How Good Is the Torque/Drag Model?

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Abstract
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.

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