Improved Prediction of ECD with Drill Pipe Rotation
Authors: Hemphill Terry , Ravi Krishna 
Published: May 2016

Abstract
The ability to accurately predict the effect of drill pipe rotation on Equivalent Circulating Density (ECD) remains a challenge for those involved in engineering today's complex wells. In many of these challenging wells (extended-reach drilling, deepwater, HPHT, etc.), the safe drilling window between hole collapse and fracturing is often narrow. Improvement in prediction of the effect of drill pipe rotation speed on ECD would allow better optimization of operational parameters in the drilling process, and hopefully reduce the incidence of operating beyond the safe drilling window.
Field ECD measurements made with downhole annular pressure tools at different circulation rates and drill pipe rotation speeds (e.g., 'fingerprinting' exercises) were modeled and presented to the industry. The presented results constituted the best-available correlation with direct downhole pressure measurements, but still showed improvement was needed. As a bridge between the earlier complex model and the one discussed in this paper, a simplified correlation was developed to give quick method of calculation of increase in ECD as a function of changing drill pipe rotation speed.
Since the development of the simplified correlation, more work has been done to improve modeling accuracy of the data set used earlier. It was found that the reduced accuracy of the earlier complex model was the result of under-prediction of the annular shear rates resulting from the axial and tangential flows that, when coupled together, produce helical flow. In this paper, a new shear rate calculation is used and the increased accuracy of the modeling is shown using the measured field data presented earlier. The improved correlation helps in better understanding circulating pressures incurred while drilling and how they relate to hole cleaning, particle suspension, and wellbore instability.

Introduction
In the past several years there have been many papers devoted to the calculation of annular pressure drop in a circulating annulus with drill pipe rotation. Given that in many of today's challenging wells the safe drilling windows (e.g., the pressure differential between hole collapse pressure and pressure to initiate fractures) are narrow, effective drilling engineering requires that any induced pressure drop caused by the drilling process should be fairly well understood. Heretofore, the effect of drill pipe rotation on circulating pressure drop has not been well understood. Several notable papers 1–3 over the years have predicted the circulating pressure drop will decrease with increasing drill pipe rotation, and researchers have attributed this to the 'shear thinning' nature of pseudoplastic drilling fluids, in which the fluid viscosity drops as a result of increasing shear rate. However, it is fluid shear stress at the conduit wall - and not fluid viscosity - that controls pressure drop in a circulating annulus.
Others, as recently as 20084 have presented data that shows pressure drops increasing and decreasing with drill pipe rotation in a circulating annulus. In particular, these researchers showed that when the inner pipe is concentric, measurements of pressure drop show decreases with rotation and, when the inner pipe is fully eccentric, the measurements of pressure drop show increases. Unfortunately no model was developed to fully account for these disparate results.
With the increased attention given to slim-hole drilling opportunities in the years 1990–1995, several researchers5,6 worked on the same general problem, as it was known that in small-diameter or reduced-diameter wellbores, the rotation of the inner pipe could significantly increase downhole pressure drop and could in many cases intiate turbulent flow conditions downhole. Their work showed contary results, one predicting decreased pressure drop with rotation in the laboratory while the other showed the opposite effect in the field.