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Drilling Bits
Drilling bit classification
1. Roller Cone Bit
Milled Tooth Bit
Inserted Tooth Bit Inserted Tooth Bit
2. Fixed Cutter Bit (Drag Bit)
Roller Cone Bit
Roller cone bits are comprised of two or
three cones having teeth sticking out of them
Nowadays three cone bits are being used in
the industrythe industry
Teeth apply a force exceeding the
compressive strength of the rock
Hydraulic cleaning action is achieved by
using nozzles
As the rock type gets harder, the tooth length
and cone offset must be reduced to prevent
tooth breakage
advantages
1. Can handle rough drilling conditions
2. Used in both hard and soft formations
3. Fastest penetration rate
4. Less expensive than fixed cutter bits4. Less expensive than fixed cutter bits
5. Are more sensitive to the amount of
pressure overbalance and thus better
indicator of overpressure formations,
disadvantages
Wear and tear is more as compared to PCD
bits
Bit Hours (Bit rotation time) are less
Borehole diameter (gauge)
The gauge of the hole drilled by roller cone
bits is maintained by the outside cutters
which are also known as gauge cutters
Cones are commonly heat treaded and made Cones are commonly heat treaded and made
of NiMo-steel, teeth are sometimes made of
NiCrMo-steel.
offset
Offset of the bit shows how much the axis of
the cones are out of center
Cone axis do not intersect at a common point
This offset causes the cones to stop rotating This offset causes the cones to stop rotating
periodically as the bit is turned and scraps
the hole similar to the cutting action of a
drag bit
Offset angle varies from O-4 degree
Mill Tooth Bit
Teeth are milled out of the same body the
cones consist of
These bits are very robust and tolerate severe
drilling conditions but wear out relativelydrilling conditions but wear out relatively
quickly
Inserted Tooth Bit
These are also called tungsten carbide bits,
have teeth made of tungsten carbide which
are fitted on the cone bodies.
These bits do not tolerate shock loadings but These bits do not tolerate shock loadings but
they can drill long sections before being worn
out
More expensive than milled Tooth Bits
Also called Button Bits
A new technology of insert bits coats the
teeth with a layer of diamond. These bits,
also known as tungsten carbide insert bits
(TCI), exhibit a significantly improved bit life(TCI), exhibit a significantly improved bit life
in abrasive formations.
Fixed Cutter Bit
Fixed cutter or drag bits have no moving
parts (e.g. bearings)
advantages
1. Can drill very long hole sections when theproper drilling conditions are given.
2. They exhibit higher Bit Hours in hard, abrasiveformations
3. They allow higher bit rotations which are given3. They allow higher bit rotations which are givenwhen down hole motors are applied
4. As a rule of thumb, the advantages of diamondbits over roller bits increase as the depthincreases and the borehole diameterdecreases.
disadvantages
1. Sensitive to steel at bottom of borehole (lost
junk),
2. Have to be run in carefully, borehole has to be
washed clean.washed clean.
3. Fail to drill very soft and gummy formations
(Bit balling effect)
4. Higher price.
Polycrystalline diamond bit (PCD)
Polycrystalline diamond compact (PCD) bits
have an industrially manufactured diamond disk
mounted on a tungsten carbide stud
Very expensive Very expensive
Under the proper drilling conditions, they can
drill very fast for long distances
PDC bits are manufactured in many different
shapes that determine their behavior to rather
drill directional or straight trajectories.
Diamond disc on tungsten carbide
stud
Good results have been reported in
Carbonates, Sandstone, Siltstone and Shale
Hydraulic cleaning action is achieved by
using Nozzlesusing Nozzles
Other design features are: size, Shape and
number of cutters and the angle of attack
Importance of bit selection
Bit is the most important part of the drill
string
Proper bit selection means good penetration
rate (ROP).rate (ROP).
So we have to select the most economical bit.
Selection criteria
Unfortunately, the selection of best available
bit for the job can only be determined by hit
and trial method.
The most valid criteria for comparing the The most valid criteria for comparing the
performance of various bits and selection of
most appropriate bit type for particular
formation are:
1. Cost per foot.
2. Geological formations.
3. Off set wells bits records or Dull 3. Off set wells bits records or Dull
grading
Cost per foot
The criterion for bit selection is normally based on
cost per foot by the equation.
C = B+(T+t)R/F ($/ft)
Where
B = Bit cost ($)B = Bit cost ($)
T = Trip time (h)
t = Rotating time(h)
R = Rig cost per hour ($/h)
F = Length drilled (ft)
Factors affecting ROP
1. Rig Efficiency
2. Personnel Efficiency
3. Formation Characteristics
4. Drilling Fluid Properties 4. Drilling Fluid Properties
5. Operating Conditions
6. Bit Type
7. Bit Wear
8. Bit Hydraulics