Drilling Bits

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DRILLING ENGINEERING
Drilling Bits
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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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5.1 Types 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.

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Common Types of Drilling Bits
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5.1.1 Drag Bits

• Design 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.
• Types
• (a) Steel cutter bits
• (b) Diamond bits
• (c) Polycrystalline diamond bits

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Diamond cutter drag bit - design nomenclature
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• Advantages
• 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 Best for soft, uniform
  unconsolidated formations. Now, replaced by
  other types in all area.
• 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 type
• 2. 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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5.1.2 PolyCrystalline Diamond (PCD) Bits

• Since the mid 1970s 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
collectors
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5.1.3 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 Bits

• (a) Milled tooth cutters
• (b) Tungsten 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.

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Wear Characteristic of milled-tooth bits
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Example tungsten carbide insert cutter used in
rolling cutter Bits
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Mohrs circle graphical analysis
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IADC Diamond and PCD Drill Bits
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IADC Diamond and PCD Drill Bits
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IADC Diamond and PCD Core Bits
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IADC Diamond and PCD Core Bits
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IADC Roller Cutting Bits
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IADC Roller Cutting Bits
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Tooth Design Characteristic for Roller-Cutting
Bits
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5.3 Bit Selection and Evaluation

• Determined by trial and error
• Most valid criterion drilling cost per unit
  interval drilled.

(1.16)

• Initial selection is based on formation
  characteristics and drilling cost in an area.
• Drillability a measure of how easy the formation
  is to drill.
• Abrasivenessa 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.

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5.3.1 Grading 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 Bits
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5.3.2 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 worn
• Bearing failure
• B7 Slightly loose cone
• If it cannot be detected It is estimated from
  the number of hours left in the bearing.

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Estimated Hours Left
Actual Rotating Hours
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Bearing grading guide for rolling cutter bits
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5.3.3 Grading 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.
• ExampleBit loses 0.5 inch in diameter the bit is
  graded G-O-4
• O Out of gauge bit
• I In gauge bit
• 4 4/8 of inch.

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Common Abbreviation used in describing bit
condition in dull bit evaluation.
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5.6 Termination 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 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 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.
• The effect of the selected operating conditions
  on the cost per foot for the bit run question and
  on subsequent bit runs.
• The effect of the selected operating conditions
  on crooked hole problems.

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• The max. desired penetration rate for the fluid
  circulating rate and mud processing rates
  available and for efficient kick detection.
• Equipment limitations on the available bit weight
  and rotary size.

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5.7.4 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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d
Rotary Speed
c
e
b
a
Weight on bit