Several oil industry operations, such as plugging and abandonment wells, cementing operations, and restarting flows of gelled pipes, use the displacement of one fluid by another in annular tubes. There are many variables that can affect the displacement efficiency, namely the rheological behavior of the fluids, the density ratios, and the well geometry. The interaction of these parameters can lead to instabilities at the interface between fluids that will result in inefficient flow displacement processes. In these circumstances, an effective procedure is required to prevent issues that could result in dangers and significant financial losses. In this work we present experimental results of displacement efficiency through an annular tube. The analysis is done for low Reynolds numbers, with a Newtonian (water) fluid being displaced by a non-Newtonian (xanthan gum solution) one, to simulate a plug & abandonment operation. The experimental study is done using a test rig that consists of a steel tubing and a transparent casing with diameters that reflect a real-case scenario. We study the displacement process using two different inclinations, concentric and eccentric geometries, and tubing vibration. To assess the impact of these parameters on displacement efficiency, we perform flow visualization at the experimental rig and evaluate interface instability.