Casing drilling is used as an alternative to conventional drilling with drillpipe in order to reduce non-productive time. The smaller annular space in casing drilling elevates the annular pressure loss considerably at similar flow rates in conventional drilling. Consequently, the Equivalent Circulating Density (ECD) is more affected by annular drilling fluid dynamics in casing drilling than the conventional drilling. The higher ECD experienced in casing drilling brings concerns about exceeding fracture gradient which can lead to induced lost circulation. However, several field observations demonstrate successful application of casing drilling in combating lost circulation and strengthening the wellbore.

Smearing effect theory backed by smaller cuttings at the shale shaker, eccentric casing wear, and discrepancy between analytical and field measurements are three main evidences for potential significant eccentricity in casing drilling operations. This paper demonstrates the inherent eccentricity of casing drilling as one of the parameters that controls the annular pressure losses. Eccentricity reduces the velocity in the narrow section of annulus. Similarly, it reduces the annular pressure losses considerably. In addition, controlling the fluid rheological properties as well as the flow rate are recommended to manage the casing drilling hydraulics. This comprehensive study of pressure loss and velocity profile at various annular sizes can help analyzing several field observations and designing the hydraulics of drilling operations.

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