Effective mud removal of drilling fluids from the wellbore is still a major problem in cementing. Although good pipe centralization has been known for years to be one of the keys to the success of the operation, most current design procedures do not allow pipe standoff to be taken into account.
When attempting to displace a mud in an eccentric annulus with a fluid thought to be in turbulent flow, it is shown that the displacing fluid can channel through the mud. An explanation for this phenomenon is given and a solution is proposed. When turbulent flow displacement cannot be achieved, displacement at lower rates has then to be considered and associated criteria leading then to improved mud removal efficiency are also discussed.
These displacement guidelines, as well as other more general considerations, show the need for spacers having well controlled engineering properties: compatibility, rheology. ability to suspend weighting agent, fluid loss. Examples of such spacers are presented and the properties of laboratory and field prepared samples are compared.1
1References and illustrations at end of paper.
The most important and difficult objective to achieve during a primary cement job is to provide downhole zonal isolation, that is to ensure that no fluid movement is possible through the annular cement sheath between the different permeable zones located behind the casing. This requires that the drilling mud originally present in the annulus be completely removed and replaced by the cement slurry, and, that the cement, once set, reaches and retains over extended periods of time certain mechanical properties such as bonding, compressive strength and permeability.
Research work on cement placement started in the 1930's and as early as 1940 Jones and Berdine  identified some of the most important parameters affecting the success of a primary cementing job, They showed that casing eccentricity is one of the primary causes for channeling of the cement slurry through the mud. Since then, numerous papers describing laboratory experimental results, theoretical modeling or field application work have been published on the subject. It is now generally accepted that the mud and the cement slurry must be kept separated during the displacement process [2,3]. This is accomplished by the use of pre-flushes such as chemical washes or spacers. However no final agreement has been reached today on the criteria to select in order to optimize mud removal efficiency. In particular, the influence of the casing eccentricity is still rarely taken into account, even in a qualitative or semi-quantitative way in the design of a primary cementing operation.
This paper deals essentially with the influence of casing eccentricity on cement placement and mud removal. Its effect during the mud circulation phase is briefly described and engineering criteria aimed at minimizing channeling of the displacing fluids [preflush(es), and cement slurry] through the mud are presented. Emphasis is put on the features that spacers must exhibit to play their role that is fluids with controlled properties, placed under specific flow regimes and acting as buffers between the mud and the cement slurry.