Design Rules and Associated Spacer Properties For Optimum Mud Removal In Eccentric Annuli

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.