Flow instabilities in fluid displacement through enlarged regions in annular ducts

This work presents numerical results of flow displacement and the instabilities that appear when a viscoplastic fluid is displaced by a Newtonian one in a vertical annular duct with an enlarged region. Hydrodynamic instabilities in displacement flows may be due to viscosity and density differences, inertia effects and geometry irregularities like eccentricity and variable cross sections. If one of the fluids is non-Newtonian, the situation can be more challenging due to the dependence of the rheology on the flow kinematics. The geometry analyzed is found in the well cementation process in the oil industry, where the drilling fluid (viscoplastic behavior) is displaced by a spacer fluid (Newtonian). The drilling fluid wash out must be perfect in order to guarantee the success of the cementation operation, thus preventing the collapse of the oil well. Situations with excessive wellbore enlargement can occur, hampering the removal of the drilling fluid and the establishment of a perfect zonal isolation. The numerical solution of the problem is obtained using the finite volume technique to solve the governing conservation equations of mass and momentum. The volume of fluid method is employed to deal with the multiphase problem. The aim of this study is to look at how the governing parameters affect the flow displacement pattern and efficiency inside the enlarged region, with the goal to optimize the displacement process. The results focus on real case scenario for unstable situations, and present new findings that helps understanding the role of viscous, inertia and buoyancy effects.