Engineers’ Dilemma: When to Use Soft String and Stiff String Torque and Drag Models

Abstract

Abstract The soft string and stiff string models are different string methods that have been used by the oil industry to calculate torque and drag for years. Opinions have often varied as to which model is better. This paper discusses the intrinsic difference between these two models and proposes a criterion for determining which method would deliver the most accurate results. Although both the soft string and stiff string models are used for torque and drag calculation, the soft string model is calculated like a soft rope, without considering the influence of the hole size and radial clearance. On the other hand, the stiff string model considers the effect of stiffness, thereby accounting for the extra side contact force. In theory, the stiff string model is more accurate because more variables are considered. However, the field data is more inclined to match the soft string model as opposed to the stiff string model. In fact, many factors, including the stiffness of the string, the shape of the wellbore, and clearance, can influence the status of the string and the value of the contact side force on the string. The present stiff string model does not consider these factors, making it difficult to predict accurate results. This paper analyzes the physics statuses of the string under different scenarios in order to determine which model to apply. Results demonstrate that borehole tortuosity and the shape of the wellbore can significantly change the status of the string. All factors are relative. A string with a large-size section can be very soft in a straight wellbore, which is fit for a soft string model. Likewise, a string with small-size section can be very stiff in a wellbore with severe tortuosity, which is a better fit for a stiff string model. To accurately estimate the drag force, the stiffness, as well as the wellbore shape and its clearance, should be considered. Extensive simulations have been performed and are reviewed in this paper. Results confirm that the soft string model is a better choice when the string is slimmer, the wellbore is in a lower curvature shape, and the clearance is larger. On the contrary, the stiff string model is more useful when the string is stronger, the wellbore is in a high curvature shape, and the clearance is lower. When to use the models depends on the bending shape of the string in the wellbore. Since neither model is equipped to handle all scenarios, combining the two methods provides better results.