Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
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.
- 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.