Page 216 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 192

[ ] Well Programming
2.4.5

The ultimate objective is to be able to describe a formation purely
using sets of numbers for the various properties that can then be
manipulated mathematically by an expert system to give optimum bit
features to drill the desired interval length at the lowest cost. Given suf-
ficiently detailed offset bit runs to analyze, bit performance could per-
haps be predicted with enough accuracy to recognize how worn the bit
is while drilling. Therefore, even in nonhomogenous formations, the
bit could be pulled when its efficiency (specific energy required to
destroy a certain volume of a particular rock, ideal for that bit ÷ actu-
al) drops to a level that justifies a round trip. Changes in drillability
due to increasing pore pressure could also give better kick prediction.
The major bit vendors will draw up bit proposals based on log
information, which they interpret for the rock mechanical and other
properties, together with offset run data. Examining proposals from
several vendors for a well can give you a good feeling for the require-
ments of the best overall bit to use.

2.4.5. Types of Drillbits

Mill tooth bits (“rock bits”). Mill tooth bits are most useful in soft
formations, usually the top sections of the hole. One large bit may drill
top hole for several wells. It is also possible to have the cutting struc-
ture built up again (by a specialist firm) and extend the bit life, as long
as the bearings are still okay.
Some operators use mill tooth bits to drill the shoetrack prior to
running a PDC bit, although PDC bits are routinely used to drill out
plugs. (For a plug to be PDC drillable, it should contain no aluminum
that tends to wrap around the cutters, which then fail through over-
heating. The nonrotating type of plug is also preferable.)
Mill tooth bits are also used in formations that contain harder nod-
ules (e.g., chert and conglomerates), which may cause TCI bit insert or
PDC cutter breakage due to shock loading. Running a shock sub will
be helpful here. Some mill tooth bits are designed with bearings and
cutting structures to be used with mud motors at higher speeds.
Certain strong, “elastic” shales can be drilled better with a mill
tooth bit than a PDC or insert.

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