New Approach to Stiff-String Torque and Drag Modeling for Well Planning

Abstract

During the process of drilling, the drillstring inadvertently comes in contact with the wellbore generating frictional losses in rotating moment (torque) and axial force (drag). These losses reduce the rotational power available at the drill bit, thus making adequate torque and drag modeling a critical piece in the drilling puzzle. The simplifying assumptions of the widely used soft-string model for torque and drag modeling make it less accurate for new complex well designs, therefore creating the need for the use of the more robust stiff-string model. This work focuses on a new approach for developing a stiff-string model that can be easily implemented for well planning. The stiff-string model addresses the pitfalls of the soft-string model by using cubic splines for its well-path trajectory and solving three coupled, non-linear ordinary differential equations that describe the motion of the drillstring at each survey point to account for the shear forces and bending stiffness. The stiff-string model is then applied to design 4 horizontal wells. In comparison with the soft-string model, results show that the stiff-string model is able to capture the extra contact loads as the drillstring goes through bends, consequently predicting higher hookload and torque values than the soft-string model. This paper also highlights how both models can be applied to well planning for improved results.