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