Experimental study on the drag coefficient of single bubbles rising in static non-Newtonian fluids in wellbore

•We adopt a vertical, cylindrical wellbore mimic in the experiment.•Drag coefficient increases with increased fluids viscosity, surface tension.•Drag coefficient increases with increased bubble diameters.•A new drag coefficient model for bubbles rising in wellbore was proposed.

Using an empirical drag coefficient model to investigate the laws of bubble rising in non-Newtonian fluids is important for calculating the safe cycle time in avoiding typhoon, tripping operation in offshore drilling engineering. Herein, the drag coefficient for a single bubble rising steadily in a static non-Newtonian fluid within a wellbore was experimentally investigated using a vertical, cylindrical wellbore mimic. The effects of viscosity, density and surface tension of the fluids, as well as bubble diameters on the drag coefficient were studied. The experimental results indicate that the drag coefficient increases with the increase of the fluid viscosity, surface tension and bubble diameters, while the solution density has little effect on the drag coefficient. Meanwhile, the relationship between Reynolds number (Re) and drag coefficient and that between Re and Eötvös numbers (Eo) were analyzed, respectively. The result suggests that surface motion leads to a transition of drag coefficient. A new correctional model for the drag coefficient of single bubbles rising in static non-Newtonian fluids in a wellbore was obtained, which showed a good agreement with the experimental data.

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