In oilwell primary cementing and side track / abandonment plug operations, effective fluid displacement is a major issue. In such operations, the drilling fluid is often displaced by wash and spacer fluids and finally by the cement slurry. Fluid displacement efficacy is in large measure determined by the shape of the interface between the two fluids: elongate profiles tend to result in fluid channelling while flat profiles normally promote efficient displacement. Chemical incompatibility between the synthetic drilling fluid and the cement slurry is another source of potentially severe operational problems. Optimization of the rheological properties of each fluid used in cementing or plugging operations is essential for obtaining success. To better understand the interrelation between fluid properties and flow profile to effective displacement both experimental and theoretical research efforts were conducted. Medium and large scale experiments were performed where the efficiency of the displacement was determined by measurements of physical properties of the injected and displaced fluids over the time of pumping. These physical measurements are compared to the expected values assuming a flat interface profile (no mixing). The following interface propagation processes were monitored: drilling fluid / spacer, spacer / cement slurry and cement slurry / spacer. A Computational Fluid Dynamics (CFD) approach supported the analysis. Based on the results of these analyses, guidelines are provided both for primary cementing and side track / abandonment plug operations in order to minimize the contamination of the cement slurry, and consequently to increase the quality of the cementing and plug operations

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