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.