Torque and Drag

What is torque and drag?

Torque and drag (T&D) are the friction losses between the drill string and the wellbore. Drag is the axial friction: it adds to the load when pipe is pulled out of the hole and takes away from it when pipe is run in or slid down. Torque is the rotational friction: it adds to the turning load the top drive has to supply. Both grow with depth, inclination and doglegs.

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What causes torque and drag?

• Contact force: in a deviated well the string lies on the low side of the hole, and where the hole curves, tension pulls the string against the wall.
• Friction factor: how slippery the contact is. It depends on the mud, on cased or open hole, on cuttings and on the state of the wellbore.
• Well path: doglegs and tortuosity add contact points.
• String weight and stiffness: heavier and stiffer pipe presses harder on the wall.

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How is torque and drag modelled?

Most models build on the soft-string model by Johancsik, Friesen and Dawson (SPE 11380, 1984). It treats the string as a flexible cable lying along the well path and adds friction at every contact as friction = friction factor × contact force. Stiff-string models also account for the bending stiffness of the pipe, which matters in tight doglegs and with large BHA components.

Typical friction factors are 0.15 to 0.3 in open hole and 0.125 to 0.4 in cased hole with water-based mud, and lower ranges with oil-based mud (Drilling Manual). The real value changes with depth and hole condition, so engineers calibrate it against measured hookload and torque.

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What is a broomstick plot?

A broomstick plot compares measured hookload with modelled values for picking up, slacking off and rotating off bottom, at several friction factors. Plotted against depth, the lines fan out like a broom. Readings that drift away from the expected lines can point to poor hole cleaning, ledges or a changing wellbore (Drilling Metrics).

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Why does torque and drag matter?

• Weight transfer: drag takes part of the weight set at surface before it reaches the bit. The weight on bit page covers how large that loss can be.
• Reach: in extended reach and long lateral wells, torque or drag can set the length limit, when the top drive or the pipe runs out of capacity, or the string can no longer slide.
• Buckling: when compression gets too high, the pipe buckles, first into a wave and then into a helix, which adds contact and drag.
• Sliding with a motor: drag makes it hard to keep weight on the bit and hold toolface.

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How is torque and drag reduced?

• Lubricants and mud choice: lower the friction factor.
• Hole cleaning: removes cuttings beds that add friction.
• A smoother well path: fewer and gentler doglegs.
• Rotating the string: turns much of the axial friction into torque, so the string slides more easily.
• Agitators: axial oscillation tools that break static friction while sliding, so weight reaches the bit more steadily (AADE-11-NTCE-47).

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Where does NexTitan fit?

Drag limits how much of the weight set at surface reaches the bit. NexTitan applies weight on bit from inside the BHA by gripping the wellbore, so the force at the bit does not depend on pushing weight through the drag of the string above it.

To estimate what steadier weight at the bit is worth on your section, use the drilling cost calculator.