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Rotary Drilling Bits
University of Petroleum, Beijing
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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
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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.
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1. Various Bit Types Available
• 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.
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1. Various Bit Types Available
• Drag bits
• Polycrystalline
Diamond(PDC) Bits
• Rolling Cutter Bits
• Standard Classification
of Bits
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1. Various Bit Types Available
• 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.
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1.1 Drag Bits
Including bits with steel
cutters,diamond bits,and
PDC bits.
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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
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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
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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
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1.2 Polycrystalline Diamond Bits
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.
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1.2 Polycrystalline Diamond Bits
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.
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1.2 Polycrystalline Diamond Bits
The side-rake assist in pushing the cuttingsformed to the side of the hole, much like the
action of the plow.
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1.2 Polycrystalline Diamond Bits
• 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.
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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.
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1.3 Rolling Cutter Bits
• 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.
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1.3 Rolling Cutter Bits
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)
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1.3 Rolling Cutter Bits
• 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.
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1.3 Rolling Cutter Bits
• 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
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1.3 Rolling Cutter Bits
• 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
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1.3 Rolling Cutter Bits
• 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.
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1.3 Rolling Cutter Bits
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.
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1.3 Rolling Cutter Bits
• The milled tooth bits designed for hard
formation are usually case hardened by
special processing and heat treating thecutter during manufacturing.
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1.3 Rolling Cutter Bits
This case-hardened steel should wear bychipping and tend to keep the bit tooth sharp
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1.3 Rolling Cutter Bits
• As for the tungsten carbide insert cutters:
• short and have a hemispherical end (button
bits) — hard formation• Long and chisel-shaped end – soft formation
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1.3 Rolling Cutter Bits
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1.3 Rolling Cutter Bits
• Position of the teeth
inner rows of teeth: positioned on different
cones ---intermesh
outer row of teeth: hard work — penetrate
and gauge protection
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1.3 Rolling Cutter Bits
• 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 )
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1.3 Rolling Cutter Bits
Sealed bearing assembly--intermediate-cost:
The bearings are maintained in a grease
environment by grease seals, a greasereservoir, and a compensator plug.
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1.3 Rolling Cutter Bits
journal bearing assembly--advanced
• In this type: the roller bearings are
eliminated and the cone rotates in contactwith the journal bearing pin.
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1.3 Rolling Cutter Bits
• 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.
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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.
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1.4 Standard Classification of Bits
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.
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1.4 Standard Classification of Bits
Third – feature number(structure feature
code of bit)
The feature number are interpreteddifferently depending on the general
type of bit being described.
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1.4 Standard Classification of Bits
• 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.
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1.4 Standard Classification of Bits
• 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
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1.5 Rock Failure Mechanisms
• Wedging
• Scraping and grinding
• Erosion by fluid jet action
• Percussion or crushing
• Torsion or twisting
All above are interrelated
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1.5 Rock Failure Mechanisms
• 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:
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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
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Wedging action of drag bit
The cuttingsare shared off
in a share planeat an initialangle to thethrust planethat isdependent onthe propertiesof the rock
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1.5 Rock Failure Mechanisms
• 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
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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
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Mohr theory of failure
tannc
Where
frictioninternalof angle planefailureat thestressnormal
materialtheof resistencecohesive
failureatstressshear
n
c
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Mohr theory of failure
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.
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Mohr theory of failure
• Equations that are
represent the Mohr
circle is given by
2sin2
131
The angle of internal friction, 90tosummust2and,
2cos2
1
2
13131 n
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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.
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Example for further understand
• 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
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Example for further understand
• 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
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1.5 Rock Failure Mechanisms
• 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.
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Apparatus used by Maurer
This apparatus allowed the borehole pressure, rock pore pressure,
and rock confining pressure to varied independently.
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1.5 Rock Failure Mechanisms
• 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.
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1.5 Rock Failure Mechanisms
• 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.
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1.5 Rock Failure Mechanisms
• 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
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1.5 Rock Failure Mechanisms
• 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.
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1.5 Rock Failure Mechanisms
• An example of
ejection of the rock
fragments from the
crater.
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1.5 Rock Failure Mechanisms
• For drilling bits with a
large offset: the
situation is more
complex.
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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
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2. Criteria for selecting the best bit
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
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2. Criteria for selecting the best bit
• 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.
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2. Criteria for selecting the best bit
• 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.
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2. Criteria for selecting the best bit
• 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:
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2. Criteria for selecting the best bit
• 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.
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2. Criteria for selecting the best bit
• 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.
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2. Criteria for selecting the best bit
• 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.
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2. Criteria for selecting the best bit
• 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.
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Inversion theory
input mode output
Feedbackinformation
Input: the drill-string structures anddrilling parameters
Output: mud log data,well trajectory surveyand well logging etc.
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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
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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)
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Tooth wear guide chart
The penetration rate of the bit just before pulling thebit should not influence the tooth wear evaluation.
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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.
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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.
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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?
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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.
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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
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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.
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4.1factors affecting tooth wear
• 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.
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4.1factors affecting tooth wear
• 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.
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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
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4.1.1 effect of tooth height on rate of tooth wear
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
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4.1.1 effect of tooth height on rate of tooth wear
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
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4.1factors affecting tooth wear
• 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
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4.1factors affecting tooth wear
• 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
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4.1factors affecting tooth wear
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
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4.1factors affecting tooth wear
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 :
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4.1factors affecting tooth wear
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
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4.1factors affecting tooth wear
• P146 table 5.7
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4.1factors affecting tooth wear
• 4.1.3 Rotary speed---for milled-tooth bits
designed for use in soft formations.
1
60
H
s
N
dt
dh
dt
dh
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4.1factors affecting tooth wear
• 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
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4.1factors affecting tooth wear
• 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
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4.1.5 tooth wear equation
• Recommended values of H1,H2, and (W/db)m
are shown as follows:
• P146 TABLE5.8
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4.1.5 tooth wear equation
• 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
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4.1.5 tooth wear equation
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
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4.1.5 tooth wear equation
• Solving for the abrasiveness constant τH
gives
2/222 f f
b H
h H h J
t
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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.
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An example to tooth wear equation
• 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
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An example to tooth wear equation
• 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
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An example to tooth wear equation
• 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
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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.
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4.2factors affecting bearing wear
• 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.
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4.2factors affecting bearing wear
• 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
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4.2factors affecting bearing wear
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
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4.2factors affecting bearing wear
• Define a bearing wear
parameter J3 using
21460
3
B
b
B
W
d
N J
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4.2factors affecting bearing wear
Integration of the equation above yields
f Bb b J t 3Solving for the bearing constant
gives
f
b
Bb J
t
3
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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
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An example to bearing wear equation
• Using equation in
page 105 we obtain
820.0
30
875.74
70
600.16.1
3
J
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An example to bearing wear equation
• Solving for the bearing constant
using bf =6/8 yields
hours 104)0.820(0.75
hours 64 B
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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
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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
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
• 4.3.3 drilling fluid properties
density
rheological flow propertiesfiltration characteristics
solids content and size distribution
chemical composition
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4.3factors affecting penetration rate
• 4.3.4 operating conditions
• Penetration rate vs. Bit weight
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
• Rotary speed vs.
penetration rate
4 3f ff i i
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4.3factors affecting penetration rate
• 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.
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4 3f ff i i
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
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
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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
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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
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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
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4.3factors affecting penetration rate
• 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
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4.3factors affecting penetration rate
• 4.3.6 bit hydraulics. Relation between bit
hydraulics and penetration rate
4 3factors affecting penetration rate
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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
4 3factors affecting penetration rate
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4.3factors affecting penetration rate
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4 3factors affecting penetration rate
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4.3factors affecting penetration rate
speedrotary
and blades,of numbereffictive element.cuttingeach
of n penetratioeffective
N
n
Lwhere
N n L R
be
pe
be pe
4 3factors affecting penetration rate
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4.3factors affecting penetration rate
• 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.
4 3factors affecting penetration rate
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4.3factors affecting penetration rate
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
4 3factors affecting penetration rate
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4.3factors affecting penetration rate
292.1
67.0
p pcb
d
cbe
p pe
L Ld d sC n
and L L
An example
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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
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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
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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
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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 3factors affecting penetration rate
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4.3factors affecting penetration rate
• 4.3.7 penetration rate equation
• For rolling cutter bits
))...()()()(( 4321 n f f f f f R
5 Bit Operation
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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
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5 Bit Operation
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5. Bit Operation
5 Bit Operation
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5. Bit Operation
• 5.2 prevention of accidental bit damage
1) breaker plate
2) tight spots3) establish fluid circulation first
5 Bit Operation
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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
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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
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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
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Bit Operation
• Cost-per-foot table