Drillstring Analysis With a Discrete Torque/Drag Model

Abstract

Summary The only standard drillstring model in use today is the torque/drag model because of its simplicity and general availability. Field experience indicates that this model generally gives good results but sometimes performs poorly. For example, some friction loads predicted for casing running in horizontal wells have not been consistent with field data. In the standard torque/drag model, the drillstring shape is taken as the wellbore shape. However, given that the most-common method for determining the wellbore shape is the minimum-curvature method, this assumed wellbore shape forces the bending moment to be discontinuous at survey points. This defect is dealt with by neglecting the bending moment. A different approach assumes that the drillstring position corresponds with the minimum-curvature wellbore only at discrete points. The obvious choice for these discrete points is at the positions of the tool joints in the drillstring. Although these tool joints are fixed in position, they are allowed to rotate within the wellbore. These extra degrees of freedom allow solution of the bending-moment problem, because continuity of bending moment can now be ensured at each tool joint. This paper gives a complete description of the drillstring calculation. Typical torque/drag problems are studied to compare the two torque/drag formulations. These studies give comparisons in drag forces and torques for the two models, and for the new formulation, the magnitude of the bending moments.

Novel Torque and Drag Model for Drilling Two-Dimensional High-Angle Wells

“…According to the yield-power law (Herschel−Bulkley) model, the shear stress (τ) of drilling fluid subjected to a shear rate of (γ) due to the rotation of a D/S with a diameter D in the hole with diameter D h was presented by Kenny et al; 29 τ τ γ = + K n y (5) where τ y = yield point (lb/100 in. 2 ), K = fluid consistency (cp), and n = fluid behavior index (dimensionless).…”

“…On the other hand, drag is the axial force generated only when the pipe is moved in an axial direction as a result of frictional contact with the wellbore. 2 It always has an opposite direction to that in which the pipe is moved. During tripping in and out operations, when the drill string (D/S) is not rotated, the drag forces are higher, while, when the drill string is rotated, the drag forces are significantly reduced.…”

“…Torque is the rotational force required to overcome resistance to rotation from several sources within the wellbore such as geometric, bit, drive system frictional, and viscous torque. On the other hand, drag is the axial force generated only when the pipe is moved in an axial direction as a result of frictional contact with the wellbore . It always has an opposite direction to that in which the pipe is moved.…”

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).…”