Effects of dosage, inorganic salts and polymers on the high-temperature rheological characteristics of bentonite, attapulgite and sepiolite suspensions

Understanding the high-temperature rheological behavior of mud-forming clays is critical for designing drilling fluids suitable for deep wells. The rheological properties of bentonite (BET), sepiolite (SEP), and attapulgite (ATP) are systematically investigated from 40 to 220 °C using a high-temperature rheometer, with flow data fitted via the Bingham, Herschel–Bulkley, and Casson models. The results reveal that the viscosity of all three clay slurries increases significantly with clay concentration (4–10 wt%) yet decreases substantially at 220 °C. Fitting comparisons indicate that the Herschel–Bulkley model best describes the flow behavior of ATP and SEP suspensions, while the Casson model is optimal for BET slurries. NaCl lowers the initial viscosity of ATP slurries but triggers a viscosity increase crosser above 190 °C, rendering salt-containing ATP slurries more viscous than freshwater counterparts. For SEP, NaCl improves viscosity and delivers outstanding retention even in saturated brine. For BET, NaCl induces flocculation and elevates viscosity across all temperatures. The anionic polymer Driscal-D increases clay suspension viscosity following the order BET > SEP > ATP, and maintains a relatively high Casson yield stress at 220 °C. Fibrous ATP and SEP exhibit superior resistance to high temperature and high salinity compared with layered bentonite. This work delivers a systematic rheological comparison of the three representative clays, and is expected to provide valuable reference data and theoretical support for the analysis and regulation of drilling fluid rheology in high-temperature drilling engineering.