Advances in the Calculation of Circulating Pressure Drop with and without Drillpipe Rotation

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Abstract Annular pressure measurements have consistently shown the direct relationship between increasing drillpipe rotation speed and increasing pressure drop. A hydraulic model that accurately predicts the hydraulic effects of drillpipe rotation can improve the pressure drop calculations. However, conventional pressure drop modeling does not take into account 100% of the calculated pressure drop that is produced with the drillpipe rotation moving the fluid in helical flow. Hence a need to better refine calculation methods is warranted. The pressure drop calculations for flowing drilling fluids were calculated using equations previously published. Calculations for axial and helical flow were performed separately, as in the axial flow case an area of plug flow is assumed, while in the helical flow case 100% of flow area is assumed to be sheared. Conventional pressure drop modeling uses geometry, pump rate, and drilling fluid rheological properties to calculate pressure drop across the annular gap. However, these results usually account for approximately 95% of the real pressure drop. What has been missing up to this point is the pressure drop that arises between the slipping fluid layers moving in helical flow. Secondly, the effects of drillpipe rotation are usually not taken into account, even when the drillpipe is rotating. Theoretical calculations using the proposed solution for total pressure drop show that: 100% of the pressure drop can now be theoretically accounted for, both with and without the effects of drillpipe rotation.With drillpipe rotation taken into account, the pressure drop in the slipping fluid layers is higher than that for axial cases. A method is shown that explains how to calculate this extra pressure for both axial and helical flow cases.Pressure drops in eccentric geometry cases can rise quickly in the more-narrow annular regions. Hence when drillpipe rotation speeds are in the normal operating range of ±100 rev/min, calculated pressure drops approximate those calculated conventionally (axial flow with no drillpipe rotation). With these extra pressure drop calculations, the real pressure drops for fluid moving in helical flow can be obtained, something especially important in areas where safe drilling window margins are very narrow. As a result, better equivalent circulating density (ECD) predictions can be made.