08. drilling bits.pdf

8/12/2019 08. Drilling Bits.pdf

1/22

Table of Contents

January 1997 Confidential Directional Drilling 8-i

8 Drilling Bits Page8.1 B ITS ....................................................................................................................................8-1

8.1.1 Bit Selection..............................................................................................................8-28.1.2 IADC bit grading system .................. .................. .................. ................. .................. .8-58.1.3 Dulling characteristics ................ .................. .................. ................. .................. ....... 8-8

List of Figures PageFigure 8-1 Fixed cutter bit components (PDC, TSP, & natural diamonds) .................. ........... 8-1Figure 8-2 Roller Cone Bit Components..................... ................. .................. .................. ........ 8-2Figure 8-3 Roller cone (a) and PDC (b) bits. ................. ................. .................. .................. ..... 8-3Figure 8-4 TSP diamond (a) and natural diamond (b) bits. ............... .................. .................. .. 8-3Figure 8-5 IADC dull bit grading system.................... ................. .................. .................. ........ 8-6Figure 8-6 Two thirds rule and how to measure gauge.. ................. .................. .................. ..... 8-7Figure 8-7 Broken cone................. .................. .................. .................. ................. .................. .. 8-8Figure 8-8 Bond failure ................. .................. .................. .................. ................. .................. .. 8-9Figure 8-9 Broken teeth............... ................. .................. .................. ................. .................. ..... 8-9Figure 8-10 Balled up bit ....................................................................................................... 8-10Figure 8-11 Cracked cone ...................................................................................................... 8-10Figure 8-12 Cone dragged.. ................. .................. .................. ................. .................. ............ 8-11Figure 8-13 Cone interference............. .................. .................. ................. .................. ............ 8-11Figure 8-14 Cored bit .................. ................. .................. .................. ................. .................. ... 8-12Figure 8-15 Chipped teeth/cutters ................ .................. ................. .................. .................. ... 8-12Figure 8-16 Erosion............ ................. .................. .................. ................. .................. ............ 8-13Figure 8-17 Flat crested wear................. .................. .................. ................. .................. ......... 8-13Figure 8-18 Heat checking ............... .................. .................. .................. ................. ............... 8-14Figure 8-19 Junk damage ................. .................. .................. .................. ................. ............... 8-14Figure 8-20 Lost cone.................... .................. .................. .................. ................. .................. 8-15Figure 8-21 Lost nozzle............... ................. .................. .................. ................. .................. ... 8-15Figure 8-22 Lost teeth/cutters ................................................................................................ 8-16Figure 8-23 Off center wear ................ .................. .................. ................. .................. ............ 8-16Figure 8-24 Pinched bit ................. .................. .................. .................. ................. .................. 8-17Figure 8-25 Plugged nozzle.............. .................. .................. .................. ................. ............... 8-17Figure 8-26 Rounded gauge ................ .................. .................. ................. .................. ............ 8-18Figure 8-27 Shirttail damage.... ................. .................. .................. ................. .................. ...... 8-19

Figure 8-28 Self sharpening wear ................ .................. ................. .................. .................. ... 8-19Figure 8-29 Tracking.......... ................. .................. .................. ................. .................. ............ 8-19Figure 8-30 Washed out bit ................. .................. .................. ................. .................. ............ 8-20Figure 8-31 Worn teeth or cutters ................ .................. ................. .................. .................. ... 8-20

List of Tables PageNo list of tables.


2/22

Drilling Operations

January 1997 Confidential Directional Drilling 8-1

8 Drilling BitsAbout 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. Themove toward integrated steerable systems makes it imperative that the DD has someinput in the bit selection process because if the steerable BHA is to perform as expectedthe bit must not only achieve an acceptable penetration rate, but must also last for thedesired 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 thenext 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 followingexercise:

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

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


3/22

Drilling Operations


Figure 8-2 Roller Cone Bit Components

8.1.1 Bit SelectionThe cutting action of the various types must be the first consideration when selecting abit. Each type of bit "makes" hole in a different manner.

The Roller Cone Bit crushes, gouges and deforms the rock (Figure 8-3). The drillingefficiency is most effected by WOB. Roller Cone Bits have moving parts which mustfunction at the desired rotary speed.


4/22

Drilling Operations


A PDC Bit (Polycrystalline Diamond Compact) removes formation from the rock faceby shearing the rock in a similar manner to the way a machinists tool removes metalfrom 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 TSPis 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 sameas for the PDC and TSP Bits but the size of the diamonds dictate that very small amountsof 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 howeach one removes some wood with each rub but the courser (largest cutters) sandpaperremoves the most wood with each pass similar to how the different bits remove differentamounts of rock with each revolution. PDC, TSP and Natural Diamond Bits drill moreefficiently with less WOB than a Roller Cone Bit but are more sensitive to the rotaryspeed. 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 beconsidered. In hole sizes 12-1/4" and smaller, bits with sealed friction bearings or fixedcutter type bits should be run on downhole motors. The usually higher than normal rotaryspeeds (the surface rotary + the speed of the downhole motor) encountered on downholemotor runs can lead to premature bearing failure and in some cases parts of the bit can belost in the hole.


5/22

Drilling Operations


In hole sizes larger than 12-1/4" the bearing surface area is usually large enough toprevent damage from the excess rotary speed. Downhole motor runs usually mean thatthe boreholes path is being deflected which causes more stress to be shifted from theface of the bit to the gauge 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 gauge 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 gauge length. The longer the gauge section, the better the bit willdrill straight ahead. Hence, if we want to steer our hole to a different direction, weshould choose a bit with a shorter gauge section.

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

1. Appropriate cutting structure for the formation.

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

3. Gauge protection.4. Bit Profile

The best indicator of how a bit will drill in a given location is from bit records of pastperformance in close offset wells. In order to do this one should become familiar withthe three-digit IADC code used to identify the various types of Roller Cone Bits so thatthe examination of bit records will yield information pertinent to bit type and not bitmanufacturer.

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 Formations2 Medium Formations3 Hard Formations

4 thru 8 Insert Types

4 Very Soft5 Soft6 Medium7 Hard8 Very Hard

The second digit is called type and represents a further classification of the formationhardness designation by the first digit:

1 Softest in its group2 Soft in its group3 Medium in its group4 Hardest in its group


6/22

Drilling Operations


The third digit is called the feature classification:

1 thru 5 Anti-friction roller bearing bits

1 Standard, non-sealed2 Air-lubricated bearing for air circulation drilling3 Standard non-sealed w/cutters/reinforced gauge

4 Sealed roller bearing5 Sealed roller bearing w/cutters/reinforced gauge

6 and 7 Sealed friction (journal) bearing

6 Sealed bearing with standard gauge7 Sealed bearing with Insert reinforced gauge

8 and 9 Reserved for future use

8 Directional9 Special application

Using this convention it is now possible to gather bit records from other wells drilled inthe area and determine which types of bits (not specific brands ) were used to drill thevarious formations. By using the bit records, one can determine what was successful andwhat was not. By following this convention one also learns much about how the presentwell is progressing and can use this information as part of his comprehensive bitselection criteria.

8.1.2 IADC bit grading systemThe IADC Dull Grading System (Figure 8-5) can be applied to all types of roller conebits 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 toroller cone and fixed cutter bits.

1. Column 1 ( I-Inner ) is used to report the condition of the cutting elements nottouching the wall of the hole (Inner). The change from inner 2/3 of the cuttingstructure 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 thattouch 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 gauge and heel conditionto good bit performance.

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

A measure of combined cutting structure reduction due to lost, worn and/or brokeninserts/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 orbreakage with the remaining teeth on the inner rows having a 50% reduction inheight due to wear, should be graded a 6 in column 1. If the inserts on the outer rowsof the bit were all intact but were reduced by wear to half of their original height, theproper grade for column 2 would be 4.


7/22

Drilling Operations


IADC DULL BIT GRADING SYSTEM

CUTTING STRUCTUREINNER OUTER DULL CHAR. LOCATION

I O D L B G O R

BEARINGS/ SEALS

GAGEOTHER

DULLCHAR.

REASONPULLED

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 cuttingstructure according to the following:

STEEL TOOTH BITS

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

08

NO LOSS OF TOOTH HEIGHTTOTAL LOSS OF TOOTH HEIGHT

INSERT BITS

A measure of total cutting structure reduction dueto lost, worn and / or broken inserts.0

8

NO LOST, WORN AND / OR BROKENINSERTSALL INSERTS LOST, WORN AND / ORBROKEN

FIXED CUTTER BITS

A measure of lost, worn and / or broken cuttingstructure.0

8

NO LOST, WORN AND / OR BROKENCUTTING STRUCTUREALL OF CUTTING STRUCTURE LOST, WORNAND / OR BROKEN

D DULL CHARACTERISTICS (Use only cutting structure related codes)BC

BFBTBUCCCDCICRCTERFCHCJDLCLNShow cone # or #' under location 4.

Broken Cone

Bond FailureBroken Teeth / CuttersBalled Up BitCracked ConeCone DraggedCone InterferenceCoredChipped Teeth / CuttersErosionFlat Crested WearHeat CheckingJunk DamageLost ConeLost Nozzle

*

**

*

*

LT

OCPBPN

RGROSDSS

TRWOWTNO

Lost Teeth / Cutters

Off-Center WearPinched BitPlugged Nozzle / Flow PassageRounded GageRing OutShirttail DamageSelf-SharpeningWearTrackingWashed Out BitWorn Teeth / CuttersNo DullCharacteristics

L LOCATION

ROLLER CONENMGA

Nose RowMiddle RowGage RowAll Rows

CONE #123

FIXED CUTTERCNTSGA

ConeNoseTaperShoulderGageAll Areas

B BEARING SEALS

NON-SEALED BEARINGSA linear scale estimatingbearing life used. ( 0 - Nolife used, 8 - All life used,i.e. no bearing liferemaining.)

E

FNX

seals effective

seals failednot able to gradefixed cutter bit

FIXED CUTTER

G GAGE

I1/162/164/16

in gage1/16" out of gage1/8" out of gage1/4" out of gage

O OTHER DULL CHARACTERISTICS Refer to Column 3 codes

R REASON BEING PULLED OR RUN TERMINATED

BHA

CMCPDMFDPDSFDSTDTFFMHPHR

Change Bottom HoleAssemblyCondition MudCore PointDownhole Motor FailureDrill PlugDrill String FailureDrill Stem TestingDownhole Tool FailureFormation ChangeHole ProblemsHours on Bit

LIHLOGPPPRRIGTD

TQTWWC

Left in HoleRun LogsPump PressurePenetration RateRig RepairTotal Depth / Casing DepthTorqueTwist OffWeather Conditions

Figure 8-5 IADC dull bit grading system

3. Column 3 (D-Dull Characteristic - Cutting Structure) uses a two-letter code toindicate the major dull characteristic of the cutting structure. Figure 8-5 lists thetwo-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 theface of the bit where the cutting structure dulling characteristic occurs. Figure 8-5lists the codes to be used for describing locations on bits.

Note G (gauge area) replaces H for this version.


8/22

Drilling Operations


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 gauge.

All - All RowsCone 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 atthe cutting structure with the bits sitting on the pin.

5. Column 5 (B-Bearing-Seals) uses a letter or a number code, depending on bearingtypes, to indicate bearing condition of roller cone bits. For non - sealed bearing rollercone bits, a linear scale from 0-8 is used to indicate the amount of bearing life thathas 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 of the seal. An E indicates an effective seal, an "F" indicates a failed seal(s), and anN indicating "not able to grade" has been added to allow reporting whenseal/bearing condition cannot be determined.

6. Column 6 ( G-Gauge ) is used to report on the gauge of the bit. The letter I (IN)indicates no gauge reduction. If the bit does have a reduction in gauge it is to berecorded in increments of 1/16". The Two Thirds Rule" is correct for three-conebits.

Note The Two Thirds Rule, as used for three cone bits, requires that the gauge 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 gauge ring ismultiplied by 2/3 and rounded to the nearest 1/16th of an inch to give the correctdiameter reduction (Figure 8-6).


9/22

Drilling Operations


7. Column 7 (O-Other Dull Characteristics) is used to report any dullingcharacteristic of the bit, in addition to the cutting structure dulling characteristiclisted in column 3 (D). Note that this column is not restricted to cutting structuredulling 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.

8.1.3 Dulling characteristicsFollowing is a discussion, with photographs where possible, of the dulling characteristicscommon to roller cone and fixed cutter bits. While the possible causes listed and possiblesolutions for problem wear modes are not presumed to be exclusive, they do representsituations commonly encountered in the field.

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


10/22

Drilling Operations


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 duringmanufacture (Figure 8-8)

Figure 8-8 Bond failure

BT (Broken Teeth ) - In some formations, BT is a normal wear characteristic of tungstencarbide insert bits and is not necessarily an indicator of any problems in bit selection oroperating practices (Figure 8-9) . However, if the bit run was of uncommonly shortduration, 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 notconsidered a normal wear mode for steel tooth roller cone bits. It may indicate improperbit 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 the

inner 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


11/22

Drilling Operations


BU (Balled-Up) - A balled-up bit will show tooth wear due to skidding, caused by acone, 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 stillgood. 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 manyof 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. Dropped drill string. Hydrogen sulfide embrittlement.


12/22

Drilling Operations


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 CI (Cone Interference) - Cone interference often leads to cone grooving and brokenteeth and is sometimes mistaken for formation damage (Figure 8-13). Broken teethcaused by cone interference are not an indicator of improper bit selection.

Figure 8-13 Cone interference


13/22

Drilling Operations


Some of the causes of cone interference are: Bit being pinched. Reaming under gauge 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 oftenbecome broken teeth. A tooth is considered chipped, as opposed to broken, if asubstantial part of the tooth remains above the cone shell ( Figure 8-15).

Figure 8-15 Chipped teeth/cutters


14/22


15/22


16/22

Drilling Operations


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 canresume. Some of the causes of lost cones are:

Bit hitting bottom or a ledge on a trip or connection. Dropped drill string. Bearing failure (causing the cone retention system to fail). Hydrogen sulfide embrittlement.

Figure 8-20 Lost cone

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


17/22

Drilling Operations


LT (Lost Teeth/Cutters) - This dulling characteristic leaves entire tungsten carbideinserts or PDC Cutters in the hole which are far more detrimental to the rest of the bitthan are broken insert (Figure 8-22). Lost teeth often cause junk damage. Lost teeth aresometimes 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 inan oversize hole. Off center wear can be recognized on the dull bit by wear on the coneshells between the rows of cutters, more gauge wear on one cone, and by a less thanexpected penetration rate. This can often be eliminated by changing bit types and thuschanging 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.


18/22


19/22

Drilling Operations


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 Gauge) - This dulling characteristic describes a bit that has experiencedgauge wear in a rounded manner, but will still drill a full size hole (Figure 8-26). Thegauge inserts may be less than nominal bit diameter but the cone backfaces are still atnominal diameter. Rounded Gauge can be caused by:

Drilling an abrasive formation with excessive RPM. Reaming an under gauge 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 bodyof the bit by the formation. Excessive pump pressure while on bottom with a decrease inpressure back to the expected value upon pulling off bottom is a good downholeindicator 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. Somepossible 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 acutting structure dulling characteristic (Figure 8-27). Shirttail wear can lead to sealfailures. 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.


20/22

Drilling Operations


Figure 8-27 Shirttail damage

SS (Self-Sharpening Wear) - This is a dulling characteristic which occurs when cutterswear 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 intothe bottomhole pattern (Figure 8-29). The cutter wear on a bit that has been tracking willbe on the leading and trailing flanks.

Figure 8-29 Tracking


21/22

Drilling Operations


The cone shell wear will be between the cutters in a row. Tracking can sometimes bealleviated by using a softer bit to drill the formation and/or by reducing the hydrostaticpressure 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 butcan 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 ornot closed, then the bit will start to washout as soon as circulation starts. Often the weldsare closed but crack during the bit run due to impact with bottom or ledges onconnections. When a crack occurs and circulation starts through the crack, the washout isestablished 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 WTis noted for steel tooth bits, it is also often appropriate to note self sharpening (SS) or flatcrested (FC) wear.

Figure 8-31 Worn teeth or cutters


22/22

Drilling Operations

NO (No Dull Characteristics) This code is used to indicate that the dull shows no signof the outer dulling characteristics described. This is often used when a bit is pulled aftera short run for a reason not related to the bit, such as a drill string washout. Next we willgrade a dull roller cone bit, and discuss some possible interpretations of the wear as itrelates to bit selection and application. It should be noted that there may be more thanone "correct" dull grading for any bit. This can happen if two persons should disagree onthe primary cutting structure dulling characteristic or on what the other dullingcharacteristic should be. Regardless, the new IADC dull grading system provides theman 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 dullbits on your location, you should be able to make sound recommendations as to the bestbit selections.

08. drilling bits.pdf

Documents