Drilling Engineering
Drilling Engineering – PE 311
An Introduction to Drilling Drill Bits
Prepared by: Tan Nguyen
Drilling Engineering
Introduction
A drilling bit is the cutting tool which is made up on the end of the drillstring. The
bit drills through the rock by scraping, chipping, gouging or grinding the rock at the
bottom of the hole. Drilling fluid is circulated through passageways in the bit to
remove the drilled cuttings.
There are however many variations in the design of drill bits and the bit selected
for a particular application will depend on the type of formation to be drilled. The
drilling engineer must be aware of these design variations in order to be able to
select the most appropriate bit for the formation to be drilled. The engineer must
also be aware of the impact of the operating parameters on the performance of the
bit.
Prepared by: Tan Nguyen
Drilling Engineering
Classification of Drilling Bits
Classification of Drilling Bits
Prepared by: Tan Nguyen
Drilling Engineering
Classification of Drilling Bits
Drag Bit
Drag bits were the first bits used in rotary drilling, but are no longer in common
use. A drag bit consists of rigid steel blades shaped like a fish-tail which rotate as
a single unit. The decline in the use of drag bits was due to:
•
The introduction of roller cone bits, which could drill soft formations more
efficiently
•
If too much WOB was applied, excessive torque led to bit failure or drill pipe
failure
•
Drag bits tend to drill crooked hole, therefore some means of controlling
deviation was required
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Drilling Engineering
Classification of Drilling Bits
Drag Bit
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bits
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bits
This drill bit was invented by Howard Hughes. The roller-cone bit has conical
cutters or cones that have spiked teeth around them.
As the drilling-string is rotated, the bit cones roll along the bottom of the hole in a
circle. New teeth come in contact with the bottom of the hole, crushing, scraping,
and gouging the rock immediately below and around the bit tooth.
The high-velocity fluid jet strikes the crushed rock chips to remove them from the
bottom of the hole.
As this occurs, another tooth makes contact with the bottom of the hole and creates
new rock chips.
Thus, the process of chipping the rock and removing the small rock chips with the
fluid jets is continuous.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bits
Advantages of roller cone bits are:
•
Improved cleaning action by using jet nozzles
•
Using tungsten carbide for hardfacing and gauge
protection
•
Introduction of sealed bearings to prevent the mud
causing premature failure due to abrasion and corrosion
of the bearings.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit - Geometry
The geometric design features that determine
cutting action are the journal angle and the offset.
Journal Angle: is the angle formed by the
intersection of a line perpendicular to the axis (or
center line) of the journal and the center line of
the bit.
Soft formation bits have smaller journal angles
than hard formation bits.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit - Geometry
Offset is the horizontal distance between a
vertical plane of the center line of the bit and
a vertical plane through the center-line of the
journal. This Figure shows this offset as a
positive displacement in the direction of
rotation (some bit companies measure offset
in inches, while others measure it in degrees.
Typical bit offsets range from 0° to 5°)
The greater a bit's offset, the more scraping
is its cutting action.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit – Cutting Elements
The two basic categories of rolling cutter bits are
defined by their cutting elements. A bit may either have
milled steel teeth or tungsten carbide inserts.
Milled steel tooth cutters are an integral part of the bit
cone. Soft formation bits have long, relatively thin teeth
that are spaced widely apart on the cone
This configuration promotes a gouging/scraping action
that results in high penetration rates with minimal
weight on bit. Unfortunately, these long teeth are
especially susceptible to breakage in harder rock.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit – Cutting Elements
Milled steel tooth (cont.)
Teeth are positioned on the cone in rows, with the
inner rows on each cone meshing with one another.
This tooth arrangement provides the optimum design
space for a given hole size, promotes self-cleaning
of the teeth as the bit turns, and provides maximum
hole coverage
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit – Cutting Elements
Tungsten carbide inserts, as their name implies, are not part of the cone
material. Rather, they are separate elements, pressed into specially machined
holes in the cone. They can be placed either as gauge inserts (along the outside
of the cone) or inner row inserts.
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Drilling Engineering
Classification of Drilling Bits
Roller Cone Bit – Cutting Elements
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Drilling Engineering
Classification of Drilling Bits
Diamond Bits
Diamond has been used as a material for cutting rock for many years. The
hardness and wear resistance of diamond made it an obvious material to be
used for a drilling bit. The diamond bit is really a type of drag bit.
A new generation of diamond bits known as polycrystalline diamond compact
(PDC) bits were introduced in the 1980’s. PDC bits have been run very
successfully in many areas around the world.
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Drilling Engineering
Classification of Drilling Bits
Diamond Bits - PDC
The PDC drill bits were introduced to the drilling industry in 1967. A PDC cutter
consists of a stud covered by an artificial diamond layer bonded in a high
pressure/high temperature sintering process.
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Drilling Engineering
Classification of Drilling Bits
Diamond Bits - PDC
The ability of the PDC bits to drill different formations with excellent efficiency
was proven to the industry (Millheim, 1986). The main advantages of the PDC
bits are:
•
Longer life (Better wear resistance)
•
Higher average ROP
•
Better drilling economics ( $/m)
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Drilling Engineering
Classification of Drilling Bits
Classification - Roller Cone Bits
The IADC categorizes for rolling cutter bits using a four-character code: 517X
The first character in the classification code indicates the cutting structure
series.
•
The digits 1 - 3 are for steel tooth bits in the soft, medium and hard formation
categories
•
The numbers 4 - 8 are for insert bits in the soft, medium, medium hard, hard
and extremely hard formation categories.
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Drilling Engineering
Classification of Drilling Bits
Classification - Roller Cone Bits
The second character further specifies the cutting structure type within each series
classification: 517X
The third character indicates bearing type and whether or not the bit is gauge-protected.
The fourth character designates additional special features and applications:
A. Air Application
E. Extended Jet
R. Reinforced Welds
Y. Conical Insert
C. Center Jet
G. Extra Gage Protection
S. Standard Steel Tooth
Z. Other Insert Shape
D. Deviation Control
J. Jet Deflection
X. Chisel Insert
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Drilling Engineering
Classification of Drilling Bits
Classification - Roller Cone Bits
Example: A Smith F2 bit has an IADC classification of 517X:
•
51 indicates that the Smith F2 has tungsten carbide inserts, designed for use
in soft formations with low compressive strength;
•
7 indicates that the cones on this bit have sealed friction bearings, and that
the bit is designed for protection against gauge wear;
•
X indicates that the inserts have a chisel tooth configuration (as opposed, for
example, to a conical shape).
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Drilling Engineering
Classification of Drilling Bits
Classification – Fixed Cutter Bits
The IADC also categorizes for fixed cutter bits using a four-character code: M431
The first character indicates the type of body material and cutting elements: M431
•
S for steel body PDC bits;
•
M for matrix body PDC bits;
•
D for natural diamond bits;
•
T for TSP bits (Thermal Stable Polycrystalline: Natural diamond).
The next three numbers indicate the following:
•
Formation Type To Be Drilled: M431
•
Cutting Structure: M431
•
Bit Profile: M431
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Drilling Engineering
Classification of Drilling Bits
Classification – Fixed Cutter Bits
Geological formation type to be drilled are classified in the following manner: M431
1 or 2: Soft and soft sticky-Highly drillable formations such as clay, marl, gumbo and
unconsolidated sands.
3: Soft-medium-Low compressive strength sands, shales and anhydrites with hard layers
intermixed.
4: Medium-Moderate compressive strength sand, chalk, anhydrite and shale.
6: Medium hard-Higher compressive strength with non or semi-sharp sand, shale, lime and
anhydrite.
7: Hard-High compressive strength with sharp layers of sand or siltstone.
8: Extremely hard-Dense and sharp formations such as quartzite and volcanic rock.
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Drilling Engineering
Classification of Drilling Bits
Classification – Fixed Cutter Bits
PDC cutting structure is denoted in the following way: M431
2 - This bit has mostly 19mm cutters
3 - This bit has mostly 13 mm cutters
4 - This bit has mostly 8 mm cutters
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Drilling Engineering
Classification of Drilling Bits
Bit profile: M431
1 - Short Fishtail
2 - Short Profile
3 - Medium Profile
4 - Long Profile
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Drilling Engineering
Classification of Drilling Bits
For example: M431
M – Matrix body PDC bit
4: This bit will perform well in medium formations.
3: Cutting Structure consists of primarily 13 mm PDC cutters.
1: Short fishtail bit profile
For example: M612
M = Matrix body PDC bit
6 - This bit will perform well in Medium hard-Higher compressive strength
1 - Cutting Structure consists of natural diamond,
2 - Short Profile
Prepared by: Tan Nguyen