Development of a High-Temperature and High-SalinityResistant Branched Polymer for Controlling Fluid Loss of Water-Based Drilling Fluids

Conventional fluid-loss reducers lack thermal and salt resistance and exhibit a pronounced viscosity-enhancing effect in bentonite-based drilling fluids. These issues will cause severe problems during drilling operations. Herein, a branched fluid-loss reducer, BPJ, was synthesized using BPEI1800 as the branching core and DMAA, AMPS, DMDAAC, and AN as monomers. BPJ showed good thermal stability, with less thermal degradation at 235 °C. BPJ was thermally stable up to 220 °C in drilling fluids, and it could tolerate saturated NaCl, 10% KCl, and 5% CaCl2. Compared with commercial fluid-loss additives, BPJ had better fluid-loss reduction and lower viscosity enhancement. Under 220 °C and 36 wt% NaCl conditions, BPJ adsorbed strongly onto bentonite surfaces. Sulphonate groups increased electrostatic repulsion and maintained good colloidal stability of the bentonite. It further inhibited aggregation and detrimental dispersion of bentonite particles. BPJ was synergistic with bentonite, helping to form an impermeable filter cake and resulting in efficient filtration loss control. The study presents a novel approach to developing fluid-loss reducers for high-temperature, high-pressure drilling fluids in deep and ultra-deep formations.