In this work we perform an experimental study of the buoyancy-driven exchange flow of the cement plug placement in oil wells, most commonly for abandonment and sidetracking. In this process, a denser fluid is placed above a less dense one, generating a highly unstable situation that can lead to unsuccessful operations when their positions invert before the cement curing. To analyze the exchange flow we perform visualization experiments with real drilling fluids positioned above a lower-density liquid in a Plexiglas vertical tube. The use of a drilling fluid, instead of a real cement plug at the top, was chosen in order to eliminate the curing influence on the preliminary tests. Thus, the same density difference between cement and other well fluid can be evaluated. A sliding gate ensures an initially flat interface that evolves with time immediately after the gate opening. The interface shape and front speed is recorded with the aid of a digital camera, and measured through image processing and analysis. Since in the real situations the well diameters are larger than the ones used in the laboratory, the interfacial tension effects are naturally negligible. Hence, surfactants are used to reduce the influence of interfacial tension on the experiments. The effects of the governing parameters on the inversion velocity and flow regime are analyzed for different combinations of fluids. Drilling fluids and cement plugs have non-Newtonian viscoplastic behavior, and careful rheological tests are performed to obtain the rheological parameters. Therefore, yield stress, viscosity levels, geometric parameters and density ratios between the fluids can lead to different flow patterns that will contribute to a successful or unsuccessful operation. The analysis of the experimental results will allow one to identify, for a given pair of fluids, the operational window in the governing parameter space within which the inversion velocity is sufficiently low (or zero) to ensure the heavier fluid placement success.