Drilling Engineering – PE 311 An Introduction to Drilling Drill Bits

Drilling Engineering

Prepared by: Tan Nguyen

Drilling Engineering – PE 311

An Introduction to Drilling Drill Bits



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.



Classification of Drilling BitsClassification of Drilling Bits



Classification of Drilling Bits

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

Drag Bit



Classification of Drilling BitsDrag Bit



Classification of Drilling BitsRoller 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.

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.

Roller Cone Bits




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.

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.

Roller Cone Bit - Geometry




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.

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





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.




Classification of Drilling BitsRoller Cone Bit – Cutting Elements




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.

Diamond Bits




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.

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)

Diamond Bits - PDC




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.

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

R. Reinforced Welds

C. Center Jet

S. Standard Steel Tooth

D. Deviation Control

X. Chisel Insert

E. Extended Jet

Y. Conical Insert

G. Extra Gage Protection

Z. Other Insert Shape

J. Jet Deflection





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).





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

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.





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





Bit profile: M431

1 - Short Fishtail

2 - Short Profile

3 - Medium Profile

4 - Long Profile




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

Documents

Drilling Engineering – PE 311 An Introduction to Drilling Drill Bits