Torque And Drag

'Torque and drag' is a broad term which refers to the effects which the geometry and other aspects of the drill hole may have on the turning and pulling of the drill string.

In regard to the drill string there are two drilling modes; sliding and rotating. In the 'sliding' mode the drill string is not rotated and torque is low; however axial drag is high and lock-up is possible.

Lock-up is the buckling of a section of the drill string within the drill hole and prevents the transmission of force to the bit or BHA.

In the rotational' mode the drill string is rotated at a rate which reduces the drag to a trivial force; lock-up is thought to be impossible; however, torque is high.

Other aspects of torque and drag are the maximum drill string weight available for the drill bit, drill string buckling (lock-up), friction factors, and maximum available torque for the drill bit.

Torque Drag Drilling Equation Manual

ORIGIN OF DRAG

The maximum tolerable drilling drag is governed by the strength of the wall of the drillpipe and tooljoints and connections, The strengths after the consideration of wear of these components may be found in API Recommendation RP 7G. The factors which induce drag on the drill string are the following:

  1. Doglegs not only increase drag but consume part of the strength of the drill string by bending it. Doglegs increase the contact force between the drill hole and the drill string.
  2. Sharp shoulders on the components of the drill string. Often even in open holes drillpipe rubbers reduce drag and torque.
  3. Thick filter cakes and especially if filled with drill cuttings.
  4. Ledges and sharp turns in the hole. Especially course correction turns which are not smooth doglegs and are sufficiently short, bind sections of the BHA. The sketch captures the idea. This factor is also thought to cause uneven stabilizer blade wear. Usually the upper or lower end of the blades wear more than the center of the blades.
  5. A mud without lubricity. Sometimes such things as walnut hulls help.
  6. Cuttings beds and they can be even more troublesome if while pulling out of the hole, the bed is "pushed up" above the BHA. Cuttings beds are classified as stable and unstable. If when circulation is stopped the bed slides down the hole, the bed is said to be unstable; if not, it is stable. The sliding of the bed can cause immediate sticking of the drill string.
  7. Swelling beds can be a surprise in horizontal holes if it is not known that higher mud weights must be used over those in vertical holes.
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