A stiff-String model for torque and drag analysis in directional wells

Abstract

Overcoming friction forces generated from the contact of tubulars with the wellbore is one of the most important challenges in directional drilling, hence the need to accurately predict these friction forces. This paper undertakes the issue of tubular friction modeling in directional wells and presents a new approach for calculating torque and drag which relies on the beam theory for large deformations in the vertical plane. A comparative study using the soft-string model, a commercial stiff-string model, and the proposed model is carried out. To effectively test the validity and limitations of the proposed model, two wells with different geometries are studied. Results indicate that the proposed model gives similar predictions of the axial force and torque compared to the other industry-validated models where the difference of values at surface did not exceed 1% in all operations for smooth trajectories. Opposed to the soft-string model, the proposed model is able to predict the contact side and provide a profiling of the bending moment across the drill string. It is also able to carry out radial clearance analysis which has been shown to be of great importance when tortuosity is an issue where an increase of up to 10.05% of axial force at surface was observed when the hole diameter was reduced from 0.40 to 0.18 m for a tortuous well. The proposed model suffers, however, from the inability to model the post-buckling behavior of tubulars, which could generally be solved by pairing it with already available buckling models