Page 441 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 417

Drill Bits [ ]
3.6.6

For sealed bearing bits, try to turn the cones. If there is any play
or roughness, or if the cones have skidded (bearing locked), then
grade F for a failed seal. If the cone is OK, then grade E for an effec-
tive seal. If unsure, grade Q for questionable. If unable to grade for any
reason, use an N.
The amount of bearing wear actually present can only be accurate-
ly gauged by cutting the bit open and measuring the bearing surfaces.
Grade each cone individually, if possible, starting with cone #1.
Digit six (gauge wear). Grade the reduction in bit diameter. Enter
1
1
the number of /16 in. For /8 in undergauge, enter a 2. For 1 /2 in under-
1
gauge, enter 24.
Digit seven (other major dull characteristic). The third digit allows
you to enter a code referring to the most important dull characteristic
of the cutting structure. The seventh digit allows you to enter the next
most important dull characteristic, whether of the cutting structure or
anywhere else on the bit.
For instance, you may have a Tungsten Carbide insert bit with
many broken teeth, some heat checking of the cutters, and junk dam-
age. The third digit would be BT (as the most significant cutting struc-
ture dull characteristic) and the seventh digit would be the most sig-
nificant of HC or JD, whichever would have had the greatest impact in
reducing the bit life.
The codes used in the seventh digit are the dull bit characteristic
codes shown in Table 3-3 under digit three (cutting structure char-
acteristics).
Digit eight (reason pulled). The reason that the bit was pulled is very
important for future evaluation. If the bit was pulled at the end of its
economic life (usually due to rate of penetration or torque indications
of bearing failure), and bit performance was maximized throughout its
run, then this will give an excellent guide for future bit choice at the
same place in offset wells. It also serves as a good guideline for how
many hours this bit could be run given similar size/WOB/RPM. See
Table 3-4 for reason pulled codes.
If the bit were pulled for other reasons then the information,
although still valuable, has to be considered in light of why it was
pulled and how much more performance might have been expected
from the bit.

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