PETE 203

DRILLING ENGINEERING

Chapter 5Chapter 5

Drilling BitsDrilling Bits

Contents:

• Various bit types available (classification).

• Criteria for selecting the best bit for a given situation.

• Standard methods for evaluating dull bite

• Factors affecting bit wear and drilling speed.

• Different Rock Failure mechanisms

• Optimization of bit weight and rotary speed

• Relative Costs of Bits

Ideal Bit (Lowest Cost $/ft)

• High drilling rate

• Long life

• Drill full-gauge, straight hole

• Moderate cost

Selecting Ideal Bit

• Drilling Cost per foot

• Drillability

• Abrasiveness

• Type of Reservoir

• Cost of the Bit

• Depth

Improving Bit Performance

• Stabilize the Bit

• Maintain minimum mud weight, sand & solids

• Maintain adequate bottom hole cleaning

• Protect the seals - avoid pressure surges

• Thoroughly inspect bit before re-running

• Keep oil from the mud, and from the seals

• Follow manufacturers recommendations (e.g.

6,000 lb/in of diameter and 40-60 RPM)

Common Types of Drilling Bits

Types of Bits1. Drag Bits: Fixed cutter blades

Consist of fixed cutter blades that are

integral with the body of the bit and rotate

as a unit with the drill string (19th century).

2. Rolling Cutter Bits: Rock bits - with cones

(1909) have two or more cones containing

the cutting elements which rotate about

the axis of the cone as the bit is rotated at

the bottom of the hole.

Drag Bits

• Design Features

– Number & shape of cutting blades/stones

– Size and location of the water courses

– Metallurgy of the bit and cutting elements

Drag Bits– Drilling is achieved by physically blowing

cuttings from the bottom of the bore-hole

AdvantagesAdvantages

– No rolling parts which require strong clean

bearing surfaces

– Because it is made from one solid piece of steel

there is less chance of bit breakage, which

would leave junk in the bottom of the hole

Drag Bits

Fishtail Drag Bit

Drag Bits

• Cutter may be made from:

– Steel

– Tungsten carbide

– Natural diamonds

– Polycrystalline diamonds

(PDC)

Drag Bits

• Types of Drag BitsTypes of Drag Bits

– Steel cutter bits (Steel cutter bits (Fishtail Type)

– Diamond bitsDiamond bits

– Polycrystalline diamond bitsPolycrystalline diamond bits

• Best for

– Soft formations

– Uniform formations

– unconsolidated formations

• Now, replaced by other types in all

area

Steel Cutter BitsSteel Cutter Bits

When Using Natural Diamond Bit?

• Penetration rate of rock bit < 10 ft/hr

• Hole diameter < 6 inches

• When it is important to keep the bit and pipe

in the hole

• When bad weather precludes making trips

• When starting a side-tracked hole

• When coring

• When a lower cost/ft would result

– Best for hard non-brittle formations

– The face or crown of the bit consists of many

diamonds set in a tungsten carbide matrix

– Fluid courses are provided in the matrix to

direct the flow of drilling fluid over the face of

the bit.

Diamond BitsDiamond Bits

• Shape of crown profit:

– Step type

– Long taper (straight hole, high wt.)

– Short taper (easier to clean)

– Non taper (directional drilling)

Diamond BitsDiamond Bits

• Size and number of diamonds, depend on

the hardness of the formation

– Hard formations:

• Many small stones

• 0.07-0.125 carrot

– Soft formations:

• Few large stones

• 0.75-2.0 carrot

Diamond BitsDiamond Bits

• Pressure drop across the face of the bit

• Pump pressure measured with the bit off

bottom-pump pressure with the bit drilling =

500 : 1000 psi

• Manufacturer usually provide estimate of

approximate circulating rate required establishing

the needed pressure drop across the bit.

Diamond BitsDiamond Bits

Top View of Diamond Bit

Side View of Diamond Bit

Natural Diamond Bits PDC Bits

Natural Diamond

bit

junk slotcuttings

radial flow

high p across face

Soft

Formation

Diamond bit

Larger diamonds Fewer diamonds Pointed nose

Hard

Formation

Diamond bit

Smaller diamonds More diamonds Flatter nose

PolyCrystalline Diamond (PCD) Bits

• Since mid 1970’s a PDC Bits has been made

possible by the introduction of a sintered

polycrystalline diamond drill blanks, as a bit cutter

element

• The drill blanks consist of a layer of a synthetic

polycrystalline diamond about 1/64 in. thick that is

bonded to a cemented tungsten carbide substrate in

a high-pressure high-temperature (HPHT) process

PDC bits

Courtesy Smith Bits

PolyCrystalline Diamond (PCD) Bits

• It contains many small diamond crystals bonded

together

• The PCD is bonded either to a tungsten carbide

bit-body matrix or to a tungsten carbide stud that

is mounted in a steel bit body

• They perform best in soft, firm, and medium-hard,

non-abrasion formations (not gummy)

PolyCrystalline Diamond (PCD) Bits

• Good results are obtained in carbonates or

evaporates that are not broken up with hard shale

stringers.

• Also good in a sandstone, siltstone, shale

• Design of crown profile is important, double-cone

and flat profile

• Size, shape, number of cutters and angle of attack

back rake, side rake and exposure

PolyCrystalline Diamond (PCD) Bits

• Advantages:

– Increase penetration rates in oil & gas wells

– Reduce drilling time and costs

– Cost 5-15 times more than roller cone bits

– 1.5 times faster than those 2 years earlier

– Work better in oil based muds; however, these

areas are strictly regulated

At about $10,000-150,000 a piece, PDC bits cost five to 15 times more than roller cone bits

PDC Bits

Relative Costs of Bits

Diamond WC Insert Milled Bits Bits Tooth Bits

$/Bit

• Diamond bits typically cost several times as much as tri-cone bits with tungsten carbide inserts (same bit diam.)• A TCI bit may cost several times as much as a

milled tooth bit.

Grading of Worn PDC Bits

CT - Chipped CutterLess than 1/3 of cutting element is gone

BT - Broken CutterMore than 1/3 of cutting element is broken to the

substrate

Grading of Worn PDC Bits – cont’d

LT - Lost CutterBit is missing one or more cutters

LN - Lost NozzleBit is missing one or more nozzles

Table 7.7 - Commonly Used Bit Sizes For Running API Casing

Casing Size Coupling Size Common Bit (OD in.) (OD in.) Sizes Used (in.)

4 1/2 5.0 6, 6 1/8, 6 1/4

5 5.563 6 1/2, 6 3/4

5 1/2 6.050 7 7/8, 8 3/8

6 6.625 7 7/8, 8 3/8, 8 1/2

6 5/8 7.390 8 1/2, 8 5/8, 8 3/4

7 7.656 8 5/8, 8 3/4, 9 1/2

7 5/8 8.500 9 7/8, 10 5/8, 118 5/8 9.625 11, 12 1/4

9 5/8 10.625 12 1/4, 14 3/4

10 3/4 11.750 1513 3/8 14.375 17 1/2

16 17 2020 21 24, 26