing to the charge interaction on the molecular chain of PAMN was shielded by sodium chloride electrolyte, which resulted in the increase of the radius of gyration (Rg) of the PAMN molecular chain. Additionally, the salt response mechanism of zwitterionic polymer PAMN had been revealed by molecular simulation technology, which simulated the radius of gyration, radial distribution function (RDF), mean square displacement (MSD) and energy of molecular chain in water and salt water, respectively. This work demonstrates that the approach can be extensively generalized to a large class of synthetic zwitterionic polymers and can promote the development of various salt-responsive intelligent drilling fluids. (C) 2020 Elsevier B.V. All rights reserved.