Rheological Behavior of Deep-Well Water-Based Herschel–Bulkley Drilling Fluids Under CO₂ Contamination

During ultra-deepwell drilling operations, the invasion of CO2 into the wellbore can significantly alter the rheological properties of drilling fluids, leading to reduced accuracy in wellbore pressure prediction and posing new challenges for precise pressure control. To address this issue, this study systematically conducted high-temperature and high-pressure rheological experiments on water-based Herschel-Bulkley drilling fluid under CO2 contamination. Experimental data were obtained under conditions with temperatures ranging from 20 to 180 °C, pressures from 0.1 to 172.36 MPa, and varying CO2 concentrations. Based on these data, nine commonly used rheological models were evaluated for their applicability. The results indicate that the shear stress of the drilling fluid varies distinctly with changes in temperature, pressure, and CO2 contamination level. CO2 contamination significantly increases shear stress, and a single rheological model is insufficient to accurately describe the fluid’s behavior under such complex conditions. Under the studied conditions, the Herschel-Bulkley model proved to be the best fit for CO2-contaminated water-based drilling fluid at temperatures below 100 °C, while the hyperbolic model provided more accurate descriptions and better applicability above 100 °C. Accordingly, a new four-parameter rheological model was proposed, which accurately characterizes the rheological behavior of water-based Herschel-Bulkley drilling fluid under different temperatures, pressures, and CO2 contamination levels, achieving a coefficient of determination (R2) of 0.994. The findings of this study can serve as a valuable reference for further research on wellbore pressure control in high-temperature, high-pressure drilling environments containing CO2.