Performance of Self-healing microgel incorporating Nano-Silica as plugging material for drilling fluid

Lost circulation severely threatens the safety and efficiency of oil drilling operations, especially at high temperatures. Accordingly, many plugging materials have also been studied and applied, such as inert bridge materials, inorganic cement, and organic gel. However, these materials have certain limitations. For example, inert materials are difficult to form dense plugging slugs and have weak cementing strength with the formation; cement slurry is easily diluted and dispersed in the formation, and the waiting process required for cement greatly increases the non-productive time. Comparatively, polymer gel is widely used in lost circulation control because of its good deformability and water absorption capacity. However, conventional polymer gel usually has low mechanical strength and weak retention capacity in the leakage layer. In addition, there is a lack of sufficient interaction between plugging materials and the formation, which makes it difficult to form a highly dense plugging layer. Under high pressure conditions, a large amount of leakage still occurs, and even repeated leakage occurs under frequent pressure fluctuations, resulting in plugging failure. Herein, we synthesized zwitterionic polymer/nano-silica microgels (P(MPTC-co-NaSS)@SiO2) with dynamic crosslinking ability through a facile onestep reversed phase emulsion polymerization. They exhibited spherical particles of narrow size distribution around 40 & mu;m in aqueous dispersion. The strong hydrogen bonds, electrostatic interaction and cation-& pi; endowed microgels excellent self-healing ability, strong mechanical strength and sufficient high-temperature stability. Although high temperature and salinity decreased the particle size and rheological property as well as the particle quantity, P(MPTC-co-NaSS)@SiO2 microgels maintained fairly constant rheological properties and achieved significant plugging performance in the temperature range of 70---150 degrees C and simultaneously with a salinity tolerance up to 7.5 wt% NaCl contamination. P(MPTC-co-NaSS)@SiO2 microgels were well dispersed in water-based drilling fluid (WBDF) and generated a bulk gel when concentrated in pores and cracks, which was applicable to plugging drilling fluids. A conventional WBDF with remarkable plugging property was formulated by facilely supplementing with P(MPTC-co-NaSS)@SiO2, which has a great prospect for applications in oil field. This study builds a bridge between fundamental research on microgels and the applications of soft particles as plugging materials in oil industry.