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Drilling BitsDrilling Bits

DRILLING ENGINEERINGDRILLING ENGINEERING

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Topics of Interest:Topics of Interest:

• Various bit types available (classification).

• Criteria for the selection for the best bit for a given situation.

• Standard methods for evaluating dull bite.

• Factors affecting bit wear and drilling speed.

• Optimization of bit weight and rotary speed.

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5.15.1 Types of BitsTypes of Bits

1. 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 (19th century).

2. Rolling Cutter Bits: (1909) have two or more cones containing the cutting elements which rotate about the axis of the cone as the bit is rotated at the bottom of the hole.

4Common Types of Drilling BitsCommon Types of Drilling Bits

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5.1.1 Drag Bits5.1.1 Drag Bits

• Design FeaturesDesign Features:: Number and shape of the cutting blades or stones. Size and location of the water courses. Metallurgy of the bit and cutting elements.

Drilling is achieved by physically blowing cuttings from the bottom of the bore-hole.

TypesTypes

(a) Steel cutter bits(a) Steel cutter bits

(b) Diamond bits(b) Diamond bits

(c) Polycrystalline diamond bits(c) Polycrystalline diamond bits

6Diamond cutter drag bit - design nomenclatureDiamond cutter drag bit - design nomenclature

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AdvantagesAdvantages No rolling parts which require strong clean bearing surfaces Because it is made from one solid piece of steel there is less

chance of bit breakage, which would leave junk in the bottom of the hole.

Steel Cutter Bits:Steel Cutter Bits: Best for soft, uniform unconsolidated formations. Now, replaced by other types in all area.

• Diamond Bits:Diamond Bits: Best for hard non-brittle formations. The face or crown of the bit consists of many diamonds set in

a tungsten carbide matrix. Fluid courses are provided in the matrix to direct the flow of

drilling fluid over the face of the bit.

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• Shape of crown profit is important

1. Step type2. Long taper (straight hole, high wt.)3. Short taper (easier to clean)4. Non taper (directional drilling)

• Size and number of diamonds, depend on the hardness of the formation.

• For hard formations: many small stones (0.07-0.125 carrot)• For soft formations: few large stones (0.75-2.0 carrot)• Pressure drop across the face of the bit

Pump pressure measured with the bit off bottom-pump pressure with the bit drilling = 500 – 1000 psi

• Manufacturer usually provide estimate of approximate circulating rate required establishing the needed pressure drop across the bit.

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5.1.25.1.2 PolyCrystalline Diamond PolyCrystalline Diamond (PCD) Bits(PCD) Bits

Since the mid 1970’s a new family of drag bits has been made possible by the introduction of a sintered polycrystalline diamond drill blanks, as a bit cutter element.

The drill blanks consist of a layer of a synthetic polycrystalline diamond about 1/64 in. thick that is bonded to a cemented tungsten carbide substrate in a high-pressure high-temperature process.

It contains many small diamond crystals bonded together.

The PCD is bonded either to a tungsten carbide bit-body matrix or to a tungsten carbide stud that is mounted in a steel bit body.

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They perform best in soft, firm, and medium-hard, non-abrasion formations that are not gummy.

Good results are obtained in carbonates or evaporates that are not broken up with hard shale stringers. Also good in a sandstone, siltstone, shale.

Design of crown profile is important, double-cone and flat profile.

Size, shape, number of cutters and angle of attack back rake, side rake and exposure: -20

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Diamond cutter drag bit- radial and feeder Diamond cutter drag bit- radial and feeder collectorscollectors

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5.1.3 ROLLING CUTTER BITS5.1.3 ROLLING CUTTER BITS

• The three-cone rolling cutter bit is by far the most common bit.

• Available with a large variety of tooth design and bearing types.

• Maximum use is made of limited space.

• Cone offset to stop rotating periodically to scrape the hole like (PCD) bits.

• It increases drilling speed but tooth wears faster. (4 for soft, 0 for hard)

• Shape of teeth: long widely spaced steel teeth are used for drilling soft formations.

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• As the rock type gets harder the tooth length and cone offset must be reduced to prevent tooth breakage.

• Tooth action = Scraping and twisting

• Zero offset cones action = Crushing

• Smaller tooth allows more room for the construction of stronger bearings

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Classification of Tricone BitsClassification of Tricone Bits

(a)(a) Milled tooth cuttersMilled tooth cutters(b)(b) Tungsten carbide insert cuttersTungsten carbide insert cutters

Hard facing on one side of the tooth allows self sharpening

Chipping tends to keep tooth sharp. Intermeshing is advantageous. Heel teeth = outer-raw very difficult job it wears it

leads to out of gauge bit (hole).

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Cheapest bearing assembly consist of:

Roller-type outer bearing Ball-type intermediate bearing Friction-type nose bearing

All standard bearings are lubricated by drilling fluids.

Intermediate cost bearing assembly is the sealed bearing assembly-lubricated by grease.

Expensive assembly: Journal bearing must have effective grease seals. It gives long bearing life.

16Wear Characteristic of milled-tooth bitsWear Characteristic of milled-tooth bits

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Example tungsten carbide insert cutter Example tungsten carbide insert cutter used in rolling cutter Bitsused in rolling cutter Bits

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Mohr’s circle Mohr’s circle

graphical analysisgraphical analysis

19IADC Diamond and PCD Drill BitsIADC Diamond and PCD Drill Bits

20IADC Diamond and PCD Drill BitsIADC Diamond and PCD Drill Bits

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IADC Diamond and PCD Core BitsIADC Diamond and PCD Core Bits

22IADC Diamond and PCD Core BitsIADC Diamond and PCD Core Bits

23IADC Roller Cutting BitsIADC Roller Cutting Bits

24IADC Roller Cutting BitsIADC Roller Cutting Bits

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Tooth Design Characteristic for Roller-Cutting Tooth Design Characteristic for Roller-Cutting BitsBits

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55.3.3 Bit Selection and EvaluationBit Selection and Evaluation• Determined by trial and error • Most valid criterion: drilling cost per unit interval drilled.

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Initial selection is based on formation characteristics and drilling cost in an area.Drillability: a measure of how easy the formation is to drill.Abrasiveness:a measure of how rapidly the tooth of milled tooth bit will wear when drilling the formation.Rules of Thumb:Table 5.5: Bit types often used in various formation types.

(1.16)

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55.3.1 .3.1 Grading Tooth WearGrading Tooth Wear Tooth wear of milled tooth bits is graded in terms of fractional tooth

height that has been worn away and is reported to the nearest eighth.

Example: Half original tooth height has been worn away, the bit will be graded as T4, i.e. the teeth are 4/8 worn.

“BT”: Broken teeth in a remarks column.

The average wear of the row of teeth with the most severe wear is reported.

Measure the height before and after the bit run. Rapid visual estimates with experience. Tooth wear of Insert bits is reported as the fraction of the total

number of inserts that have been broken or lost to the nearest eighth. Example: Half the inserts broken or lost it would be graded T4. i.e.

4/8 of the inserts are broken or lost.

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Tooth Wear guide chart for milled-tooth BitsTooth Wear guide chart for milled-tooth Bits

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5.3.2 Grading Bear Wear5.3.2 Grading Bear Wear

Difficult to evaluate in the field. Must be disassembled.

Bearing failure results in; Cones do not rotate “locked” Extremely loose cones.

– Code B8 : Bearings are 8/8 wornCode B8 : Bearings are 8/8 worn

Bearing failureBearing failure

B7: Slightly loose coneB7: Slightly loose cone

If it cannot be detected: It is estimated from the number of hours left in the bearing.

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B1B1

B4B4

B7B7

Actual Rotating Hours

Est

imat

ed H

ours

Lef

t

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Bearing grading guide for rolling cutter bitsBearing grading guide for rolling cutter bits

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55.3.3 .3.3 Grading Gauge WearGrading Gauge Wear

• When wear is in the base area of the rolling cones the bit will drill under sized hole.

• A Ring Gauge and a Ruler are used to measure the amount of gauge wear.

Example:Bit loses 0.5 inch in diameter the bit is Example:Bit loses 0.5 inch in diameter the bit is graded G-O-4graded G-O-4

O = Out of gauge bitO = Out of gauge bitI = In gauge bitI = In gauge bit4 = 4/8 of inch.4 = 4/8 of inch.

33Common Abbreviation used in describing bit Common Abbreviation used in describing bit

condition in dull bit evaluation.condition in dull bit evaluation.

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5.65.6 Termination A Bit RunTermination A Bit Run• There is always uncertainty about the best time to terminate a bot run

and begin tripping operations.

• Tooth and Bearing wear equations give at best a rough estimate of when the bit will be completely worn.

• It is helpful to monitor the torque needed to rotate the bit. The torque increases or fluctuates when a cone become locked due to worn bearing.

• If a sharp decrease in penetration rate is noticed it is advisable to pull the bit before it is completely worn.

• If the lithology is uniform, the total drilling cost can be minimized by minimizing the cost of each bit run.

• Keep a current estimate of cost/ft for the bit run, when it starts to increase pull the bit even if significant life remains.

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5.7 Factor Affecting Penetration Rate5.7 Factor Affecting Penetration Rate

Bit type Formation Characteristics Drilling Fluid properties Bit operation conditions (bit weight. and speed) Bit tooth wear Bit Hydraulics.

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5.8.3 Selection of Bit Weight and Rotary 5.8.3 Selection of Bit Weight and Rotary Speed Speed

The weight applied to the bit and the rotational speed of the drilling sting have a major effect on the both the penetration rate and the life of the bit.

Consideration must be given to the following items when selecting the bit weight and rotary speed.

1. The effect of the selected operating conditions on the cost per foot for the bit run question and on subsequent bit runs.

2. The effect of the selected operating conditions on crooked hole problems.

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3. The max. desired penetration rate for the fluid circulating rate and mud processing rates available and for efficient kick detection.

4. Equipment limitations on the available bit weight and rotary size.

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5.7.4 Operating Conditions5.7.4 Operating Conditions• The bit weight and rotary speed have a tremendous effect on

rate of penetration.

• As shown in the fig, No significant penetration rate is obtained until the threshold

bit weight is applied (Point a). Penetration rate then increases rapidly with increasing values

of bit weight for moderate values of bit weight (Segment ab). A linear curve is often observed at moderate bit weight,

subsequent increase in bit weight causes only slight improvement in the penetration rate (segment cd)

In some cases, a decrease in penetration rate is observed at extremely high values of bit weight (Segment de). This behavior is called bit floundering. It is due to less efficient bottom hole cleaning at higher rates of cutting generation.

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Weight on bit

Rot

ary

Spe

ed

a

b

c

d

e