Overview of Drill Bits

8/9/2019 Overview of Drill Bits

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Rotary Drilling Bits

University of Petroleum, Beijing



Rotary Drilling Bits

1. Various bit types available

2. Criteria for selecting the best bit for agiven situation

3. Standard methods for evaluating dull bits

4. Factors affecting bit wear and drillingspeed

5. Optimization of bit weight and rotaryspeed



1. Various Bit Types Available

• The process of drilling a hole in the ground

required the use of drilling bits. Indeed, the

bit is the most basic tool used by drillingengineer, and the selection of the best bit

and bit operating conditions is one of the

most basic problems that he faces.




• An extremely large variety of bits are

manufactured for different situations

encountered during rotary drillingoperations. It is important for the drilling

engineer to learn the fundamentals of bit

design so he can understand fully thedifference among the various bits available.




• Drag bits

• Polycrystalline

Diamond(PDC) Bits

• Rolling Cutter Bits

• Standard Classification

of Bits




• All drag bits consist of fixed cutter blades that are

integral with the body of the bit and rotate as a

unit with the drill-string.• Usually have two or more cones containing the

cutting elements.



1.1 Drag Bits

Including bits with steel

cutters,diamond bits,and

PDC bits.



1.1 Drag Bits

advantages: have no rolling parts; less chance

of bit breakage

formation: steel cutter elements — (fishtail

bit) uniformly soft, unconsolidated;

diamond bit — non-brittle formations with

plastic mode of failure



1.1 Drag Bits

• Design features:

The number and shape of the cutting stones,thesize and location of the water courses,and

metallurgy of the bit and cutting elements.For diamond bit: important design — crown profile. The size and number of diamonds used ina diamond bit depends on the hardness of the

formation.Hard — 0.07~0.125carat;soft — 0.75~2carat



1.2 Polycrystalline Diamond Bits

Other important design features:cutterorientation in terms of back rake, side rake,

and chip clearance or cutter exposureformation: soft, firm, and medium-hardnonabrasive that are not “gummy”.

small back-rake angles — soft formationside rake — move cuttings




The exposure of the cutter provides roomfor the cutting to peel off the hole bottomwithout impacting against the body andpacking in front of the cutter.




A negative back-rake angle of 20° isstandard on many steel-body PDC bits.

However, smaller back-rake angle, whichare better-suited for soft formations, arealso available.




The side-rake assist in pushing the cuttingsformed to the side of the hole, much like the

action of the plow.




• Cutter orientation must be properly matched to thehardness of the formation being drilled.

• Soft, nonabrasive---set to emphasize aggressive

cutting. Otherwise use a less-aggressive cuttingorientation.

• The cutter orientation also depends on theexpected cutter velocity, which in turn depends on

the distance of the cutter location from the centerof the hole.



1.3 Rolling Cutter Bits

• The three-cone rolling cutter bit is the most

common bit type currently used in rotary

drilling operations. This general bit type isavailable with a large variety of tooth

design and bearing types.

• The drilling action of a rolling cutter bitdepends to some extent on the offset of the

cones.




• Offset of the cones: a measure of how much the

cone axes moved so that their axes do not

interact at a common point of the centerline ofthe hole.




Advantage: cause the cone to stop rotating periodically as the bit is turned and scrape

the hole bottom much like a drag bit. Tendsto increase drilling speed in most formationtypes.

Disadvantage: promotes faster tooth wear in

abrasive formation

cone offset angle 0~4°(hard — soft)




• Shape of the bit teeth also has a large effect on

the drilling action of a rolling cutter bit.

long, widely spaced steel teeth – soft formation.The long teeth easily penetrate the soft rock, and

the scraping action provided by alternate

rotation and plowing action of the offset cone

removes the material penetrated.




• The wide spacing of the teeth on the cone

promotes bit cleaning.

• Teeth cleaning is mainly provided by theintermeshing of teeth on different cones.

• As the rock type gets harder, the tooth

length and cone offset must be reduced to prevent tooth breakage




• The metallurgy requirements of the bit teeth

also depend on the formation characteristics.

• The two primary types used are (1)milledtooth, and (2) tungsten carbide insert cutters.

• Milling the teeth out of a steel cone.

• Pressing a tungsten carbide cylinder into

accurately machined holes in the cone




• The milled tooth bits designed for soft

formation usually are faced with a wear-

resistant material, such as tungsten carbide,on one side of the tooth.




The application of

hard facing onlyon one side of thetooth allows morerapid wear on oneside of the tooth

than the other,andthe tooth staysrelatively sharp.




• The milled tooth bits designed for hard

formation are usually case hardened by

special processing and heat treating thecutter during manufacturing.




This case-hardened steel should wear bychipping and tend to keep the bit tooth sharp




• As for the tungsten carbide insert cutters:

• short and have a hemispherical end (button

bits) — hard formation• Long and chisel-shaped end – soft formation




• Position of the teeth

inner rows of teeth: positioned on different

cones ---intermesh

outer row of teeth: hard work — penetrate

and gauge protection




• Bearings assemblies

standard bearing assembly — inexpensive:

consists of (1) a roller-type outer (2) a ball-

type intermediate, and (3)a friction-type

nose bearing. ( gif )




Sealed bearing assembly--intermediate-cost:

The bearings are maintained in a grease

environment by grease seals, a greasereservoir, and a compensator plug.




journal bearing assembly--advanced

• In this type: the roller bearings are

eliminated and the cone rotates in contactwith the journal bearing pin.




• High speed O ring sealed floating journal bearing assembly — recentdeveloped:

• Adding the floating sleeve betweenthe surfaces of journal bearings andadding floating button between thethrust faces. This decreases therelative linear velocity of the

bearings and reduces the

temperature of the friction surfaces,and makes bearing being applicableto high speed drilling.



1.4 Standard Classification of Bits

• IADC — three-digit code

first---bit series number and the formation seriescode

from D1 to D5 are reserved for diamond or PDC bitsin the soft, medium-soft, medium, medium-hard, andhard formation categories respectively. Series1,2,and 3 are reserved for milled tooth bits in the soft,

medium, and hard formation categories respectively.Series 5,6,7, and 8 are for insert bits in the soft,medium, hard and extremely hard formationcategories, respectively. 4 is used for future use.




Second — type number

type 0 is reserved for the PDC drag bits.

Type 1 through 4 designate a formationhardness sub-classification from the softest

to the hardest formation within each

category.




Third – feature number(structure feature

code of bit)

The feature number are interpreteddifferently depending on the general

type of bit being described.




• Kingdream roller cone bit for oil well drilling

featured innovative structure with various

types. 10 standard series, 25 sizes and morethan 500 types of bit designs are tailored for

the drilling applications in different kinds of

formation from soft to hard.

Classification:standard series with the specialstructure combinations formed into special

series.




• For example 8 1/2HJT537GLbit

• 8 ½: Bit diameter is 8 1/2 inches(215.9mm)

• HJT: Metal face sealed journal bearing,special gage protection

• 537:Insert bit for drilling in soft to medium

hard formation with low compressive strength

• G :Reinforced bit head OD

• L :Bit head OD stabilization pad



1.5 Rock Failure Mechanisms

• Wedging

• Scraping and grinding

• Erosion by fluid jet action

• Percussion or crushing

• Torsion or twisting

All above are interrelated




• For drag bits: primarily by wedging

mechanism, A twisting action also may

contribute to rock removal from the center portion of the hole.

• A schematic illustrating the wedging action

of a drag bit tooth just prior to cutting failure

is shown below:



Wedging action of drag bit(5.13)

A vertical forcefrom the bit

weight, ahorizontal forcefrom the rotarytable. The two

force definesthe plane ofthrust of thetooth or wedge



Wedging action of drag bit

The cuttingsare shared off

in a share planeat an initialangle to thethrust planethat isdependent onthe propertiesof the rock




• The depth of the cut is controlled by the plane of

thrust and is selected based on the strength of the

rock and the radius to the cut. The depth of the cutis often expressed in terms of the bottom cutting

angle. The angle is a function of the desired cutter

penetration per revolution, it can be defined by

r

L p

2tan



Mohr theory of failure

• The Mohr criterion states that yielding or

fracturing should occur when the shear stress

exceeds the sum of the cohesive resistance ofthe material c and the frictional resistance of

the slip planes or fracture plane.

• The Mohr criterion is stated mathematically by




tannc

Where

frictioninternalof angle planefailureat thestressnormal

materialtheof resistencecohesive

failureatstressshear

n

c




This is the equation of a line that is tangent to Mohr’s

circle drawn for at least two compression test madeat different levels of confining pressure.




• Equations that are

represent the Mohr

circle is given by

2sin2

131

The angle of internal friction, 90tosummust2and,

2cos2

1

2

13131 n



Example for further understand

• A rock sample under a 2,000-psi confining

pressure fails when subjected to a

compressive loading of 10,000psi along a plane which makes angle of 27°with the

direction of the compressive load. Using the

Mohr failure criterion, determine the angleof internal friction, the shear strength, and

the cohesive resistance of the material.




• Solution. The angle 90tosummust2and,

Thus ,the angle of internal friction is given by

3627290

The shear strength is computed by

236,354sin000,2000,10

2

1

2sin2

1 31




• The stress normal to the fracture plane is

computed by

3,649psi

54cos000,2000,102

1000,2000,10

2

1

2cos2

1

2

13131

n

The cohesive resistance can be computed by

psi

c n

585363,649tan-3,236

tan




• For rolling cutter bits: employ all of the

basic mechanism. Predominant — percussion

or crushing (present for the IADC series 3,7,and 8,hard brittle formation)

• Maurer have provided considerable insight

into the basic mode of failure beneath the bit tooth.



Apparatus used by Maurer

This apparatus allowed the borehole pressure, rock pore pressure,

and rock confining pressure to varied independently.




• A load is applied to a bit tooth.

The constant pressure beneath

the tooth increase until it

exceeds the crushing strength ofthe rock, finely powdered rock

formed. As the force on the

tooth increase the material in

the wedge compresses andexerts high lateral forces on the

solid rock surrounding the

wedge then fracture formed.




• The fracture propagate

along a maximum

shear surface, which

intersect the direction

of the principal

stresses at a nearly

constant angle as predicted by the Mohr

failure criteria.




• At low differential

pressure, the cuttings

formed in the zone of

broken rock are ejectedeasily from the crater. The

bit tooth then moves

forward until it reaches the

bottom of the crater, andthe process may be

repeated




• At high differential

pressure, the downward

pressure and frictional

forces between the rockfragments prevent ejection

of the fragments. As the

force on the tooth is

increased, displacementtakes place along fracture

planes parallel to the

initial fracture.




• An example of

ejection of the rock

fragments from the

crater.




• For drilling bits with a

large offset: the

situation is more

complex.



2. Criteria for selecting the best bit

• Trial and error

• The most valid criterion for comparing the

performance of various bits is the drillingcost per unit interval drilled

• the drilling cost formula is




timetrip t

run bittheduringtimerotatingnont

run bittheduringtimerotatingtotal t

unit time perrigtheof costoperatingfixedtheisC

bittheof costtheisC

depthunit percostdrilledisC

where

t

c

b

r

b

f

D

t t t C C C t cbr b f




• Since on amount of arithmetic allows us todrill the same section of hole more than

once, comparisons must be made betweensucceeding bits in a given well or between bits used to drill the same formation indifferent wells. The formations frilled with

a given bit on a previous nearby well can becorrelated to the well in progress using welllogs and mud logging records.




• Wildcat area: bit type can be made on the basis ofwhat is known about the formation characteristicsand drilling cost in an area.

• The terms usually used by drilling engineers todescribe the formation characteristics aredrillability and abrasiveness.

• drillability is a measure of how easy the formationis to drill. It is inversely related to the compressivestrength of the rock. Generally tends to decreasewith depth in a given area.




• The abrasiveness of the formation is a

measure of how rapidly the teeth of a milled

tooth bit will wear when drill the formation.Although there are some exceptions, the

abrasiveness tends to increase as the

drillability decreases. • In absence of prior bit records, several rules

of thumb often are used:




• The initial bit type and features selected should begoverned by bit cost consideration.

• Three-cone rolling-cutter bits are the mostversatile bit type available and are a good initialchoice for the shallow portion of the well

• When using a rolling-cutter bit :

• using the longest tooth size possible

• A small amount of tooth breakage should be toleratedrather than selecting a shorter tooth size.




• When enough weight cannot be applied economically to a

milled tooth bit to cause self-sharpening tooth wear, a

longer tooth size should be used.

• When the rate of tooth wear is much less than the rate of

bearing wear,select a longer tooth size, a better bearing

design, or apply more bit weight.

• When the rate of bearing wear is much less than the rate of

tooth wear, select a shorter tooth size, a more economical bearing design or apply less bit weight.




• Diamond drag bits perform best in non-

brittle formation having a plastic mode of

failure, especially in the bottom portion of adeep well, where the high cost of tripping

operation favors a long bit life, and a small

hole size favors the simplicity of a drag bit

design.




• PDC drag bit perform best in uniform

section of carbonates that are not broken up

with hard shale stringers or other brittlerock types.

• PDC drag bits should not be used in gummy

formations,which have a strong tendency tostick to the bit cutters.



Inversion theory

input mode output

Feedbackinformation

Input: the drill-string structures anddrilling parameters

Output: mud log data,well trajectory surveyand well logging etc.



3.Bit Evaluation

• IADC standard: adopted a numerical code

for reporting the degree of bit wear relative

to the (1) teeth, (2) bearings, and (3) bitdiameter (gauge wear) structure.

• Grading tooth wear

• Grading bearing wear

• Grading gauge wear



3.Bit Evaluation

• Grading tooth wear

• For milled tooth bits:• in terms of the fractional tooth height that

has been worn away, and is reported to thenearest eighth. (t-4: the tooth are 4/8 worn).

• Visual estimate are commonly usedaccording to a profile chart guide. (next page)



Tooth wear guide chart

The penetration rate of the bit just before pulling thebit should not influence the tooth wear evaluation.



3.Bit Evaluation

• For insert bit:

• The tooth wear usually is reported as the

friction of the total number of inserts thathave been broken or lost to the nearest

eighth.

• T-4, 4/8 of the inserts are broken or lost.



3.Bit Evaluation

• Grading bearing wear

• It is very difficult

• Usually based on thenumber of hours of

bearing life that the

drilling engineer

thought the bearingswill last.



3.Bit Evaluation

• Grading gauge wear

• A ring gauge and a ruler

• G-O-4: lost 4/8 in.

• Is there exist G-I-4?



3.Bit Evaluation example

This type of wear occurswhen the nose areas of the

cones are worn away orlost. This frequently occursbecause of excessive loadsbeing applied to the conetips.

Low bit weight and highrotary speeds.



4. Factors affecting bit wear and drilling speed

• 4.1 factors affecting tooth wear

• 4.2 factors affecting bearing wear

• 4.3 factors affecting penetration rate

• 4.4 terminating a bit run



4.1factors affecting tooth wear

• If the time interval of bit use is increased too much, the bit

may break apart leaving junk in the hole. This will required

an additional trip to fish the junk from the hole or may

reduce greatly the efficiency of the next bit if an attempt ismade to drill past the junk. Thus a knowledge of the

instantaneous rate of bit wear is needed to determine how

much the time interval of bit use can be increased safely.

Since practices are not always the same for the new and

old bit runs, a knowledge of how the various drilling

parameters affect the instantaneous rate of bit wear also is

needed.




• The rate of tooth wear depends primarily on

(1) formation abrasiveness. (2)tooth

geometry.(3)bit weight. (4)rotary speed, and(5)the cleaning and cooling action of the

drilling fluid.




• 4.1.1 effect of tooth height on rate of tooth

wear--Steel tooth

Steel tooth abraderate is directlyproportional to thearea of the tooth in

contact with thegrinding wheel.



4.1.1 effect of tooth height on rate of tooth wear

11 y xi ww A

The bit tooth initially hasa contact area given by

After removal of

certain tooth height,the area are given by




2

1212

12112111

121121

L

Lh

121121

i

r

hwwwwhwwwwwwww

wwhwwwhw

ww

L

Lwww

L

Lwww A

y y x x

y y x x x y y x

y y y x x x

y y

i

r y x x

i

r x y x




h H dt

dh

dt

dh

s 21

1

The simplifiedequation is

Recall that a case-hardened bit tooth or a tooth

with hard facing on one side often will have a self-sharpening type of tooth wear, a constant H2 canbe selected too




• 4.1.1 tooth height--PDC blank: the cutter contact

area is proportional to the length of the chord.

2/sin

1

c s d dt

dh

dt

dh




• 4.1.2 Bit weight: Galle and Woods published one of

the first equations for predicting the effect of bit weight

on the instantaneous rate of tooth wear. The relation is

given by

0.10andinches,indiameter bitd

unitslbm-1,000in bit weight

log1

1

b

b

b

d W

W

where

d

W dt

dh




b

s

d

W

dt

dh

dt

dh

log1

3979.0

The wear rate at various bit weight can beexpressed in terms of a standard wear rate thatwould occur for a bit weight of 4,000 lbf/in. Thus,

the wear rate relative to this standard wear rate isgiven by




bmb d

W

d

W dt

dh

1

Note that dh/dt becomes infinite forW/db=10,this equation predicts the teethwould fail instantaneously if 10,000 lbf/in. ofbit diameter were applied. Another relation isgiven by :




bmb

mb

s

d

W

d

W

d

W

dt

dh

dt

dh4

Expressing this relation in terms of astandard wear rate at 4,000lbf/in. of bit

diameter yields




• P146 table 5.7




• 4.1.3 Rotary speed---for milled-tooth bits

designed for use in soft formations.

1

60

H

s

N

dt

dh

dt

dh




• 4.1.4 Hydraulics

the effect of the cooling and cleaning action

of the drilling fluid on the cutter wear rate ismuch more important for diamond or PDC

bit than the rolling cutter bit, but no

mathematical models




• 4.1.5 tooth wear equation: the instantaneous rate

of tooth wear is given by

h H

H

d

W

d

W

d

W

N

dt

dh

bmb

mb

H

H 2

2

1

21

4

60

11



4.1.5 tooth wear equation

• Recommended values of H1,H2, and (W/db)m

are shown as follows:

• P146 TABLE5.8




• Define a tooth wear parameter J2 using

21

160

4 2

1

H N

d

W

d W

d W

J

H

mb

bmb

2




The tooth wear equation can be expressed by

b f t h

H dhh H J dt 0 0

22 1

Integration of this equation yields

2/2

22 f f H b h H h J t




• Solving for the abrasiveness constant τH

gives

2/222 f f

b H

h H h J

t



An example to tooth wear equation

• An 8.5-in. class 1-3-1 bit drilled from a depth of

8,179 to 8,404 ft in 10.5 hours. The average bit

weight and rotary speed use for the bit run was

45,000lbf and 90 rpm, respectively. When the bit

was pulled, it was graded T-5, B-4, G-I. Compute

the average formation abrasiveness for this depth

interval. Also estimate the time required to dull

the teeth completely using the same bit weight and

rotary speed.




• Solution. Using table in page 93 we obtain

H1=1.84, H2=6, and (W/db)m=8.0. using

equation in page 94 we obtain

08.0

2/61

1

90

60

0.40.8

5.8450.884.1

2

J




• Solving for the abrasiveness constant using

a final fraction tooth dullness of 5/8(0.625)

gives

hours

hours H

0.73

2/625.06625.0080.0

5.102




• The time required to dull the teeth

completely (hf =1.0) can be obtained by

hours4.23

2/16173.00.08

2/

2

2

22

f f H b h H h J t



4.2factors affecting bearing wear

• The prediction of bearing wear is much more difficult thanthe prediction of tooth wear. Like tooth wear, theinstantaneous rate of bearing wear depends on the currentcondition of the bit. After the bearing surface become

damaged, the rate of bearing wear increases greatly.However, since the bearing surface cannot be examinedreadily during the dull bit evaluation, a liner rate of bearingwear usually is assumed. For a given applied force, the

bearing life can be expressed in terms of total revolution aslong as the rotary speed is low enough to prevent an

excessive temperature increase. Thus, bit bearing lifeusually is assumed to vary linearly with rotary speed.




• The effect of bit weight on bearing life depends onthe number and type of bearings used and whetheror not the bearings are sealed.

• The hydraulic action of the drilling fluid at the bitis also thought to have some effect on bearing life.As flow rate increase, the ability of the fluid tocool the bearings also increases. It is believed thatflow rate sufficient to lift cuttings will also be

sufficient to prevent excessive temperature buildup in the bearings.




• However hydraulic horsepower values

above 4.5hp/sq in. can be detrimental to

bearing life.• A bearing wear formula frequently used to

estimate bearing life is given by




hoursconstant, bearing

andexponents,wearB

inches diameter, bitd

1,000lbf , bit weight W

rpmspeed,rotary N

, t

consumed beenhasthatlife bearingfractional b

460

1

B

21,

b

21

bearing B

hourstime

where

d

W N

dt

db B

b

B

B




• Define a bearing wear

parameter J3 using

21460

3

B

b

B

W

d

N J




Integration of the equation above yields

f Bb b J t 3Solving for the bearing constant

gives

f

b

Bb J

t

3



An example to bearing wear equation

• Compute the bearing constant for a 7.875-

in., class 6-1-6(sealed journal bearings) bit

that was graded T-5, B-6, G-I after drilling64 hours at 30,000lbf and 70 rpm.

Solution. Get B1=1.6 and B2=1.0




• Using equation in

page 105 we obtain

820.0

30

875.74

70

600.16.1

3

J




• Solving for the bearing constant

using bf =6/8 yields

hours 104)0.820(0.75

hours 64 B



4.3factors affecting penetration rate

• The most important variables affecting

penetration rate that have been identified

and studied include (1) bit type,(2)formation characteristics, (3)drilling fluid

properties, (4)bit operating conditions, (5)

bit tooth wear, and (6) bit hydraulics



4.3factors affecting penetration rate • 4.3.1 bit type

• For rolling cutter bits:long tooth and a large cone

offset angle will get high rate in soft formation• Drag bit are designed to obtain a given penetration

rate.

• The diamond and PDC bits are designed for a

given penetration per revolution by selection ofthe size and number of blades




• 4.3.2 formation characteristics

Elastic limit and ultimate strength are two

main formation properties affect the penetration rate.

permeability of the formation and the

mineral composition of the rock




• 4.3.3 drilling fluid properties

density

rheological flow propertiesfiltration characteristics

solids content and size distribution

chemical composition




• 4.3.4 operating conditions

• Penetration rate vs. Bit weight




• No significant penetration rate is obtained until thethreshold bit weight is applied (point a). Penetration ratethen increase rapidly with increasing values of bit weight(segment ab). A liner curve is often observed at moderate

bit weights (segment bc). However, at higher values of bitweight, subsequent increase in bit weight causes onlyslight improvement in penetration rate (segment cd). Insome cases, a decrease in penetration rate is observed atextremely high values of bit weight (segment de). Thistype of behavior often is called bit floundering .

f ff i i




• Rotary speed vs.

penetration rate

4 3f ff i i




• Penetration rate usually increases linearly

with rotary speed at low values of rotary

speed. At higher values of rotary speed, theresponse of penetration rate to increasing

rotary speed diminishes.



4 3f ff i i




• This theoretical relation assumes perfect

bottom-hole cleaning and incomplete bit

tooth penetration.• The theoretical equation of Maurer can be

verified using experimental data obtained at

relatively low bit weight and rotary speeds

corresponding to Segment ab in page 115

and 117

4 3f ff i i




• 4.3.5 bit tooth wear. Most bits tends to drill

slower as the bit run progresses because of

tooth wear. The tooth length of milled toothis reduced continually by abrasion and

chipping. The insert tooth fail by breaking

or losing rather than abrasion. The same as

the diamond bits.

4 3f t ff ti t ti t




• For rolling-cutter bits. Model of tooth wear on

penetration rate is

)sharpening-self (0.5exponentana

away been wornhas that

heighttoothfractionaltheh

1692815.01

7

2

7

where

hh R

a

4 3f t ff ti t ti t




hae R 7

Anther similar but less complex

relationship is given by

a7 is determined based on the observed declineof penetration rate with tooth wear for previous

bits run under similar conditions.

A l



An example

• An initial penetration rate of 20 ft/hr was observed

in shale at the beginning of a bit run. The previous

bit was identical to the current bit and was

operated under the same conditions of bit weight,rotary speed, mud density, etc. However, a drilling

rate of 12 ft/hr was observed in the same shale

formation just before pulling the bit. If the

previous bit was graded T-6, compute the

approximate value of a7.

A l



An example

• Solution. The value of h for the previous bit just

before the end of the bit run is 6/8 or 0.75. The

value of h for the now bit is zero. Thus, for the

relation given we have

75.00 77

7

12 and20 aa

ha

Ke Ke

Ke R

A l



An example

• Dividing the first equation by the second yields

775.0

1220 ae

Taking the natural logarithm of both sides and

solving for a7 gives

68.0

75.0

1220ln7 a

4 3f t ff ti t ti t




• 4.3.6 bit hydraulics

• drilling practice showed that significant

improvement in penetration rate could be

achieved through an improved jetting action at the

bit. The improved jetting action promoted better

cleaning of the bit teeth as well as the hole bottom.

4 3f t ff ti t ti t




• 4.3.6 bit hydraulics. Relation between bit

hydraulics and penetration rate

4 3factors affecting penetration rate




• Eckel found that penetration rate could be correlated to

a Reynolds number group given by

1-a

Re

seconds10,000atfluiddrilling

of iscosityapparent v anddiameter,nozzle rateflow

densityfluiddrilling constantscalinga

d v

K where

vd K N

a





speedrotary

and blades,of numbereffictive element.cuttingeach

of n penetratioeffective

N

n

Lwhere

N n L R

be

pe

be pe





• The equations were derived for a simplified modelwhich assumed the following.

1. The bit has a flat face that is perpendicular tothe axis of the hole.

2. Each blade is formed by diamonds laid out asa helix.

3. The stones are spherical is shape.

4. The diamonds are spaced so that the cross-sectional area removed per stone is a maximumfor the design depth of penetration.





5. The bit is operated at the design depth of

penetration.

6. The bit hydraulics are sufficient for perfect bottom-hole cleaning.

For these conditions, the effective

penetration and the effective number of blades are given by





292.1

67.0

p pcb

d

cbe

p pe

L Ld d sC n

and L L

An example



An example

• An 8.625-in. diamond bit containing 2700.23-in.-diameter stones of 1.00 carat isdesigned to operate at a depth of penetration

of 0.01 in. Estimate the penetration ratethat could be obtained with this bit if theformation characteristics are such that anacceptable bit weight and torque for this penetration could be maintained at a rotaryspeed of 200 rpm.

An example



An example

• Solution. Ignoring the bit contouring required

for proper hydraulic action and gauge protection,

the bit is assumed to have a flat face that is

perpendicular to the axis of the hole. Thus .

in.stones/sq621.4

625.84

270

2

d

c

s

C

An example



An example

• The effective number of blades is given by

3.59

01.001.023.0625.84.6211.92

92.1

2

2

p pcb

d

cbe

L Ld d

s

C n

The effective penetration is given by

.in0067.001.067.0 pe L

An example



An example

• The penetration rate at a rotary speed of 200

rpm is given by

ft/hr 24

2006059.312

0067.0

N n L R be pe





• 4.3.7 penetration rate equation

• For rolling cutter bits

))...()()()(( 4321 n f f f f f R

5 Bit Operation



5. Bit Operation

• Items of primary concern include:(1)selection of bottom-hole assembly

(2)prevention of accidental bit damage

(3)selection of bit weight and rotary speed

(4)bit run termination

proper attention to all of these items must be

given to approach a minimum-cost drilling

operation



5 Bit Operation



5. Bit Operation

5 Bit Operation



5. Bit Operation

• 5.2 prevention of accidental bit damage

1) breaker plate

2) tight spots3) establish fluid circulation first

5 Bit Operation



5. Bit Operation

• 5.3 selection of bit weight and rotary speed.

In selecting the bit weight and rotary speed to

be used in drilling a given formation,

consideration must be given to these items;• (1)the effect of the selected operating

conditions on the cost per foot for the bit run

in question and on subsequent bit runs.

Bit Operation



Bit Operation

• (2) the effect of the selected operating

conditions on crooked hole problems.

• (3) the maximum desired penetration ratefor the fluid circulating rates and mud

processing rates available and for efficient

kick detection, and

Bit Operation



Bit Operation

(4) equipment limitations on the available bit

weight and rotary speed.

• One straightforward technique that can beused to determine the best constant

weight/speed schedule is to generate a cost-

per-foot table.

Bit Operation



Bit Operation

• Cost-per-foot table

Overview of Drill Bits

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Transcript of Overview of Drill Bits