Visco-plastic fluid displacements in near-vertical narrow eccentric annuli: prediction of travelling-wave solutions and interfacial instability

Abstract

We present new results on the laminar displacement of Herschel–Bulkley fluids along narrow eccentric annuli. We adopt a Hele-Shaw modelling approach and consider the possibility that a long displacement finger should advance along the wide side of the annulus. We deduce conditions under which this cannot happen. We also analyse local instability of the interface on wide and narrow sides of the annulus using the Muskat approach. We thus show that it is possible to have both steady and unsteady travelling-wave solutions, for which the interface is locally stable.

We show how steady stable displacements arise from an increase in effective viscosity difference between displacing and displaced fluids and also analyse effects of buoyancy on the displacement. As opposed to many studies of Hele-Shaw displacements, the principle focus is on identifying stable steady displacements. Finally, we show how predictions of our model, derived from the Navier–Stokes equations using well-defined scaling arguments, compare with some of the ad hoc rule-based design systems that are currently used in the oil industry for design of primary cementing displacements.