Are Casing Centralization Calculations Really Conservative?

Abstract

Abstract Since the early 1940s casing centralization has been identified as being key to efficient mud removal and therefore to successful primary cementing. Prior to a job it is common for field engineers to spend time optimizing casing centralization using commercially available or proprietary software, in particular for highly deviated wells. However predicted casing centralizations suffer several limitations due a number of possible assumptions. For example, the mechanical response of centralizers is usually characterized in a single hole size, and different rules are used to extrapolate their behavior in different hole sizes. Needless to say, there are not too many holes that are perfectly in gauge and therefore centralization calculations in out-of-gauge sections can be questioned. With the recent introduction of a new generation of cement evaluation tools making use of flexural waves propagating vertically along casing and radiating into the annular space, it may be possible to measure the time of arrival of the cement/formation interface echo and therefore to measure casing eccentering. Therefore, one can now compare the calculated casing centralization to the one measured downhole. The objective of this paper is to review some of the current limitations of casing centralizer calculations as they are being performed in the field on a day to day basis. The analysis is supported by a comparison between measured and predicted casing centralizations on a number of field cases. Examples showing the effects of the observed discrepancies on cement placement using a displacement simulator are discussed. Recommendations to improve centralization predictions are proposed.