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Drilling Engineering > Unit 4 - Drilling Bits > Flashcards

Unit 4 - Drilling Bits Flashcards

1
Q

Performance of the bit is a function of?

A
  • WOB - RPM - mud properties - hydraulic efficiency
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2
Q

What types of drilling bits are available?

A
  • drag bits - roller cone bits - diamond bits
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3
Q

Drag bits (description)?

A
  • consist of rigid steel blades in fish-tail shape
  • no longer commonly used
  • were used in soft formations
  • because of dragging/scraping action of bit → high RPM but low WOB
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4
Q

Why was there a decline in the use of drag bits?

A
  • introduction of roller cone bits → could drill soft formations more easily
  • too much WOB → excessive torque → bit- /drillpipe failure
  • drag bits tend to drill sloping holes → deviation control was necessary
  • limited to drilling through soft, uniformly unconsolidated formations without abrasive layers
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5
Q

Roller Cone Bits (Rock bits) - description?

A
  • most common bit used worldwide
  • cutting by cones with either steel teeth or tungsten carbide inserts
  • classified in milled tooth bits and insert bits
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6
Q

How are the cones fixed to the main body of the bit?

A

mounted on bearing pins, which extend from bit body

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7
Q

Why are 3 cones used for Roller cone bits?

A
  • even weight distribution - balanced cutting structure - centered hole (compared to 2 cone bit)
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8
Q

What is the advantage of having sealed bearings on roller cone bits?

A
  • prevent the mud causing premature failure by abrasion/corrosion of the bearings
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9
Q

Name the elements of a rock bit!

A
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10
Q

Description of a natural diamond bit ?

A
  • Type of drag bit (no moving cones)
  • scraping cutting action
  • diamonds are bonded into matrix on steel body
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11
Q

Why are natural diamonds used for a bit ?

A

Have a high wear resistance and are very hard

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12
Q

What are the disadvantages of natural diamond bits?

A
  • Cost ( up to 10x of similar sized Rock bit)
  • very sensitive to shock and vibration
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13
Q

When are natural diamond bits very cost effective?

A
  • When formations require long rotation hours (200-300 hours per bit)

→ diamond bits have no moving parts → longer lifetime than roller cone bits → long bit runs = less round trips

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14
Q

What does PDC - Bit stand for?

A

Polycrystalline Diamond compact bit

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15
Q

What is the difference of a PDC bit to a Natural Diamond bit?

A
  • use small discs of synthetic diamonds to provide a scraping cutting surface → these discs are not sensitive to failure along cleavage planes like natural diamonds
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16
Q

When are PDC- bits especially successful?

A
  • for long bit runs with a high ROP
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17
Q

When are PDC bits not used?

A
  • when very large bit diameter is needed
  • in extremly hard formations (granite, basalt, crystalline rock → would destroy PDC-bit quickly)
  • junk drilling
  • drilling out casing shoe
  • bridge plugs
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18
Q

What does TSP-Bit stand for?

A

Thermally Stable Polycrystalline Bit

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19
Q

TSP-Bit (Description)

A
  • Further development of PDC Bit (can tolerate way higher temperatures)
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20
Q

Which bits are the most common ones?

A
  • roller cone bit
  • pdc bit
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21
Q

Which are the 4 principle elements considered in the Roller Cone Bit Design?

A
  • bearing assemblies
  • cones
  • cutting elements
  • fluid circulation
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22
Q

Which types of bearings are used in Roller Cone Bits?

A
  • Roller bearings
  • Ball bearings
  • Friction bearing
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23
Q

What is the task of the roller bearing?

A
  • It forms the outer assembly → supports the radial loading (WOB)
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24
Q

What is the task of the ball bearings?

A

Resist longitudinal or thrust loads → help secure cones on journal

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25
Q

What is the most important factor in design of a bearing assembly ?

A
  • Space availability → bearings should be large enough to support applied load but balanced against journal strength and cone shell
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26
Q

What are journal bearing bits?

A
  • they don’t have roller bearing → cones are directly mounted onto journal

→contact area is specially treated and inlaid with alloys to increase wear resistance

→ only little lubrication needed → ball bearings still used to retain cones on journal

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27
Q

What is the advantage of a journal bearing bit?

A
  • larger contact area over which load is transmitted from cone to journal
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28
Q

What is one of the basic factors to be decided for the cone design?

A

journal or pin angle → since all three cones fit together, journal angle specifies the outside contour of the bit

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29
Q

What is the journal or pin angle ? (Graphik)

A
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30
Q

what is an important factor, which affects the journal angle of a bit?

A

The degree of meshing or interfit

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31
Q

Which aspects of the bit design are affected by the amount of interfit?

A
  • allows bigger space for tooth depth, more space for bearings and greater cone thickness
  • allows mechanical cleaning of grooves → helps prevent bit balling
  • provides space for one cone to extend across centre of hole → prevent coring effects
  • helps cutting action of cones by increasing cone slippage
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32
Q

What is an advantage if the cone is designed so that it slips during rotation ? (cone slippage)

A
  • allows rock bit to drill with normal grinding and crushing action + scraping action
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33
Q

What are the two ways on how cone slippage can be designed?

A
  1. cones with two profiles (outer + inner cone profile) → inner cone has to scrape while outer cone is forced to gauge → good for soft formations but excessive wear in hard formations
  2. offsetting axes of cones → journals have to be angled slightly away from centre → often in soft formations
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34
Q

What is the main factor on selecting milled teeth or inserts on a bit?

A

Mainly hardness of formation → main consideration then in design of cutting structure is height and spacing of teeth or inserts

35
Q

What kind of teeth would you select for soft formations?

A

Deep penetration → long, thin and widely spaced to prevent bit balling

36
Q

How does bit balling happen?

A

When soft formations are drilled and soft material accumulates on surface of bit → prevents the teeth from penetrating the rock

37
Q

Why would you choose many short and stubby teeth when drilling a harder formation?

A
  • teeth need enough strength to withstand the crushing and chipping action
  • large number of teeth reduces unit load
38
Q

Why is the outer edge of the cutting structure always protected by tungsten carbide hardfacing?

A
  1. Otherwise it could erode because of abrasive formation → decrease of hole diameter → undergauged section would have to be reamed out → waste of drilling time
  2. if gauge area is worn away → redistribution of thrust forces throughout bearing assembly → leading to possible bit failure
39
Q

How is mud nowadays ejected on bit?

A

Through three jet nozzles around the outside of the bit body → the turbulence created by the jet streams is enough to clean cutters and allow efficient drilling to continue

40
Q

What material are jet nozzles made of and why ?

A

Of tungsten carbide → to prevent fluid erosion

41
Q

What are the 7 major components of PDC bit design?

A
  • Cutting material
  • Bit body material
  • cutter rake
  • bit profile
  • cutter density
  • cutter exposure
  • fluid circulation
42
Q

What is the cutter material of a PDC-bit?

A
  • Polycristalline Diamond (PDC) → 90-95% pure diamond, which is manufactured into compacts, which are set into body of bit
43
Q

What is a problem with the PDC bits?

A

High friction temperatures generated with these bits leads to break up of the polycrystalline diamond

44
Q

What does TSP-Bit stand for and why were they introduced?

A
  • Thermally Stable Polycristalline Diamond → were introduced after seeing, that PDC-bits sometimes chipped during drilling
45
Q

Why do the cutters of PDC-bits sometimes chip?

A

-internal stresses that are caused by differential expansion of the diamond and binder material (binder is mostly cobalt and expands faster than diamond)

46
Q

Why do TSP-diamond bits not chip like PDC bits ?

A
  • they don’t have a cobalt binder → no internal stress due to different expansion
47
Q

What are the two types of bit body for PDC-bits?

A
  1. entirely steel body
  2. steel shell with tungsten carbide matrix on surface
48
Q

What is an advantage of a steel body bit?

A

Can be removed and replaced if damaged

49
Q

What is an advantage of a Matrix body bit?

A

is erosion and abrasion resistent + matrix pocket provides impact resistance for the cutter

50
Q

Which angle determines the size of a cutting that is produced?

A

The back rake angle → the smaller the rake angle, the larger the cutting and greater the ROP for a given WOB → but cutter is more vulnerable to breakage

51
Q

What is the task of the side rake angle?

A

To direct the cuttings towards the flank of the bit and into the annulus

52
Q
  • Three types of PDC bit crown profiles?
A
  • flat or shallow cone
  • tapered or double cone
  • parabolic
53
Q

What is the cutter density?

A

number of cutters per unit area on face of the bit

54
Q

What is the cutter exposure?

A

It is the amount by which cutters protrude from the bit body! → must be high enough to allow good cleaning of bit face but not too high to reduce mechanical strength of cutter

55
Q

How is fluid circulation supposed to be designed?

A
  • to remove cuttings efficiently and cool bit surface

→ can be satiesfied with fluid flowrate increasement or design of water courses that run across the face of bit → but increased fluid flow may also cause excessive erosion of the face and therefore cause premature failure

56
Q

What is used to classify Roller cone drillbits?

A

comparison charts by the IADC (International Association of Drilling Contractors)

57
Q

An IADC-Code consists of?

A

Three numbers and one character

58
Q

What do the numbers of the IADC-code describe?

A

column 1 - series → particular formation drillability

column 2 - each series is subdivided into 4 types according to drillability of formation

row 1 - features → are defined on horizontal axis of system

  • additional table → additional feature design
59
Q

How are soft formations to be drilled?

A
  • unconsolidated clays and sands
  • low WOB
  • high RPM
  • large flow rates → to clean hole effectively , but excessive flow rates may cause washouts
60
Q

How are medium hard formations to be drilled?

A
  • may include shales, gypsum, shaley lime, sand and siltstone
  • low WOB
  • high rotation speeds → soft sandstone can also be drilled with these parameters
  • high flow rates recommended
61
Q

How are hard formations to be drilled?

A
  • limeston, anhydrite, hard sandstone → rocks have high compressive strength and contain abrasive material
  • high WOB
  • slower rotary speeds
62
Q

How are Fixed Cutter Bits classified?

A

Four character classification

63
Q

” Rock Bit evaluation “ means, that each bit is pulled from the hole, its physical appearance is inspected and graded to the wear is has sustained. Why is that useful?

A
  • improve bit type selection
  • identify effects of WOB, RPM, etc.
  • allow drilling personnel to improve ability to recognise when a bit should be pulled
  • to evaluate the bit performance and help to improve the design
64
Q

By which criteria is the bit performance judged? What is the goal?

A
  • how much footage is drilled
  • how fast (rpm)
  • cost per foot of hole drilled
    goal: lowest cost per foot drilled
65
Q

Penetration rate of a roller cone bit is a function of ?

A
  • WOB
  • RPM
  • mud properties
  • hydraulic efficiency
66
Q

If you increase the WOB, the penetration rate will increase linearly but there are certain limitations?

Name them!

A
  • HHP at bit
  • Type of formation
  • Hole deviation
  • bearing life
  • tooth life
67
Q

Why is HHP at bit a limitation factor for the penetration rate?

A
  • If HHP at bit is not sufficient, to ensure good bit cleaning, then ROP is reduced by either
    1. bit balling → grooves between teeth of bit are clogged by formation cuttings (mostly in soft formations)
    2. bottom hole balling → hole gets clogged up with fine particles (mostly with grinding action of hard formation bits)

→ if this happens → no increase in ROP from increase of WOB unless HHP is improved

68
Q

Formula to calculate the HHP?

A

(Pb x Q)/1714

Pb = Pressure drop across nozzles of bit (psi)

Q = flow rate through bit (gpm)

69
Q

How can you increase the HHP?

A

higher Pb (smaller nozzles) or Q (faster pump speed or larger liners) → but could mean change to other drilling factors → may not be beneficial

70
Q

Why is the type of formation a limitation factor for the penetration rate?

A

in soft formations, excessive WOB would bury teeth into rock → would increase the torque but not the ROP

71
Q

Why is the hole deviation a limitation factor for the penetration rate?

A
  • in some areas WOB will produce bending in the drillstring → crooked hole
72
Q

Why is the bearing life a limitation factor for the penetration rate?

A

the greater the load on bearings, the shorter their operational life → optimising ROP will depend on compromise between WOB and bearing wear

73
Q

Why is the tooth life a limitation factor for the penetration rate?

A
  • In hard formations (high compressive strength) excessive WOB will cause teeth to break
74
Q

The applied RPM is a function of?

A
  1. Type of bit → lower RPM for insert bits than milled tooth bits → give inserts more time to penetrate formation
  2. Type of formation → harder formations are less easily penetrated → require low RPM → high RPM may cause damage to bit or the drill string
75
Q

Why do you have a filter cake at the bottom of the hole?

A

overbalance of mud forces liquid portion of mud (the filtrate) into the formation and solids form a filter cake on wall of the borehole → also happens on the bottom of borehole

76
Q

What happens when the tooth of a bit penetrates the surface of a rock?

A
  • compressive strength of rock is exceeded and cracks develop, which loosen small fragments or chips from formation
77
Q

What is the problem with a filter cake at bottom of Hole?

A
  • between the successive teeth, the filter cake covers up the cracks and therefore prevents the mud pressure being exerted below the chip → differential pressure on chip tends to keep the chip against the formation → static chip hold down effect
78
Q

How can you reduce the chip hold down effect?

A
  • reduction of positive differential pressure by lowering the mud weight
  • reduction of solid content from mud → both, clay and drilled solids
79
Q

Why does the dynamic chip hold down effect occur?

A
  • Cracks form around the chip → mud enters the cracks to equalise the pressure → but by doing that, a pressure drop is created which keeps the chip pressed against the bottom of the hole

→ the longer the tooth penetration, the greater the hold down pressure

80
Q

What is the general problem caused by static and dynamic hold down effects’?

A

They cause bit balling and bottom hole balling

81
Q

With which settings should PDC-bits be used?

A
  • low WOB and high RPM
  • highest RPM possible should be used
82
Q

What are reasons for improved performance of PDC-bits with oil based muds?

A
  • increased lubricity
  • decreased cutter wear temp.
  • preferential oil wetting of bit body
83
Q
A

Drilling Engineering (13 decks)

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