The process of plugging oil well is very common in the oil industry and practiced for several reasons such as abandonment or recompletion. In this process, a cement plug is placed on the drilling uid to seal and abandon the well. The density of the cement paste is typically greater than that of the drilling fluid, so that the cement paste tends to sink into the drilling fluid. This situation is highly unstable due to action of the gravitational forces (density difference). The objective of this study is to determine the operation window in the space of governments parameters where the inversion velocity is low enough to ensure the success of the plugging operation during the cement curing. If the displacement occurs in excess, the contamination between the fluids will result in a unsuccessful operation. The efficiency and safety of the fluid replacement process are related to cross contamination between the fluids. An experimental investigation of the displacement of a more dense fluid above a less dense in the interior of a vertical pipe was performed. In the first stage of the experiment, a plug of water (denser and less viscous fluid) is located above the mineral oil Lubrax GL-05 and a digital camera was used to capture images of the interface during flow. In the second stage of the experiment, a plug of aqueous solution with 45 per cent ethanol (denser and less viscous fluid) is located above the soybean oil and a digital camera was used to record the interface during flow. In both cases, it is expected that the denser fluid moves to the bottom of the tube. The parameters to be investigated are the density ratio, the ratio between the height of the column of heavier fluid and the well diameter, and the rheological properties of the two fluids. The results are presented through dimensionless parameters in order to improve understanding of the problem.