Bit selection and dull grading

Quick refresher

The IADC code of a roller-cone bit is three digits and a letter. The first digit is the series. Series 1–3 have milled steel teeth (teeth cut out of the steel of the cone), and series 4–8 have tungsten carbide inserts (studs of very hard metal). The higher the digit, the harder the rock. The second digit is the type, 1–4: it refines the hardness within the series. The third digit gives the bearing and the gauge protection. The letter is an additional feature.
The code of a fixed-cutter bit is a letter and three digits. The letter is the body material (M for matrix, a hard tungsten carbide composite; S for steel). The digits are the cutter density (1–4 for PDC bits, the bits with synthetic-diamond cutters), the cutter size and the profile (the shape of the bit face). The code describes how the bit is built, not where to use it.
The mud presses on the rock that the bit breaks. When the mud pressure is above the pore pressure (the pressure of the fluid in the rock) by $\Delta p$ (the overbalance), the strength under the bit is $C_0 + q\,\Delta p$ . This is more than the unconfined strength (the strength of a rock sample with no pressure on its sides). The heavier the mud, the “harder” the same rock is.
We choose a bit by weighing the properties of the rocks to be drilled, the experience of offset wells (what drilled most cheaply and why it was pulled), the drive and the well profile (the shape of the path of the hole). We check the final choice against the cost per metre (the total cost of a run divided by the metres drilled).
The IADC dull code has eight positions. They give the wear of the inner rows and of the outer rows of cutters or teeth (each 0–8), the main characteristic of wear, its location, the bearing (X for fixed-cutter bits), the loss of diameter in sixteenths of an inch, a second characteristic of wear, and the reason for pulling the bit.
Read the wear together with the operating parameters (the load on the bit, the rotary speed, the flow rate) and the rocks that were drilled. Even wear means normal work. Chips and breakages mean shocks and vibrations. Balling (clay stuck to the bit) means poor cleaning. A failed bearing means that it has used up its life in revolutions.

Key relationships

C_{\text{conf}} = C_0 + q\,\Delta p, \qquad q = \dfrac{1 + \sin\varphi}{1 - \sin\varphi}

strength of the rock under the bit at an overbalance $\Delta p$ (the Mohr–Coulomb criterion)

k_{\text{c}} = 8\,\dfrac{h}{d_{\text{c}}}

wear grade of a cutter on a scale of 0–8: the worn height divided by the cutter diameter; the code takes the average over a zone, rounded to a whole number

\Delta d = \tfrac{2}{3}\,s

loss of diameter of a three-cone bit from the gap $s$ between the ring gauge and the third cone

n = 60\,N\,t

number of revolutions of the bit in a run: a measure of how much of the bearing life of a roller-cone bit has been used

Common pitfalls

Treating the IADC code as a recommendation for use. Two bits with the same code can work quite differently.
Comparing the loads on bits of different diameters in kilonewtons, instead of kilonewtons per millimetre of diameter.
Reading a dull code without the reason for pulling. A bit pulled at total depth (TD) could have drilled further. A bit pulled because its rate of penetration (how fast it drills) fell, code PR, is usually already used up.
Measuring the diameter of a roller-cone bit with a ring that has the tolerance of a new bit, or forgetting the two-thirds rule.
Mixing up “I” (bit in gauge) and “1” (a loss of 1/16 inch) in the sixth position.

After this lesson you can

decode the IADC code of a roller-cone bit (series, type, bearing and gauge protection, additional feature) and of a fixed-cutter bit (body, cutter density, cutter size, profile), and relate them to the designation of roller-cone bits under GOST 20692-2003
estimate the strength of the rock under the bit when the mud pressure is above the pore pressure (the overbalance), and explain how strength, abrasiveness, ductility (squashing without breaking) and interbedding (hard and soft layers that alternate) decide the choice of the cutting structure
describe how a bit is chosen from the properties of the rocks and from the data of offset wells, that is, from bit records and dull codes
record the wear of a pulled bit in the eight IADC positions: calculate the grade of the cutting structure from the cutters and inserts, find the loss of diameter by the two-thirds rule, and choose the codes for the kind and location of wear and for the reason for pulling
read a dull code and draw conclusions from it for the next run

Why bits have codes

Dozens of manufacturers make bits, and each has its own model names. To make the bits comparable, IADC introduced classification codes: the same code means a similar design for similar rocks. People use the codes to build tables that compare the bits of different manufacturers, and they write the codes in the bit records (the reports of bit runs). The records of offset wells then help us choose a bit for a new well. How the bits themselves are built (the cutting structure, bearings, cutters, profile and junk slots, which are channels that carry the cuttings, the rock chips, away) is covered in the lesson Drill bits: types and design. Here we look at how bits are described, chosen and graded after a run.

There are three kinds of code. The first is the classification of roller-cone bits. The second is the classification of fixed-cutter bits, the bits whose cutters are fixed to the body. They include PDC bits (cutters faced with synthetic diamond), bits with natural diamonds, and impregnated bits (fine diamonds in a hard matrix). The third is one system of dull grading for all bits. All three are agreements of the industry, adopted and refined in the 1980s and 1990s. IADC issued an updated manual on dull grading in 2026 (see the advanced section).

IADC code of a roller-cone bit

The code of a roller-cone bit is three digits and, if needed, a letter, for example 617 or 447X. The first digit is the series. It tells us about the cutting structure and about the hardness of the rocks that the bit is meant for. Series 1–3 are bits with milled steel teeth (teeth cut out of the steel of the cone), and series 4–8 are bits with tungsten carbide inserts. Within each group, the higher the digit, the harder the rock.

IADC series of roller-cone bits

Series: 1
Cutting structure: milled steel teeth
Rocks: soft, with low compressive strength and high drillability

Series: 2
Cutting structure: milled steel teeth
Rocks: medium to medium-hard, with high compressive strength

Series: 3
Cutting structure: milled steel teeth
Rocks: hard, semi-abrasive and abrasive

Series: 4
Cutting structure: tungsten carbide inserts
Rocks: soft, with low compressive strength and high drillability

Series: 5
Cutting structure: inserts
Rocks: soft to medium, with low compressive strength

Series: 6
Cutting structure: inserts
Rocks: medium-hard, with high compressive strength

Series: 7
Cutting structure: inserts
Rocks: hard, semi-abrasive and abrasive

Series: 8
Cutting structure: inserts
Rocks: very hard and abrasive

Series	Cutting structure	Rocks
1	milled steel teeth	soft, with low compressive strength and high drillability
2	milled steel teeth	medium to medium-hard, with high compressive strength
3	milled steel teeth	hard, semi-abrasive and abrasive
4	tungsten carbide inserts	soft, with low compressive strength and high drillability
5	inserts	soft to medium, with low compressive strength
6	inserts	medium-hard, with high compressive strength
7	inserts	hard, semi-abrasive and abrasive
8	inserts	very hard and abrasive

Drillability is how easily a bit breaks the rock. It falls as the strength and abrasiveness of the rock rise. An abrasive rock is one that wears away teeth and cutters quickly.

The second digit is the type (1–4): the step of hardness within the series, with 1 for the softest rocks of the series and 4 for the hardest. The higher the type, the shorter and blunter the cutting structure and the smaller the cone offset (how far the axes of the cones are shifted from the axis of the bit). The third digit describes the bearing of the cones (the part on which a cone turns) and the gauge protection (the gauge is the outer edge of the bit, which sets the diameter of the hole):

Third digit of the code: bearing and gauge protection

Digit: 1
Bearing: open, on roller bearings
Gauge protection: none

Digit: 2
Bearing: on roller bearings, cooled by air, for air drilling
Gauge protection: none

Digit: 3
Bearing: open, on roller bearings
Gauge protection: yes

Digit: 4
Bearing: sealed, on roller bearings
Gauge protection: none

Digit: 5
Bearing: sealed, on roller bearings
Gauge protection: yes

Digit: 6
Bearing: sealed journal (sliding) bearing
Gauge protection: none

Digit: 7
Bearing: sealed journal (sliding) bearing
Gauge protection: yes

Digit	Bearing	Gauge protection
1	open, on roller bearings	none
2	on roller bearings, cooled by air, for air drilling	none
3	open, on roller bearings	yes
4	sealed, on roller bearings	none
5	sealed, on roller bearings	yes
6	sealed journal (sliding) bearing	none
7	sealed journal (sliding) bearing	yes

Gauge protection means tungsten carbide inserts or hardfacing (a hard metal layer welded on) on the back cone, the tapered surface at the base of each cone. It protects the diameter of the hole. The lesson on roller-cone bits explains how the bearings are built and why, at high rotary speeds, they limit the length of a run.

The letter at the end is an additional feature. There are sixteen of them. A bit may have several features, but the code usually shows the main one.

Additional features (the fourth character of the code)

Letter: A
Feature: for drilling with air

Letter: B
Feature: special bearing seal

Letter: C
Feature: centre jet

Letter: D
Feature: for controlling deviation of the hole

Letter: E
Feature: extended nozzles

Letter: G
Feature: extra protection of the gauge and the body

Letter: H
Feature: for horizontal and steered drilling

Letter: J
Feature: jet-deflection nozzle

Letter: L
Feature: lug pads (wear pads on the legs)

Letter: M
Feature: for use with a downhole motor

Letter: S
Feature: standard bit with steel teeth

Letter: T
Feature: two-cone bit

Letter: W
Feature: enhanced cutting structure

Letter: X
Feature: mostly chisel-shaped (wedge) inserts

Letter: Y
Feature: conical inserts

Letter: Z
Feature: inserts of another shape

Letter	Feature
A	for drilling with air
B	special bearing seal
C	centre jet
D	for controlling deviation of the hole
E	extended nozzles
G	extra protection of the gauge and the body
H	for horizontal and steered drilling
J	jet-deflection nozzle
L	lug pads (wear pads on the legs)
M	for use with a downhole motor
S	standard bit with steel teeth
T	two-cone bit
W	enhanced cutting structure
X	mostly chisel-shaped (wedge) inserts
Y	conical inserts
Z	inserts of another shape

How to read a code: three examples

Code: 117M
Meaning: steel teeth, soft rocks, the softest type of the series; sealed journal bearing with gauge protection; for use with a downhole motor

Code: 447X
Meaning: tungsten carbide inserts for soft rocks, the hardest type of series 4; journal bearing with gauge protection; chisel-shaped inserts

Code: 617
Meaning: inserts for medium-hard rocks with high compressive strength, the softest type of series 6; journal bearing with gauge protection

Code	Meaning
117M	steel teeth, soft rocks, the softest type of the series; sealed journal bearing with gauge protection; for use with a downhole motor
447X	tungsten carbide inserts for soft rocks, the hardest type of series 4; journal bearing with gauge protection; chisel-shaped inserts
617	inserts for medium-hard rocks with high compressive strength, the softest type of series 6; journal bearing with gauge protection

Codes are written in a row (617) or with hyphens (6-1-7).

IADC code of a fixed-cutter bit

Fixed-cutter bits (the PDC bits, the bits with natural diamonds and the impregnated diamond bits) are described by four characters: a letter and three digits, for example M323. The code tells us three things: the material of the body (a matrix, which is a hard composite of tungsten carbide powder and a metal binder, or steel); how many cutters sit on the bit and how large they are; and the profile (the shape of the bit face). In other words, it describes how the bit looks.

Characters of the code of a fixed-cutter bit

Character: 1
What it stands for: body material
Values: M — matrix, S — steel

Character: 2
What it stands for: cutter density
Values: 1–4 for PDC bits, from a sparse layout of cutters (a light set) to a dense one (a heavy set); 6–8 for bits with surface-set diamonds, by the size of the diamonds; 0, 5 and 9 are not defined

Character: 3
What it stands for: cutter size; for diamond bits, the kind of diamond
Values: for PDC: 1 — larger than 24 mm, 2 — 14–24 mm, 3 — 8–14 mm, 4 — smaller than 8 mm; for diamond bits: natural diamonds, thermally stable polycrystalline (TSP) diamonds (synthetic diamond that withstands more heat than PDC), a mix of them, or an impregnated layer (fine diamonds spread through the matrix)

Character: 4
What it stands for: profile
Values: 1 — flat (for PDC, the “fishtail” profile; also flat diamond and TSP bits), 2 — short, 3 — medium, 4 — long

Character	What it stands for	Values
1	body material	M — matrix, S — steel
2	cutter density	1–4 for PDC bits, from a sparse layout of cutters (a light set) to a dense one (a heavy set); 6–8 for bits with surface-set diamonds, by the size of the diamonds; 0, 5 and 9 are not defined
3	cutter size; for diamond bits, the kind of diamond	for PDC: 1 — larger than 24 mm, 2 — 14–24 mm, 3 — 8–14 mm, 4 — smaller than 8 mm; for diamond bits: natural diamonds, thermally stable polycrystalline (TSP) diamonds (synthetic diamond that withstands more heat than PDC), a mix of them, or an impregnated layer (fine diamonds spread through the matrix)
4	profile	1 — flat (for PDC, the “fishtail” profile; also flat diamond and TSP bits), 2 — short, 3 — medium, 4 — long

The code says nothing about the hydraulics (the number and position of the nozzles, the area of the junk slots), the tilt of the cutters, the depth-of-cut limiters or the grade of the cutters themselves. How these elements work is explained in the lesson on PDC bits.

Examples of PDC bit codes

Code: S123
Meaning: steel body, light set, cutters of 14–24 mm, medium profile
Where it fits: soft and sticky rocks: large cutters and big slots

Code: M323
Meaning: matrix body, set above medium, cutters of 14–24 mm (for example 16 mm), medium profile
Where it fits: medium rocks

Code: M433
Meaning: matrix body, heavy set, cutters of 8–14 mm (for example 13 mm), medium profile
Where it fits: hard and abrasive layers

Code	Meaning	Where it fits
S123	steel body, light set, cutters of 14–24 mm, medium profile	soft and sticky rocks: large cutters and big slots
M323	matrix body, set above medium, cutters of 14–24 mm (for example 16 mm), medium profile	medium rocks
M433	matrix body, heavy set, cutters of 8–14 mm (for example 13 mm), medium profile	hard and abrasive layers

The higher the second and third digits, the more small cutters the bit carries. Each cutter then carries less load and the bit lasts longer, but it drills more slowly in soft rocks (why).

Designation of roller-cone bits under GOST 20692-2003

In the CIS countries, roller-cone bits for full-face drilling (drilling out the whole bottom of the hole, with no core left) are designated under GOST 20692-2003, “Roller-cone bits. Specifications”. This is an interstate standard: several countries agreed on it, and the standards bodies of Kazakhstan also voted for its adoption. The designation consists of the number of cones (in a Roman numeral), the diameter in millimetres, the type of the bit by rock and, after a hyphen, the kind of flushing and bearing. The standard uses letters of the Russian alphabet. Here we write them with the matching Latin letters (transliteration).

Bit types by rock

Type: M
Cutting structure: steel teeth
Rocks: soft

Type: MZ
Cutting structure: tungsten carbide inserts
Rocks: soft, abrasive

Type: MS
Cutting structure: steel teeth
Rocks: soft, with thin layers of medium hardness

Type: MSZ
Cutting structure: tungsten carbide inserts
Rocks: soft, abrasive, with thin layers of medium hardness

Type: S
Cutting structure: steel teeth
Rocks: medium

Type: SZ
Cutting structure: tungsten carbide inserts
Rocks: abrasive, of medium hardness

Type: ST
Cutting structure: steel teeth
Rocks: medium, with thin layers of hard rock

Type: T
Cutting structure: steel teeth
Rocks: hard

Type: TZ
Cutting structure: tungsten carbide inserts
Rocks: hard, abrasive

Type: TK
Cutting structure: steel teeth and tungsten carbide inserts
Rocks: hard, with thin layers of strong rock

Type: TKZ
Cutting structure: tungsten carbide inserts
Rocks: hard, abrasive, with thin layers of strong rock

Type: K
Cutting structure: tungsten carbide inserts
Rocks: strong

Type: OK
Cutting structure: tungsten carbide inserts
Rocks: very strong

Type	Cutting structure	Rocks
M	steel teeth	soft
MZ	tungsten carbide inserts	soft, abrasive
MS	steel teeth	soft, with thin layers of medium hardness
MSZ	tungsten carbide inserts	soft, abrasive, with thin layers of medium hardness
S	steel teeth	medium
SZ	tungsten carbide inserts	abrasive, of medium hardness
ST	steel teeth	medium, with thin layers of hard rock
T	steel teeth	hard
TZ	tungsten carbide inserts	hard, abrasive
TK	steel teeth and tungsten carbide inserts	hard, with thin layers of strong rock
TKZ	tungsten carbide inserts	hard, abrasive, with thin layers of strong rock
K	tungsten carbide inserts	strong
OK	tungsten carbide inserts	very strong

The letter “Z” in the type means tungsten carbide inserts and abrasive rocks. The letter “K” means strong rocks, which rank one step above the hard ones.

Bearing and flushing in the designation

Letters: V, VU
What they mean: bearing on roller bearings (thrust journal bearings may be present); U — the bearing is sealed

Letters: N, NU
What they mean: one radial journal bearing, the rest are roller bearings; U — sealed

Letters: A, AU
What they mean: two or more radial journal bearings; U — sealed

Letters: Ts, G, TsG
What they mean: flushing through the centre, side-jet flushing (the mud leaves through nozzles at the side as fast jets), combined flushing

Letters: P, PG
What they mean: central and side blowing with air

Letters	What they mean
V, VU	bearing on roller bearings (thrust journal bearings may be present); U — the bearing is sealed
N, NU	one radial journal bearing, the rest are roller bearings; U — sealed
A, AU	two or more radial journal bearings; U — sealed
Ts, G, TsG	flushing through the centre, side-jet flushing (the mud leaves through nozzles at the side as fast jets), combined flushing
P, PG	central and side blowing with air

For example, III 215.9 TZ-GAU is a three-cone bit with a diameter of 215.9 mm, with tungsten carbide inserts for hard abrasive rocks, with side-jet flushing and a sealed journal bearing. III 393.7 M-GV is a three-cone bit of 393.7 mm with steel teeth for soft rocks, with side-jet flushing and a roller bearing.

What rock the bit sees

We choose a bit by the properties of the rocks. Five of them matter most for breaking the rock:

strength: the stress (force per unit area) needed to break the rock. We measure it on a core sample (a cylinder of rock cut from the well) as the unconfined compressive strength (UCS). We can also estimate it from logs (measurements made in the well) calibrated against cores (how this is done);
abrasiveness: how fast the rock wears away the cutters and inserts. It is set mainly by quartz and its cement. Quartz sandstones and quartzites are abrasive, and limestones and clays hardly at all;
ductility: the ability of the rock to squash out of shape without breaking into chips. Pressing a tooth into clays and salt at depth hardly breaks them, so it is better to shear (cut) them off;
stickiness: the tendency of clay cuttings to stick to the bit and cover the cutting structure;
interbedding: soft and hard layers that alternate, and also cherts (very hard rock made of silica) and conglomerates (rock made of cemented pebbles). At the boundaries the bit takes shocks, and operating parameters that suit one layer are wrong for the next.

The strength measured on a sample with no side pressure is not yet the strength that the bit meets. At the bottom of the hole the mud presses on the rock. Here the overbalance matters: it is the excess of the mud pressure over the pore pressure. The mudcake (the thin layer of solids that the mud leaves on the rock) passes this pressure difference on to the rock skeleton (the solid framework of grains). The skeleton then behaves as if it had a side pressure in a triaxial test (a laboratory test in which a rock sample is squeezed from the sides as well as along its axis). By the Mohr–Coulomb criterion, the larger the angle of internal friction of the rock (a number that shows how well its grains resist sliding on each other), the more the side pressure strengthens it. The result is the confined compressive strength (CCS).

Strength of the rock under the bit at an overbalance

C_{\text{conf}} = C_0 + q\,\Delta p, \qquad q = \frac{1 + \sin\varphi}{1 - \sin\varphi}, \qquad \Delta p = 0.00981\,(\rho_m - \rho_{\text{p}})\,z

where:

$C_{\text{conf}}$ confined compressive strength of the rock (CCS), MPa
$C_0$ unconfined compressive strength (UCS), MPa
$q$ slope of the Mohr–Coulomb line: how many megapascals the strength grows for each megapascal of confining pressure, —
$\varphi$ angle of internal friction of the rock, °
$\Delta p$ overbalance: the mud pressure minus the pore pressure at the same depth, MPa
$\rho_m,\ \rho_{\text{p}}$ mud density and the equivalent mud weight of the pore pressure (the density of the mud that would just balance the pore pressure), g/cm³
$z$ true vertical depth, m

Units: MPa; 0.00981 is the pressure of a column of liquid with a density of 1 g/cm³ and a height of 1 m, in MPa

Holds when

a permeable rock with a mudcake: the whole pressure difference acts on the skeleton as an effective confining stress. In tight clays and salts the pore pressure under the bit changes as the rock breaks, and the simple formula works only as an estimate (see the advanced section);
a Mohr–Coulomb straight line with constant $C_0$ and $\varphi$ , as in the lesson on sand production. At high confining pressures the strength grows more slowly, and the formula overestimates it;
when we drill with circulation, $\Delta p$ also includes the pressure lost to friction in the annulus (the space between the pipe and the wall of the hole). We express it as the equivalent circulating density (ECD): the mud density that would give the same pressure at the bottom as the moving mud does. The example takes the mud density, which slightly underestimates the confining pressure.

For the same rock, the strength under the bit grows with the overbalance. A heavy mud makes the rock “harder” and drilling slower. This gives a practical rule: keep the overbalance no higher than well control requires. That also reduces how hard the mud presses the cuttings onto the bottom of the hole (what the jets do at the bottom).

Worked exampleStrength of limestone and dolomite under the bit

Given (assumed data)

Depth: 4560 m true vertical: the 215.9 mm interval of the 4800 m well from the lesson on drill string design
Mud: 1.40 g/cm³; the pore pressure is 1.30 g/cm³ as an equivalent mud weight
Limestone: $C_0$ = 90 MPa, $\varphi$ = 35°
Dolomite: $C_0$ = 160 MPa, $\varphi$ = 38°
For comparison: the same at a normal pore pressure of 1.05 g/cm³ with the same mud

Solution

Overbalance: $\Delta p = 0.00981\cdot(1.40 - 1.30)\cdot 4560 = 4.47$ MPa.
Slope of the line: for limestone $\sin 35° = 0.5736$ , $q = (1 + 0.5736)/(1 - 0.5736) = 3.690$ ; for dolomite $\sin 38° = 0.6157$ , $q = 4.204$ .
Strength under the bit: limestone $90 + 3.690\cdot 4.47 = 106.5$ MPa (18% more than the unconfined strength), dolomite $160 + 4.204\cdot 4.47 = 178.8$ MPa (12%).
At a normal pore pressure the overbalance would be $0.00981\cdot 0.35\cdot 4560 = 15.66$ MPa, and the strength would be: limestone $90 + 3.690\cdot15.66 = 147.8$ MPa (+64%), dolomite $160 + 4.204\cdot15.66 = 225.8$ MPa (+41%).

Answer: The bit sees limestone with a strength of about 106.5 MPa and dolomite of about 178.8 MPa. With the same mud at a normal pore pressure, they would be 147.8 and 225.8 MPa.

What it means. Here the mud is heavy because the pore pressure is high too, so the overbalance is small, and the rock gets only 12–18% stronger. In an interval with normal pressure, the same mud density would make the same rocks 41–64% stronger. We would have to choose a bit “one step harder”, and drilling would be slower. To choose a bit, we compare exactly this strength under the bit. Here dolomite is 1.7 times as strong as limestone. If the rocks alternate in thin layers, we choose the cutting structure for the dolomite layers. The same dolomite appears again in the lesson Bit performance and cost per metre, where we compare the strength under the bit with the mechanical specific energy (the energy the bit spends to break each unit volume of rock). The data in this example are assumed for illustration.

How a bit is chosen

The general principles (which bit for which rocks) are covered in the lesson on drill bits. In practice the choice goes step by step and relies on data: the logs and cuttings of offset wells, their bit records and their dull codes.

Order of selection

1
The rocks to be drilled
From the logs, cuttings and cores of offset wells, we pick out rock units (groups of similar beds). We estimate the strength of each under the bit, and its abrasiveness, ductility and interbedding. The strongest and most abrasive layers set the requirements for the cutting structure. The softest and stickiest ones set the requirements for flushing.
2
Bit records of offset wells
We collect the bit runs in the same interval: the bit code, depths, hours, operating parameters, dull code and reason for pulling. We look for what limited each run: wear of the cutting structure, the bearing, shocks, balling or outside causes (total depth, logging, motor failure).
3
Drive and trajectory
The drive is one of four kinds: a rotary table (it turns the whole string from the surface); a positive displacement motor (a motor in the string that the mud turns); a turbodrill (a turbine in the string that the mud turns); or a rotary steerable system (a tool that steers the bit while the string rotates). The drive sets the rotary speed and the requirements for the gauge and for how well the bit can be steered (Deflection tools and downhole motors). At high rotary speeds a roller-cone bit needs a bearing made for use with a motor (the letter M).
4
Flushing and mud
The area of the slots and the position of the nozzles must be able to clear the cuttings of the softest units. We then choose the flow rate and the nozzles by calculation (Bit hydraulics optimisation). An oil-based mud reduces the sticking of clay (Oil-based muds).
5
Candidates and economics
We choose two or three bits from different manufacturers by their codes and recommendations. We estimate the expected footage (the metres drilled) and speed, and compare the bits by cost per metre (Bit performance and cost per metre). We keep a spare option in case the bit does not work.
6
Feedback
We grade the pulled bit with the IADC code and compare the grade with the operating parameters and the rocks. Then we correct the choice and the parameters for the next run and the next well.

What in the rocks changes the design of the bit

In the rocks: Stronger rock
PDC bit: more blades and cutters, smaller cutters, a larger tilt of the cutters
Roller-cone bit: a higher series and type: shorter inserts, smaller cone offset

In the rocks: Abrasiveness
PDC bit: wear-resistant cutters, protection of the shoulder and gauge, moderate rotary speed
Roller-cone bit: tungsten carbide inserts, gauge protection (G)

In the rocks: Ductile, sticky clays
PDC bit: few blades, large cutters, big slots, nozzles on the blades
Roller-cone bit: long teeth, large offset, a centre jet (C)

In the rocks: Interbedding, cherts
PDC bit: tough cutters, depth-of-cut limiters, a stable profile; hybrid bits (cones and PDC cutters on one body)
Roller-cone bit: inserts, moderate load, enhanced cutting structure (W)

In the rocks: Motor or RSS
PDC bit: short gauge for steerability, a bit made for high rotary speed
Roller-cone bit: a bearing made for high rotary speed (M), or a PDC bit

In the rocks	PDC bit	Roller-cone bit
Stronger rock	more blades and cutters, smaller cutters, a larger tilt of the cutters	a higher series and type: shorter inserts, smaller cone offset
Abrasiveness	wear-resistant cutters, protection of the shoulder and gauge, moderate rotary speed	tungsten carbide inserts, gauge protection (G)
Ductile, sticky clays	few blades, large cutters, big slots, nozzles on the blades	long teeth, large offset, a centre jet (C)
Interbedding, cherts	tough cutters, depth-of-cut limiters, a stable profile; hybrid bits (cones and PDC cutters on one body)	inserts, moderate load, enhanced cutting structure (W)
Motor or RSS	short gauge for steerability, a bit made for high rotary speed	a bearing made for high rotary speed (M), or a PDC bit

This is a summary of the way bits work. Every manufacturer has its own specific solutions. RSS is the rotary steerable system.

We compare the loads on bits of different diameters per millimetre of diameter. For example, 100 kN on a 215.9 mm bit is 0.46 kN/mm (about 2.6 thousand pounds per inch of diameter). A 393.7 mm bit at the same load per millimetre would get about 180 kN. The bit record (the report of a bit run) lists:

the bit: diameter, manufacturer, model, IADC code, serial number, nozzles and their total area;
the run: depths in and out, footage, hours of drilling, average rate of penetration;
the parameters: weight on bit, rotary speed (with a motor, also its speed), flow rate, standpipe pressure (the pump pressure read at the surface), type and density of the mud;
the dull grade by the IADC code, the reason for pulling and remarks: rock units, problems, vibrations.

Number of bit revolutions in a run

n = 60\,N\,t

where:

$n$ number of revolutions of the bit in a run; usually given in thousands, rev
$N$ rotary speed of the bit: that of the rotary table or the top drive (the motor that turns the string from above) plus that of the shaft of the downhole motor, rpm
$t$ time of drilling with the bit on the bottom, h

Units: rpm × h × 60 min/h

Holds when

the rotary speed is constant during the run. If the parameters change, we add up the revolutions over the periods of time;
the life of the bearing also depends on the load, the temperature and the mud. The number of revolutions is a simple measure of how much of it is used, but not the only one.

The bearing of a roller-cone bit wears with every revolution of the cones under load. So it is more convenient to measure its life not in hours but in revolutions. In bit records we can see after how many thousands of revolutions, in a given rock and under a given load, the bearings begin to fail. With a motor, the bit collects the same number of revolutions several times faster. PDC bits have no bearings, and the wear of the cutters limits the run.

Worked exampleWhat the bit records of offset wells tell us

Given (assumed data)

Interval: 215.9 mm hole, limestones with thin layers of abrasive sandstone and dolomite; two offset wells, 12 and 15
Run 1 (well 12): roller-cone bit 617: 3920–4105 m in 30.5 h, 80 rpm, 130–150 kN; dull grade 2-3-BT-G-F-2/16-SD-TQ
Run 2 (well 12): PDC M323, 16 mm cutters: 4105–4540 m in 36.5 h, 120 rpm, 90–100 kN; dull grade 1-4-WT-S-X-I-BT-PR
Run 3 (well 15): PDC M433, 13 mm cutters: 4180–4800 m in 59.0 h, 120 rpm, 100–110 kN; dull grade 1-2-WT-S-X-I-NO-TD

Solution

Footage and average rate of penetration: run 1 — 185 m, $185/30.5 = 6.07$ m/h; run 2 — 435 m, 11.92 m/h; run 3 — 620 m, 10.51 m/h.
Specific load (load per millimetre of diameter) at the middle of the range: run 1 — $140/215.9 = 0.65$ kN/mm; run 2 — 0.44; run 3 — 0.49 kN/mm. The PDC bits drill with a smaller load than the roller-cone bit.
The roller-cone bit of run 1 made $60\cdot 80\cdot 30.5 = 146.4$ thousand revolutions. Its bearing failed (F), it lost 2/16 inch of its diameter, the inserts of the gauge row were broken (BT, G), and it was pulled because of torque (TQ), which is a sign of a bearing that is breaking down. The life of the bearing is what limited this run.
The bit of run 2 drilled faster than the others. But when it was pulled, the outer third of its cutters was worn halfway (4) and a cutter on the shoulder was broken (BT). It had larger cutters and a sparser set than the bit of run 3. It could not stand the abrasive thin layers, and it was pulled because its rate of penetration fell (PR).
The bit of run 3 had a dense set of small cutters. It drilled a little slower, but it drilled the most, and it was pulled at total depth (TD) with the wear grade 1-2. Its life was not used up.

Answer: For this interval it is reasonable to take a bit like the one in run 3 (a heavy set, cutters of about 13 mm, protection of the shoulder and gauge), with a load of about 100–110 kN and 120 rpm. A roller-cone bit with inserts is the spare option for the units where the PDC bit does not work.

What it means. We do not compare the reports by one number. The fastest bit (run 2) did not reach total depth, and the longest-lasting one (run 3) could have drilled further. The dull codes show what limited each run and suggest what to change. In the end we compare the options by cost per metre. The same calculation for these three runs is in the lesson Bit performance and cost per metre. The wells, depths and dull grades are assumed. In a real analysis we take more runs and remember that the rocks and the parameters in neighbouring wells differ a little. The data in this example are assumed for illustration.

The eight positions of the IADC dull code

We wash a pulled bit and examine it in good light. We photograph its face and gauge and write down its condition in eight positions. The first four describe the cutting structure, the fifth the bearing, the sixth the gauge, and the seventh and eighth the remarks.

Positions of the dull code

No.: 1
Position: I — inner rows
What is recorded: wear of the cutting structure that does not touch the wall of the hole (for fixed-cutter bits, the inner 2/3 of the radius), 0–8

No.: 2
Position: O — outer rows
What is recorded: wear of the cutting structure that touches the wall (for fixed-cutter bits, the outer third of the radius), 0–8

No.: 3
Position: D — dull characteristic
What is recorded: the main sign of wear of the cutting structure, a code of two letters

No.: 4
Position: L — location
What is recorded: where this sign is: the row or the number of the cone; for fixed-cutter bits, the zone of the profile

No.: 5
Position: B — bearing
What is recorded: for open bearings, a scale of 0–8; for sealed ones, E, F or N; for fixed-cutter bits always X

No.: 6
Position: G — gauge
What is recorded: I if the diameter has not decreased; otherwise the loss of diameter in sixteenths of an inch

No.: 7
Position: O — other characteristic
What is recorded: the second most important sign of wear, any one, not only of the cutting structure; NO if there is none

No.: 8
Position: R — reason pulled
What is recorded: a code of two or three letters

No.	Position	What is recorded
1	I — inner rows	wear of the cutting structure that does not touch the wall of the hole (for fixed-cutter bits, the inner 2/3 of the radius), 0–8
2	O — outer rows	wear of the cutting structure that touches the wall (for fixed-cutter bits, the outer third of the radius), 0–8
3	D — dull characteristic	the main sign of wear of the cutting structure, a code of two letters
4	L — location	where this sign is: the row or the number of the cone; for fixed-cutter bits, the zone of the profile
5	B — bearing	for open bearings, a scale of 0–8; for sealed ones, E, F or N; for fixed-cutter bits always X
6	G — gauge	I if the diameter has not decreased; otherwise the loss of diameter in sixteenths of an inch
7	O — other characteristic	the second most important sign of wear, any one, not only of the cutting structure; NO if there is none
8	R — reason pulled	a code of two or three letters

The code is written with hyphens: 1-4-WT-S-X-I-BT-PR. The second and seventh positions are both marked with the letter O (outer and other). We tell them apart by their place in the code.

SchematicZones of the bit in the dull code (a diagram, not to scale). Left: half of the profile of a PDC bit, from the axis to the gauge: cone (C), nose (N), taper (T), shoulder (S), gauge (G). The cutters of the inner 2/3 of the radius give the first position of the code, and the cutters of the outer third give the second. Right: one cone of a roller-cone bit, with nose (N), middle (M) and gauge (G) rows. The gauge rows touch the wall of the hole and give the second position. The other rows give the first.

Inner and outer rows (I, O). We grade the wear of the cutting structure on a linear scale from 0 (no wear) to 8 (the cutting structure is completely lost). A grade of 4 means half. What exactly is measured depends on the bit:

steel teeth: the loss of tooth height from wear and breakage;
tungsten carbide inserts: the total reduction of the cutting structure from lost, broken and worn inserts. If a quarter of the cutting structure is lost, the grade is 2;
PDC cutters and diamonds: the worn part of the cutter. A wear flat over half the diameter of the cutter is 4, and a cutter broken off flush with the body is 8. We grade each cutter and take the average for the zone.

Wear grade of a cutter of a fixed-cutter bit

k_{\text{c}} = 8\,\frac{h}{d_{\text{c}}}, \qquad I = \frac{1}{n_I}\sum_{r < 2R/3} k_{\text{c}}, \qquad O = \frac{1}{n_O}\sum_{r \ge 2R/3} k_{\text{c}}

where:

$k_{\text{c}}$ wear grade of an individual cutter on a scale of 0–8, —
$h$ worn height of the cutter: how far the wear flat has cut into the cutter across its diameter; measured across the diamond layer, mm
$d_{\text{c}}$ diameter of the cutter, mm
$r,\ R$ radius of a cutter on the face of the bit and radius of the bit, mm
$n_I,\ n_O$ number of cutters inside and outside two thirds of the radius (on all blades), —

Units: any identical units of length

Holds when

a cutter broken off flush with the body, or a lost cutter, counts as completely worn: 8;
the code takes the averages, rounded to a whole number;
the manufacturer often grinds the gauge cutters flat in advance. This cut is not wear.

The average over a zone smooths out single breakages. So we also mark a single broken cutter with a wear-characteristic code (BT, LT) in the third or the seventh position.

Dull characteristic (D) and other characteristic (O). We fill both positions with codes from one list. The third position takes the main sign of wear of the cutting structure, the one that would limit the further work of the bit most. The seventh takes the second most important sign, which may concern any part of the bit.

Codes of the wear characteristics

Code: BC
Characteristic: broken cone

Code: BF
Characteristic: bond failure: a cutter or a diamond layer has come off along the braze (the soldered joint)

Code: BT
Characteristic: broken teeth or cutters

Code: BU
Characteristic: balling up: the bit is covered with cuttings

Code: CC
Characteristic: cracked cone

Code: CD
Characteristic: cone dragged: the cone seized and was dragged along the bottom

Code: CI
Characteristic: cone interference: the cones rubbed against each other

Code: CR
Characteristic: cored: the centre of the bit is worn out, and a column of rock is left on the bottom

Code: CT
Characteristic: chipped teeth or cutters

Code: ER
Characteristic: erosion of the body or of the base of the cutters

Code: FC
Characteristic: flat crested wear: the tops of the teeth are worn flat

Code: HC
Characteristic: heat checking: a network of thermal cracks from heating by friction

Code: JD
Characteristic: junk damage: damage from metal on the bottom

Code: LC
Characteristic: lost cone

Code: LN
Characteristic: lost nozzle

Code: LT
Characteristic: lost teeth or cutters

Code: NO
Characteristic: no dull characteristic

Code: OC
Characteristic: off-centre wear: wear from the bit turning about a point other than its own axis

Code: PB
Characteristic: pinched bit: the bit was squeezed in a narrowed hole

Code: PN
Characteristic: plugged nozzle or passage

Code: RG
Characteristic: rounded gauge

Code: RO
Characteristic: ring out: a ring-shaped groove worn along the profile

Code: SD
Characteristic: shirttail damage: damage to the shirttail, the outer lower edge of a bit leg

Code: SS
Characteristic: self-sharpening wear: the teeth wear but keep a sharp edge

Code: TR
Characteristic: tracking: the cutting structure follows the same grooves, as in a rut

Code: WO
Characteristic: washed out bit: the flow of mud has eaten a hole in the bit

Code: WT
Characteristic: worn teeth or cutters

Code	Characteristic
BC	broken cone
BF	bond failure: a cutter or a diamond layer has come off along the braze (the soldered joint)
BT	broken teeth or cutters
BU	balling up: the bit is covered with cuttings
CC	cracked cone
CD	cone dragged: the cone seized and was dragged along the bottom
CI	cone interference: the cones rubbed against each other
CR	cored: the centre of the bit is worn out, and a column of rock is left on the bottom
CT	chipped teeth or cutters
ER	erosion of the body or of the base of the cutters
FC	flat crested wear: the tops of the teeth are worn flat
HC	heat checking: a network of thermal cracks from heating by friction
JD	junk damage: damage from metal on the bottom
LC	lost cone
LN	lost nozzle
LT	lost teeth or cutters
NO	no dull characteristic
OC	off-centre wear: wear from the bit turning about a point other than its own axis
PB	pinched bit: the bit was squeezed in a narrowed hole
PN	plugged nozzle or passage
RG	rounded gauge
RO	ring out: a ring-shaped groove worn along the profile
SD	shirttail damage: damage to the shirttail, the outer lower edge of a bit leg
SS	self-sharpening wear: the teeth wear but keep a sharp edge
TR	tracking: the cutting structure follows the same grooves, as in a rut
WO	washed out bit: the flow of mud has eaten a hole in the bit
WT	worn teeth or cutters

The codes that concern the cones (BC, CC, CD, CI, LC) and the shirttail (SD) apply only to roller-cone bits. For BC, CC, CD and LC the number of the cone (or the numbers) goes in the fourth position. BF and RO are not used for roller-cone bits.

Location of wear (the fourth position)

Bit: roller-cone
Location codes: N — nose rows, M — middle rows, G — gauge rows (they touch the wall), A — all rows; the cone number 1, 2, 3 for the characteristics BC, CC, CD and LC (No. 1 is the cone with the insert nearest to the centre)

Bit: fixed-cutter
Location codes: C — cone, N — nose, T — taper, S — shoulder, G — gauge, A — all zones; several zones may be given

Bit	Location codes
roller-cone	N — nose rows, M — middle rows, G — gauge rows (they touch the wall), A — all rows; the cone number 1, 2, 3 for the characteristics BC, CC, CD and LC (No. 1 is the cone with the insert nearest to the centre)
fixed-cutter	C — cone, N — nose, T — taper, S — shoulder, G — gauge, A — all zones; several zones may be given

Bearing, gauge and reason for pulling

Bearing (B). For an open bearing we grade the life used on a scale of 0–8: 0 is new, and 8 means that the life is used up (the cone has seized or is lost). For a sealed bearing we record the state of the bearing with its seal: E — still working, F — failed, N — cannot be judged. If part of the bearing is visible, or the cone clearly wobbles on its journal (the short shaft it turns on), we count the bearing as failed. Fixed-cutter bits have no bearings, and here X is always written.
Gauge (G). If the bit has kept its diameter, we write I (in gauge). Otherwise we write the loss of diameter in sixteenths of an inch (1/16″ ≈ 1.59 mm), rounded to the nearest sixteenth: 1/16, 2/16 and so on, often just 1, 2. We write the letter I clearly, so that nobody can take it for the digit 1.

We check the diameter with a ring gauge of the nominal diameter. We do not use the ring that accepts new bits. New roller-cone bits are made with a plus tolerance on the diameter (they may be a little larger than nominal), and a ring with this tolerance would give a wrong grade. For fixed-cutter bits we use their own nominal ring, because new bits of this type have a minus tolerance, not a plus one. On a three-cone bit we put the ring so that it touches two cones, and we measure the gap to the third. The loss of diameter is two thirds of this gap.

Loss of diameter of a roller-cone bit from the gap at the ring gauge

\Delta d = \frac{2}{3}\,s \ \ (\text{three cones}), \qquad \Delta d = s \ \ (\text{two cones}), \qquad n_{16} = \frac{\Delta d}{1.5875}

where:

$\Delta d$ reduction of the diameter of the bit, mm
$s$ gap between the ring pressed to two cones and the gauge cutting structure of the third cone, mm
$n_{16}$ loss of diameter in sixteenths of an inch; the code takes the nearest whole number, —

Units: mm; 1.5875 mm = 1/16 inch

Holds when

the cones have no play. In a bit with a broken-down bearing the gap depends on how the cone is pressed, and the measurement is unreliable;
we measure at the gauge inserts or at the inserts of the back cone, whichever are closer to the nominal diameter.

The factor 2/3 comes from the geometry of three cones set 120° apart. If the gauge of each cone is worn by $\delta$ , the ring pressed to two cones shifts towards the third by $2\delta$ by itself. The gap at the third cone then equals $3\delta$ , while the loss of diameter is $2\delta$ , which is two thirds of the gap. In a two-cone bit the cones stand opposite each other, and the gap equals the loss of diameter.

A bit that is pulled out of gauge leaves a narrowed hole behind it. We run the next bit of full diameter carefully, and we ream the last interval (work it again with the string rotating to widen it). Otherwise the bit may jam in the narrowed hole, and then it gets the code PB.

Reasons for pulling (the eighth position)

Code: BHA
Reason: change of the bottom hole assembly (BHA, the lower part of the string just above the bit)

Code: CM
Reason: conditioning the mud

Code: CP
Reason: coring

Code: DMF
Reason: failure of the downhole motor

Code: DP
Reason: drilling out a cement plug or another plug

Code: DSF
Reason: failure of the drill string

Code: DST
Reason: drill stem test (testing a formation on the drill pipe)

Code: DTF
Reason: failure of other downhole equipment, for example, of the MWD tool (the measurement-while-drilling tool that sends measurements from the hole to the surface)

Code: FM
Reason: change of formation

Code: HP
Reason: hole problems

Code: HR
Reason: hours on the bit

Code: LIH
Reason: bit left in the hole

Code: LOG
Reason: logging

Code: PP
Reason: standpipe pressure

Code: PR
Reason: rate of penetration

Code: RIG
Reason: rig repair

Code: RS
Reason: retrieving a survey instrument (an inclinometer, which measures the direction of the hole)

Code: TD
Reason: planned total depth, or the depth for running the casing (the steel pipe that lines the hole)

Code: TQ
Reason: torque

Code: TW
Reason: twist-off: the drill string breaks

Code: WC
Reason: weather conditions

Code: WO
Reason: washout in the drill string: mud has eaten a hole through the pipe

Code	Reason
BHA	change of the bottom hole assembly (BHA, the lower part of the string just above the bit)
CM	conditioning the mud
CP	coring
DMF	failure of the downhole motor
DP	drilling out a cement plug or another plug
DSF	failure of the drill string
DST	drill stem test (testing a formation on the drill pipe)
DTF	failure of other downhole equipment, for example, of the MWD tool (the measurement-while-drilling tool that sends measurements from the hole to the surface)
FM	change of formation
HP	hole problems
HR	hours on the bit
LIH	bit left in the hole
LOG	logging
PP	standpipe pressure
PR	rate of penetration
RIG	rig repair
RS	retrieving a survey instrument (an inclinometer, which measures the direction of the hole)
TD	planned total depth, or the depth for running the casing (the steel pipe that lines the hole)
TQ	torque
TW	twist-off: the drill string breaks
WC	weather conditions
WO	washout in the drill string: mud has eaten a hole through the pipe

The reason for pulling is the key to reading the whole code. With TD, LOG, RIG or BHA the bit was not pulled because of its condition, so its footage is not the limit of what it can do.

Dull grading in examples

Worked exampleDull grade of the PDC bit after run 2

Given (assumed data)

Bit: PDC M323, 215.9 mm, five blades, 16 mm cutters, as in the lesson Drill bits: types and design; radius 107.95 mm
Cone, nose, taper (inside 2/3 of the radius): 6 cutters, wear 0.80 mm on average; 5 cutters, 2.24 mm; 3 cutters, 3.20 mm
Shoulder and gauge (outside): the shoulder has 12 cutters: 11 of them are worn by 8.40 mm on average, and one is broken off flush with the blade; the gauge has 5 cutters, 3.20 mm
Other: the bit passes through the ring (in gauge); the nozzles are intact; it was pulled because the rate of penetration fell

Solution

Boundary of the zones: $\tfrac{2}{3}\cdot 107.95 = 71.97$ mm. The cone, nose and taper are inside, and the shoulder and gauge are outside.
Grade of a cutter $8h/16 = h/2$ . Averages by zone: cone 0.40, nose 1.12, taper 1.60, shoulder 4.20 (a broken cutter counts as 8), gauge 1.60.
Inner rows (14 cutters): $(6\cdot 0.40 + 5\cdot 1.12 + 3\cdot 1.60)/14 = 12.80/14 = 0.914$ . This goes into the code as 1.
Outer rows (17 cutters): $(11\cdot 4.20 + 8 + 5\cdot 1.60)/17 = 62.20/17 = 3.66$ . This goes into the code as 4.
The main characteristic is even wear of the cutters (WT) at the shoulder (S). The bearing is X, the gauge is I, the other characteristic is a broken cutter (BT), and the reason for pulling is the rate of penetration (PR).

Answer: Dull grade: 1-4-WT-S-X-I-BT-PR.

What it means. The outer third is worn four times as much as the inner two thirds. On the shoulder the cutters travel the longest way over the rock and cut the abrasive thin layers. The broken cutter points to impacts on hard layers. The outer third has lost almost half of its cutting structure on average, so the rate would have fallen even faster later. The conclusion for the next run is a bit with a denser set of smaller cutters and protection of the shoulder, and a moderate rotary speed in the sandstones. The average over a zone hides the scatter: the figure shows that the one broken cutter added about a quarter of a point to the outer grade. The data in this example are assumed for illustration.

CalculatedWear grade of each of the 31 cutters of the bit from the example, against the radius at which it sits. The dashed vertical line is two thirds of the radius. The horizontal segments are the averages by zone. After rounding they give the first and the second positions of the code.

Worked exampleDull grade of the roller-cone bit after run 1

Given (assumed data)

Bit: roller-cone 617, 215.9 mm; drilled for 30.5 h at 80 rpm
Inner rows: 96 inserts: 6 are broken, and the rest are worn by 20% of the height on average
Gauge rows: 54 inserts: 13 are broken, 4 are lost, and the rest are worn by 10% on average
Bearing: cone No. 2 is loose (it has play), and the seal does not hold the lubricant
Gauge: the nominal ring touches two cones, and the gap to the third is 4.6 mm
Other: the hardfacing on the shirttail of the leg of cone No. 2 is worn down to bare metal; the bit was pulled because of sudden jumps in torque

Solution

Inner rows: we count broken inserts as lost, and worn ones by the fraction of their height: $8\cdot(6 + 0.20\cdot 90)/96 = 8\cdot 24/96 = 2.0$ . This goes into the code as 2.
Gauge rows: $8\cdot(13 + 4 + 0.10\cdot 37)/54 = 8\cdot 20.7/54 = 3.07$ . This goes into the code as 3.
The main characteristic is broken inserts (BT) in the gauge rows (G).
The bearing is sealed and the seal has failed: F.
Gauge: $\Delta d = \tfrac{2}{3}\cdot 4.6 = 3.07$ mm $= 3.07/1.5875 = 1.93$ sixteenths. This goes into the code as 2/16.
The other characteristic is damage to the shirttail (SD). The reason for pulling is torque (TQ).
Number of revolutions in the run: $60\cdot 80\cdot 30.5 = 146.4$ thousand.

Answer: Dull grade: 2-3-BT-G-F-2/16-SD-TQ. The bit made about 146.4 thousand revolutions.

What it means. The failed seal and the jumps in torque show a bearing that is breaking down. A little later the cone could have seized or been lost (LC), and the metal on the bottom would have ruined the next bit, especially a PDC bit. The broken inserts of the gauge row and the shirttail worn against the wall are marks of impacts in rocks that alternate in thin layers. The last interval was drilled with a diameter smaller than nominal by about 3.07 mm (cone No. 2 has play, so the gauge measurement is approximate), and the next bit reams it. For these rocks it is useful to limit the runs of roller-cone bits to a smaller number of revolutions, with a margin below the 146.4 thousand at which the bearing failed, or to change to PDC bits. The rule for counting the inserts is a convention of the example. The standard describes the scale in words, and different inspectors grade a little differently. The data in this example are assumed for illustration.

What the wear tells us

The dull code is valuable not on its own but as evidence. We compare it with the parameters of the run, with the rocks seen in cuttings and logs, and with the plots of load, torque and pressure. Some signs show normal work. Others show that the bit or the parameters did not suit the rock.

Sign of wear, likely causes and what to change

Sign: WT — even wear
Likely causes: normal work in an abrasive rock
What to change: nothing, if the cost per metre is acceptable; otherwise a more wear-resistant cutting structure

Sign: BT, CT at the shoulder and gauge
Likely causes: impacts on hard thin layers, vibrations, whirl of the bit (the bit rotating off its own centre)
What to change: tougher cutters, depth-of-cut limiters, operating parameters that avoid vibration

Sign: BT in the middle rows of the cones
Likely causes: the load is too high for this cutting structure
What to change: less load, or a harder type

Sign: BU — balling
Likely causes: sticky clay, poor cleaning of the bottom
What to change: bigger slots, fewer blades, nozzles on the blades, a higher flow rate, an inhibited mud (a mud treated so that clay does not swell) or an OBM

Sign: ER — erosion
Likely causes: high velocity of the jets, abrasive cuttings
What to change: larger nozzles, a matrix body

Sign: HC — thermal cracks
Likely causes: heating by friction: a high rotary speed with heavy wear, not enough cooling
What to change: lower rotary speed, higher flow rate

Sign: RO, CR — ring or central groove
Likely causes: a lost cutter, an abrasive thin layer, vibrations
What to change: profile and layout of the cutters, operating parameters

Sign: F, LC, BC — bearing and cones
Likely causes: bearing life used up, overload, impacts
What to change: limit the revolutions per run, a different bearing, or a PDC bit

Sign: RG, loss of diameter
Likely causes: abrasive wall, a long run
What to change: stronger gauge protection; reaming the hole with the next bit

Sign: JD — junk damage
Likely causes: metal on the bottom: pieces of inserts, a nozzle
What to change: clean the bottom (with a magnet or a milling tool) before running a PDC bit

Sign: PN, LN, WO
Likely causes: plugging, poor installation of the nozzles, washout
What to change: check the nozzles and seals, and the filter on the line

Sign: TR, OC
Likely causes: unstable work: a rut, rotation off the axis
What to change: a different layout of the cutting structure, a more stable BHA

Sign	Likely causes	What to change
WT — even wear	normal work in an abrasive rock	nothing, if the cost per metre is acceptable; otherwise a more wear-resistant cutting structure
BT, CT at the shoulder and gauge	impacts on hard thin layers, vibrations, whirl of the bit (the bit rotating off its own centre)	tougher cutters, depth-of-cut limiters, operating parameters that avoid vibration
BT in the middle rows of the cones	the load is too high for this cutting structure	less load, or a harder type
BU — balling	sticky clay, poor cleaning of the bottom	bigger slots, fewer blades, nozzles on the blades, a higher flow rate, an inhibited mud (a mud treated so that clay does not swell) or an OBM
ER — erosion	high velocity of the jets, abrasive cuttings	larger nozzles, a matrix body
HC — thermal cracks	heating by friction: a high rotary speed with heavy wear, not enough cooling	lower rotary speed, higher flow rate
RO, CR — ring or central groove	a lost cutter, an abrasive thin layer, vibrations	profile and layout of the cutters, operating parameters
F, LC, BC — bearing and cones	bearing life used up, overload, impacts	limit the revolutions per run, a different bearing, or a PDC bit
RG, loss of diameter	abrasive wall, a long run	stronger gauge protection; reaming the hole with the next bit
JD — junk damage	metal on the bottom: pieces of inserts, a nozzle	clean the bottom (with a magnet or a milling tool) before running a PDC bit
PN, LN, WO	plugging, poor installation of the nozzles, washout	check the nozzles and seals, and the filter on the line
TR, OC	unstable work: a rut, rotation off the axis	a different layout of the cutting structure, a more stable BHA

OBM is an oil-based mud. The table is a general guide. The same sign may have different causes, and we check it against the data of the run.

How to read the code 1-2-WT-S-X-I-NO-TD (run 3)

1
1 and 2
The centre is hardly worn (1), and the outer third is worn by a quarter (2). The bit worked evenly, and the cutters on the shoulder wear faster, as they should.
2
WT, S
The main characteristic is even wear of the cutters on the shoulder: there are no breakages and no grooves.
3
X, I
There are no bearings. The bit is in gauge, so the hole is of full diameter.
4
NO, TD
There are no other characteristics, and the bit was pulled at total depth. Its life was not used up. Such a bit can be run again, or we can assume that this type will drill more.

Terms and symbols

IADC bit code: A classification code. For a roller-cone bit it gives the series, the type, the bearing with gauge protection and a feature letter. For a fixed-cutter bit it gives the body, the cutter density, the cutter size and the profile.
Series and type: The first and second digits of the code of a roller-cone bit. The series gives the cutting structure and the hardness of the rocks, and the type gives the step of hardness within the series.
Cutter density: How many cutters a fixed-cutter bit carries, from a light set (a sparse layout of large cutters) to a heavy set.
Confined compressive strength (CCS): The strength of the rock under a side pressure equal to the overbalance: $C_0 + q\,\Delta p$ . The bit has to overcome it.
IADC dull grade: A code of eight positions: I, O, the characteristic and location of wear, the bearing, the gauge, the second characteristic and the reason for pulling.
Inner and outer rows: For a roller-cone bit, these are the rows that do not touch and that do touch the wall of the hole. For a fixed-cutter bit, they are the inner 2/3 and the outer third of the radius.
Ring gauge: A ring of the nominal diameter for checking the loss of diameter of a bit. For a three-cone bit the loss is 2/3 of the gap to the third cone.
Bit record: A record of a bit, its run, parameters, wear and reason for pulling. We use such records to choose bits for new wells.
Revolutions in a run: Found as $60\,N\,t$ and usually given in thousands. It measures how much of the bearing life of a roller-cone bit has been used.

Common mistakes and misconceptions

Wrong

“Bits with the same IADC code perform the same.”

In fact

The code describes a class of design. The shape and layout of the cutting structure, the hydraulics and the materials differ between models, so bits with the same code can drill the same rock quite differently.

Wrong

“The strength of a rock is its unconfined strength from the core.”

In fact

At the bottom of the hole the overbalance presses on the rock, and the bit has to overcome the confined compressive strength $C_0 + q\,\Delta p$ . As the overbalance grows, the strength grows $q$ times as fast. For the lesson's rocks that is 3.7–4.2 times.

Wrong

“The bit was pulled almost unworn, so its type does not suit the rock: it drilled little.”

In fact

Look at the reason for pulling. With TD, LOG, BHA or RIG the bit was not pulled because of its condition. A short footage with little wear and the reason PR points more to a strong formation, balling or poorly chosen operating parameters than to a bad cutting structure.

Wrong

“The loss of diameter of a three-cone bit equals the gap at the ring.”

In fact

For a three-cone bit the loss of diameter is two thirds of the gap between the third cone and the ring pressed to two cones. It equals the gap only for a two-cone bit.

Wrong

“A single broken cutter is not worth mentioning: the average grade has already taken it into account.”

In fact

The average over a zone smooths out a breakage almost to nothing. So we mark broken and lost cutters with a wear-characteristic code (BT, LT). They point to impacts and may leave metal on the bottom.

In depth: advanced detail

The new IADC manual of 2026 and the limits of the eight-position code

The eight-position code was created when people inspected a bit on the catwalk (the walkway at the rig) and wrote the report by hand. It has known weaknesses. It explains why the bit was pulled better than why it wore out. The grade depends on the experience of the inspector. It has few characteristics for damage to PDC cutters, and it does not describe the bottom hole assembly at all. So for years many companies have added their own unofficial codes, for example, whether the bit is fit to be run again.

In 2021 IADC began to update the system. In April 2026 it issued the IADC Bit and BHA Dull Grading Manual, prepared by a joint working group of IADC and the Society of Petroleum Engineers (SPE). According to published descriptions of the project, the manual keeps a quick check at the rig, close to the present one. It adds a detailed check in the workshop, down to individual cutters. It also adds new kinds of damage to PDC cutters: damage to the chamfer (the bevelled edge of the cutter), chips along the axis, and wear of the substrate (the carbide base under the diamond layer). It adds finer zones of the bit, codes for damage to the parts of the BHA, and rules for storing codes and photographs in digital form. This lesson teaches the classic code. Bit records of several decades are written in it, and we use them to choose bits for new wells.

When the confined strength is calculated differently

The formula $C_0 + q\,\Delta p$ assumes that the pressure difference acts on the rock skeleton through the mudcake, and that the pore pressure under the cake stays equal to the formation pressure. In permeable sandstones and limestones this is reasonable. In tight clays almost no fluid filters through. When the rock breaks, its volume changes faster than the pore fluid can flow, and the pore pressure in the zone of failure may differ a lot from the formation pressure. So for clays we calculate the confined strength with care. There the confinement depends not only on the overbalance, but also on how the pore pressure changes in the zone of failure.

There are other simplifications. The rock under the bit is not loaded along one axis only. Near the bottom the stresses are concentrated and depend on the overburden pressure, and the strength itself grows with confinement more slowly than along a straight line. The failure under a cutter happens very fast, while strength is measured on samples under slow loading. So the confined strength is an estimate for comparison. It is convenient for comparing rock units and wells and for checking the mechanical specific energy (Bit performance and cost per metre), but it is not an exact value.

Connections to other topics

Drill bits: types and design — how roller-cone bits and PDC bits are built and how they break rock; the general choice of type for the rocks to be drilled.
Bit performance and cost per metre — rate of penetration, the drill-off test, specific energy and cost per metre: a comparison of the same runs in money.
Sand production: types, prediction and cost of the problem — rock strength, the Mohr–Coulomb criterion and strength estimates from logs.
Bit hydraulics optimisation — flow rate and nozzles of the bit: cleaning of the bottom against balling and erosion.
Deflection tools and downhole motors — downhole motors and rotary steerable systems: the rotary speed and the requirements for the gauge of the bit.
Drill string design — the weight on bit that the bottom hole assembly can supply, and the 4800 m well of the examples.
Pore pressure and overburden pressure — pore pressure and overbalance, on which the strength of the rock under the bit depends.

Check yourself

1
What does the code 537 of a roller-cone bit mean?
1. A.Steel teeth, soft rocks, an open roller bearing
2. B.A PDC bit with a steel body and a medium profile
3. C.Tungsten carbide inserts for soft to medium rocks, one of the harder types of the series, a sealed journal bearing with gauge protection
4. D.Tungsten carbide inserts for very hard rocks, a sealed roller bearing
Show the answer
Correct answer: C. Tungsten carbide inserts for soft to medium rocks, one of the harder types of the series, a sealed journal bearing with gauge protection
Series 5 is tungsten carbide inserts for soft to medium rocks with low compressive strength. Type 3 is the third of the four steps of hardness within the series. Digit 7 is a sealed journal bearing with gauge protection. PDC bits have a code of a letter and three digits.
2
How does the PDC bit with the code S223 differ from the bit M433?
1. A.It does not differ: the last digits are the same
2. B.A steel body, a sparser layout and larger cutters (14–24 mm): it is meant rather for softer rocks
3. C.Only in the hydraulics: S223 has more nozzles
4. D.A matrix body and a denser set: it is meant for harder rocks
Show the answer
Correct answer: B. A steel body, a sparser layout and larger cutters (14–24 mm): it is meant rather for softer rocks
S is a steel body. The 2 is a lighter set than the 4. The 2 in the third character means cutters of 14–24 mm, against 8–14 mm. The last digit, 3, is the same in both: a medium profile. The code does not describe the hydraulics at all.
3
A limestone ( $C_0$ = 80 MPa, $\varphi$ = 35°, $q$ = 3.69) at a depth of 3000 m was drilled with a mud of 1.25 g/cm³. Then the mud was made heavier, up to 1.45 g/cm³. The pore pressure did not change. How did the strength of the rock under the bit change?
1. A.It fell: a heavy mud holds the walls better
2. B.It did not change: strength is a property of the rock
3. C.It grew by about 6 MPa, by the increase of the overbalance
4. D.It grew by about 22 MPa
Show the answer
Correct answer: D. It grew by about 22 MPa
The overbalance grew by $0.00981\cdot 0.20\cdot 3000 \approx 5.89$ MPa, and the confined strength grows $q$ times as fast: $3.69\cdot 5.89 \approx 21.7$ MPa. So when the mud is made heavier, the rate of penetration falls markedly, even in the same rock.
4
A three-cone bit of 215.9 mm was checked with the nominal ring: the gap to the third cone is 6.2 mm. What do we write in the sixth position of the dull code, and what do we do with the next bit?
Show the answer
The loss of diameter is $\Delta d = \tfrac{2}{3}\cdot 6.2 \approx 4.13$ mm, or $4.13/1.5875 \approx 2.6$ sixteenths of an inch, so we write 3/16. The last interval was drilled narrower than the nominal size (undergauge). So we run the next bit of full diameter carefully and ream this interval. Otherwise the bit may jam in the hole.
5
A PDC bit was pulled in a unit of clays because its rate of penetration fell, with the code 0-0-BU-A-X-I-NO-PR. What happened, and what should be changed in the next run?
Show the answer
The cutters are not worn (0-0), but the whole bit is covered with clay (BU, A). The cuttings could not leave the bottom, the cutters stopped touching the rock, and the rate fell. It helps to use a bit with fewer blades and bigger slots and with nozzles aimed at the blades, and to raise the flow rate and the hydraulic power. In chemically reactive clays (clays that swell or break up in water), an inhibited mud or an oil-based mud helps too. After cleaning, the bit itself can most likely be run again.
6
The bit record of a roller-cone bit shows the code 3-5-LC-3-F-3/16-JD-TQ. What happened, and why must a PDC bit not be run next?
Show the answer
Cone No. 3 is lost (LC, 3). With it a third of the cutting structure is gone, so the inner rows are also graded no lower than 3 ( $8/3 \approx 2.7$ ). The bearing has failed (F). The bit lost about 3/16 inch of diameter (this is an approximate measurement from the two remaining cones, because the two-thirds rule cannot be used without the third cone). It carries marks of impacts on metal (JD), and it was pulled for jumps of torque (TQ). The cone and its fragments stayed on the bottom, and a PDC bit would break its cutters on them. First we clean the bottom with a fishing tool (a tool that catches lost objects), a magnet or a milling tool. Then we ream the narrowed interval, and only after that we run a new bit.

Updated: 28 October 2026