β-Cyclodextrin Hydrothermal Carbon Microspheres/Polymer Composites for Filtration Loss Control in Ultrahigh-Temperature Water-Based Drilling Fluids

Extreme high-temperature and high-salinity environments pose severe challenges for the field application of water-based drilling fluids. The thermal degradation of conventional polymer loss control agents is a key factor limiting their performance. The incorporation of nanomaterials offers a novel approach to enhance the high-temperature resistance of polymer loss control agents, making it a research hotspot in this field. In this study, a novel composite additive (DANT/HCM) was developed by physically blending β-cyclodextrin (β-CD) hydrothermal carbon microspheres (HCMs) with a comb-like polymer (DANT). Thermogravimetric analysis showed that DANT/HCM has excellent thermal stability. Structural characterization results reveal that oxygen-containing functional groups on the HCM surface form stable interfacial interactions with polymer segments via hydrogen bonding, conferring unique structural synergistic effects to the system. Scanning electron microscopy images showed that the HCM maintained an intact spherical structure after compositing. Performance tests showed that after aging at 240 °C, the filtration loss of the base mud with 1.5 wt % DANT/HCM was only 9.0 mL, significantly lower than the 20.8 mL of DANT alone. Additionally, this composite material exhibits excellent salt resistance, maintaining a filtration volume of 27 mL even in 30 wt % NaCl drilling fluid. Combined with the zeta potential, particle size distribution, and SEM analyses, the performance enhancement was mainly due to the enhanced thermal stability of the composite system, improved particle dispersion, and the formation of a dense, uniform filter cake structure. This study achieves highly efficient filtration loss reduction by introducing thermally stable, nanosized, water-based, thermally carbonized microspheres and combining them with long-chain comb polymers, thereby forming a unique synergistic loss reduction mechanism. It provides a theoretical basis and practical reference for the development of high-performance, high-temperature-resistant filtration loss reduction additives suitable for ultradeep wells and high-temperature environments and has a good prospect for engineering applications.