Friday, 3 November 2017


Drill Collars drilling string bottom hole assemply

Drill Collars

Drill collars are large diameter-small bore steel pipes which posses a great weight and great  stiffness.  Drill  collars  are  run  above  the  Drilling Bit  and  make  up  the  predominant component of the Drilling BHA.

What is the functions of Drill Collars:

  • Provide weight on the bit (WOB)
  • Minimize bit stability problems from vibrations, wobbling and jumping
  • Minimize directional control problems by providing stiffness to the BHA.

Drill Collar Thread

Since the drill collars have such a large wall thickness Tool Joints are not necessary and the connection threads can be machined directly onto the body of the collar. The weakest point in the drill collars is the connection and therefore the correct make up torque must be applied to prevent failure. The external surface of a regular collar is round (slick), although other profiles are available.

Drill Collar Lengths

Drill collars are normally supplied in Range 2 lengths (30-32 ft). The collars are manufactured from chrome-molybdenum alloy, which is fully heat treated over the entire length. The bore of the collar is accurately machined to ensure a smooth, balanced rotation.

Drill Collar Sizes

Drill collars are produced in a large range of sizes with various types of joint connection. The sizes and weight per foot of a range of drill collar sizes are shown in Table 14. The weights that are quoted in Table 14 are the “weight in air” of the drill collars. It is very important that proper care is taken when handling drill collars. The shoulders and threads must be lubricated with the correct lubricant (containing 40- 60% powdered metallic-zinc or lead).

Stress subjected to Drill Collars:

  • Buckling and bending forces
  • Tension
  • Vibrations
  • Alternate compression and tension.
However, if properly made up, the shoulder/shoulder connection will be sufficient to resist these stresses. Figure below shows how numbered connections should be selected to provide an efficient seal, and adequate strength.

Drilling Collar Types

drill collar types

Anti-wall stick / Spiral Drill Collar

When drilling through certain formations the large diameter drillcollars can become stuck against the borehole (differential sticking). This is likely to happen when the formation is highly porous, a large overbalance of mud pressure is being used and the well is highly deviated. One method of preventing this problem is to reduce the contact area of the collar against the wellbore. Spiral grooves can be cut into the surface of the collar to reduce its surface area.

spiral drill collar

Square Drill collars

These collars are usually 1/16” less than the Drilling Bit size, and are run to provide maximum stabilisation of the Drilling Bottom Hole Assembly (BHA).

Monel Drill Collars

These collars are made of a special non-magnetic steel alloy. Their purpose is to isolate directional Survey Instruments from magnetic distortion due to the steel Drill String.

Related Topic

Drill Collar Size Selection

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