Page 439 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 415

3.6.6
Drill Bits [ ]

3.6.6. Post-Drilling Bit Analysis

Proper analysis of the bit run is important to improve future per-
formance. A large problem is that different people will grade a particu-
lar bit differently. The IADC 8 point grading scheme is vastly better
than the old TBG grading, however, if gradings are not done with care,
it will mislead future drillers.
Record grading details and comments on the bit report. Update
recommendations in the field operational notes. Make more extensive
comments and recommendations in the end of section report for inclu-
sion in the final well report.
The IADC 8 point grading should always be used to grade bits. The
first four digits refer to the cutting structure. The last four digits refer
to other characteristics.
IADC 8 point grading scheme.
Digits one and two—tooth wear (cutting structure, tooth/cutter
wear). The first digit relates to the teeth which do not touch the bore-
hole wall. The second digit relates to the remaining teeth. Grade the
tooth with the most wear in eighths, where 0 is no wear and 8 is total-
ly worn down to the cone.
The IADC recommends that tooth breakage be taken into account
when grading wear, but this should be done with care. Tooth wear and
tooth breakage are two different characteristics that have different
causes and should not be confused. If, for example, 50% of the teeth
were broken (perhaps due to bit bouncing on bottom) and the other
50% were worn to two-eighths, then under the IADC the grading
would be a 6 or 7. Someone looking at this grading later would assume
that the 6 or 7 meant that the teeth were almost worn away and may
change the bit selection based on that. Knowing that the tooth wear
was moderate but breakage was present, it may be concluded that the
cutting structure was broadly suitable for the application, but that
drilling parameters and/or the BHA should be changed (such as by run-
ning a shock sub).
Digits three and four (cutting structure, characteristics, and location).
If more than one major cutting structure wear characteristic is present,
enter here the most severe (that which has the greatest effect on bit per-
formance). If tooth wear and breakage are both present, do not enter
WT here (the fact that the teeth are worn will be obvious from the first

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