Effects of Geometrical Irregularities on Displacement Flow in Annuli: An Experimental Approach

Abstract Effective primary cementing is essential for ensuring well integrity, extending operational life, and mitigating environmental risks. Existing research on the displacement of drilling mud by cementing fluids (wash, spacer, slurry), has focused extensively on displacement flow in a uniform eccentric annular space between the casing and wellbore wall. Very few investigations have focused specifically on irregular wellbore geometries such as ovality, rugosity and washout sections. Eccentricity has long ago been identified as the most critical factor influencing mud removal. Experimental studies reveal that parameters like flow rates, viscosity ratios, and density ratios significantly affect displacement front stability. For instance, significant density differences (buoyancy) can stabilize highly eccentric annular displacement flows in vertical annuli. Little experimental research has been yet carried out on other irregularities. Experiments have shown that eccentricity often dominates the effects of local enlargements or obstructions, with residual fluid accumulation and localized displacement effects noted. Recent computational advances have incorporated realistic geometries derived from caliper data to better understand flow behaviors in irregular wellbores. Although caliper data is complex, much of the borehole variation is captured in a few low frequencies, prompting the question of how displacement flows respond to wavy walled geometries. This study introduces a novel experimental setup featuring a wavy annulus inspired by caliper-filtered wellbore data. The newly developed wavy annulus is designed for experiments under both concentric and eccentric configurations. Here we report preliminary results using Newtonian fluid pairs, to experiments explore displacement dynamics within this semi-sinusoidal geometry. The findings contribute to optimizing displacement flows in complex wellbore geometries, advancing primary cementing practices and raising industry awareness of these effects.