STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 783
the hole. This supports the decision to pull the bit based on the hours. A
bearing condition of 3 on the air bearings indicates good bearing life still
remaining. Since there are no harder bits available, and the dull grade indicates
that a softer bit would not be appropriate, this seems to have been a proper
bit application.
The third dull bit is a 12$-in. IADC 5-1-7-X bit and was graded O,O,NO,
A,E,I,LN,PP (see Table 4-95). Since there is no evidence of any cutting structure
dulling, the O,O,NO,A is used to describe the cutting structure. If this bit had
been run for a long time before losing the nozzle, this dull grading would
indicate that a softer bit (possibly a milled tooth bit) might be better suited to
drill this interval. If the run was very short, then the indication is that the nozzle
was not the proper one, or that it was improperly installed. If this was the case,
then no other information concerning the proper or improper bit application
can be determined.
Steel Tooth Bit Selection
The decision to run a specific bit can only be based on experience and
judgment. Usually, a bit manufacturer provides qualitative recommendations on
selection of his bits.
General considerations are:
1. Select a bit that provides the fastest penetration rate when drilling at
shallow depths.
2. Select a bit that provides maximum footage rather than maximum penetration
rate when drilling at greater depths where trip time is costly.
3. Select a bit with the proper tooth depths, as maximum tooth depth is
sometimes overemphasized. When drilling at 200 rpm at a rate of 125 ft/hr,
only + of the hole is cut per revolution of the bit. Bits are designed with
long teeth and tooth deletions for tooth cleaning.
4. Select a bit with enough teeth to efficiently remove the formations, as that
often can be more important than using a bit with maximum tooth depth.
5. Select a bit with enough gage tooth structure so that the gage structure
will not round off before the inner-tooth structure is gone.
6. Select a bit with tungsten carbide inserts on gage if sand streaks are
expected in the formation. Do not depend on gage hardfacing alone to
hold the hole to gauge.
Crooked hole considerations are:
1. Select a bit with less offset.
2. Select a bit with open gage teeth to straighten hole.
3. Selecting a bit with more teeth and with shorter crested teeth results in
smoother running and reduced rate of tooth wear.
4. Selecting a bit with "T"-shaped gage teeth reduces the tendency for the
bit walk.
Pinching considerations are:
1. Select a bit with less offset and harder formation type (more vertical gage
angle).
2. Do not select a bit with reinforced gage teeth unless excessive gage tooth
rounding is the reason for pinching.