Roller Cone Bits

GAGE CHISEL CONICAL TOP FLAT

Fig. 7—23 Tungsten carbide inserts for Smith bits (Courtesy Smith Tool Co.)

High Speed FP51A FP52 5!

HP-SM FP53/FP53A FP62 FP62S FP62X

HP-MH FP63

ti ffl ffl ffiÖD

HP H PP72 FP73 FP74 FPX3

Fig. 7-24 Tungsten carbide inserts for Reed bits (Courtesy Reed Rock Bit Co.)

TO

T2

T4 T6

A

A

A A

A A

jfe^

  1. 7-25 Tooth wear (Courtesy Smith Tool Co.)
  2. 7-25 Tooth wear (Courtesy Smith Tool Co.)

Class:

1-1

2-2 1-3 1-4 2-

2 2—3 3

Bearing c

apacity

—.

\ >

S /

> \

s

s

\

. Offset

Formation

Soft

Medium

Hard

Fig. 7-26 Bearing capacity

Table 7-10 General Bearing Relationships Between Several Bit Types

IADC Code Bearing Capacity**

Table 7-10 General Bearing Relationships Between Several Bit Types

IADC Code Bearing Capacity**

1,1,1

1.00

1,2,1

Lis

1,3,1

1.20

2,1,1

1.35

2,2,1

1.45

3,1,1

1 ;45

  • Relative values
  • Relative values as shown in Fig. 7-27, aids in rapid dissipation and transfer of heat from the sliding surfaces of the cone and journal to the outer cone. These inlaid materials are then ground to a precision finish, and each leg is individually matched to each cone with close tolerance.

A solid journal hearing of this design and manufacture can withstand higher unit loading than roller bearings, and the bit can run longer before journal wear or fatigue is experienced. The inner bearing in the solid journal-bearing design retains the same type of ball bearing structure and a similar type of friction

Roller Cone Bit

Fig. 7-27 Feature #7 indicates a special alloy inlay in the cone that dissipates heat rapidly away from sliding surfaces of journal and cone (Courtesy Smilh Tool Co.)

bearing structure. Thrust loading capability, however, is increased by eliminating the pilot pin bushing in the cone, casehardening the inner bearing bore, and inlaying hard metal in the leg journal thrust face.

Bearing grading of used bits is important for future bit runs. Bearings are graded according to eighths of the bearing wear. For example, a grade of S—3

indicates % of the bit bearing iife was used. Unlike teeth grading, bearing evaluation is highly subjective. The IADC has established some guidelines that serve as a reference:

Used Bearing Life Code Comment

% B-2 Tight bearings

4/s B-4 Tight but worn bearings

% B-6 Loose bearings

% B-8 Locked or lost bearings

The IADC has also established other abbreviations relating to bearing conditions, as follows:

Broken rollers

BR

Lost bearings

LB

Lost rollers

LR

Seals effective

SE

Seal failure

SF

Seals questionable

Diamonds embedded in a matrix have been used since the early Chinese dynasties; however, modern usage for mineral exploration came into common use in the early 1900s. The diamond's natural wear resistance makes it competitive in today's drilling practices, even at their higher prices. Diamond hits are used in conventional rotary, turbine, and coring operations.

Like any other product, diamond bit designs and applications must be understood to obtain optimum results at economical rates. The combination of fluid hydraulics blended wilh the selection and arrangement of diamonds enables this goal to be met in most formation types.

Diamond hits are structured differently than roller cone bits (see Fig. 7-28). A matrix structure is embedded with diamonds and contains waterways from the bit throat to the exterior of the bit. The drilling fluid should flow across the face of the bit to clean the cuttings and cool the diamonds.

Rock failure mechanism with a diamond bit is slightly different than steel tooth or insert bits. The diamond is embedded in the formation and then dragged

Shank Bore

Weld

Gage Broaches

Junk Slot

Pin Chamfer

Iadc Bit Classification Chart

Connection (Upper Section)

High Pressure Feeders

Cone Angle

Shank Bore

Gage Broaches

Junk Slot

Pin Chamfer

Bit Breaker Slots (Hairpin) (Wrap-Around)

Diamond Pad with Diamonds Low Pressure Collectors

High Pressure Feeders

Cone Angle

Connection (Upper Section)

Alignment Threads

Weld

Fig. 7-28 Diamond bit nomenclature (a) with hard and soft formation diamond bit (b, c) and turbine drilling bit (d) (Courtesy NL Hycalog)

Diamond Bit Christensen

across the face of the rock without being lifted and re-embedded, as would be the case with roller cone bits. This shearing mechanism can be used in soft, medium, and hard rock.

Several physical characteristics of diamond bits are important when considering their use. The primary considerations are the low shear strength and resistance to impact loading. As a result, lower hit weights are required, and it becomes necessary to ensure that the drillstring is properly stabilized to minimize bending and shearing forces.

Diamond bits are typically used to achieve a lower drilling cost per foot. The reduced cost occurs when the diamond bit achieves greater drilling rates than roller bits in certain conditions. Reductions in trips to change bits is also a positive factor in diamond bit use. Manufacturers can be consulted for a cost estimate/performance evaluation of diamond bits for a given well.

Many types of diamond bit structures are manufactured to address a broad range of operating systems. The selection is often based on economics, the formations to be drilled, and the type of rotary drill bit (cone type) normally used in an area. Most diamond bit manufacturers have developed comparison guides between their products and roller cone bits. Table 7-11 shows a guide for Christensen products.

The IADC has established a classification chart for diamond products similar to the charts for roller bits (see Fig. 7—18). The code uses a three-character group preceded by the letter D to indicate diamond products. Table 7-12 illustrates the chart for NL Hycalog's diamond products.

Evaluation of Used Diamond Bits. As with every other tool, the diamond bit will show signs of wear eventually. Some wear is considered failure; other wear must be qualified. Failure can be thought of as the breakdown in operation or function. In many cases this definition applies because the diamond bit has stopped drilling or performing its function. Failure can also be defined as falling short. This is sometimes the case where the ROP was too slow, pressure drop was too high, or some combination whereby the continued running of the diamond bit was not economical or practical. Failure is also mentioned when the bit has performed economically for its specified interval yet the observer of the used bit can detect pronounced wear. This could be localized fluid erosion, missing or worn diamonds, or some other phenomenon that did not detract from the economic or overall performance of the bit but, in final condition, the bit is decidedly not rerunable.

The discussion of diamond bit wear can be broken down into three categories: 1) diamonds, 2) matrix, and 3) others.

Diamond Wear. Diamond wear can result from any one or a combination of the following modes of failure:

  • graphitization
  • oxidation
  • gross breakage
  • abrasion or microscopic chipping

Graphitization of a diamond occurs when it is subject to temperatures exceeding 2,500°F in an inert atmosphere. (In other environments this temperature may be lower.) The diamond crystal at this time degenerates to graphite crystal, which is more stable. Graphitization apparently occurs on the surface of the diamond while it is drilling. It also occurs inside the diamond with heat sufficient to cause the carbon atoms inside the diamond to crystallize, allowing the diamond to reform or graphitize. Because the graphite crystal occupies a greater space, the diamond's outer crystal lattice cracks to accommodate the graphite crystal's growth. This is one cause of stone breakage and should not be confused with thermal stress cracking.

Oxidation of the diamond begins to occur at temperatures between 900 and 1,400°F in the presence of oxygen. The carbon of the diamond and free oxygen are converted into carbon dioxide and carbon monoxide, both of which are gases. Free oxygen is present in the drilling mud; since the temperatures needed for oxidation are present, oxidation downhole most certainly occurs. Some feel that oxidation might be the most significant form of diamond wear.

Gross breakage occurs when the strength of the diamond is exceeded or when a fracture or cleavage plane is oriented such that some impact or tangential force on the diamond is of a magnitude that failure results. Since most diamonds used in drill bits contain fracture planes, this type of failure is likely, although infrequent, on a large scale.

Abrasion, or microscopic chipping of the diamond, is caused by the continual pounding of the diamond by the formation, the sand that has been dislodged, and the solids being carried by the drilling mud. When this impacting has caused a small piece of the diamond to chip away, the sharp exposed areas are then ground back down. This repeated chipping and grinding causes the diamond to lose mass and a wear flat begins to develop.

Matrix Wear. The most predominant form of matrix wear comes in the form of fluid erosion. All diamond bits will show fluid erosion to some extent after they have been run. The amount of this wear is proportional to the length of time the bit was run and the hydraulic conditions under which it was subjected.

Fluid erosion is caused by the drilling fluid passing over the face of the bit at high energy levels with solids pecking away at the matrix of the bit. As the velocity of the fluid is increased, the energy expended also increases. High energy levels help drill faster by tending to destroy and remove the formation, but at the same time they also accelerate the rate of the matrix attrition. Rate of matrix attrition can also be increased by adding fine abrasive solids to the drilling mud. These solids can be so small (-200 mesh) thai they are not detectable on a mud report but are nevertheless present and highly detrimental to the life of the bit. However, with today's construction materials, erosion is rarely a severe problem.

Table 7-11 Christensen's Diamond Bit Selection Guide Compared to Formation Types and Roller Bit Styles

Rock

Rock

Step

Compact

Dia,

BallaSet

Downhole

Formation

Type

Bits

Bits

Bits

Bits

Bits

Motor Bits

Core Bits

Soft formation with

Gumbo

S3S

R481(R4I)

RC473 (RC5)

sticky layers and low

Clay

J11

R448(R401)

R448 (R40I)

RC444 (RC4)

compressive strength

Marl

Y12

K810

R381 (R32)

K810

Soft formation with low

Marl

OSCIG

D38

RC476 (RC6)

compressive strength

Salt

F2

R382(R31)

D341

K820

CIS

and high drillability

Anhydrite

FP51

K820

R426 (Rl)

D262

K831

Shale

CJ2

R482

D503

( R40LF)

Soft to medium forma

Sand

J33

K831

D3.ll

R419 (R4)

RC476 (RC6)

tion with low com

Shale

S88F

R422

T18

C18

pressive strength in-

(R26LF)

terbedded with hard

Chalk

FP53

R422

layers

(R26LF)

F3

K839

R419(R4)

K839

C22

Medium to hard forma

Shale

J44

D331

R419 (R4)

tion with high com

Mudstone

F4

R486

K851

pressive strength and

(R40H)

S225 (D187)

small abrasive layers

Lime-

CJ3

K852

Rock Rock Step Compact Dia. BallaSet Downhole

Type Bits Bits Bits Bits Bits Motor Bits Core Bits

Sandstone FP62 S225 (D187) S226 (D185)

SC226 (C201B)

Hard and dense forma

Lime

J55

K851

D331 S226 (D185)

S225 (D187)

C23

tion with very high

stone

compressive strength,

Dolomite

M89F

K852

T51

SC226

but non-abrasive

(C201B)

Anhydrite

FP63

K859

F57

K859

D41 S246

T52

(D331B)

Hard and dense forma

Siltstone

J77

D24

SC 276

tion with very high

(C23B)

compressive strength,

Sandstone

Y31

K869

S249 (S240)

and some abrasive

Mudstone

H88

S 249 (S240)

T54

formation layers

F7

SC249

(SC240)

Extremely hard and

Quartzite

J 99

S 249 (S240)

S 249

SC249

abrasive formation

(S240)

(SC240)

Volcanic

H100

Courtesy Chrisler sen Diamond Products

Courtesy Chrisler sen Diamond Products

Table 7-12 IADC Diamond Drill Bit Classification Chart

Bit Design Features

IADC Stone Series Size, Step

Num- Car- Type Long Taper Short Taper Formation ber ats 1 2 3

Soft 0 JETPAX

Weakly bonded shales, 1 1 vi-2

sands, evaporites, 2 I'A 901

and other formations 3 1 901

with low compres- 4 -A 901 sive strengths

Medium Soft 0 IETPAX

Medium bonded in- 1 % 901 MSI 730

terbedded sands and 2 V2-V1 901 MSI 730

shales, granular D2 3 '/s 901 MSI 730

limestones, and 4 most unaltered precipitates

Medium

Hard shales, sandy shales, dolomites, crystalline limestone, and other formations of similar compressive strengths

D3

0

1

'/a

901

730

2

W-'/a

901

730

3

901

730

4

l/4

901

Calcareous sands, sili- 1 lA 901 730

and pressure com- 4 Vi 901 730

pacted formations

Hard 0

Well-cemented quartz-

1

m

901 730

itic sands, schists,

2

m

901 730

altered siltstones, D5

3

Vis

901 730

and other rocks with

4

<Vii

high compressive strengths high compressive strengths

Courtesy NL Hycalog with NL Hycalog Nomenclature

Bit Design Features

Non- Downhole Side-taper Motor track

TURBOPAX

Base Core Ejector Other 7 8 9

ST WM

90 IS 90 IS

90 ICE 90 ICE

204 BI CNTR 204 BI CNTR

TURBOPAX

MS IT 901 730T ST 901S 901CE 204 BI CNTR

MS IT 901 730T ST 901S 90ICE 204 BI CNTR

MS1T 901 730T ST WM 90IS 901CE 204 BI CNTR

MIT 90IT 730T MIT 90IT 730T MIT 90IT 730T M1T901T 730T

ST 901S

ST 901S

ST 90 IS

ST WM 90IS

90ICE 730CE 90ICE 730CE 90ICE 730CE 90ICE 730CE

204 BI CNTR 204 BI CNTR 204 BI CNTR 204 BI CNTR

525 501 MIT 90IT 730T ST 901S 730CE 204 BI CNTR

525 501 M1T901T 730T ST 901S 730CE 204 BI CNTR

525 501 M1T901T 730T ST 901S 730CE 204 BI CNTR

525 501 M1T901T 730T ST WM 901S 730CE 204 BI CNTR

525 501 MIT 90IT 730T

525 501 MIT 90IT 730T

525 501 MIT 90IT 730T

525 501 MIT

ST 901S 730CE

ST 901S 730CE

730CE

Matrix degradation can also be caused by heat. When for some reason the drilling fluid cannot reach some part of the bit to cool it, the heat buildup cannot be dissipated. This heat buildup can result in heat checking or spalling and can adversely affect life and ROP.

Other Wear. The remaining forms of wear are in a general classification and cover not only wear but also damage to parts of the bit that are not directly involved in formation excavation and removal functions. These include the shank, API connection, results of drill string washout, and the mechanical functions of the drilling operation that might have prematurely caused the bit to be pulled (e.g., LCM, twistoff, dropping).

Dull Bit Analysis. Diamond bits tend to clog when soft, sticky shales and limestones are being drilled with low hydraulic energy levels. The fluid courses may clog and the fluid that normally would (low through is diverted elsewhere. The impacted formation tends to build up and pack tighter, in low-horsepower applications, it is not uncommon to pull a bit that has as much as one-third of the fluid courses clogged, with the matrix around these areas showing heat cracks. The shale acts as a bearing at these points and heat is generated that cannot be dissipated. Subsequently, the matrix expands and cracks.

Loss of Gauge. Gauge loss on a diamond bit can have many causes. One could be reaming to the bottom too quickly with too much weight. Since a diamond bit has no hydraulic distribution system until it reaches bottom, there is no efficient way to cool the diamonds on the gauge. If too much weight or rotary is used or long sections are reamed, the gauge diamonds can be overloaded and destroyed even before the bit reaches the bottom. Another cause of loss of the gauge diamonds is when the diamonds set on the outer diameter row (ODR) arc too large and not numerous enough. Because the angular speed of the bit is at a maximum at gauge, these diamonds tend to be overworked and undernourished. Mud flow is harder to control as it nears the gauge, so the diamonds are not cooled as efficiently. Fortunately, gauge stones do not do much work in most cases.

The best way to protect against these problems is to minimize the amount of reaming done with the diamond and to be careful to ream slowly when it is necessary, in addition, if gauge problems are encountered, the use of smaller diamonds on the ODR and gauge section will increase the resistance to wear.

Cored Center. In the past, center coring was a common cause of bit failure. Center coring occurs when the diamonds in and around the apex are destroyed or lost. The formation begins to wear away the matrix material and, essentially, the bit tries to cut a small core. Penetration rate slows or stops, and perhaps an increase in standpipc pressure is evident. Generally, the causes are I) improper design, 2) incorrect diamonds (too large), 3) broken or fractured formation, 4) poorly bonded bedding planes, 5) improper stabilization, and 6) extraneous metal.

The use of small diamonds in the apex area will usually solve this problem. A core ejector modification may also help. Improved stabilization is always recommended.

Ring-out. A ring-out failure looks like an O-ring groove appearing anywhere on the drilling face. The most popular site is just inside the nose, but it may occur anywhere. A ring-out is recognizable on the surface by the rapid increase oT the standpipc pressure accompanied by a decrease in ROP,

Improper stabilization is a prime factor. Broken formations, junk, erosion, and too few diamonds also participate in the development of ring-out failures. A modified design, smaller diamonds, increased density, and judicious use of carbonado diamonds generally alleviate this undesirable side effect.

Rounded Gauge. A new bit has a perfectly cylindrical gauge section. Occasionally a used bit will be pulled and the gauge will be rounded. The bit is not necessarily out of gauge, but it most likely is. This results from random wobble and is associated with inadequate bottom-hole stabilization and pendulum hook-ups. If improved stabilization is not an option, a more densely set gauge and possibly an extended gauge will provide relief.

Matrix Erosion. Erosion of the bit matrix is almost always apparent in used bits. Heavy erosion is normally associated with high hydraulic energy levels used to drill soft to medium-soft formations. The area just inside the nose generally shows the highest amount of fluid erosion. This is because the fluid is undergoing acceleration and extreme turbulence due to a directional change of the fluid path. This maximizes the energy imparted to the matrix and results in the high fluid erosion. This is all compounded with the increase in undesirable solids content (especially fine abrasive sand) that fast drilling generally produces. Erosion is normal, but a distinction must be made: is it a normal amount or is it excessive? Due to the many factors involved and the complicated nature of the erosion mechanism, it is not always easy to tell. Depending on the apparent cause, one of the following solutions may apply: lower hydraulic horsepower, cleaner mud, and alternate bit design, or less diamond exposure.

Broken Diamonds. A broken diamond is the result of excessive tangential force. When the strength of the diamond is exceeded, a fracture along a cleavage plane(s) occurs and all or most of the exposed stone separates. On the used bit, this is recognizable by the generally flat, highly rellective surfacc. Insignificant, random breakage occurs on most bits, but occasionally it is widespread and can affect performance. Causes arc stones too large, broken formation, junk, too few stones, WOB excessive, improper stabilization, and erosion.

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Responses

  • omar
    Does the api reguler roller bit connection bore have a tolerance?
    7 years ago
  • Hugo
    What 111 IADC bit means?
    5 years ago

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