A salt responsive, high temperature resistant polymer that spontaneously forms a network as a viscosifier for solid free saturated CaCl2 reservoir drill-in fluid

To address the challenges of limited solubility, inadequate temperature stability, and insufficient viscosity enhancement exhibited by current treatment agents in solid-free CaCl 2 brine drill-in fluids, a zwitterionic monomer, 3-(3-vinyl-1-imidazolio)-1-propanesulfonate (SBVI), was synthesised using 1,3-propane sultone and 1-vinylimidazole. A high temperature resistant zwitterionic polymer viscosifier SEOD for divalent CaCl 2 brine drill-in fluids was synthesized using this monomer with N,N-dimethylacrylamide (DMAA), 2-Acrylamido-2-methylpropane sulfonic acid (AMPS), Sodium p-Styrene Sulfonate (SSS). As the brine density changed, SEOD showed excellent salt responsiveness and the viscosity increasing effect became more and more obvious. In saturated brine, SEOD can form intermolecular association structures, and its molecular chains entangled with each other to form a network, thereby significantly increasing the viscosity of the brine solution. The 1.0   wt% SEOD in 1.4   g/cm³ CaCl₂ brine showed exceptional thermal stability, with an initial apparent viscosity of 30.5 mPa·s, which remained at 29.5 mPa·s after aging at 200 °C for 16   hours. As a novel anti-high temperature viscosifier, SEOD effectively overcomes the thermal stability degradation and inadequate viscosity enhancement of conventional additives in divalent brine fluids. It improved the rock cuttings suspension and wellbore cleaning performance of solid free divalent brine fluids under high temperature/high pressure (HTHP) conditions in complex formations.