Dynamic Model for Stiff String Torque and Drag

Abstract

A dynamic stiff string torque and drag (T&D) model is presented that assumes steady state motion of the drillstring as its basis for calculations. Results are compared to previously published T&D models that are based on static equilibrium. The novelty of the new dynamic model is the ability to solve T&D operations of the entire drillstring from bit to top drive in reasonable time using standard engineering computers.The new approach is based on a 3D dynamic model of drillstring and BHA in an elastic borehole. It considers bending stiffness, torsional stiffness, contact forces, and friction with localization of contact points. A numerical method is described that has proven to have excellent convergence. Complete governing equations are provided and the method is described in detail to permit readers to replicate results.The dynamic model is compared to two static stiff string models. Comparisons are also provided for three conventional soft string models including the Lubinski-Paslay-Cernocky bending stress magnification factor. Three field case studies are presented for horizontal wells. One well is short radius with dogleg severity over 50 deg/100 ft and two wells are unconventional shale wells with doglegs up to 15 deg/100 ft. Predictions for surface torque and drag up and down for the new dynamic stiff string model are compared to the static stiff and soft string models. In many situations modeled the top-level results for surface torque and drag up/down are close enough for all six models to be within the uncertainty range associated with the commonly used, lumped-parameter friction factor. However, some major differences in hook load for sliding and slack off operations are observed, which are shown to be caused by differences in location and magnitude of contact force between the drillstring and wellbore. Further, significantly lower surface torque is predicted by the new dynamic stiff string compared to other models for one case history because of lower contact forces in the vertical section of the well. In fact, the key finding of this paper is that major differences are observed for contact forces for the new dynamic stiff string model compared to all five other models, including the two static stiff string models. These differences in contact forces are most significant when the drillstring has helically buckled or when doglegs in the wellbore are high. Contact forces have a large impact on local stress behavior, which is important for predictions of casing and drillpipe wear, drillstring fatigue, and failure points in the drillstring.Although several previous papers have published stiff string models there is no industry standard formulation. The main problem holding back the development of an industry standard stiff model is perhaps the complexity of the numerical algorithm and substantial running time. To address this problem, some previous stiff string models account for bending stiffness of the drillstring but not for radial clearance while others appear to model only portions of the drillstring ...

A New Model of High-Frequency Torsional Oscillations (HFTO) of Drillstring in 3-D Wells

“…The assumption is widely used in static analysis of drillstring torque and drag in a 3D well [12]. This assumption was proved in paper [18] while comparing contact forces of the drillstring in the well calculated based on the "soft" model not accounting for the string bending stiffness with those calculated based on the "stiff" model accounting for the same.…”

Modeling contact of pipes within elliptic and spiralled wellbores using finite element analysis

“…Various models are used in oil industry to estimate the forces on drillpipes [1][2][3][4][5][6]. These tools provide various benefits, such as estimating torsional and axial drag, bending moments, load distribution along the drillpipe and critical buckling loads for a given hole-pipe-wellbore combination.…”

“…Identify the closest well point. 3. Obtain the radius of the wellbore (r w ) and the offset of the wellbore center due to spiralling at that point (x o w and y o w ).…”

Drilling Systems Automation: Current State, Initiatives and Potential Impact

“…Cayeux et al (2009) have achieved a piecewise approach using hydraulic, mechanical and thermal models. Torque and drag models have recently seen significant development (see Tikhonov et al 2013, Mitchell et al 2013, and Ødegård et al 2013) and should soon be available to supplement earlier models for real-time automation applications. Additional models are under development for drillstring dynamics and other issues (see for example, Dashevskiy et al 2003).…”