Mud Displacement By Slurry During Primary Cementing Jobs - Predicting Optimum Conditions
Authors: Martin Madeleine , Latil Marcel , Vetter Pierre 
Published: December 2007

Abstract
This paper deals with determining the optimum conditions for the best chances of success during primary cementing jobs. primary cementing jobs. Results obtained from laboratory experiments carried out in pilot-plant models with a limited length may not always be applied to actual wellbores. Consequently, a theoretical approach was adopted, resulting in the development of a mathematical model. The method is based on a formulation similar to the one used in multiphase flow through porous media. Moreover, it takes into account the non-Newtonian and possibly thixotropic behavior of the fluids. possibly thixotropic behavior of the fluids. The results emphasize how strongly displacement efficiency is affected by mud thixotropy and spacer characteristics. They also show that the choice of an optimum displacement flow rate, for slow flow or turbulent flow, depends on all the other conditions, namely, on the density and viscosity ratios between mud and slurry.
The validity of the proposed mathematical model has been verified by comparing actual and computer-predicted results, both in experimental annuli 5 m long and in field cementing jobs with casing-shoe depth varying from 600 to 3,600 m.

Introduction
The channeling of cement slurry through mud is a common cause of poor quality primary cementing jobs. Mud displacement depends on a great many parameters, none of which can be ignored: annulus geometry (diameters, eccentricity, inclination, depth); spacer and slurry volumes; flow rates; properties of each fluid (mud, spacer, slurry) including density, flow characteristics and thixotropy; pump shutdowns during which the gel strength could increase; and casing movements.
The experimental study of this problem requires great care in interpreting the results. Consequently, a theoretical approach has been adopted, resulting in the development of a mathematical model.

EXPERIMENTAL APPROACH LIMITATIONS
An initial limitation comes from the non-Newtonian nature of the fluids. Because the laws of similarity do not apply to them, the actual values of the different parameters governing cementing must be maintained. But in pilot-plant models, although it is possible to maintain full-size diameters, flow rates, and fluids properties, the length must be limited (often to not more than a few meters).
This involves a second limitation because the full effect of gravity cannot be taken into account when the separation area between mud and slurry does not remain horizontal, especially when the annulus is eccentric and the mud to be removed is relatively thixztropic. Actual wellbores are more than several hundred or a thousand meters deep. As illustrated Fig. 1, with a level difference Delta H (greater than the pilot-plant model height), the slurry which is pilot-plant model height), the slurry which is heavier than mud may modify the interface shape by making it flatter.

THEORETICAL APPROACH
Previously published mathematical cementing models either assimilate the eccentric annulus with a set of parallel tubes manifolded together or assume that the fluids obey a given rheological law. The present model is not bound by these limitations. Moreover, it takes into account the different parameters mentioned at the beginning of this paper parameters mentioned at the beginning of this paper (with the exception of casing rotation), i.e., in particular spacer characteristics, mud thixotropy, particular spacer characteristics, mud thixotropy, and reciprocating movement.

Flow Assumptions
Fluids (mud, spacer, and slurry) are considered as immiscible and separated by sharp interfaces, with no mixing zone. A cross-section of the flow domain is an annulus, bounded by two circles (sections of the casing and the wellbore).