Summary
Residual bend exists in every coiled tubing string. When the coiled tubing
is placed inside a wellbore, its initial configuration is not straight but has
a certain residual bend. This paper’s objective is to explain how this residual
bend in coiled tubing affects its extended reach.
This paper presents a new modeling technique to account for the effect of
residual bend on coiled-tubing posthelical buckling behavior and maximum
penetration. The initial configuration of the coiled tubing is assumed to be
the form of a helix along the wellbore. By using the energy method, a set of
solutions is obtained for the helix configuration and the radial contact force
under the conditions of applied axial force and initial residual bend. These
solutions are used to analyze axial load transfer and extended-reach capability
of coiled tubing on horizontal wells. It is shown that in a horizontal well,
with a relatively small residual bend radius, the existence of residual bend
significantly reduces the maximum depth the coiled tubing can reach. Results of
this work can be used to enhance job design for extended-reach
applications.
Introduction
Residual bend exists in every coiled-tubing string. During storage and
transportation, a coiled-tubing string is plastically deformed (bent) as it is
spooled on a reel. During operations, the tubing is unspooled (bent) from the
reel and bent on the gooseneck before entering into the injector and the
wellbore. After leaving the injector, the coiled tubing has a residual bending
radius typically ranging from 150 to 400 in. After entering into the wellbore,
the initial configuration of the coiled tubing is not straight; it retains a
residual bending curvature. The objective of this paper is to explain how this
residual bend affects coiled-tubing posthelical buckling behavior and
lockup.
The subject of buckling and lockup for long tubing strings has been
researched extensively in the oilfield industry.P1-11P In his
pioneering work on the helical buckling of a long tubing string in the
wellbore, Lubinski first established the relationship between the pitch of the
helix and the applied forces (Lubinski et al. 1962). Thereafter, most of
the research focused on determining the critical buckling loads of a long
tubing string in horizontal or inclined wells (Paslay and Bogy 1964; Dawson and
Paslay 1984; Chen et al. 1990; Wu and Juvkam-Wold 1995). A few researchers
looked at the effect of torsion on helical buckling (Wu 1997; He et al. 1995;
Miska and Cunha 1995; Mitchell and Miska 2006) or the effect of connectors
(Mitchell and Miska 2006). Mitchell (1986) further established the relationship
between the radial contact force and the applied axial force on a helically
buckled string, which has since been widely used to evaluate the effect of
helical buckling on lockup depth.
These works typically assumed a straight tubing string. Qiu et al. (1997)
developed a new model to evaluate the effect of the tubing’s initial
configuration (curvature) on sinusoidal and helical buckling. They assumed that
the initial configuration of the coiled tubing was sinusoidal with the coiled
tubing lying on the low side of the wellbore. They concluded that the initial
configuration had a significant effect on the least axial force required to
produce coiled-tubing helical buckling in the wellbore.
In this paper, a new modeling technique is presented to account for the
effect of coiled-tubing residual bend on posthelical buckling and lockup. The
initial configuration of a coiled-tubing string is assumed to take the form of
a helix. By using the energy method, the formulae for helix pitch and radial
contact force is derived, accounting for the initial residual bend and the
applied axial force. These relationships are used to derive the axial load
transfer along the coiled tubing with residual bend. The axial load transfer
solution allows us to evaluate the effect of residual bend on coiled tubing
helical buckling and lockup. It is shown that in a horizontal well, the effect
of residual bend could significantly reduce the coiled tubing maximum reach.
Results of this work can be used to help design coiled-tubing jobs for
extended-reach applications.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
19 June 2006
- Manuscript approved:
20 June 2006
- Version of record:
20 February 2007
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