Development and Field Application of New Downhole External-Casing Mud-Removal Technology for Primary Cementing
Authors: Dillenbeck Robert L. , Simpson Stewart 
Published: December 2007

Abstract
During the course of developing techniques and applications for better primary cementing results, it has become universally recognized that conventional drilling fluids and their residue must be removed from the annular space of a well before an effective hydraulic seal with cement can be achieved. To adequately protect the annular integrity, this hydraulic seal must be effective at both the cement to formation interface, and also at the cement to casing interface. To combat the problem of mud residue on the formation face, a vast array of external casing devices such as formation wipers and scratchers have been developed to help remove this residue through mechanical means. These devices, while effective at mechanically assisting chemical spacers and flushes in mud residue removal from the formation face, do nothing to help clean the exterior of the casing.
Due to its impermeable nature, casing suspended in a well is not subject to the same mud solids build up due to filtrate loss that exposed formations are. However, due to low shear rates at the casing wall and tough film coating tendencies, some mud systems can build impressive gel strengths at the casing boundary, and become totally immobile at this point.
Experience has shown that this immobile layer can resist even the abrasive flow of cement slurries pumped down the inside of casings, and resulted in the development of the bottom wiper plug commonly used to remove mud residue from the I.D. of the casing on many cement jobs pumped today. When this same immobile layer of mud adheres to the exterior of the casing through the placement of cement, the cement can have a very difficult time bonding to the casing and creating the necessary hydraulic seal.
The authors have tested a new mechanical device under controlled test conditions and also in the field, which allows drilling fluid residue to be mechanically removed from the casing exterior both before and during the actual placement of primary cement in the casing annulus. For reference in this abstract, the External Casing Wiper Tool will be referred to as ECWT.
The first prototype tool was developed in 1991. The first model was made of simple circular rubber "O" ring type wipers incased inside rigid metal rings similar to "finger type" external casing scratchers. These were mounted around the casing at such points that would need good cement to casing bonds. The idea was for downhole lateral casing loads on the casing string to hold the tool tightly against the borehole wall. When the casing was reciprocated before the start of cement operations, and also during the first stages when spacers and cement slurry were still inside the casing, the pipe would pass back and forth through the wiper elements. After the initial tools were made, testing and improvements have produced the current version with a more rigid tool body structure to hold the tool stationary in the hole during casing reciprocation and purpose designed elastomer wipers. A switch to a tool body of a rigid (Non-Bow Spring) type centralizer with angled vanes was also made. Having a slightly longer body than most external casing scratchers, this allows for two individual wiper elements to be fitted in one tool body. The tool body also incorporated drain holes between the two wiper elements to allow for any mud that might otherwise collect between the two wiper elements to escape out of the tool and away from the casing.
In the latest field test applications, the tools have been run on the same casing string with "conventional" non-wiper type rigid casing centralizers, so that a comparison between sections can be made. Even when casing reciprocation is halted up to 1.5 hours prior to cementing, a marked improvement in the casing cement bond log can be observed between those sections using the ECWT's and those that did not. The results of select field trials have convinced the authors that the technology is viable and can be applied in the field without problems.