The wellbore friction, torque and drag, between drill string and the wellbore wall is one of the most critical issues which limits the drilling industry to go beyond a certain measured depth. For this reason many studies on torque & drag modeling have been performed. In this regard different approaches have been used; the difference in these approaches is often on how to include bending stiffness and shearing forces in the T&D calculations. These approaches are (1) the effect of shear forces calculations with the assumption of continuous contact of the wellbore wall and the drillstring as well as constant curvature trajectory, (2) the effect of bending stiffness and shearing forces calculations with assuming of clearance between drillstring and wellbore-wall and (3) the effect of bending moment and shearing forces, assuming continuous contact of the pipe and wellbore-wall and using a non-constant curvature trajectory in which the first and second derivatives of the curve exist (i.e. spline type trajectories).
This paper consists of two main parts: first a review on literatures on the subject of stiff-string T&D will be considered. Secondly, an approach will be discussed in which despite of the assumption of constant curvature trajectory and full-contact of pipe and wellbore wall, the effect of bending stiffness on torque and drag calculations are considered. The simple and robust stiff-string model is implemented in two example wells in order to find out the importance of the stiff-string calculations.