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.
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:
C = Cb + (R + Tt )Cr F
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where: |
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C = |
overall cost per foot ($/foot) |
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Cb = |
cost of bit ($) |
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R = t |
rotating time with bit on bottom (hrs) |
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T = t |
round trip time (hrs) |
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C = r |
cost of operating rig ($/hrs) |
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.
Since penetration rate is one of the most significant factors in the assessment of bit performance this will be studied in greater depth.