1

PETE 411Well Drilling

Lesson 7Drilling Bits - Drag Bits

2

Contents

The Ideal Bit Drag Bits

Fishtail Type Natural Diamond Polycrystalline Diamond Compact

(PDC)

Relative Costs of Bits

3

Read: ADE, Ch.5 (bits)

HW # 3: due 9 -18- 2002

4

The purpose of Chapter 5 (ADE) is to introduce the student

to the:

Rotary Drill bits

selection and operation

of rotary drilling bits.

5

Rotary Drilling Bits

Bit types available Criteria for selecting the best bit for a

given situation Standard methods for evaluating dull bits Factors affecting bit wear and drilling

speed Optimization of bit weight and rotary

speed

6

Bit types available

Drag bits (fixed cutter blades) Fishtail bit Natural diamond bits PDC Bits (Polycrystalline Diamond Compact)

Rolling cutter bits (rock bits - with cones) Mill tooth bits Tungsten carbide bits

7

The Ideal Bit *

1. High drilling rate

2. Long life

3. Drill full-gauge, straight hole

4. Moderate cost

* (Low cost per ft drilled)

8

The Ideal Bit

Hardness (soft, medium, hard) abrasiveness cuttings stickiness other considerations … e.g. cost

“The Ideal Bit” will depend on the type of formation to be drilled

9

Drag Bits

Drag bits drill by physically “plowing”

or “machining” cuttings from the

bottom of the hole.

10

Drag Bits

Cutter may be made from:

Steel Tungsten carbide Natural diamonds Polycrystalline diamonds (PDC)

Drag bits have no moving parts, so it is less likely that junk will be left in the hole.

11Fishtail type drag bit

12Natural Diamond Bits PDC Bits

13

Natural Diamond

bit

junk slotcuttings

radial flow

high p across face

14

Soft Formation

Diamond bit

Larger diamonds Fewer diamonds Pointed nose

15

Hard Formation

Diamond bit

Smaller diamonds More diamonds Flatter nose

16

Natural Diamonds

The size and spacing of diamonds on a bit determine its use.

NOTE: One carat = 200 mg precious stones

What is 14 carat gold?

17

Natural Diamonds

2-5 carats - widely spaced diamonds are used for drilling soft formations such as

soft sand and shale

1/4 - 1 carat - diamonds are used for drilling sand, shale and limestone formations of varying (intermediate) hardness.

1/8 - 1/4 carat - diamonds, closely spaced,

are used in hard and abrasive formations.

18

When to Consider Using a Natural Diamond Bit?

1. Penetration rate of rock bit < 10 ft/hr. 2. Hole diameter < 6 inches. 3. When it is important to keep the bit and

pipe in the hole. 4. When bad weather precludes making trips. 5. When starting a side-tracked hole. 6. When coring.* 7. When a lower cost/ft would result

19

Top view of diamond bit

20

Side view of diamond bit

21

PDC bits

Courtesy Smith Bits

22

PDC Cutter

23

24At about $10,000-150,000 apiece, PDC bits cost five to 15 times more than roller cone bits

PDC Bits

25The Rise in Diamond Bit Market Share

26

Coring bit

PDC + natural

diamond

27

Bi-Center bit

Courtesy Smith Bits

28

Relative Costs of Bits

Diamond WC Insert Milled Bits Bits Tooth Bits

$/Bit

• Diamond bits typically cost several times as much as tri-cone bits with tungsten carbide inserts (same bit diam.)• A TCI bit may cost several times as much as a

milled tooth bit.

29

PDC BitsRef: Oil & Gas Journal, Aug. 14, 1995, p.12

Increase penetration rates in oil and gas wells

Reduce drilling time and costs Cost 5-15 times more than roller cone bits 1.5 times faster than those 2 years earlier Work better in oil based muds; however,

these areas are strictly regulated

30

PDC Bits

Parameters for effective use include

weight on bit mud pressure flow rate rotational speed

31

PDC Bits

Economics• Cost per foot drilled measures Bit performance economics

• Bit Cost varies from 2%-3% of total cost, but bit affects up to 75% of total cost

• Advantage comes when - the No. of trips is reduced, and when - the penetration rate increases

32

PDC Bits

Bit Demand U.S Companies sell > 4,000 diamond drill

bits/year

Diamond bit Market is about $200 million/year

Market is large and difficult to reform

When bit design improves, bit drills longer

33

PDC Bits

Improvements in bit stability, hydraulics, and cutter design => increased

footage per bit

Now, bits can drill both harder and softer formations

Formations in US are not as conducive to PDC bits as formations in some other areas

Bit Demand, cont’d

34

PDC Bits

Improvements are a result of the following: Research Good Engineering Practices Competition with other PDC bit manufacturers/rock bit industries

Bit Design General Electric introduced PDC in 1973

Product Life = 2 years

35

PDC Bits

Bit Design, cont’d

Now, a speciality tool

PDC bit diameter varies from 3.5 in to 17.5 in

Goals of hydraulics: clean bit without eroding it

clean cuttings from bottom of hole

36

PDC Bits

Factors that limit operating range and economics:

Lower life from cutter fractures

Slower ROP from bad cleaning

Bit design, cont’d

37

PDC Bits

Cutters• Consist of thin layer of bonded diamond

particles + a thicker layer of tungsten carbide

• Diamond• 10x harder than steel• 2x harder than tungsten carbide• Most wear resistant material

but is brittle and susceptible to damage

38

PDC Bits

• Diamond/Tungsten Interface

• Bond between two layers on cutter is critical

• Consider difference in thermal expansion coefficients and

avoid overheating

• Made with various geometric shapes to reduce stress on diamond

Cutters, cont’d

39

PDC Bits

• Various Sizes

• Experimental dome shape

• Round with a buttress edge for high impact loads

• Polished with lower coefficient of friction

Cutters, cont’d

40

PDC Bits

Bit Whirl (bit instability)• Bit whirl = “any deviation of bit rotation

from the bit’s geometric center”• Caused by cutter/rock interaction forces

• PBC bit technology sometimes reinforces whirl

• Can cause PDC cutters to chip and break

41

PDC Bits

Preventing Bit Whirl

• Cutter force balancing

• Bit asymmetry

• Gauge design

• Bit profile

• Cutter configuration

• Cutter layout

42

PDC Bits

Applications

• PDC bits are used primarily in

• Deep and/or expensive wells

• Soft-medium hard formations

43

PDC Bits

Advances in metallurgy, hydraulics and cutter geometry

• Have not cut cost of individual bits

• Have allowed PDC bits to drill longer and more effectively

• Allowed bits to withstand harder formations

Application, cont’d

44

PDC Bits

Application, cont’d• PDC bits advantageous for high rotational speed drilling and in deviated hole section drillings

• Most effective: very weak, brittle formations (sands, silty claystone, siliceous shales)

• Least effective: cemented abrasive sandstone, granites

45

Grading of Worn PDC Bits

CT - Chipped CutterLess than 1/3 of cutting element is gone

BT - Broken CutterMore than 1/3 of cutting element is broken to the substrate

46

Grading of Worn PDC Bits – cont’d

LT - Lost CutterBit is missing one or more cutters

LN - Lost NozzleBit is missing one or more nozzles

47

Table 7.7 - Commonly Used Bit Sizes For Running API Casing

Casing Size Coupling Size Common Bit (OD in.) (OD in.) Sizes Used (in.)

4 1/2 5.0 6, 6 1/8, 6 1/4

5 5.563 6 1/2, 6 3/4

5 1/2 6.050 7 7/8, 8 3/8

6 6.625 7 7/8, 8 3/8, 8 1/2

6 5/8 7.390 8 1/2, 8 5/8, 8 3/4

7 7.656 8 5/8, 8 3/4, 9 1/2

7 5/8 8.500 9 7/8, 10 5/8, 118 5/8 9.625 11, 12 1/4

9 5/8 10.625 12 1/4, 14 3/4

10 3/4 11.750 1513 3/8 14.375 17 1/2

16 17 2020 21 24, 26

48

ENDof

Lesson 7 - Drag Bits -