The ability to accurately predict the effect of drillpipe 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. Accurate prediction of the effect of drillpipe rotational speed could allow better optimization of operational parameters in the drilling process, and hopefully reduce the incidence of violation of the safe drilling window.
Previously ECD results measured at different circulation rates and drillpipe rotational speeds were modeled and presented to the industry in SPE110470, presented at the 2007 SPE ATCE in Anaheim, California. The calculation methods involved several complex factors including estimated drillpipe eccentricity, nonlinear shear rate modeling coefficients, drillpipe geometry correction factors, etc. The presented results constituted the best-available correlation with direct downhole pressure measurements, but still showed improvement was needed.
After a review of the field data in hand, a new approach was taken to try to reduce the complexity of the calculations into a more easily-usable form. In this new predictive model, the diameter ratios and the rotation speed are used to predict pressure drop increase caused by the rotation of the drillpipe. This new calculation method is presented in the discussion. The modeled results show that this calculation technique can produce better prediction of field measurements of downhole pressure changes while rotating and is much simpler to use. Not only can it help in navigating through the safe drilling window, but it also can be used to separate pure rotation effects from other coupled wellbore events, such as hole cleaning and barite sag.