Flow Behavior Of An Unset Cement Plug In Place
Authors: Beirute Robert M. 
Published: December 2007

Abstract
The unstable flow behavior of an unset cement slurry plug, resting on top of a mud lighter than the slurry, is discussed in this paper.
From experimental studies, it is concluded that by carefully controlling the rheology of the fluids, this unstable flow behavior can be changed to a condition of slow, stable flow, or even to a situation of no flow. The stable flow behavior of the unset cement plug in place is analyzed by means of a detailed mathematical model. Laboratory experiments are used to obtain the pressure gradient governing the stable flow of the plug.
The analysis gives guidelines that can be used to design cement slurries with minimum "fall back" due to density differences between the cement and the mud left in the hole. The distance that a cement plug "falls" before it sets is calculated by the analysis.
A discussion of field cases using the model is also given in this paper.

Introduction
It is a well known fact throughout the oil industry that cement plugs are often hard to get. For example, it has been reported that sometimes as many as five or six attempts are needed before a satisfactory plug is obtained. The interesting thing is that, often, there is no real explanation for the failure of the job, since everything seems to go well at the surface.
A survey of the literature revealed several fine papers related to slurry design and placement techniques on setting cement plugs. However, placement techniques on setting cement plugs. However, we found no paper particularly concerned with the study of the behavior of the liquid cement plug once it is left alone in the hole. We, therefore, felt the need to investigate this situation.
If one reflects for a minute on the phenomenon, it is easy to visualize that the cement-mud phenomenon, it is easy to visualize that the cement-mud system could tend to flow at the lower interface due to the density difference (cement is almost always heavier than the mud). This tendency to flow is similar in nature to the behavior observed when a film of water is carefully placed on top of an oil phase such as diesel. The water, being heavier than the diesel, promptly flows to the bottom of the container, displacing the diesel toward the top. Obviously, this swapping effect occurs at a lower rate when the cement-mud system is considered, since the fluids have quite a bit more consistency than diesel and water.
The problem of instabilities of interfaces has been given a good deal of attention in the literature, particularly in regard to flow displacement phenomena in porous media. Thus, as an example, Dumore studied the stability problems associated with the downward miscible problems associated with the downward miscible displacement of one Newtonian fluid by another of lesser density and viscosity in a porous media. He developed a criteria for the critical velocity dividing the stable and unstable regimes for this situation. Cooney, in his studies on ion exclusion operations, also showed that flow instabilities do occur at the interface of fluids as described by Dumore's work. Dumore defines stability as the ability to suppress fluid protrusions at the interface. Consequently, on an protrusions at the interface. Consequently, on an unstable system, these protrusions, fingers or tongues tend to grow, causing the fluids to bypass each other in irregular patterns.
In a preliminary effort to study the flow characteristics of a liquid cement plug in place, several laboratory experiments were conducted where a heavy phase was placed on top of a light phase in a pipe or graduated cylinder, and the phase in a pipe or graduated cylinder, and the flow process was allowed to take place. In order to better simulate well conditions, the walls of the pipe were initially wetted with the light phase. The heavy phase consisted of a water base phase. The heavy phase consisted of a water base bentonite and dispersant mud weighted with barite, and the light phase consisted of a transparent dispersion of two polymers in water.