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Abstract

This paper studies the wall contact force, frictional drag, and slackoff load transmission for tubulars in inclined and horizontal wellbores The wall contact force of helically buckled tubulars in horizontal and inclined wellbores shows to have the same expression as that in vertical wells. New wall contact force of sinusoidally buckled tubulars is modeled and expressed as half of that of helically buckled tubulars. Equations for the axial load distribution of buckled tubular and the transmitted tubular bottom load are also presented. It is shown that less load is transmitted to the tubular bottom, and helical buckling (or even "lockup") occurs much earlier, when the frictional drag from sinusoidal buckling is considered. The results can be used to improve the frictional drag and load transmission calculation for tubular operations (drillstring, tubing, and coiled tubing) in horizontal and inclined wells.

Introduction

Frictional drag could be a big problem to deal with when sliding tubulars in inclined and horizontal wells. The surface slackoff load may not be fully transmitted to the tubular bottom due to friction. The situation may become even worse, when the tubular buckles. The tubular may even experience difficulty reaching the well bottom. Although the frictional drags for unbuckled tubulars and helically buckled tubulars have been discussed, no method has been presented to calculate the frictional drag for sinusoidally buckled tubulars.

The frictional drag caused by sinusoidally buckled tubular was previously assumed to be very small and therefore ignored. This could be acceptable for the initial sinusoidal buckling status, but may not for large-deflection sinusoidal buckling. It is well accepted that the wall contact force due to helical buckling of tubular is very large. The sinusoidal buckling with large deflection close to the helical buckling should also result in a wall contact force large enough to be considered. It is thus important to study this sinusoidal buckling wall contact force for tubulars in horizontal and high-angle inclined wellbores where a long portion of tubular may buckle sinusoidally, in order to predict the frictional drag and slackoff load transmission more precisely for the tubular operations.

THEORETICAL STUDY
Wall Contact Force

For straight, unbuckled tubular in straight, inclined and horizontal wellbores, the wall contact force between tubular and wellbore wall is just the tubular weight in the lateral direction:

(1)

However, if the tubular buckles under large axial compressive load, an additional wall contact force will be generated between tubular and wellbore wall. The tubular first buckles sinusoidally under the sinusoidal buckling load Fcr, and then develops into helical shape as the axial compressive load increases to the helical buckling load Fhel. The sinusoidal buckling load to cause the sinusoidal buckling is:

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