Drill bits: types and design

Quick refresher

A roller-cone bit breaks rock by indentation. A tooth or insert pressed down by the load crushes the rock beneath it, cracks spread out from the crushed zone, and the rock chips out, leaving a crater. In soft rocks the offset cone axes make the cutting structure slip along the bottom, which adds scraping.
A PDC bit shears the rock like the cutting tool of a lathe. In one revolution each cutter removes a layer. The thickness of the layer is the penetration per revolution divided by the number of cutters on its track (the ring of the bottom that it cuts). Shearing works best in ductile (plastic) and medium rocks, but impacts and overheating easily damage the cutters.
An impregnated bit grinds very hard, abrasive rock (rock that wears steel away quickly) with fine diamonds embedded in the hard matrix of the bit. The matrix wears away and exposes new grains. Such bits work at high rotary speeds, driven by a turbodrill (a turbine that the mud turns, just above the bit) or by a high-speed motor.
A roller-cone bit has moving parts: the bearings of the cones. At high rotary speeds their life often limits the run. A PDC bit has no moving parts. Wear and chipping of the cutters limit its run.
In softer, more ductile rock, a PDC bit has fewer blades (the ribs that carry the cutters) and larger cutters, and a roller-cone bit has longer teeth. The bit also needs bigger junk slots, the channels between the blades through which mud and cuttings leave. In harder, more abrasive rock, a PDC bit has more cutters on each track, smaller cutters and stronger protection of the gauge (the outer edge that keeps the hole to size). In very hard abrasive rocks, roller-cone bits with inserts and impregnated bits are used.
Nozzles and junk slots are part of the design of a bit. The cuttings must leave the bottom of the hole before the next cutter passes over it. Choosing the nozzles is covered in the lesson on bit hydraulics.

Key relationships

\delta = \dfrac{1000\,R}{60\,N}, \qquad \delta_{\text{c}} = \dfrac{\delta}{z}

penetration per revolution, mm ( $R$ in m/h, $N$ in rpm) and the depth of cut of a PDC cutter with $z$ cutters on a track

v = \dfrac{2\pi\,r\,N}{60}

sliding speed of a cutter at radius $r$ : it is greatest at the gauge

v_{\text{js}} = \dfrac{Q}{A_{\text{js}}}

average velocity of the mud in the junk slots of the bit

\omega = \dfrac{\Omega}{\sin\beta}

rotation of a cone relative to the body in pure rolling, that is, rolling without slipping ( $\beta$ is the journal angle)

Common pitfalls

Thinking that a PDC bit suits every rock. In interbedded rocks with hard stringers (thin hard layers), in conglomerates (rock made of cemented pebbles) and in cherts (very hard rock made of silica), its cutters chip. In very abrasive rocks they overheat and wear away.
Running a bit with closely spaced cutters and narrow slots in sticky clays. The cuttings cannot get away, and clay clogs the bit (bit balling).
Raising the rotary speed of a roller-cone bit without thinking about the bearings. The cones turn noticeably faster than the bit itself.
Confusing the penetration per revolution with the depth of cut of one cutter. If there are two cutters on a track, each removes half.
Choosing a bit by hardness alone. Abrasiveness, interbedding, the drive and the requirements for the direction of the hole matter too.

After this lesson you can

explain how a roller-cone bit (indentation, crushing and chipping, and, with offset cones, scraping), a PDC bit (shearing) and an impregnated bit (grinding) break rock, and why the mechanism decides the weight and rotary speed the bit needs
describe how a roller-cone bit is built: the legs and journals, the cones, the cutting structure (milled teeth and tungsten carbide inserts), the bearings and their seals, the gauge row and the nozzles
describe how a PDC bit is built: the cutters, the steel or matrix body, the blades, the cutter back rake, the profile, the gauge pads, the nozzles and the junk slots
calculate the penetration per revolution, the depth of cut of a PDC cutter and the speed of the cutters at the gauge, and explain how these change with the rate of penetration, the rotary speed and the cutter layout
match the types of bits, in general terms, to the rocks — soft, medium, hard, abrasive, interbedded (hard and soft layers alternating) — and to the bit drive (what turns the bit: the rotary table or a downhole motor)

How a bit breaks rock

A bit can break rock in three ways, and each type of bit relies mainly on one of them.

Indentation: a hard tooth or insert is pressed into the rock under load. Under it the rock is crushed to fine powder. This is the crushed zone. Cracks spread from it in all directions, and when they reach the surface a piece of rock chips out and leaves a crater. The next tooth breaks the ridge between the craters. This needs a high contact pressure, which means a high weight on the bit. It works best on brittle, hard rocks (rocks that crack rather than bend). Roller-cone bits work this way.
Shearing: the sharp edge of a cutter moves along the bottom of the hole and removes a layer of a set thickness, like the cutting tool of a lathe. The rock in front of the cutter breaks by shear (one layer slides over another) and leaves along the cutter face as a large chip. Per unit volume this usually takes less energy than crushing. This is especially true in ductile rocks, which are squashed under a tooth rather than chipped. PDC bits work this way.
Grinding: very many tiny cutting points — diamond grains — scratch shallow grooves in the rock. Each grain removes very little, so speed comes from the number of grains and from a high rotary speed. Impregnated and diamond bits work this way in the hardest, most abrasive rocks.

SchematicThree ways in which a bit breaks rock (schematic, not to scale): indentation by an insert of a roller-cone bit, with the crushed zone, cracks and chipped fragments; shearing of a layer by a PDC cutter; grinding of the rock by the diamond grains of an impregnated bit.

From this follow the main rules. The more ductile the rock — clays and salt at great depth, ductile mudstones (hardened clay rock) — the worse indentation works, because the rock is squashed rather than chipped. Shearing then pays off more. The harder and more abrasive the rock, the faster it wears a cutter that slides over it. Indentation or grinding then pays off more. Soft, loose rocks are also partly washed away by the mud jets themselves. On every bit the fragments must be cleared from the bottom of the hole at once. The pressure of the mud presses them against the bottom, and cuttings that are not cleared are ground again by the bit (what the jets do at the bottom of the hole).

Types of bits

The main types of bits

Type: Roller-cone with milled teeth
How it breaks rock: indentation and scraping
Rocks: soft, sticky
Strengths: cheap, cleans well
Limitations: teeth wear quickly

Type: Roller-cone with tungsten carbide inserts
How it breaks rock: indentation and chipping
Rocks: medium to very hard, abrasive, interbedded
Strengths: durable cutting structure, withstands impacts
Limitations: bearing life, rotary speed

Type: PDC
How it breaks rock: shearing
Rocks: soft to hard, slightly abrasive; salt
Strengths: high speed, long runs, no moving parts
Limitations: impacts, vibration, abrasiveness

Type: Impregnated
How it breaks rock: grinding
Rocks: very hard, abrasive
Strengths: life in the hardest rocks
Limitations: low speed, needs high rotary speed

Type: Hybrid
How it breaks rock: shearing and indentation
Rocks: interbedded, hard
Strengths: smoother torque (turning force), less vibration
Limitations: more expensive, more complex

Type	How it breaks rock	Rocks	Strengths	Limitations
Roller-cone with milled teeth	indentation and scraping	soft, sticky	cheap, cleans well	teeth wear quickly
Roller-cone with tungsten carbide inserts	indentation and chipping	medium to very hard, abrasive, interbedded	durable cutting structure, withstands impacts	bearing life, rotary speed
PDC	shearing	soft to hard, slightly abrasive; salt	high speed, long runs, no moving parts	impacts, vibration, abrasiveness
Impregnated	grinding	very hard, abrasive	life in the hardest rocks	low speed, needs high rotary speed
Hybrid	shearing and indentation	interbedded, hard	smoother torque (turning force), less vibration	more expensive, more complex

An overview, not a selection rule: each group has bits for soft rocks and bits for hard rocks. The lesson Bit selection and dull grading covers matching a bit to the rocks to be drilled and the classification codes.

A bit is named by its diameter, which is the nominal diameter of the hole: 215.9 mm (8½ in), 295.3 mm (11⅝ in), 444.5 mm (17½ in) and so on. At the top the bit has a pin, a threaded end with a REG (“regular”) tool-joint thread. For a 215.9 mm bit it is 4½ in REG. The pin screws into a bit sub (a short connector with a threaded socket, called a box, at both ends), into a downhole motor (a motor on the string that turns the bit) or into a rotary steerable system (a tool that steers the bit while the string keeps turning) (subs, Deflection tools and downhole motors).

Roller-cone bits

The body of a roller-cone bit is welded from three legs. Each leg ends in a journal, a short shaft set at an angle to the bit axis. A cone — a conical body with rows of teeth — rotates on bearings on the journal. The bit turns with the string, and the cones roll over the bottom of the hole, so their cutting structure is pressed into the rock again and again. The mud leaves through nozzles in the body between the legs.

SchematicRoller-cone bit (schematic, not to scale). Top: side view — pin, body, legs with shirttails (the lower outer edges of the legs), cones with the gauge (heel) row and inner rows, nozzle. Bottom: cross-section of a leg and cone — the journal, set at an angle

\beta

to the horizontal, the roller, ball (locking) and sliding bearings, the seal, the lubricant reservoir with the compensator, and the passage through which the balls are loaded.

Cutting structure. Bits for soft rocks have milled teeth: long, thin, widely spaced steel teeth cut directly out of the cone and covered with a layer of hard alloy welded on (hardfacing). They sink deep into soft rock and clean themselves. Bits for medium and hard rocks have tungsten carbide inserts (TCI): small cylinders of tungsten carbide pressed into holes in the cone. The harder the rock, the shorter and blunter the inserts and the closer together they sit. Wedge-shaped and long inserts are for soft rocks. Conical and semi-round (domed) ones are for hard rocks.

The teeth sit in circular rows. The rows of neighbouring cones mesh together, so the cones clean each other of rock that sticks to them. One of the cones reaches past the centre of the bottom, so that no column of uncut rock is left there.
The gauge row (heel row) cuts the corner between the bottom and the wall and keeps the hole to its full diameter. If it wears down, the hole becomes narrower, and the next bit has to be run in with reaming (cutting the hole back to full size). Forces that the design did not plan for also begin to act on the bearings. So the gauge is protected with extra inserts, some of them diamond-coated, and with hardfacing.
The shirttail of the leg, its lower outer edge, rubs against the wall, so it is also hardfaced. If it wears away, mud with cuttings reaches the seal of the bearing.

Cone geometry. The journal axis is tilted from the horizontal by the journal angle $\beta$ . It is usually 30–33° in bits for soft rocks and 34–39° in bits for hard ones. If the tip (apex) of the cone lies on the bit axis, the cone rolls over the bottom without slipping (see the advanced section). In bits for soft rocks the cone axes are offset (cone offset) from the bit axis, by up to about 9–10 mm (3/8 in), and each cone is built from several conical parts with different angles. Pure rolling is then impossible. The cone slips, so the teeth do not only indent but also scrape and gouge the rock. In hard rocks, slipping only wears the cutting structure. So in bits for hard rocks the offset is small, about a millimetre, or there is none.

Bearings. A cone sits on its journal on three groups of bearings. The roller bearing at the base of the journal and the sliding bearing, a sleeve on the nose of the journal, carry the sideways (radial) load from the weight on the bit. The ball bearing holds the cone on the journal. The balls are loaded through a passage in the leg, which is then closed with a plug. Together with the thrust button, a hard disc at the tip of the journal, the ball bearing takes the forces along the journal axis (axial forces). The room for bearings is limited: the larger the bearings, the thinner the wall of the cone and the shorter the teeth. So bits for soft rocks, which have a long cutting structure, have smaller bearings.

Bearings of roller-cone bits

Bearing: Open, rolling-element (rollers and balls)
Lubricant: drilling mud
Where used: shallow intervals, short runs; cheap

Bearing: Sealed, rolling-element (rollers and balls)
Lubricant: grease
Where used: allows higher rotary speeds

Bearing: Sealed journal (sliding) bearing
Lubricant: grease
Where used: heavy loads, long runs at moderate rotary speeds

Bearing	Lubricant	Where used
Open, rolling-element (rollers and balls)	drilling mud	shallow intervals, short runs; cheap
Sealed, rolling-element (rollers and balls)	grease	allows higher rotary speeds
Sealed journal (sliding) bearing	grease	heavy loads, long runs at moderate rotary speeds

The mud lubricates and cools an open bearing, but the abrasive particles in the mud wear it out quickly. A sliding bearing passes the load over a large contact area. A sealed bearing is filled with grease from a reservoir in the leg. A flexible diaphragm, the compensator, makes the pressure of the grease equal to the pressure of the mud in the well, so that the seal is not pushed in or out. The seals are rubber rings or, for high rotary speeds and temperatures, metal face seals.

Here is how a roller-cone bit wears out. The teeth wear down and break, the gauge wears, and the bearings fail. When a bearing is destroyed, the cone jams on the journal or wobbles. In the worst case it breaks off and stays at the bottom of the hole, and it has to be recovered by fishing (catching and pulling out lost objects with special tools). So the bearings often limit the run before the cutting structure does. The lesson Bit selection and dull grading covers how the wear of a bit pulled from the hole is described and how the next bit is chosen.

PDC bits

A PDC cutter (polycrystalline diamond compact) is a cylinder of tungsten carbide, called the substrate. A layer of polycrystalline diamond a few millimetres thick is sintered (bonded under heat and pressure) onto its end. The layer is made of fine crystals of synthetic diamond, fused together at high pressure and temperature with a cobalt binder. The cutters are usually 13, 16 or 19 mm in diameter (less often 8–11 mm, and 22 mm and larger). The edge of the diamond layer is usually bevelled with a chamfer, so that it chips less on impact. The cutters are brazed (joined with a melted filler metal) into pockets on the blades of the bit body.

The body is either steel or matrix. A steel body is machined from alloy steel. It is strong and allows tall blades and large junk slots, but mud with cuttings erodes it, so it is hardfaced. A matrix body is sintered from tungsten carbide powder infiltrated with a copper alloy (the melted alloy fills the gaps between the powder grains). It resists erosion and abrasion, but it is more brittle and more expensive.
Blades and cutter layout. Bits for soft rocks have three or four blades with large cutters. Each cutter removes a thick layer, and there is a lot of room between the blades for cuttings. Bits for hard rocks have six blades or more, small cutters, and several cutters on one circular track of the bottom. Each cutter carries less load and the bit lasts longer, but in soft rocks it is slower.
The bit profile is the shape of its face in cross-section: the cone in the centre, the nose, the shoulder and the gauge. A short, flat profile spreads the load evenly among the cutters and is easier to steer. A long, parabolic profile gives more room for cutters on the shoulder, where they wear fastest, and runs more steadily with a motor.
Back rake. A cutter is set tilted backwards (back rake; in cutting theory this is a negative rake angle). Its axis leans away from the direction of motion, up towards the bit body, by an angle $\theta$ . The angle is usually about 15–20°, and larger at the gauge and in bits for hard rocks. The cutting face leans by the same angle from the perpendicular to the bottom of the hole, and the side surface of the cutter behind the edge is lifted off the rock. The larger the angle, the blunter the edge that meets the rock. The cut is then thinner and the penetration slower, but the cutter is stronger and chips less. With a small angle the cutter is more aggressive: it cuts deeper. Side rake sends the cuttings towards the junk slots.
Gauge pads are the continuations of the blades along the wall of the hole, reinforced with hard alloy or diamond. A long gauge keeps the bit steady and the hole smooth. A short gauge makes it easier to steer the bit with a motor or a rotary steerable system (deflection tools).
Depth-of-cut control elements are projections behind the cutters. They stop the cutters from digging in deeper than planned and smooth out the changes in torque.

SchematicPDC bit (schematic, not to scale). Top: the face of a bit with five blades, seen from the bottom of the hole (so the rotation of the string appears counter-clockwise): cutters on the leading edge of the blades, nozzles between them, junk slots at the gauge, and gauge pads. Bottom: a cutter at work. It is tilted back by the angle

\theta

(its side surface is lifted off the bottom by this angle, and its face leans from the vertical by it). It shears a layer of thickness

\delta_{\text{c}}

, and the sheared rock rises in front of its face.

Penetration per revolution and the depth of cut of a cutter

\delta = \frac{1000\,R}{60\,N}, \qquad \delta_{\text{c}} = \frac{\delta}{z}

where:

$\delta$ penetration of the bit per revolution, mm/rev
$R$ rate of penetration, m/h
$N$ rotary speed of the bit, counting the speed of the motor if there is one in the bottom hole assembly (BHA, the tools just above the bit), rpm
$\delta_{\text{c}}$ thickness of the layer removed by one cutter per revolution (depth of cut), mm
$z$ number of cutters passing over one circular track of the bottom, —

Units: metric: $R$ in m/h, $N$ in rpm, $\delta$ in mm

Holds when

The cutters of one track are evenly spaced round the circumference and remove equal shares of the penetration.
The bit penetrates uniformly over the whole bottom, without rocking or vibration.
The tracks of neighbouring cutters overlap, so the depth of cut varies across the width of a cutter. The formula gives the greatest depth of cut.

At the same rate of penetration, a higher rotary speed makes the cut thinner, and a higher rate of penetration at the same rotary speed makes it thicker. The depth of cut sets the forces on the cutter: the greater it is, the greater the torque and the load on each cutter.

In other unit systems (1)

Oilfield units

\delta\,[\text{in/rev}] = \frac{12\,R\,[\text{ft/h}]}{60\,N} = \frac{R}{5N}

$R$ in feet per hour, $N$ in rpm, penetration in inches per revolution.

12 inches in a foot, 60 minutes in an hour.

Worked exampleHow much rock one cutter removes

Given (assumed data)

Bit: 215.9 mm PDC, five blades, 16 mm cutters
Operating conditions: 20 m/h at 150 rpm; for comparison, 300 rpm with a motor at the same rate of penetration
Cutter layout: one cutter per track in the centre and on the cone, two on the shoulder
Innermost cutter: at a radius of 30 mm

Solution

Penetration per revolution: $\delta = 1000\cdot 20/(60\cdot150) = 2.22$ mm.
A cutter that has its track to itself removes a layer of 2.22 mm per revolution, which is 14% of its diameter. On the shoulder there are two cutters per track, so each removes $2.22/2 = 1.11$ mm.
Sliding speed of a cutter at the gauge (radius 107.95 mm): $v = 2\pi\cdot 0.10795\cdot 150/60 = 1.70$ m/s. For the cutter at a radius of 30 mm, $2\pi\cdot 0.030\cdot 150/60 = 0.47$ m/s, which is 3.6 times lower. In an hour the cutter at the gauge travels over the rock $1.70\cdot 3600/1000 = 6.1$ km.
With a motor, at 300 rpm and the same rate of penetration: penetration per revolution 1.11 mm, speed at the gauge 3.39 m/s.

Answer: In one revolution the bit penetrates 2.22 mm, and a single cutter shears a layer of the same thickness. The cutter at the gauge slides over the rock at 1.70 m/s, which is 3.6 times faster than the innermost one.

What it means. A depth of cut of a few millimetres is what makes a bit “aggressive”. The fewer the blades and the cutters on a track, the thicker the cut at the same rate of penetration. This gives a higher speed in soft rocks, but also a greater load on each cutter and more torque. Cutters on the shoulder and gauge travel kilometres in an hour, so they heat up and wear more. That is why more cutters are placed there. At the same rate of penetration, a motor halves the depth of cut but doubles the sliding speed. In soft and medium rocks, the margin in depth of cut allows faster drilling. In hard abrasive rocks, the high rotary speed heats and wears down the cutters. The lesson Bit performance and cost per metre covers how the rate of penetration depends on weight and rotary speed. The data in this example are assumed for illustration.

Diamond, impregnated and hybrid bits

Natural diamond bits: diamonds set into the surface of a matrix crown. Each diamond ploughs a shallow groove in the rock. These bits were used to drill the hardest rocks and to cut core (a column of rock taken from the well for study). Today impregnated bits and PDC bits have almost entirely replaced them.
Thermally stable polycrystalline diamond (TSP): pieces of sintered diamond from which the cobalt binder has been removed. They withstand a much higher temperature than ordinary PDC cutters, but they are smaller and weaker. They are set in the matrix like natural diamonds, in impregnated bits and on the gauge.
Impregnated diamond bits: fine grains of synthetic diamond are spread through the matrix of the blades or of separate segments. The grains grind the rock, the matrix wears away gradually, and new grains are exposed, so the bit “sharpens itself”. The rate of penetration is low, but the bit lasts a long time in very hard abrasive rocks — quartzites (almost pure quartz rock), quartz sandstones, and crystalline rocks such as granite. It needs high rotary speeds, so it is run with a turbodrill or a high-speed motor (downhole motors).
Hybrid bits have blades with PDC cutters and cones on one body. The cutters shear the rock. The cones crush it, which prepares the way for the cutters, and they steady the bit, so the torque changes less than with a pure PDC bit. Hybrid bits are used in interbedded and hard rocks.

The same principles apply to core bits (also called core heads), the bits that cut core. They are ring-shaped bits with PDC cutters, natural diamonds or impregnated diamonds. They break the rock only in a ring and leave a column of core in the centre (Core analysis).

Nozzles and junk slots

Early roller-cone bits sent the mud out through a central passage. The jet hit the cones rather than the bottom of the hole, the cuttings stayed under the bit, and clay clogged the cutting structure. Modern bits are cleaned through jet nozzles: replaceable tungsten carbide sleeves with a hole of a set diameter (the diameter is given in 1/32 in). A roller-cone bit has three nozzles in the body between the legs, sometimes also a central one. Extended nozzles bring the jet closer to the bottom. A PDC bit usually has more nozzles, up to eight or more. They are aimed along the blades, clean and cool the cutters, and push the cuttings towards the junk slots.

Junk slots are the channels between the blades at the gauge. The mud and cuttings leave the bottom of the hole through them and enter the annulus (the space between the string and the wall of the hole). Their total area, the junk slot area (JSA), is an important characteristic of a fixed-cutter bit. Bits for soft rocks have few blades and large slots (an “open face”), so large cuttings leave freely. Closely set bits for hard rocks, which have many cutters packed together, have smaller slots. In sticky clay the cuttings may not get away in time. They stick to the blades and cutters, and the bit balls up (bit balling): the cutters stop touching the rock and the rate of penetration falls. Large slots, well-placed nozzles and enough flow rate help. In chemically active clays, inhibited muds (muds that keep clay from swelling) and oil-based muds help too (Oil-based muds).

Worked exampleWhere the cuttings have least room: the junk slots of the bit and the annulus

Given (assumed data)

Bit: 215.9 mm PDC; junk slot area 100 cm² (assumed)
Flow rate: 30 L/s, as in the well of the lesson on circulating pressure losses
Above the bit: 158.8 mm drill collars (heavy, thick-walled tubes), above them 127 mm drill pipe
Rate of penetration: 20 m/h

Solution

Area of the bottom of the hole: $\pi\cdot 21.59^2/4 = 366.1$ cm². The slots take up 27% of it.
Average mud velocity in the slots: $v_{\text{js}} = 0.030/0.0100 = 3.0$ m/s.
For comparison, take the annulus above the bit. At the drill collars the annular area is $\pi(0.2159^2 - 0.1588^2)/4 = 0.0168$ m², so the velocity is $0.030/0.0168 = 1.79$ m/s. At the drill pipe it is 1.25 m/s.
A closely set bit for hard rocks, with slots of 70 cm², would have a velocity in the slots of $0.030/0.0070 = 4.3$ m/s.
In an hour the bit drills out $0.0366\cdot 20 = 0.73$ m³ of rock, and all of these cuttings pass through the slots.

Answer: The mud moves through the slots at about 3.0 m/s, which is 1.7 times faster than in the annulus at the drill collars. The slots are the narrowest passage on the way of the cuttings from the bottom of the hole.

What it means. The smaller the slots, the faster the flow in them, and the easier it is for large or sticky cuttings to plug them. In soft rocks the bit makes a lot of large cuttings, so large slots matter more than extra cutters. In hard rocks there are fewer cuttings and they are finer, so slot area can be given up to fit more cutters. We calculate the pressure drop across the nozzles, the jet power and the choice of nozzles separately (pressure drop at the bit, Bit hydraulics optimisation). The data in this example are assumed for illustration.

How the rock selects the type of bit

To choose a bit, we start with the rocks to be drilled (the geological section): how hard and how ductile they are, how abrasive they are (quartz wears away steel and tungsten carbide alike), whether they are interbedded, and whether they contain cherts and conglomerates. Then we consider the drive (rotary table, motor, turbodrill, rotary steerable system), the requirements for the direction of the hole, the type of mud and the economics of the run. The table below is an overview, not a recipe.

The rocks to be drilled and the typical choice of bit

Rocks: Soft sticky clays, loose sands
What is usually chosen: PDC with 3–4 blades, large cutters and large slots; roller-cone with long teeth
Why: a lot of large cuttings, good cleaning is needed

Rocks: Medium: mudstones, sandstones, limestones
What is usually chosen: PDC with 5–6 blades
Why: shearing is faster, long run

Rocks: Salt
What is usually chosen: PDC
Why: ductile rock shears well

Rocks: Hard abrasive sandstones, dolomites
What is usually chosen: PDC with many blades and gauge protection; roller-cone with inserts
Why: cutters and gauge wear quickly

Rocks: Interbedding with hard stringers, cherts, conglomerates
What is usually chosen: roller-cone with inserts, hybrid
Why: impacts chip PDC cutters

Rocks: Very hard abrasive rocks: quartzites, crystalline rocks
What is usually chosen: impregnated with a turbodrill; roller-cone with inserts
Why: shearing cutters overheat

Rocks	What is usually chosen	Why
Soft sticky clays, loose sands	PDC with 3–4 blades, large cutters and large slots; roller-cone with long teeth	a lot of large cuttings, good cleaning is needed
Medium: mudstones, sandstones, limestones	PDC with 5–6 blades	shearing is faster, long run
Salt	PDC	ductile rock shears well
Hard abrasive sandstones, dolomites	PDC with many blades and gauge protection; roller-cone with inserts	cutters and gauge wear quickly
Interbedding with hard stringers, cherts, conglomerates	roller-cone with inserts, hybrid	impacts chip PDC cutters
Very hard abrasive rocks: quartzites, crystalline rocks	impregnated with a turbodrill; roller-cone with inserts	shearing cutters overheat

An overview based on general principles. We choose the specific bit from the experience of offset wells (nearby wells already drilled) and the manufacturer's recommendations. Classification codes and dull grading (describing the wear of a used bit): Bit selection and dull grading. Comparing runs by cost per metre: Bit performance and cost per metre.

Drive. The high rotary speed of a motor or turbodrill suits PDC and impregnated bits. The bearing life of roller-cone bits falls as the rotary speed rises.
Direction of the hole. Roller-cone bits turned by the rotary table usually make the hole drift to the right (bit walk). PDC bits do so less, and more often to the left (how a bit turns the hole). For directional drilling we choose bits with a suitable gauge length and a suitable side-cutting aggressiveness (how strongly the bit cuts sideways).
Mud. In oil-based muds less clay sticks to the bit, so PDC bits work better in clays.
Economics. An expensive bit pays off if it drills more in a run and saves trips. We compare not the price of the bit but the cost per metre.

Terms and symbols

Roller-cone bit: A bit with cones that rotate on journals. It breaks rock by indentation and chipping.
Leg, journal, cone: The leg is a part of the bit body. The journal is the inclined shaft at the end of the leg. The cone is the toothed body that rotates on the journal.
Cutting structure: The teeth or inserts of a bit. Milled teeth are made of steel, and tungsten carbide inserts are made of tungsten carbide.
Gauge row (heel row): The outermost row of cone teeth. It cuts the corner of the bottom and keeps the hole to its full diameter.
Cone offset: The shift of the journal axis away from the bit axis. It makes the cone slip and scrape soft rock.
Cone bearing: The bearings on which a cone rotates on the journal. They can be rolling-element or sliding, open or sealed.
PDC bit: A bit with fixed cutters made of polycrystalline diamond on a tungsten carbide substrate. It shears the rock.
Back rake: The backward tilt of a PDC cutter. By this angle the cutter axis leans away from the direction of motion, and the cutting face leans from the perpendicular to the bottom of the hole. The angle sets how aggressive and how strong the cutter is.
Bit profile: The shape of the face of a PDC bit: the cone, the nose, the shoulder and the gauge.
Junk slot: A channel between the blades at the gauge. The cuttings leave the bottom of the hole through it.
Impregnated diamond bit: A bit with fine diamonds in the matrix. It grinds very hard abrasive rocks at high rotary speed.
Bit balling: Cuttings stick to the bit, so that the cutting structure no longer touches the rock.

Common mistakes and misconceptions

Wrong

“A PDC bit is always faster than a roller-cone bit.”

In fact

In ductile and medium rocks, usually yes. But in interbedded rocks with hard stringers, in conglomerates and in cherts the PDC cutters chip, and in very abrasive rocks they overheat and wear away. There, roller-cone bits with inserts, hybrid bits or impregnated bits may drill more in a run.

Wrong

“The more blades and cutters, the better the bit.”

In fact

More cutters mean a thinner cut and less load on each cutter. The bit lasts longer in hard rocks, but it is slower in soft rocks and cleans worse, because less room is left for the slots. In soft rocks, bits with three or four blades and large cutters do best.

Wrong

“The rotary speed of a roller-cone bit can be raised without limit — the bearings will take it.”

In fact

The cones turn noticeably faster than the bit itself. In pure rolling at a journal angle of 33° they turn almost twice as fast. At high rotary speeds the bearings heat up and wear. Bits with bearings built for high rotary speeds are made for motors, but PDC bits are used more often.

Wrong

“An impregnated bit is run at a high weight and low rotary speed, like a roller-cone bit.”

In fact

An impregnated bit grinds the rock with tiny diamond grains. Each grain penetrates very shallowly, so the speed comes from a high rotary speed, with a turbodrill or a high-speed motor.

Wrong

“Nozzles and slots are a matter for hydraulics and have nothing to do with bit selection.”

In fact

The size of the slots and the number and position of the nozzles are built into the design of the bit. In sticky clays a bit with narrow slots balls up, however well the nozzle diameters are chosen.

In depth: advanced detail

How a cone rolls and why it slips

Picture the cone as a perfect cone. Its tip (apex) lies on the bit axis at the bottom of the hole, and the lowest straight line along its surface (the generator) lies on the bottom. The bit rotates at angular velocity $\Omega$ . A point of the generator at a distance $s$ from the bit axis moves with the bit body at the speed $\Omega s$ . To keep this point from sliding over the bottom, the cone surface at this point must move, relative to the body, at the same speed in the opposite direction. The distance from this point to the cone axis is $s\sin\beta$ , because the angle between the cone axis and the generator equals the journal angle $\beta$ . So $\omega\,s\sin\beta = \Omega s$ . The distance $s$ cancels. The condition is met along the whole generator at once, and the cone rolls without slipping.

Rotation of a cone in pure rolling

\omega = \frac{\Omega}{\sin\beta}

where:

$\omega$ rotary speed of the cone relative to the bit body, rpm
$\Omega$ rotary speed of the bit — the same quantity as $N$ in the section on PDC bits; here it is paired with the $\omega$ of the cone, rpm
$\beta$ journal angle to the horizontal — also the angle between the cone axis and the generator lying on the bottom, degrees

Units: $\omega$ and $\Omega$ in the same units

Holds when

The cone is a single cone, its apex lies on the bit axis, and there is no axis offset.
The bottom of the hole is flat and the bit does not rock.

Suppose the apex of the cone is not on the bit axis, because the axes are offset or because the cone is built from conical parts with different angles. Then the equality $\omega s\sin\beta = \Omega s$ cannot hold for all points at once. The cone turns at some intermediate speed, some rows lag behind or run ahead, and their teeth scrape the rock. This is exactly what is wanted in bits for soft rocks.

Worked exampleHow fast the cones rotate

Given (assumed data)

Journal angle $\beta$: 33° (bit for soft rocks) and 36° (bit for hard rocks)
Rotary speed of the bit: 90 rpm with the rotary table; 200 rpm with a motor
Gauge row: 20 inserts

Solution

The ratio $\omega/\Omega = 1/\sin\beta$ : at 33°, 1.836; at 36°, 1.701.
At 90 rpm the cones rotate relative to the body at $90\cdot1.836 = 165$ and $90\cdot1.701 = 153$ rpm. With a motor at 200 rpm, they rotate at $200\cdot1.836 = 367$ rpm (angle 33°).
A gauge row of 20 inserts at 165 rpm strikes the bottom $165\cdot 20/60 = 55$ times a second.

Answer: The cones and their bearings rotate 1.7–1.8 times faster than the bit: at 90 rpm, about 165 rpm, and with a motor, about 367 rpm.

What it means. This is the ideal case of pure rolling. A real cone, with offset and several cone angles, turns at a slightly different speed, but the size is about the same. The bearings run at almost twice the rotary speed of the bit, and they take a shock with every strike of an insert. That is why their life limits the run of a roller-cone bit at high rotary speeds. The data in this example are assumed for illustration.

Why heat is dangerous for PDC cutters

A cutter that slides over rock is heated by friction. This is worse when the cutter already has a wear flat, a flat spot worn on its edge. Here is a rough estimate. A worn cutter at the gauge is pressed against the rock with a force of 2 kN. The friction coefficient is 0.3 and the speed is 1.7 m/s. The cutter then releases $2000\cdot0.3\cdot1.7 \approx 1$ kW of heat on a flat of a few square millimetres (the numbers are assumed). The mud carries heat away, but the edge of the cutter becomes much hotter than the bottom of the hole.

The weak point is the cobalt left between the diamond crystals after sintering. When heated, it expands several times more than diamond. The coefficients of linear expansion at room temperature are about $13\cdot10^{-6}$ and $1\cdot10^{-6}$ per degree. On heating the difference gets smaller, but it stays several-fold. Above about 750 °C the bonds between the crystals break, and the cobalt also speeds up the change of diamond back into graphite. The cutter cracks and wears away quickly. For this reason the cobalt is leached out (dissolved with acid) from the working layer at the edge. These are called leached cutters. Such a layer withstands more heat, while the rest of the diamond and the substrate keep their strength. Diamond with no cobalt at all is even more heat-resistant but weaker. These are the TSP elements.

The practical conclusion is the same as in the example of the lesson. In hard abrasive rocks, high rotary speeds heat the PDC cutters on the shoulder and gauge, and the bit “burns out” before it wears away mechanically. There we reduce the rotary speed, use more heat-resistant cutters, or switch to impregnated and roller-cone bits.

Connections to other topics

Bit hydraulics optimisation — the flow rate and nozzles of the bit, jet power and jet impact force.
Circulating pressure losses — the pressure drop across the bit nozzles and the standpipe pressure.
Deflection tools and downhole motors — downhole motors, turbodrills and rotary steerable systems, and how a bit turns the hole.
Drill string design — how the string creates weight on the bit and where the neutral point lies.
Drill string: pipe, tool joints, drill collars and heavy-weight drill pipe — the bit sub, the stabilisers and the whole BHA above the bit.
Bit selection and dull grading — bit classification codes, choosing a bit for the rocks to be drilled, and describing the wear of a bit pulled from the hole.
Bit performance and cost per metre — rate of penetration, the effect of weight and rotary speed, cost per metre and the moment to pull the bit.
Abnormal formation pressures: origin, prediction and recognition — the d-exponent, a number that shows how easily the rock drills and helps to spot abnormal pressure, and why we draw its trend again after the type of bit changes.
Core analysis — core bits and coring.

Check yourself

1
How does a PDC bit mainly break rock?
1. A.By grinding with tiny diamond grains
2. B.By indentation and chipping
3. C.By erosion from the jets of the nozzles
4. D.By shearing a layer of rock with the edge of a cutter
Show the answer
Correct answer: D. By shearing a layer of rock with the edge of a cutter
A PDC cutter moves along the bottom of the hole and shears a layer of a set thickness, like the cutting tool of a lathe. Roller-cone bits work by indentation, and impregnated bits by grinding. The jets help to clean the bottom of the hole, but it is the bit that breaks the rock.
2
A 215.9 mm PDC bit drills 12 m/h at 120 rpm. There are two cutters on each track on the shoulder. What layer does a cutter on the shoulder remove per revolution? What changes if the rotary speed is raised to 240 rpm at the same rate of penetration?
Show the answer
The penetration per revolution is $\delta = 1000\cdot12/(60\cdot120) \approx 1.67$ mm. A cutter on the shoulder removes half of this, about 0.83 mm. At 240 rpm the penetration per revolution is half as much, 0.83 mm, and the depth of cut of a cutter on the shoulder is about 0.42 mm. The load on a cutter is smaller, but the sliding speed and the path of the cutter over the rock are twice as great. So in abrasive rocks the heating and the wear increase.
3
Why are the cone axes of roller-cone bits for hard rocks hardly offset from the bit axis, while in bits for soft rocks the offset is large?
1. A.In hard rock slipping only wears the inserts, and indentation does the work; in soft rock scraping speeds up drilling
2. B.Offset makes the hole wider, which is not needed in hard rocks
3. C.Without offset the bit pushes the hole off course less, which matters more in hard rocks
4. D.Offset reduces the load on the bearings, and in hard rocks the load is small
Show the answer
Correct answer: A. In hard rock slipping only wears the inserts, and indentation does the work; in soft rock scraping speeds up drilling
Offset moves the apex of the cone off the bit axis, and the cone slips. The teeth then do not only indent but also scrape the rock. In soft rock this speeds up drilling. Hard rock is hardly touched by scraping: it breaks by indentation and chipping, and slipping only wears the inserts. So bits for hard rocks have an offset of about a millimetre or none.
4
An interval of very hard abrasive quartzites is drilled with a turbodrill at high rotary speeds. Which bit suits best?
1. A.Impregnated
2. B.Roller-cone with long milled teeth
3. C.PDC with three blades and large cutters
4. D.Drag bit
Show the answer
Correct answer: A. Impregnated
An impregnated bit grinds the rock with many small diamonds, works at high rotary speeds and exposes new grains by itself as the matrix wears. Milled teeth would wear away at once in quartzite. Large PDC cutters would overheat and chip at high rotary speeds.
5
Why do the cutters on the shoulder and gauge of a PDC bit wear faster than those at the centre?
Show the answer
The sliding speed and the path of a cutter over the rock are proportional to the radius. At the gauge of a 215.9 mm bit at 150 rpm a cutter travels about 6 km an hour, and near the centre several times less. The cutters at the gauge also cut the corner of the bottom and the wall. So more cutters per track are placed on the shoulder, the back rake is increased, and wear-resistant elements are used at the gauge.
6
In a bed of sticky clay a five-blade PDC bit was replaced by a bit of the same diameter with seven blades and closely spaced cutters. The rate of penetration fell sharply, and the bit pulled out of the hole is covered with clay. What happened and what can be done?
Show the answer
The bit balled up. A closely set bit has a smaller junk slot area, so the clay cuttings could not leave the bottom of the hole. They stuck to the blades, and the cutters stopped touching the rock. What helps is a bit with fewer blades and larger slots, nozzles placed so that they clean the blades, enough flow rate, and, in active clays, an inhibited mud or an oil-based mud.

Updated: 19 October 2026