How Good is the Torque-Drag Model?

Abstract

Abstract Perhaps the only "standard" drillstring model in use today is the torque-drag model originally developed by Johancsik (1973) in 1984, and put in a standard form by Sheppard (1987). Because of the simplicity and general availability of this model, it has been used extensively for planning and in the field. This model is known to be an approximation of real drillstring behavior, in particular, that 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. Field experience indicates that the friction coefficient that matches torque data does not match drag data. Why does this model work well in some circumstances, yet not so well in other cases? Part of the problem may lie in the description of the wellbore trajectory and the wellbore tortuosity. However, this study concentrates on the actual model formulation. The torque-drag model formulation has been reviewed in the context of a large displacement equilibrium analysis and it was determined that the model satisfies all of the force equilibrium equations, but only one of the moment equilibrium equations. The remaining two moment equations therefore provide objective criteria for evaluating specific torque-drag problems. Several example problems with different inclinations and wellbore curvatures are studied to show how these criteria can be used to validate torque-drag models. Some general observations are made about torque-drag model applicability, but the real value of this study is the test criteria themselves.