A Comparative Study of Soft String Vs Stiff String Models Application in Torque and Drag Analysis

Abstract

Torque and drag models estimate downhole forces, torques and moments acting in wellbores and drillstring elements during drilling and completion operations. A comparison was made between soft string and stiff string torque and drag model using conventional survey data. Survey data needed for torque and drag modeling are provided by field surveys. Field survey can be conventional survey or continuous survey. Conventional survey is carried out every 90 to 100ft interval or more and only gives a partial representation of the actual wellpath, micro-doglegs and micro-tortuosities may not be fully captured with this survey. Continuous survey is carried out between 1 to 5ft intervals of the wellbore using high resolution survey tools and captures more the micro-doglegs and micro-tortuosities but more expensive than the conventional survey. Torque and drag simulations were performed using both Soft and Stiff String models for comparison using a novel software package. Data provided includes deviational survey data from conventional survey, drillstring/BHA data, and fluid rheological data. The torque and drag simulation produced results for hook loads and buckling while running-in-hole (RIH) and pulling-out-of-hole (POOH). Results from this study show that prior to buckling, results from soft string and stiff string model are almost identical with minimal differences within the range of 0.8% to 1.6% and these were achieved as open-hole friction factors (CHFF) from 0.1 to 0.25. High buckling risk was detected for OHFF of 0.3. When buckling occurs, the differences in results between the two models become very apparent. This paper showed that in order to use stiff string torque and drag model for a more realistic, representative and more accurate pre-buckling and post-buckling operations in a highly deviated well, a high resolution continuous survey is needed; this will capture more readily, the micro-doglegs and micro-tortuosities in the wellbore paths.

Investigation of the viscous fluid effect on torque and drag modeling in highly deviated wells

“…However, because of so many factors in the well, the amount of torque generated by the top drive at the surface does not get fully transmitted to the bits as there are torque losses along the wellbore. The degree of losses is a function of so many factors like the geometry of the well, the doglegs and tortuosity of the wellpath, the fluid weight, the friction factors in the well, the stiffness and bending parameters of the string, the radial clearance between the string and the borehole, the degree of eccentricity of the string in the borehole, etc (Wang and Yao, 2017;Ohia et al, 2021). The deviated sections of the borehole generate greater torque losses than straight sections because of the greater contact between the string and the walls of the well in these curved well sections.…”

“…They highlighted the difference between the two models and developed appropriate criteria on the application of the models. Ohia et al (2021) conducted an extensive study on the comparative analyses of stiff string and soft string models on standard survey data. They defined standard survey data to be the conventional deviational data done every 90-95 ft intervals.…”