08. Drilling Bits.pdf

Table of Contents

8

Drilling Bits

Page

8.1 8.1 BITS ....................................................................................................................................8-1 8.1.1 Bit Selection..............................................................................................................8-2 8.1.2 IADC bit grading system .................. .................. .................. ................. .................. . 8-5 8.1.3 Dulling characteristics .................................. ................ .................. .................. ................. .................. ....... 8-8

List of Figures Figure 8-1 Figure 8-2 Figure 8-3 Figure 8-4 Figure 8-5 Figure 8-6 Figure 8-7 Figure 8-8

Page

Fixed cutter bit components (PDC, TSP, & natural diamonds) .................. ........... 8-1 Roller Cone Bit Components..................... ................. .................. .................. ........ 8-2 Roller cone (a) and PDC (b) bits. ................. ................. .................. .................. ..... 8-3 TSP diamond (a) and natural diamond (b) bits. ................................. ............... .................. .................. .. 8-3 IADC dull bit grading system.................... ................. .................. .................. ........ 8-6 Two thirds rule and how to measure gauge................... gauge.. ................. .................. .................. ..... 8-7 Broken cone................. cone ................. .................. .................. .................. ................. .................. .. 8-8 Bond failure ................. .................. .................. .................. ................. .................. .. 8-9

Figure 8-9 Broken teeth............... ................. .................. .................. ................. .................. ..... 8-9 Figure Figure 8-10 Balled up bit .................. ........................... .................. .................. .................. .................. .................. .................. .................. .................. ............. .... 8-10 Figure Figure 8-11 Cracked cone .................. ........................... .................. .................. .................. .................. ................... ................... .................. .................. ........... .. 8-10 Figure 8-12 Figure 8-13 Figure 8-14 Figure 8-15 Figure 8-16 Figure 8-17 Figure 8-18 Figure 8-19 Figure 8-20 Figure 8-21 Figure Figure 8-22 Figure 8-23 Figure 8-24 Figure 8-25 Figure 8-26 Figure 8-27 Figure 8-28 Figure 8-29 Figure 8-30

Cone dragged................... dragged.. ................. .................. .................. ................. .................. ............ 8-11 Cone interference............. .................. .................. ................. .................. ............ 8-11 Cored bit .................. ................. .................. .................. ................. .................. ... 8-12 Chipped teeth/cutters .................................. ................ .................. ................. .................. .................. ... 8-12 Erosion............ ................. .................. .................. ................. .................. ............ 8-13 Flat crested wear................. wear ................. .................. .................. ................. .................. ......... 8-13 Heat checking ................................. ............... .................. .................. .................. ................. ............... 8-14 Junk damage ................. .................. .................. .................. ................. ............... 8-14 Lost cone.................... .................. .................. .................. ................. .................. 8-15 Lost nozzle............... ................. .................. .................. ................. .................. ... 8-15 Lost teeth/cutt teeth/cutters ers .................. ........................... .................. .................. .................. .................. ................... ................... .................. .............. ..... 8-16 Off center wear .................................. ................ .................. .................. ................. .................. ............ 8-16 Pinched bit ................. .................. .................. .................. ................. .................. 8-17 Plugged nozzle.............. .................. .................. .................. ................. ............... 8-17 Rounded gauge .................................. ................ .................. .................. ................. .................. ............ 8-18 Shirttail damage..................... damage.... ................. .................. .................. ................. .................. ...... 8-19 Self sharpening wear .................................. ................ .................. ................. .................. .................. ... 8-19 Tracking.......... ................. .................. .................. ................. .................. ............ 8-19 Washed out bit................. bit ................. .................. .................. ................. .................. ............ 8-20

Figure 8-31

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

List of Tables

Page

No list of tables.

January 1997

Confidential 

Directional Drilling 8-i

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Drilling Operations

8

Drilling Bits About this chapter

The first thing in any drilling assembly is the bit. This chapter will help the DD gain the knowledge 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 bit’s bit’s 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, diamond, TSP, and PDC Bit. 2. Explain the criteria for bit selection. 3. Inspect a dull bit bit and fill out a dull grading form. 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 driller’s knowledge.

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Drilling Operations

Figure 8-2

8.1.1 Bit Selection

Roller Cone Bit Components 

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Drilling Operations A PDC Bit (Polycrystalline Diamond Compact) removes formation from the rock face by shearing the rock in a similar manner to the way a machinist’s 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 a slower 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

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Drilling Operations 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 borehole’s path is being deflected which causes more stress to be shifted from the face 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 Roller Cone 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 will drill straight ahead. Hence, if we want to steer our hole to a different direction, we should choose a bit with a shorter gauge 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 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 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 2 3

Soft Formations Medium Formations Hard Formations

4 thru 8

Insert Types

4 5 6 7 8

Very Soft Soft Medium Hard Very Hard

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

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Drilling Operations The third digit is called the feature classification: feature classification: 1 thru 5

Anti-friction roller bearing bits

1 2 3 4 5

Standard, non-sealed Air-lubricated bearing fo for air ci circulation dr drilling Standard non-sealed w/cutters/reinforced gauge Sealed roller bearing Sealed roller bearing w/cutters/reinforced gauge

6 and 7

Sealed friction (journal) bearing

6 7

Sealed bearing with standard gauge Sealed bearing with Insert reinforced gauge

8 and 9

Reserved for future use

8 9

Directional 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. I-Inner) is used to report the condition of the cutting elements not 1. Column 1 (I-Inner) touching the wall of the hole (Inner). The c hange 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 gauge 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.

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Drilling Operations IADC DULL BIT GRADING SYSTEM CUTTING STRUCTURE INNER

OUTER

I

O

DULL CHAR.

LOCATION

D

BEARINGS/  SEALS

GAGE

OTHER DULL CHAR.

B

G

O

L

I

 —

INNER CUTTING STRUCTURE (All Inner rows)

O

 —

OUTER CUTTING STRUCTURE (Gage row only)

L

REASON PULLED R

 — LOCATION N M G A

In columns 1 and 2 a linear scale from 0 to 8 is used to describe the condition of the cutting structure according to the following:

 —  —  —  —

ROLLER CONE Nose Row CONE # Middle Row 1 Gage Row 2 All Rows 3

FIXED C  — N  — T  — S  — G  — A  —

STEEL TOOTH BITS 

CUTTER Cone Nose Taper Shoulder Gage All Areas

A measure of lost tooth height due to abrasion and / or damage. 0  — 8  —

NO LOSS OF TOOTH HEIGHT TOTAL LOSS OF TOOTH HEIGHT

B  —

BEARING SEALS

NON-SEALED BEARINGS A linear scale estimating bearing life used. ( 0 - No life used, 8 - All life used, i.e. no bearing life remaining.)

INSERT BITS 

A measure of total cutting structure reduction due to lost, worn and / or broken inserts. 0  — NO LOST, WORN AND / OR BROKEN INSERTS 8  — ALL INSERTS LOST, WORN AND / OR BROKEN G  —

GAGE

FIXED CUTTER BITS 

I 1/16 2/16 4/16

A measure of lost, worn and / or broken cutting structure. 0  — NO LOST, WORN AND / OR BROKEN CUTTING STRUCTURE 8  — ALL OF CUTTING STRUCTURE LOST, WORN AND / OR BROKEN D

 —

 —  —  —  —  —  —  —  —  —  —  —

Broken Cone Bond Failure Broken Teeth / Cutters Balled Up Bit Cracked Cone Cone Dragged Cone Interference Cored Chipped Teeth / Cutters Erosion Flat Crested Wear

LT OC PB PN

 —

RG RO SD SS

 —

TR

 —

 —  —  —

 —  —  —

Lost Teeth / Cutters Off-Center Wear Pinched Bit Plugged Nozzle /  Flow Passage Rounded Gage Ring Out Shirttail Damage Self-Sharpening Wear Tracking

 —  —  —  —

in gage 1/16" out of gage 1/8" out of gage 1/4" out of gage

O  —

OTHER DULL CHARACTERISTICS Refer to Column 3 codes

R  —

REASON BEING PULLED OR RUN TERMINATED

DULL CHARACTERISTICS (Use only cutting structure related codes) * BC BF BT BU * CC * CD CI CR CT ER FC

FIXED CUTTER E  — seals effective F  — seals failed N  — not able to grade X  — fixed cutter bit

BHA  — CM CP DMF DP DSF DST DTF

 —  —  —  —  —  —  —

Change Bottom Hole Assembly Condition Mud Core Point Downhole Motor Failure Drill Plug Drill String Failure Drill Stem Testing Downhole Tool Failure

LIH LOG PP PR RIG TD

 —

TQ TW

 —  —

 —  —  —  —

Left in Hole Run Logs Pump Pressure Penetration Rate Rig Repair Total Depth /  Casing Depth Torque Twist Off

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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 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) ( 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 effe ctive 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. G-Gauge) is used to report on the gauge of the bit. The letter “I” (IN) 6. Column 6 (G-Gauge) indicates no gauge reduction. If the bit does have a reduction in gauge 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 gauge ring be pulled  so that it contacts two of the cones at their outermost points.

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Drilling Operations 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 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 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 87). 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. c ones.

•

Bit hitting a ledge on a trip or connection. c onnection.

•

Dropped drill string.

•

Hydrogen sulfide embrittlement.

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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 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 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

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Drilling Operations  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 810) . 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 f ormation 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).

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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.

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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.

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Drilling Operations 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.

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Drilling Operations 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 

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Drilling Operations  LC (Lost ( 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 c onnection.

•

Dropped drill string.

•

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

•

Hydrogen sulfide embrittlement.

Figure 8-20

Lost cone 

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Drilling Operations  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.

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Drilling Operations

•

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 gauge (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.

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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 experienced gauge wear in a rounded manner, but will still drill a full size hole (Figure 8-26). The gauge inserts may be less than nominal bit diameter but the cone backfaces are still at nominal diameter. Rounded Gauge can be caused by: •

Drilling an abrasive formation with excessive RPM.

•

Reaming an under gauge hole.

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Drilling Operations

Figure 8-27

Shirttail damage 

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Drilling Operations 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.

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Drilling Operations  NO (No Dull Characteristics) This code 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