Page 217 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 193

2.4.5
Drill Bit Selection, Parameters, and Hydraulics [ ]

Mill tooth bits are the only ones capable of drilling on small pieces
of steel junk. Large amounts of junk will of course cause serious dam-
age to the bits and may create even more junk by knocking off cones if
they are not run carefully.
Tungsten carbide insert (TCI) bits. Bit teeth made of sintered
tungsten carbide revolutionized tricone bit drilling since they were
introduced. The materials and techniques used today have made these
cutters very long lasting. Diamond coatings can be applied to make
them even more wear resistant, which is especially useful for gauge
protection in abrasive sands. These bits are more expensive than mill
tooth bits using the same bearing structure and are far more durable
and certainly more popular in medium, hard, and very hard forma-
tions. The tungsten carbide inserts, while extremely hard, are also brit-
tle and break under shock loading.
When using tricone bits in softer, sticky formations (shales) which
may ball the bit, center jets are often helpful. Extended jets that take
the flow closer to the bottom also help bottom cleaning. In a bit with
three nozzles, a “dead zone” of little fluid flow in the bit center can
result from running three equally sized nozzles. Running one large and
two smaller nozzles of a different size gives a cross-flow effect on bot-
tom, which may improve cleaning, especially when drilling fast. Some
operators blank off one nozzle completely to give a very strong cross-
flow effect. This does not seem to have an adverse effect on bearing life
(due to overheating) as might be expected; the blanked off area will
experience strong flow upwards from the two nozzles.
Polycrystalline diamond compact (PDC) bits. PDC bits use a thin
wafer of diamond mounted on a stud. This cuts efficiently like a lathe
tool. They are good in plastic formations (e.g., medium shales and salt)
and can give fast ROP over long intervals. Early PDC bits used in water-
based muds tended to ball the cutters, then overheat, which delaminat-
ed the diamond. Better water mud technology has overcome this.
PDC bits use either a steel or a matrix body. The manufacturer can
refurbish some steel-bodied bits as long as the bodies are in good
shape, giving you “new” bits for a reduced cost.
PDC bits have a wide choice of cutting structures. Large cutters work
aggressively in softer formations, removing large cuttings at high rates of
penetration. Bits for harder formations will be heavier set with a greater
number of smaller cutters. Gauge protection is often with natural dia-
monds. They are not suitable for very hard or abrasive formations.

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