PDC Bit Performance
The performance of a bit may be judged on the following criteria:
- How much footage it drilled (ft).
- How fast it drilled (ROP).
- How much it cost to run (the capital cost of the bit plus the operating costs of running it in hole) per foot of hole drilled .
Cost Per Feet
Since the aim of bit selection is to achieve the lowest cost per foot of hole drilled the best method of assessing the bits’ performance is the last of the above. This method is applied by calculating the cost per foot ratio, using the following equation:
This equation relates the cost per foot of the bit run to the cost of the bit, the rate of penetration and the length of the bit run. It can be used for:
- Post drilling analysis to compare one bit run with another in a similar well.
- Real-time analysis to decide when to pull the bit. The bit should be pulled theoretically when the cost per foot is at its minimum.
PDC Bit Performance
- PDC bits tend to drill faster with low WOB and high RPM.
- They are also found to require higher torque than roller cone bits.
- The general recommendation is that the highest RPM that can be achieved should be used.
- Although the torque is fairly constant in shale sections the bit will tend to dig in and torque up in sandy sections.When drilling in these sandy sections, or when the bit drills into hard sections and penetration rate drops, the WOB should be reduced but should be maintained to produce a rotary torque at least equal to that of a roller cone bit.
- Too low a WOB will cause premature cutter wear, possible diamond chipping and a slow rate of penetration.
The best ROP results have been achieved with oil based muds but a good deal of success has been achieved with Water Based Mud.
Reasons for the improved performance in oil based muds has been attributed to increased lubricity, decreased cutter wear temperature and preferential oil wetting of the bit body. The performance
of PDC bits in respect to other mud properties is consistent with that found with Roller Cone Bits i.e. increase in mud solids content or mud weight decreases ROP.
The effects of increased hydraulic horsepower at the bit are similar to their effect on Roller Cone Bits. However manufacturers will often recommend a minimum flow rate in an attempt to ensure that the bit face is kept clean and cutter temperature is kept to a minimum. This requirement for flowrate may adversely affect optimization of HHP.