Self-Assembling Low-Viscosity-Increment Zwitterionic Polymer for Rheological Control in High-Temperature and High-Salinity Water-Based Drilling Fluids

Abstract

Under harsh high-temperature and high-salinity (HTHS) conditions, water-based drilling fluids (WBDFs) face a critical technical bottleneck in rheological control: the difficulty in simultaneously achieving yield point (YP) and gel strength enhancement without excessive viscosity increase while preventing the deterioration of rheological structures under severe thermal and saline stresses. In this work, a self-assembling low-viscosity-increment rheology modifier (SARM) based on a zwitterionic copolymer was synthesized via inverse emulsion polymerization, with a small amount of Zr4+ introduced as an auxiliary coordination reinforcement component. Its molecular structure and thermal stability were systematically characterized, and its rheological regulation behavior and mechanism in HTHS environments were investigated in detail. SARM retained rheological regulation capability after hot rolling at 180°C under 7 wt% KCl and saturated 36 wt% NaCl conditions. In high-density WBDFs, SARM regulated rheological behavior with a relatively low viscosity increment and achieved enhanced YP values and favorable YP/plastic viscosity (PV) characteristics compared with representative commercial rheology modifiers. Mechanistic analysis indicates that SARM promotes the formation of a shear-responsive 3D self-assembled network in drilling fluids through the synergistic effects of thermally stable and conformationally constrained polymer chains and salt-responsive zwitterionic structures, with Zr4+ coordination interactions providing additional network reinforcement. This design helps alleviate the common trade-off between enhancing YP and gel strength, and increasing viscosity in conventional rheology modifiers while improving the dispersion state of bentonite particles and the compactness of filter cakes. This work provides a potential material strategy for precise rheological regulation of WBDFs under HTHS conditions, particularly for drilling in long horizontal sections.