Equivalent-Circulating-Density Fluctuation in Extended-Reach Drilling
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 115149, "Fluctuation of Equivalent Circulating Density in Extended Reach Drilling With Repeated Formation and Erosion of Cuttings Bed," by Shigemi Nafanawa, SPE, U. of Tokyo, and Komei Okatsu, SPE, Japan Oil, Gas, and Metals Natl. Corp., originally prepared for the 2008 IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition, Jakarta, 25-27 August. The paper has not been peer reviewed.
Management of equivalent circulating density (ECD) is one of the key issues in designing and drilling extended-reach wells (ERWs). The full-length paper focuses on the ECD fluctuation that occurs because of the repeated formation and erosion of a cuttings-deposit bed while drilling an ERW. While the ECD fluctuation is thought to be caused primarily by the mud pumps being turned on and off, calculations showed that repeated formation and erosion of a cuttings-deposit bed, because of the repetition of drilling and drilling interruption for making connections, cause a significant level of ECD fluctuation.

Introduction
ECD management is one of the key issues in designing and drilling ERWs. High ECDs experienced while drilling a long highly inclined or horizontal section increase the risk of drilling problems such as lost circulation, formation failure, and formation damage. In some situations, the reduction in mud-flow rate to control ECD and to eliminate circulation losses may cause insufficient hole cleaning and negative effects on torque and drag. ECD is thought to be a more significant issue in drilling ERWs because of the following.Long measured depth relative to the vertical depth.ERWs generally are shallow so that the formations often have little integrity.ERWs generally use larger-diameter drillpipe for hydraulics reasons and/or prevention of buckling.A more aggressive flow rate generally is required for sufficient hole cleaning.
As well as the high static ECD, fluctuation of ECD is known to cause borehole instability analogous to formation fatigue failure. Borehole instability induced by ECD fluctuation could be a serious problem especially in drilling brittle formations and shallow ERWs in which the formations tend to have little integrity.
It was reported in the field of extended-reach drilling (ERD) that the actual ECD measured by pressure while drilling was higher than the predicted ECD from modeling and that this high ECD was attributed to poor hole cleaning. While the cause of ECD fluctuation is thought to be primarily a result of the mud pumps being turned on and off, hole cleaning should be a vital factor affecting ECD. From previous numerical studies on cuttings transport, the authors came to suppose that repeated formation and erosion of a cuttings-deposit bed, as a result of the repetition of drilling and drilling interruption for making a connection, may cause a significant level of ECD fluctuation, and this kind of ECD fluctuation could not be estimated or detected from conventional steady-state-hydraulics calculations.