Validation of Drillpipe Rotation Hydraulics Using Drillpipe Eccentricity as a Key Factor

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Abstract The theoretical study of the calculation of pressure drop with and without drillpipe rotation was recently presented to the industry. In the study, fluid velocities, drilling fluid shear rates, and their corresponding shear stresses were used to model increases in annular hydraulics caused by drillpipe rotation in non-Newtonian fluid flow. No extra modifications (i.e., added exponents, multipliers, etc.) were used in the modeling of circulating shear rates. It has been known for some time that the position of the drillpipe (in the annular center vs. off-center) has a significant effect on calculated pressure drop. Yet, most conventional pressure drop calculations neglect both drillpipe rotation speeds and drillpipe eccentricity and assume the drillpipe is always in the center of the annulus. It is known that moving the drillpipe from the center of the hole to an off-center position can reduce circulating annular pressure by up to 30%, and these reduced pressures can be increased significantly using increased drillpipe rotation speeds. Validation of this purely theoretical approach is presented using available field fingerprinting information, where annular pressure changes with fluid velocity and drillpipe rotation speeds were measured using downhole pressure tools. Levels of equivalent circulating density (ECD) were calculated from the changes in pressure drop using standard equations. Hence, this kind of validation is quite different than what has been used previously, for which only some success was achieved in matching the data. The results presented here show that small ranges in values of drillpipe eccentricity can describe the changes in measured pressure drop for the following: Vertical to near-vertical wellbores (one range set)Deviated wellbores (a second range set) Using the small ranges in drillpipe eccentricity presented in this study, drilling engineers can better calculate the hydraulic effects of drillpipe rotation in a given drilling situation where no experimental fingerprinting data is available.