Application of Tea Polyphenols as a Biodegradable Fluid Loss Additive and Study of the Filtration Mechanism

[Image: see text] Drilling fluids with poor filtration property are disadvantageous for well drilling, easily causing wellbore instability and formation collapse. This work reports the novel utilization of tea polyphenols (TPs) as a fluid loss additive in the bentonite–water-based drilling fluids (BT-WDFs). The influence of TP concentration and temperature on the filtration property of the fluids was described. The results showed that an increase in the TP concentration contributed to a decrease in fluid loss. Especially BT-WDFs added with 3.0 wt % TP exhibited a low fluid loss (less than or approximately 10 mL) at room temperature and high temperatures (∼150 °C), displaying better filtration property and temperature resistance than common fluid loss agents. Through the investigations on the viscosity, the particle size of TP/BT-WDFs, and micromorphology of filter cakes, the dispersion effect of TP was considered as the dominant factor for the filtration property of TP/BT-WDFs. TP molecules, containing many functional groups, could attach to the surface of bentonite platelets, improve the hydration of bentonite particles, and promote the dispersion of bentonite particles. At room temperature, TP facilitated the dispersion of hydrated bentonite. The existing “house-of-cards” structure was weakened, decreasing the particle size and viscosity of TP/BT-WDFs. At high temperature, bentonite dehydrated and aggregated, thereby increasing the particle size of bentonite particles, decreasing the viscosity of bentonite dispersion, and resulting in a high fluid loss. The addition of TP dispersed bentonite from face-to-face (FF) attraction to edge-to-face (EF) attraction, recovered the house-of-cards structure, and increased the viscosity of TP/BT-WDFs. Under the dispersion effect of TP, an appropriate grain composition of bentonite particles was formed and the pore throats were plugged to prevent the penetration of water. Finally, a compact and thin filter cake was built and the fluid loss was greatly reduced. The TP/BT-WDFs exhibited good filtration property. TP is a prospective candidate to be a high-performance and biodegradable fluid loss additive in well-drilling applications.