STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

782 Drilling and Well Completions
Example [46]
We will grade three dulled roller cone bits, and discuss some possible
interpretations of the wear as it relates to bit selection and application. It should
be noted that there may be more than one “correct” dull grading for each bit.
This can happen if two persons should disagree on the primary cutting structure
dulling characteristic or on what the other dulling characteristic should be.
Regardless, the IADC dull grading system provides the man on the rig with
ample opportunity to report what he sees when examining a dull.
The first dull bit is a 7%“ IADC 5-1-7-X bit and has been graded as a 6, 2,
BT, M, E, I, NO, PR (see Table 4-95). The bit looks to have been dulled by
encountering a harder formation than the bit was designed for. This is indicated
by the heavy tooth breakage on the inner teeth, and by the bit having been
pulled for penetration rate (the reduced penetration rate having been caused
by the tooth breakage occurring when the bit encountered the hard formation).
Excessive weight on the bit could also cause the dull to have this appearance.
If the run was of reasonable duration, then the bit application was proper as
evidenced by the lack of “other” dulling features, the effective seals, and the fact
that the bit is still in gage. However if the bit had a shorter than expected run, it
is probable that the application was improper. The bit may have been too “soft”
for the formation, or it may have been run with excessive weight on the bit.
The second dull bit is a 72-in. IADC 8-3-2-A bit that was graded 5,8,WT,
A,3,2,FC,HR (see Table 4-95). This dull grade indicates proper bit selection and
application. The tooth wear (WT is normal in the harder tungsten carbide insert
bits as opposed to chipped or broken teeth which could indicate excessive WOB
or RPM) is not a great deal more on the outer cutters than on the inner cutters,
indicating proper RPM and WOB. The bit was still drilling well when pulled as
indicated by listing HRS as the reason pulled. However the bit was slightly under
gage (+ in.) at this point and may well have lost more gage rapidly if left in
Table 4-95
I ‘ 1 . ’ ’ ! L ’
Courtesy SPE