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Provisory - 06 Dec 96 Confidential Directional Drilling

Directional Drilling Training Manual

Section 8 - Drilling Bits

Document Type UOP Template (Word 6 PC)

Software Microsoft Word 6.0 for Windows NT

Source File DDTM_08.DOC

Other Source File TM.DOT

Author Mike Smith

Author info Anadrill Technique

200 Gillingham Lane

Sugar Land TX 77478-3136

Tel: + 1 281 285 8859

Fax: + 1 281 285 8290/4155

email: [email protected]

Review & approval

Revision History 04 Dec 96 2nd Revision

06-Dec-96 Final review and approval MJS

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Table of Contents

Provisory - 06 Dec 96 Confidential Directional Drilling 8-i

8 Drilling Bits Page

8.1 BITS ....................................................................................................................................8-1

8.1.1 Bit Selection .............................................................................................................8-2

8.1.2 IADC bit grading system..........................................................................................8-5

8.1.2.1 Dulling characteristics.....................................................................................8-8

List of Figures Page

Figure 8-1 Fixed cutter bit components (PDC, TSP, & natural diamonds) ............................. 8-1

Figure 8-2 Roller Cone Bit Components.................................................................................. 8-2

Figure 8-3 Roller cone (a) and PDC (b) bits. ........................................................................... 8-3

Figure 8-4 TSP diamond (a) and natural diamond (b) bits. ..................................................... 8-3

Figure 8-5 IADC dull bit grading system................................................................................. 8-6

Figure 8-6 Two thirds rule and how to measure gauge............................................................ 8-7

Figure 8-7 Broken cone............................................................................................................ 8-8

Figure 8-8 Bond failure ............................................................................................................ 8-9

Figure 8-9 Broken teeth............................................................................................................ 8-9

Figure 8-10 Balled up bit ....................................................................................................... 8-10

Figure 8-11 Cracked cone ...................................................................................................... 8-10

Figure 8-12 Cone dragged...................................................................................................... 8-11

Figure 8-13 Cone interference................................................................................................ 8-11

Figure 8-14 Cored bit ............................................................................................................. 8-12

Figure 8-15 Chipped teeth/cutters.......................................................................................... 8-12

Figure 8-16 Erosion................................................................................................................ 8-13

Figure 8-17 Flat crested wear................................................................................................. 8-13

Figure 8-18 Heat checking ..................................................................................................... 8-14

Figure 8-19 Junk damage ....................................................................................................... 8-14

Figure 8-20 Lost cone ............................................................................................................ 8-15

Figure 8-21 Lost nozzle.......................................................................................................... 8-15

Figure 8-22 Lost teeth/cutters ................................................................................................ 8-16

Figure 8-23 Off center wear ................................................................................................... 8-16

Figure 8-24 Pinched bit .......................................................................................................... 8-17

Figure 8-25 Plugged nozzle.................................................................................................... 8-17

Figure 8-26 Rounded gage ..................................................................................................... 8-18

Figure 8-27 Shirttail damage.................................................................................................. 8-19

Figure 8-28 Self sharpening wear .......................................................................................... 8-19Figure 8-29 Tracking.............................................................................................................. 8-19

Figure 8-30 Washed out bit.................................................................................................... 8-20

Figure 8-31 Worn teeth or cutters .......................................................................................... 8-20

List of Tables Page

No list of tables.

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Provisory - 06 Dec 96 Confidential Directional Drilling 8-1

8 Drilling Bits

About this chapter

The first thing in any drilling assembly is the bit. This chapter will help the DD gain theknowledge necessary to make intelligent recommendations regarding bit selection. The

move toward integrated steerable systems makes it imperative that the DD has some

input in the bit selection process because if the steerable BHA is to perform as expected

the bit must not only achieve an acceptable penetration rate, but must also last for the

desired footage while allowing the DD to directionally control the hole.

After the bit is run the driller and the tool pusher and the DD usually grade the dull bit.

This makes it easier to evaluate the bits performance and is a valuable tool in making the

next bit selection. The second part of this chapter is dedicated to dull bit grading.

Objectives of this Chapter

On completing this chapter the directional driller should be able to do the following

exercise:

1. Name the basic parts of a tricone, diamond, TSP, and PDC Bit.

2. Explain the criteria for bit selection.

3. Inspect a dull bit and fill out a dull grading form.

4. Use the information from offset bit records.

8.1 Bits

In drilling operations the drill bit is the first thing to go in hole. A basic understanding of

the different parts of a drill bit, general guidelines to bit selection, and specific guidelines

to bit dull grading are a major part of the directional drillers knowledge.

Figure 8-1 Fixed cutter bit components (PDC, TSP, & natural diamonds)

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Figure 8-2 Roller Cone Bit Components

8.1.1 Bit Selection

The cutting action of the various types must be the first consideration when selecting a

bit. Each type of bit "makes" hole in a different manner.

The Roller Cone Bit crushes, gouges and deforms the rock (Figure 8-3). The drilling

efficiency is most effected by WOB. Roller Cone Bits have moving parts which must

function at the desired rotary speed.

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A PDC Bit (Polycrystalline Diamond Compact) removes formation from the rock face

by shearing the rock in a similar manner to the way a machinists tool removes metal

from a part being turned in a lathe (Figure 8-3b).

Figure 8-3 Roller cone (a) and PDC (b) bits

A TSP Bit (Thermally Stable PDC ) has a similar cutting action to the PDC but the TSP

is more tolerant to heat so will cut much harder rock, but the cutting element itself is

much smaller than a PDC which results in smaller cuttings being made which results in aslower penetration rate (Figure 8-4a).

Natural Diamond Bits will drill the hardest formations. The cutting action is the same

as for the PDC and TSP Bits but the size of the diamonds dictate that very small amounts

of rock are removed by each diamond (Figure 8-4b). A good analogy for the effect of

cutter size to penetration rate would be to think of various grits of sandpaper and how

each one removes some wood with each rub but the courser (largest cutters) sandpaper

removes the most wood with each pass similar to how the different bits remove different

amounts of rock with each revolution. PDC, TSP and Natural Diamond Bits drill more

efficiently with less WOB than a Roller Cone Bit but are more sensitive to the rotary

speed. Having no moving parts, the fixed cutter type bits can safely operate at high rotary

speeds for extended periods of time.

Figure 8-4 TSP diamond (a) and natural diamond (b) bits

If a bit is to be run on a downhole motor, the type or absence of bearings should be

considered. In hole sizes 12-1/4" and smaller, bits with sealed friction bearings or fixed

cutter type bits should be run on downhole motors. The usually higher than normal rotary

speeds (the surface rotary + the speed of the downhole motor) encountered on downhole

motor runs can lead to premature bearing failure and in some cases parts of the bit can be

lost in the hole.

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In hole sizes larger than 12-1/4" the bearing surface area is usually large enough to

prevent damage from the excess rotary speed. Downhole motor runs usually mean that

the boreholes path is being deflected which causes more stress to be shifted from the

face of the bit to the gage area.

For downhole motor runs the profile of the bit will greatly effect the ability of the

deflecting tool to move the well path sideways. The effective gage length of a RollerCone Bit is short which will allow it to easily be steered" to the side.

Fixed Cutter Bits come in a multitude of shapes, but the single biggest influence on

"steerability" is the gage length. The longer the gage section, the better the bit will drill

straight ahead. Hence, if we want to steer our hole to a different direction, we should

choose a bit with a shorter gage section.

Special care should be taken in selecting a drill bit for a downhole motor run that will

address:

1. Appropriate cutting structure for the formation.

2. Bearings (or lack thereof) to handle the operating speed.

3. Gage protection.4. Bit Profile

The best indicator of how a bit will drill in a given location is from bit records of past

performance in close offset wells. In order to do this one should become familiar with

the three-digit IADC code used to identify the various types of Roller Cone Bits so that

the examination of bit records will yield information pertinent to bit type and not bit

manufacturer.

The code has two parts:

The first two digits designate the formation hardness and the type of cutting

structure (milled tooth or tungsten carbide insert).

The third digit shows unique characteristics, i.e., bearing type.

The first digit indicates formation hardness and is called the formation hardness series:

1 thru 3 Milled Tooth Types

1 Soft Formations

2 Medium Formations

3 Hard Formations4 thru 8 Insert Types

4 Very Soft

5 Soft

6 Medium

7 Hard

8 Very Hard

The second digit is called type and represents a further classification of the formation

hardness designation by the first digit.

1 Softest in its group

2 Soft in its group

3 Medium in its group

4 Hardest in its group

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The third digit is called the feature classification.

1 thru 5 Anti-friction roller bearing bits

1 Standard, non-sealed

2 Air-lubricated bearing for air circulation drilling

3 Standard non-sealed w/cutters/reinforced gage

4 Sealed roller bearing

5 Sealed roller bearing w/cutters/reinforced gage6 and 7 Sealed friction (journal) bearing

6 Sealed bearing with standard gage

7 Sealed bearing with Insert reinforced gage

8 and 9 Reserved for future use

8 Directional

9 Special application

Using this convention it is now possible to gather bit records from other wells drilled in

the area and determine which types of bits (not specific brands ) were used to drill the

various formations. By using the bit records, one can determine what was successful and

what was not. By following this convention one also learns much about how the present

well is progressing and can use this information as part of his comprehensive bit

selection criteria.

8.1.2 IADC bit grading system

The IADC Dull Grading System(Figure 8-5) can be applied to all types of roller cone

bits as well as all types of fixed cutter bits. Bits with steel teeth, tungsten carbide inserts,

natural or synthetic diamond cutters can be described with this system. A description of

the dull grading system follows with each of the components explained as they apply to

roller cone and fixed cutter bits.

1. Column 1 (I-Inner) is used to report the condition of the cutting elements not

touching the wall of the hole (Inner). The change from inner 2/3 of the cutting

structure was made to reduce variations in grading and increase under-standing of

the system.

2. Column 2 (O-Outer) is used to report the condition of the cutting elements that

touch the wall of the hole (Outer). In the previous version, this was the outer 1/3 of

the cutting structure. This change reflects the importance of gage and heel condition

to good bit performance.

In columns 1 and 2, a linear scale from 0-8 is used to describe the condition of the

cutting structure as follows:

A measure of combined cutting structure reduction due to lost, worn and/or broken

inserts/teeth/cutters.

0 - No loss of cutting structure.

8 - Total loss of cutting structure.

Example: A bit missing half of the inserts on the inner rows of the bit due to loss or

breakage with the remaining teeth on the inner rows having a 50% reduction in

height due to wear, should be graded a 6 in column 1. If the inserts on the outer rows

of the bit were all intact but were reduced by wear to half of their original height, the

proper grade for column 2 would be 4.

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IADC DULL BIT GRADING SYSTEM

CUTTING STRUCTURE

INNER OUTER DULL CHAR. LOCATION

I O D L B G O R

BEARINGS/

SEALSGAGE

OTHER

DULL

CHAR.

REASON

PULLED

I INNER CUTTING STRUCTURE (All Inner rows)

O OUTER CUTTING STRUCTURE (Gage row only)

In columns 1 and 2 a linear scale from 0 to 8 isused to describe the condition of the cutting

structure according to the following:

STEEL TOOTH BITS

A measure of lost tooth height due to abrasionand / or damage.

0

8

NO LOSS OF TOOTH HEIGHT

TOTAL LOSS OF TOOTH HEIGHT

INSERT BITS

A measure of total cutting structure reduction due

to lost, worn and / or broken inserts.

0

8

NO LOST, WORN AND / OR BROKEN

INSERTS

ALL INSERTS LOST, WORN AND / OR

BROKEN

FIXED CUTTER BITS

A measure of lost, worn and / or broken cuttingstructure.

0

8

NO LOST, WORN AND / OR BROKEN

CUTTING STRUCTURE

ALL OF CUTTING STRUCTURE LOST, WORN

AND / OR BROKEN

D DULL CHARACTERISTICS(Use only cutting structure related codes)

BC

BFBT

BU

CC

CD

CI

CR

CT

ER

FC

HC

JD

LC

LN

Show cone # or #' under location 4.

Broken Cone

Bond FailureBroken Teeth / Cutters

Balled Up Bit

Cracked Cone

Cone Dragged

Cone Interference

Cored

Chipped Teeth / Cutters

Erosion

Flat Crested Wear

Heat Checking

Junk Damage

Lost Cone

Lost Nozzle

*

*

*

*

*

LT

OCPB

PN

RG

RO

SD

SS

TR

WO

WT

NO

Lost Teeth / Cutters

Off-Center WearPinched Bit

Plugged Nozzle /

Flow Passage

Rounded Gage

Ring Out

Shirttail Damage

Self-Sharpening

Wear

Tracking

Washed Out Bit

Worn Teeth / Cutters

No Dull

Characteristics

L LOCATION

ROLLER CONE

NM

GA

Nose RowMiddle Row

Gage RowAll Rows

CONE #1

23

FIXED CUTTER

CN

TS

GA

ConeNose

TaperShoulder

GageAll Areas

B BEARING SEALS

NON-SEALED BEARINGS

A linear scale estimating

bearing life used. ( 0 - Nolife used, 8 - All life used,i.e. no bearing life

remaining.)

E

FNX

seals effective

seals failednot able to gradefixed cutter bit

FIXED CUTTER

G GAGE

I

1/162/16

4/16

in gage

1/16" out of gage1/8" out of gage

1/4" out of gage

O OTHER DULL CHARACTERISTICS

Refer to Column 3 codes

R REASON BEING PULLED OR RUN TERMINATED

BHA

CM

CP

DMF

DP

DSF

DST

DTF

FM

HP

HR

Change Bottom Hole

Assembly

Condition Mud

Core Point

Downhole Motor Failure

Drill Plug

Drill String Failure

Drill Stem Testing

Downhole Tool Failure

Formation Change

Hole Problems

Hours on Bit

LIH

LOG

PP

PR

RIG

TD

TQ

TW

WC

Left in Hole

Run Logs

Pump Pressure

Penetration Rate

Rig Repair

Total Depth /

Casing Depth

Torque

Twist Off

Weather Conditions

Figure 8-5 IADC dull bit grading system

3. Column 3 (D-Dull Characteristic - Cutting Structure) uses a two-letter code to

indicate the major dull characteristic of the cutting structure. Figure 8-5 lists the

two-letter codes for the dull characteristics to be used in this column.

4. Column 4 (L-Location) uses a letter or number code to indicate the location on the

face of the bit where the cutting structure dulling characteristic occurs. Figure 8-5

lists the codes to be used for describing locations on bits.

Note

G (gauge area) replaces H for this version.

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Location is defined as follows:

Gauge- Those cutting elements which touch the hole wall.

Nose - The centermost cutting element(s) of the bit.

Middle- Cutting elements between the nose and the gage.

All- All Rows

Cone numbers are identified as follows:

The No. 1 cone contains the centermost cutting element.

Cones No. 2 and No. 3 follow in a clockwise orientation as viewed looking down at

the cutting structure with the bits sitting on the pin.

5. Column 5 (B-Bearing-Seals) uses a letter or a number code, depending on bearing

types, to indicate bearing condition of roller cone bits. For non- sealed bearing roller

cone bits, a linear scale from 0-8 is used to indicate the amount of bearing life that

has been used. A zero (0) indicates that no bearing life has been used (a new bearing)

and an 8 indicates that all of the bearing life has been used (locked or lost). For

sealed bearing journal or roller) bits, a letter code is used to indicate the condition ofthe seal. An E indicates an effective seal, an "F" indicates a failed seal(s), and an

N indicating "not able to grade" has been added to allow reporting when

seal/bearing condition cannot be determined.

6. Column 6 (G-Gage) is used to report on the gage of the bit. The letter I (IN)

indicates no gage reduction. If the bit does have a reduction in gage it is to be

recorded in increments of 1/16". The Two Thirds Rule" is correct for three -cone

bits.

Note

The Two Thirds Rule, as used for three cone bits, requires that the gage ring be pulled so

that it contacts two of the cones at their outermost points.

Figure 8-6 Two thirds rule and how to measure gauge

Then the distance between the outermost point of the third cone and the gage ring is

multiplied by 2/3 and rounded to the nearest 1/16th of an inch to give the correct

diameter reduction (Figure 8-6).

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7. Column 7 (O-Other Dull Characteristics) is used to report any dulling

characteristic of the bit, in addition to the cutting structure dulling characteristic

listed in column 3 (D). Note that this column is not restricted to cutting structure

dulling characteristics. Figure 8-5 lists the two-letter codes to be used in this column.

8. Column 8 (A-Reason Pulled) is used to report the reason for terminating the bit run.

Figure 8-5 lists the two-letter and three-letter codes used in this column.

8.1.2.1 Dulling characteristics

Following is a discussion, with photographs where possible, of the dulling characteristics

common to roller cone and fixed cutter bits. While the possible causes listed and possible

solutions for problem wear modes are not presumed to be exclusive, they do represent

situations commonly encountered in the field.

BC (Broken Cone) - This describes a bit with one or more cones that have been broken

into two or more pieces, but with most of the cone still attached to the bit (see Figure 8-

7). Broken cones can be caused in several ways. Some of the causes of BC are:

Cone interference - where the cones run on each other after a bearing failure and

break one or more of the cones.

Bit hitting a ledge on a trip or connection.

Dropped drill string.

Hydrogen sulfide embrittlement.

Figure 8-7 Broken cone

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BF (Bond Failure) - The cutter has come completely off the tungsten carbide substrate.

This is abnormal and usually indicates that the cutters were poorly bonded during

manufacture (Figure 8-8)

Figure 8-8 Bond failure

BT(Broken Teeth)- In some formations, BT is a normal wear characteristic of tungsten

carbide insert bits and is not necessarily an indicator of any problems in bit selection or

operating practices (Figure 8-9) . However, if the bit run was of uncommonly short

duration, broken teeth could indicate one or more of the following: the need for a shock

sub, too much WOB and/or RPM, or improper bit application. Broken teeth is not

considered a normal wear mode for steel tooth roller cone bits. It may indicate improper

bit application or operating practices. Some causes of BT are:

Bit run on junk.

Bit hitting a ledge or hitting bottom suddenly.

Excessive WOB for application. Indicated by broken teeth predominantly on theinner and middle row teeth.

Improper break-in or when a major change in bottomhole pattern is made.

Formation too hard for bit type

Figure 8-9 Broken teeth

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BU(Balled-Up) - A balled-up bit will show tooth wear due to skidding, caused by a cone,

or cones, not turning due to formation being packed between the cones (Figure 8-10) .

The bit will look as if a bearing had locked up even though the bearings are still good.

Some causes of balling up are:

Inadequate hydraulic cleaning of the bottomhole.

Forcing the bit into formation cuttings with the pump not running.

Drilling a sticky formation.

Figure 8-10 Balled up bit

CC (Cracked Cone) - A cracked cone is the start of a broken or lost cone and has many

of the same possible causes (Figure 8-11).

Figure 8-11 Cracked cone

Some of these causes are:

Junk on the bottom of the hole.

Bit hitting a ledge or bottom.



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Overheating of the bit.

Reduced cone shell thickness due to erosion.

Cone interference.

CD (Cone Dragged) - This dull characteristic indicates that one or more of the cones did

not turn during part of the bit run, indicated by one or more flat wear spots (Figure 8-12).Some of the possible causes are:

Bearing failure on one or more of the cones.

Junk lodging between the cones.

Pinched bit causing cone interference.

Bit balling up.

Inadequate break in.

Figure 8-12 Cone dragged

Cl (Cone Interference) - Cone interference often leads to cone grooving and broken

teeth and is sometimes mistaken for formation damage (Figure 8-13). Broken teeth

caused by cone interference are not an indicator of improper bit selection.

Figure 8-13 Cone interference

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Some of the causes of cone interference are:

Bit being pinched.

Reaming under gage hole with excessive WOB.

Bearing failure on one or more cones.

CR (Cored) - A bit is cored when its centermost cutters are worn and/or broken off(Figure 8-14). A bit can also be cored when the nose part of one or more cones is broken.

Some things that can cause bits to become cored are:

Abrasiveness of formation exceeds the wear resistance of the center cutters.

Improper breaking in of a new bit when there is a major change in bottomhole

pattern.

Cone shell erosion resulting in lost cutters.

Junk in the hole causing breakage of the center cutters.

Figure 8-14 Cored bit

CT (Chipped Teeth/Cutters) - On tungsten carbide insert bits, chipped inserts often

become broken teeth. A tooth is considered chipped, as opposed to broken, if a

substantial part of the tooth remains above the cone shell (Figure 8-15).

Figure 8-15 Chipped teeth/cutters

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Part of the diamond layer and/or part of the tungsten carbide substrate has fragmented

and left a sharply irregular cutter. Some causes of chipped teeth/cutters are:

Impact loading due to rough drilling.

Slight cone interference.

Rough running in air drilling application.ER (Erosion) - Fluid erosion leads to cutter reduction and/or loss of cone shell material.

The loss of cone shell material on tungsten carbide insert bits can lead to a loss of inserts

due to reduced support and grip of the cone shell material (Figure 8-16). Erosion can be

caused by:

Abrasive formation contacting the cone shell between the cutters, caused by

tracking, off-center wear, or excessive WOB.

Abrasive formation cuttings eroding the cone shell due to inadequate hydraulics.

Excessive hydraulics resulting in high velocity fluid erosion.

Abrasive drilling fluids or poor solids control.

Figure 8-16 Erosion

FC(Flat Crested Wear)-Flat crested wear is an even reduction in height across the entire

face of the cutters (Figure 8-17). Interpretation of the significance of flat crested wear are

numerous, and operating factors include formation, hardfacing and operating parameters.

Figure 8-17 Flat crested wear

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One of the causes of flat crested wear is:

Low WOB and high RPM, often used in attempting to control deviation.

HC(Heat Checking) - This dulling characteristic happens when a cutter is overheated

due to dragging on the formation and is then cooled by the drilling fluid over many

cycles (Figure 8-18). Some situations that can cause heat checking are:

Cutters being dragged.

Reaming a slightly under-gauge hole at high RPM.

Figure 8-18 Heat checking

JD (Junk Damage) - Junk damage can be detected by marks on any part of the bit. Junk

damage can lead to broken teeth, damaged shirttail, and shortened bit runs and therefore

can become a problem (Figure 8-19). It is necessary to clear the junk out of the hole

before continuing to drill. Some common sources of junk, and therefore causes of junk

damage are: Junk dropped in the hole from the surface (tong dies, tools, etc.).

Junk from the drill string (reamer pins, stabilizer blades, etc.).

Junk from a previous bit run (tungsten carbide inserts, ball bearings, etc.).

Junk from the bit itself (tungsten carbide inserts, etc.).

Figure 8-19 Junk damage

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LC(Lost Cone) - It is possible to lose one or more cones in many ways (Figure 8-20).

With few exceptions, the lost cone must be cleared from the hole before drilling can

resume. Some of the causes of lost cones are:

Bit hitting bottom or a ledge on a trip or connection.


Bearing failure (causing the cone retention system to fail).


Figure 8-20 Lost cone

LN (Lost Nozzle) - While LN is not a cutting structure dulling characteristic, it is an

important "Other Dulling Characteristic" that can help describe a bit condition(Figure 8-

21). A lost nozzle causes a pressure decrease which requires that the bit be pulled out of

the hole. A lost nozzle is also a source of junk in the hole. Some causes of lost nozzles

are: Improper nozzle installation.

Improper nozzle and/or nozzle design.

Mechanical or erosion damage to nozzle and/or nozzle retaining system.

Figure 8-21 Lost nozzle

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LT (Lost Teeth/Cutters) - This dulling characteristic leaves entire tungsten carbide

inserts or PDC Cutters in the hole which are far more detrimental to the rest of the bit

than are broken insert (Figure 8-22). Lost teeth often cause junk damage. Lost teeth are

sometimes preceded by rotated inserts. Lost teeth can be caused by:

Cone shell erosion.

A crack in the cone/crown that loosens the grip on the insert/cutters.

Hydrogen sulfide embrittlement cracks.

Figure 8-22 Lost teeth/cutters

OC (Off-Center Wear) - This dulling characteristic occurs when the geometric center of

the bit and the geometric center of the hole do not coincide (Figure 8-23). This results in

an oversize hole. Off center wear can be recognized on the dull bit by wear on the cone

shells between the rows of cutters, more gage wear on one cone, and by a less than

expected penetration rate. This can often be eliminated by changing bit types and thus

changing the bottomhole pattern.

Figure 8-23 Off center wear

Off center wear can be caused by:

Change of formation from a brittle to a more plastic formation.

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Inadequate stabilization in a deviated hole.

Inadequate WOB for formation and bit type.

Hydrostatic pressure that significantly exceeds the formation pressure.

PB (Pinched Bit) - Bits become pinched when they are mechanically forced to a less

than original gage (Figure 8-24). Pinched bits can lead to broken teeth, chipped teeth,cone interference, dragged cones and many other cutting structure dulling characteristics.

Some possible causes of pinched bits are:

Bit being forced into under-gauge hole.

Roller cone bit being forced into a section of hole drilled by fixed cutter bits, due

to different OD tolerances.

Forcing a bit through casing that does not drilling to the bit size used.

Bit being pinched in the bit breaker.

Bit being forced into an undersized blowout preventer stack.

Figure 8-24 Pinched bit

PN (Plugged Nozzle) - This dulling characteristic does not describe the cutting structure

but can be useful in providing information about a bit run (Figure 8-25). A plugged

nozzle can lead to reduced hydraulics or force a trip out of the hole due to excessive

pump pressure.

Figure 8-25 Plugged nozzle

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Plugged nozzles can be caused by:

Jamming the bit into fill with the pump off.

Solid material going up the drill string through the bit on a connection and

becoming lodged in a nozzle when circulation is resumed.

Solid material pumped down the drill string and becoming lodged in a nozzle.RG (Rounded Gage) - This dulling characteristic describes a bit that has experienced

gauge wear in a rounded manner, but will still drill a full size hole (Figure 8-26). The

gage inserts may be less than nominal bit diameter but the cone backfaces are still at

nominal diameter. Rounded Gage can be caused by:

Drilling an abrasive formation with excessive RPM.

Reaming an under gage hole.

Figure 8-26 Rounded gauge

RO (Ring Out) - This dull characteristic describes a bit that has lost all of its cuttingstructure in a ring around the face of the bit. A groove will actually be cut into the body

of the bit by the formation. Excessive pump pressure while on bottom with a decrease in

pressure back to the expected value upon pulling off bottom is a good downhole

indicator of a ringed bit. A ringed bit can be caused by junk in the hole and regardless of

the cause may leave junk in the hole. Care should be taken on subsequent runs. Some

possible causes of ringed bits are:

Junk in the hole.

Chert and or pyrite.

SD (Shirttail Damage) - Shirttail damage may be different than junk damage and is not a

cutting structure dulling characteristic (Figure 8-27). Shirttail wear can lead to seal

failures. Some causes of shirttail damage are:

Junk in the hole.

Reaming under-gauge hole in faulted or broken formations.

A pinched bit causing the shirttails to be the outer part of the bit.

Poor hydraulics.

High angle well bore.

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Figure 8-27 Shirttail damage

SS (Self-Sharpening Wear) - This is a dulling characteristic which occurs when cutters

wear in a manner such that they retain a sharp crest shape (Figure 8-28).

Figure 8-28 Self sharpening wear

TR (Tracking) - This dulling characteristic occurs when the teeth mesh like a gear into

the bottomhole pattern (Figure 8-29). The cutter wear on a bit that has been tracking will

be on the leading and trailing flanks.

Figure 8-29 Tracking

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The cone shell wear will be between the cutters in a row. Tracking can sometimes be

alleviated by using a softer bit to drill the formation and/or by reducing the hydrostatic

pressure if possible. Tracking can be caused by:

Formation changes from brittle to plastic.

Hydrostatic pressure that significantly exceeds the formation pressure.

WO (Washed Out Bit) - Bit washouts are not cutting structure dulling characteristics but

can provide important information when used as an "Other dulling characteristic

(Figure 8-30). This can occur at anytime during the bit run. If the bit weld is porous or

not closed, then the bit will start to washout as soon as circulation starts. Often the welds

are closed but crack during the bit run due to impact with bottom or ledges on

connections. When a crack occurs and circulation starts through the crack, the washout is

established very quickly.

Figure 8-30 Washed out bit

WT (Worn Teeth/Cutters) This is a normal dulling characteristic of the tungsten carbideinsert bits and steel tooth bits as well as for the fixed cutter bits(Figure 8-31). When WT

is noted for steel tooth bits, it is also often appropriate to note self sharpening (SS) or flat

crested (FC) wear.

Figure 8-31 Worn teeth or cutters

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NO (No Dull Characteristics) Thiscode is used to indicate that the dull shows no sign

of the outer dulling characteristics described. This is often used when a bit is pulled after

a short run for a reason not related to the bit, such as a drill string washout. Next we will

grade a dull roller cone bit, and discuss some possible interpretations of the wear as it

relates to bit selection and application. It should be noted that there may be more than

one "correct" dull grading for any bit. This can happen if two persons should disagree on

the primary cutting structure dulling characteristic or on what the other dulling

characteristic should be. Regardless, the new IADC dull grading system provides the

man on the rig with ample opportunity to report what he sees when examining a dull.

By using the information available from offset bit records and from examining the dull

bits on your location, you should be able to make sound recommendations as to the best

bit selections.

Documents

DDTM_08