Historically, a cement job was considered successful if the casing could be centralized and if high flow rates could be maintained. Today, however, there are more situations where one or both of these criteria are difficult to satisfy. Centralization in highly deviated wells is often challenging, and wells with low fracture gradients place a ceiling on pump rate. Better understanding of fluid movement through eccentric annuli over a wide range of casing standoffs and flow rates is essential for proper cement placement. In addition, being able to predict where cement slurry is located around the casing is very important. For instance, with an eccentric annulus there is no guarantee that cement returns at surface/mud line means complete cement coverage around the casing. Annular fluid velocities can be quite different between the wide and narrow portions of an eccentric annulus. There currently exists limited measured data on flow through annular sizes common to primary cementing operations. Therefore, a full suite of physical testing was performed to find differences in velocities and flow rates on the wide side vs. the narrow side of a true annulus. Nine models were built, each taller than two meters and each with unique annular geometries. The annular sizes chosen are common to primary cementing operations. The flow area was divided at the top of each model to capture wide and narrow side flow variations. The weight of recovered fluid vs. time was recorded and used to determine flow rates and velocities on both sides. In total, over 250 runs were performed in typical annular geometries with pump rates from 1 bbl/min to 7 bbl/min, fluid rheologies from water-thin to highly viscous, and standoffs from 50% to 85%. While most results verified current industry best practices, other results were quite unexpected, for instance, higher annular velocities on the narrow side under certain conditions. A comprehensive analysis of these results and an appraisal of their potential benefits are presented.
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