Axial-Force Transfer of Buckled Drillpipe in Deviated Wells
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 119861, "Axial Force Transfer of Buckled Drill Pipe in Deviated Wells," by S. Menand, SPE, H. Sellami, and A. Bouguecha, Mines Paris Tech.; P. Isambourg, Total S.A.; and C. Simon, SPE, Drillscan, originally prepared for the 2009 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 17-19 March. The paper has not been peer reviewed.
Axial-force transfer is an issue in deviated wells where friction and buckling phenomena take place. The general perception in the industry is that once drillpipe exceeds conventional buckling criteria, axial force cannot be transferred downhole. The full-length paper shows that, even though buckling criteria are exceeded, axial-force transfer still could be good if drillpipe is in rotation.

Introduction
Axial-force transfer is an issue in highly deviated wells, such as horizontal and extended-reach-drilling (ERD) wells where drag friction is significant and buckling may occur. These challenging wells are characterized by a long horizontal departure (HD) relative to the true vertical depth (TVD) of the well. The axial-force-transfer issue comes from insufficient tubular weight in the vertical, or low-deviation, section of the well to run the drillstring in the long horizontal section. To overcome this problem, drilling engineers sometimes use drill collars or heavyweight drillpipe above regular drillpipe to push the string downhole. The critical angle for which drillpipe can no longer move downhole because of its own weight is a function of the coefficient of friction, μ.
Drilling and completion running-string designs for these increasingly complex wells are based on torque-and-drag results, and more specifically on buckling criteria, to determine if the string can be run in the hole. However, some recent papers and some full-scale experiments have shown that these conventional buckling models should be revisited. Because these models fail to predict buckling correctly, they also fail to predict axial-force transfer. Drilling-equipment manufacturers have developed some torque-and-drag-reduction tools to push the limits of ERD. Even though these tools are able to reduce friction in certain situations, accurate modeling still is needed to predict the axial-force transfer correctly and the occurrence of lockup.

Tools, Techniques, and Methods
Many methods and tools can be used to reduce friction and improve axial-force transfer. Friction reduction can be obtained with liquid or solid lubricants and mud additives, mechanical tools such as rollers, and by vibrating or rotating the drillstring. The goal of these tools is to reduce friction between the drillstring and borehole to improve axial-force transfer downhole. Torque-and-drag- or friction-reduction tools can be added to drillpipe or completion-running strings. They consist generally of nonrotating sleeves (or centralizers) or low-friction slider pads that reduce friction in the axial (sliding) or tangential (rotating) direction. Even though these tools can reduce the friction coefficient as much as 50%, these special devices require additional rig time for installation and removal and generally are used only in the cased-hole section.