chapter3-new (drilling bits)

8/3/2019 Chapter3-New (Drilling Bits)

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Drilling Bits

DRILLING ENGINEERING


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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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4Common 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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6Diamond 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 unconsolidatedformations. 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 polycrystallinediamond about 1/64 in. thick that is bonded to a cementedtungsten carbide substrate in a high-pressure high-temperatureprocess.

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 backrake, 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 commonbit.

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 holelike (PCD) bits.

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

Shape of teeth: long widely spaced steel teeth are used fordrilling soft formations.


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As the rock type gets harder the tooth length and coneoffset 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 ofstronger 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 selfsharpening

Chipping tends to keep tooth sharp.

Intermeshing is advantageous.

Heel teeth = outer-raw very difficult job it wears itleads to out of gauge bit (hole).


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Cheapest bearingassembly consist of:

Roller-type outer bearing

Ball-type intermediate bearing

Friction-type nose bearing

All standard bearings are lubricated by drillingfluids.

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

Expensive assembly: Journal bearing must have

effective grease seals. It gives long bearing life.


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16Wear 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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19IADC Diamond and PCD Drill Bits


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20IADC Diamond and PCD Drill Bits


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21IADC Diamond and PCD Core Bits


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22IADC Diamond and PCD Core Bits


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23IADC Roller Cutting Bits


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24IADC 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.

D

tttCC

C

tcbb

f

)(

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 toothbit 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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5.3.1 Grading Tooth Wear

Tooth wear of milled tooth bits is graded in terms offractional tooth height that has been worn away and isreported 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 severewear 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 thenearest 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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B1

B4

B7

Actual Rotating Hours

EstimatedHoursLeft


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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.

Example:Bit loses 0.5 inch in diameter the bit is

graded G-O-4

O = Out of gauge bit

I = In gauge bit4 = 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 terminatea bot run and begin tripping operations.

Tooth and Bearing wear equations give at best a roughestimate 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 isadvisable to pull the bit before it is completely worn.

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

Keep a current estimate of cost/ft for the bit run, when itstarts 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.

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 Conditions

The bit weight and rotary speed have a tremendous effect onrate of penetration.

As shown in the fig,

No significant penetration rate is obtained until the thresholdbit weight is applied (Point a).

Penetration rate then increases rapidly with increasing valuesof 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 slightimprovement in the penetration rate (segment cd)

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


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Weight on bit

Rotary

Speed

a

b

c

d

e

chapter3-new (drilling bits)

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