High-Temperature Resistant Zwitterionic Filtration Loss Reducer for Brine-Based Drill-In Fluids

Oil exploration and development is increasingly advancing into deep, high-temperature, and high-pressure (HTHP) formations, making the control of drilling fluid solid-phase content extremely critical, which can effectively prevent the decline in reservoir permeability caused by solid phase invasion. To address the challenges of poor salt resistance, low thermal stability, and limited filtration control efficiency of filtration loss reducers used in high density saturated divalent brine drill-in fluids, a novel filtration loss reducer (THOD) was successfully synthesized using N,N-dimethylacrylamide (DMAA), diallyldimethylammonium chloride (DMDAAC) and the zwitterionic monomer 3-(1-vinyl-3-imidazolio)-propanesulfonate (SBVI) as raw materials. THOD exhibites excellent solubility and thermal stability in saturated CaCl2 (1.4 g/cm3) and CaBr2 (1.8 g/cm3) brines. Even at a dosage of 2%, the brine solutions remain clear and transparent without phase separation or flocculation, with the transmittances reaching 78% and 92.3%, respectively. The THOD polymer tightly fills the pores in the acid-soluble CaCO3 filter cake skeleton through deformation and adsorption, forming a continuous, dense membrane that reduces brine filtration loss. After hot rolling at 190 °C for 16 h, the filtration loss of the drill-in fluid system could still be controlled within 15 mL. Furthermore, after hot rolling at 200 °C for 16 h, the saturated CaBr2 brine system exhibited an API filtration loss of only 3.2 mL and an HTHP filtration loss of 15 mL. In addition, the filter cake formed by the drill-in fluids can be completely dissolved in 2% HCl solution, with a reservoir permeability recovery rate up to 85%, significantly improving the reservoir protection effect. The research results provide a new solution for the filtration control of high-temperature and high-density brine reservoir drilling fluids and is expected to provide technical support for safe and efficient drilling and reservoir protection under complex geological conditions, such as deep and ultradeep wells.