Summary
A purpose-built finite-element model (FEM) is applied to simulate radial
displacement of a casing string constrained within an outer wellbore. The FEM
represents a fully stiff-string model wherein the casing is approximated by
general-beam elements with six degrees of freedom at each node to account for
all possible physical displacements and rotations. Results predicted include
deflection of the casing centerline from the wellbore centerline, effective
dogleg curvature, bending deformation, wall-contact forces, and bending-stress
magnification. These results will provide for a more-accurate assessment of
well integrity in terms of casing-stress safety factors and centralization
before cementing, as well as more accurate prediction of running loads during
the drilling phase.
In critical-well-casing design, accurate assumptions regarding bending
stiffness may be necessary to avoid overly conservative as well as
nonconservative analysis. Challenging finite high-pressure/high-temperature
(HP/HT) and extreme-temperature wells are opportunities for increased design
efficiency by avoiding overly conservative and costly designs, which can be
crucial. Alternatively, design for extreme loads such as overpull loads in long
deviated wells may be nonconservative if severe bending stresses are not
considered.
A realistic case study is presented that demonstrates the possibility to
achieve cost efficiency by means of optimized casing design. A case study also
is presented in which a nonconservative design may result if severe bending
loads are not modeled. The purpose-built FEM code is in many ways preferable to
the use of commercial finite-element-analysis (FEA) packages because of the
time-consuming effort required to build up the detailed model.
In typical casing and tubular-stress design, a soft-string model assumes
casing strings are coincident with the wellbore centerline. The known or
assumed wellbore curvature is applied directly to the casing string. Any effect
of casing-string stiffness and allowable radial displacement within the outer
wellbore is ignored. In many cases, this results in an overly conservative
analysis. Likewise, the impact of bending-stress magnification is typically
ignored, along with the effects of centralizer placement. This may also be
nonconservative for critical overpull situations, such as in
extended-reach-drilling (ERD) and horizontal wells.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
26 July 2011
- Meeting paper published:
28 March 2011
- Revised manuscript received:
24 April 2012
- Manuscript approved:
1 May 2012
- Published online:
22 August 2012
- Version of record:
18 September 2012
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