A high-temperature resistant and high-density polymeric saturated brine-based drilling fluid

Three high-temperature resistant polymeric additives for water-based drilling fluids are designed and developed: weakly cross-linked zwitterionic polymer fluid loss reducer (WCZ), flexible polymer microsphere nano-plugging agent (FPM) and comb-structure polymeric lubricant (CSP). A high-temperature resistant and high-density polymeric saturated brine-based drilling fluid was developed for deep drilling. The WCZ has a good anti-polyelectrolyte effect and exhibits the API fluid loss less than 8 mL after aging in saturated salt environment at 200 °C. The FPM can reduce the fluid loss by improving the quality of the mud cake and has a good plugging effect on nano-scale pores/fractures. The CSP, with a weight average molecular weight of 4804, has multiple polar adsorption sites and exhibits excellent lubricating performance under high temperature and high salt conditions. The developed drilling fluid system with a density of 2.0 g/cm3 has good rheological properties. It shows a fluid loss less than 15 mL at 200 °C and high pressure, a sedimentation factor (SF) smaller than 0.52 after standing at high temperature for 5 d, and a rolling recovery of hydratable drill cuttings similar to oil-based drilling fluid. Besides, it has good plugging and lubricating performance.