Exchange flows between yield stress materials and Newtonian oils

We perform an experimental analysis of the exchange flow between an elasto-viscoplastic thixotropic material above a less dense Newtonian oil inside a vertical tube. This situation is usually found in cementing plug operations, performed in the oil industry when there is a need to abandon an oil well. The experiments show that three different flow regimes are observed, namely unstable, quasi-stable, and stable (no flow). The unstable regime is a wavy core-annular flow with the denser liquid in the core region. In the quasi-stable regime a slow plug flow starts after a time delay, which is a consequence of thixotropic and elastic effects. Through analysis of the experimental results it is possible to identify, for a given pair of fluids, the region in the governing parameter space, within which the exchange speed is sufficiently low (or zero).