Incomplete fluid–fluid displacement of yield-stress fluids. Part 2: Highly inclined pipes

•Experimental results of viscoplastic fluid displacement flows.•Characterizing the flow regimes over the full range of dimensionless parameters.•Observations of a range of exotic behaviors for slump flows at higher inclinations.•New results on the exchange flow of a Newtonian and viscoplastic fluid.•Locating the region of highly unstable flows in the dimensionless flow regime maps. We present results of an experimental study of buoyant miscible displacement flows of a yield-stress fluid (Carbopol) by a higher density Newtonian fluid along a long inclined pipe. We focus on the industrially interesting case where the yield stress is significantly larger than a typical viscous stress in the displacing fluid, but where buoyancy forces may be significant. We find that the slump and centre-type displacements identified in our earlier work on near-horizontal flows are in fact observed over the full range of pipe inclinations. Interestingly, the occurrence of these regimes is primarily governed by approximately the same ratio of Reynolds number to densimetric Froude number, Re/Fr, as in near-horizontal case. However, we do observe a range of exotic behaviour for slump flows at higher inclinations associated with the progressive break-up of the Carbopol layer. We give a detailed description of these secondary regimes and their formation. We also observe a third regime which we refer to as turbulent-mixed flow. In this case, despite the existence of the yield stress, the flow is very similar to that for a Newtonian displacement when the mixing is very efficient. In the absence of an imposed flow of the displacing fluid, we observe that flows may nonetheless develop under the action of buoyancy despite the yield stress. We discuss the probable mechanisms and potential implications for the plug cementing process.