Curing Hydrodynamic Instability During Balanced Plug Cementing

Abstract

Summary It is shown how the flow from pumping cement through an open-ended pipe very quickly changes direction and the cement flows upward. This rapid change in flow direction indicates that a diverter tool, which leads the cement slurry perpendicularly out of a closed-ended pipe, does not have any function. The placement of a balanced plug is feasible. However, a high-density fluid above a lighter fluid is not stable. The phenomenon is known as Rayleigh-Taylor instability. In principle, to be reasonably stable, the interface must be horizontal. The longer the interface is, the more unstable is this case. Thus, it is difficult, or sometimes impossible, to create a stable situation in a deviated well section, especially if the well section diameter is large. Observations show that it is possible to modify density differences, thickening time, and viscosity differences such that the success rate can be between 40% and 60%. Using a floatable cement foundation tool, this success rate increased to more than 95% in North Sea applications. The use of such a tool is described, and its performance is justified by a numerical analysis of cement flow.