Summary
Perhaps the only "standard" drillstring model in use today is the
torque/drag model originally developed by Johancsik et al. (1984) and put in a
standard form by Sheppard et al. (1987). Because of the simplicity and general
availability of this model, it has been used extensively for planning and in
the field. Field experience indicates that the model usually works very well.
This model is thought to be an approximation of real drillstring behavior—in
particular, the bending stiffness is neglected, so the torque/drag model is
often called a "soft-string" model. There have been many
"stiff-string" models developed, but there is no
"industry-standard" formulation.
Why does this model work well in some circumstances, yet not so well in
other cases? This question is difficult to answer without a comprehensive
torque/drag model, so this study concentrated on developing this model
formulation. The standard torque/drag-model formulation was then reviewed in
the context of this analysis, and it was determined that the model satisfied
all of the force-equilibrium equations but only one of the moment-equilibrium
equations. Solving the remaining two equilibrium equations required that both
torque and drag be zero. Fortunately, this condition is easy to resolve by
assuming that the string has shear forces. While the standard torque/drag model
is often termed a soft-string model, in reality, it must have shear forces.
Two field cases were studied. The first case was known to match field data
well; the second case was known to fail. Study of these two cases indicated
that part of the problem may lie in the description of the wellbore trajectory.
This study provides approximation formulae to evaluate the effects of variable
wellbore curvature and wellbore torsion, and these formulae were able to show
notable differences between the two sample problems.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 November 2006
- Meeting paper published:
20 February 2007
- Revised manuscript received:
17 April 2008
- Manuscript approved:
21 July 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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