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Selecting a drill bit

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Factors to consider during bit selection and operation

Some important rules to help guide in drilling bit selection are discussed below:

Rules of thumb for bit selection

  • Shale has a better drilling response to drill speed.
  • Limestone has a better drilling response to bit weight.
  • Bits with roller bearings can be run at a higher speed than bits with journal bearings.
  • Bits with sealed bearings have a longer life than bits with open bearings.
  • Bits with journal bearings can be run at higher weights than bits with roller bearings.
  • Diamond product bits can run at higher speeds than three-cone bits.
  • Bits with high offset may wear more on gauge.
  • Cost-per-foot analysis can help you decide which bit to use.
  • Examination of dulls can also help you decide which bit to use.

Tripping can ruin a new bit

  • Make the bit up to proper torque.
  • Hoist and lower the bit slowly through ledges and doglegs.
  • Hoist and lower the bit slowly at liner tops.
  • Avoid sudden stops. Drillpipe stretch can cause a bit to hit the hole bottom.
  • If reaming is required, use a light weight and low speed.

Establish a bottomhole pattern

  • Rotate the bit and circulate mud when approaching bottom. This will prevent plugged nozzles and clear out fill.
  • Lightly tag bottom with low speed.
  • Gradually increase speed and then gradually increase weight.

Use a drill-off test to select best weight on bit (WOB) and speed

  • Select speed.
  • Select bit weight. Depending on bit selected, refer to appropriate manufacturer’s recommended maximum speed and WOB.
  • Lock brake.
  • Record drill-off time for 5,000-lbm increments of weight indicator decrease.
  • Repeat this procedure for different speeds.
  • Drill at the weight and speed that give the fastest drill-off time.

The bit is not always to blame for low ROP

  • Mud weight may be too high with respect to formation pressure.
  • Mud solids may need to be controlled.
  • Pump pressure or pump volume may be too low.
  • Formation hardness may have increased.
  • Speed and weight may not be the best for bit type and formation. Use drill-off test.
  • Bit may not have adequate stabilization.
  • Bit may be too hard for the formation.

References

Noteworthy papers in OnePetro

External links

See also

Rotary drill bits

Roller cone bit design

PDC drill bits

PEH:Introduction to Roller-Cone and Polycrystalline Diamond Drill Bits

Page champions

Sebastian Desmette

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