Stiff String Torque and Drag: Choosing a Realistic Tortuosity Model

Abstract

Selection of the correct tortuosity model is critical when performing a pre-planning stiff string torque and drag analysis. Various tortuosity models exist in the literature however selection and calibration can be complicated. This paper presents a methodology to aid in selecting an appropriate tortuosity model as well as real case studies. The goal is to apply a suitable tortuosity to a planned trajectory that will mimic the expected tortuosity along the wellbore. Depending on the directional drilling driving system employed, the trajectory can be smooth or very tortuous with significant additional local doglegs. Overall, the effect of standard tortuosity models on a run-in-hole (RIH) analysis using a stiff string torque and drag model will be investigated. Industry standard tortuosity models such as sinusoidal, random, random with dependent azimuth and helical will be investigated to help identify when they are appropriate depending on the directional driving system employed. In today's high-cost and complex 3D wells, simulations using smooth planned trajectories together with conventional soft-string torque and drag models have shown its limits in anticipating bottom hole assembly (BHA), casing or completion strings lock-up events. Hence, new methodology shall be introduced to better predict field observations. This methodology shows that the applicable tortuosity model may change depending on the directional driving system used. The drilled path or a motor, with a slide-rotate pattern, may be significantly different to that of the cyclical path drilled by a rotary steerable system (RSS). The case study shows that the additional tortuosity that was generated by the directional driving system had a significant effect on predicted hookload margin as well as elevated forces and stresses along the liner. The stiff string torque and drag and buckling model can analytically calculate the contact points which may vary significantly, both axially and radially, in a tortuous wellbore. Ultimately the selection and calibration of an applicable tortuosity model is necessary when performing a robust stiff string torque and drag analysis. This ability to correctly apply a tortuosity model in preplanning reduces the risk of BHA, casing and/or completion strings getting stuck or locked-up when running in hole, paving the way for a reduction in non-productive time (NPT) and loss of equipment.