Selecting a drill bit

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.

See also

Rotary drill bits

Roller cone bit design

PDC drill bits

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

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Sebastian Desmette