Composite Regulation Technology of Well-Flushing Fluid and Its Application in Ultra-Deep Wells

Abstract To address the issue of wellbore contamination caused by residual drilling fluids under high-temperature and high-pressure (HTHP) conditions in ultra-deep wells, this study developed a temperature-resistant well-flushing fluid system by optimizing the ratio of a composite surfactant (SEO) and a viscosity-enhancing polymer (β-MAAD). The cleaning performance and field application effects were systematically investigated. Experiments revealed that increasing the dosage of surfactant SEO significantly improved the interfacial tension and stripping efficiency of the flushing fluid, while optimizing β-MAAD enhanced the viscosity and solid-carrying capacity of the system, forming a weakly crosslinked network structure to disperse solid particles. Field applications demonstrated cleaning efficiencies of 94.65% and 98.75% in the PS-6 well (oil-based drilling fluid) and the K2-2 well (water-based drilling fluid), respectively. The solid content in the return fluid continuously decreased during flushing, reaching 0.36–0.54% at the end of cleaning, effectively resolving issues of casing wall residue and bottomhole particle deposition. The results indicate that the composite flushing fluid, through synergistic regulation of surface activity and rheological properties, adapts to the cleaning requirements of different drilling fluid systems, providing technical support for efficient wellbore cleaning in ultra-deep wells.