High-temperature resistant zwitterionic polymer viscosifier for saturated CaCl2 brine drill-in fluids

Solid-free brine drill-in fluids are widely used in oil and gas reservoir protection operations because they use soluble salts as weighting agents and contain no insoluble solids. To address the issues of insufficient thermal stability at elevated temperatures and poor solubility of conventional viscosifiers in CaCl2brine drill-in fluids, a zwitterionic polymer viscosifier named MPOD was synthesized using N, N-Dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, Dimethyl Diallyl Ammonium Chloride, and (Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide as raw materials. MPOD dissolves well in CaCl2brines, and its viscosifying effect improves with increasing brine density. In 1.4 g/cm3CaCl2brine, the apparent viscosity of a 1.0 wt% MPOD solution is 50.5 mPa·s and the yield point is 8.5 Pa. After ageing at 150 °C, viscosity remained stable, confirming good thermal resistance. Furthermore, when combined with other treatment agents, MPOD exhibits good compatibility. The system maintains excellent rheological properties after ageing, and MPOD reduces the HTHP (high-temperature, high-pressure) fluid loss of the CaCl2brine system to less than 15 mL. MPOD exhibits excellent high-temperature resistance, enabling efficient cuttings transport and wellbore cleaning.