Unit 4 - Drilling Bits Flashcards
Performance of the bit is a function of?
- WOB - RPM - mud properties - hydraulic efficiency
What types of drilling bits are available?
- drag bits - roller cone bits - diamond bits
Drag bits (description)?
- 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
Why was there a decline in the use of drag bits?
- 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
Roller Cone Bits (Rock bits) - description?
- most common bit used worldwide
- cutting by cones with either steel teeth or tungsten carbide inserts
- classified in milled tooth bits and insert bits
How are the cones fixed to the main body of the bit?
mounted on bearing pins, which extend from bit body
Why are 3 cones used for Roller cone bits?
- even weight distribution - balanced cutting structure - centered hole (compared to 2 cone bit)
What is the advantage of having sealed bearings on roller cone bits?
- prevent the mud causing premature failure by abrasion/corrosion of the bearings
Name the elements of a rock bit!
Description of a natural diamond bit ?
- Type of drag bit (no moving cones)
- scraping cutting action
- diamonds are bonded into matrix on steel body
Why are natural diamonds used for a bit ?
Have a high wear resistance and are very hard
What are the disadvantages of natural diamond bits?
- Cost ( up to 10x of similar sized Rock bit)
- very sensitive to shock and vibration
When are natural diamond bits very cost effective?
- 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
What does PDC - Bit stand for?
Polycrystalline Diamond compact bit
What is the difference of a PDC bit to a Natural Diamond bit?
- 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
When are PDC- bits especially successful?
- for long bit runs with a high ROP
When are PDC bits not used?
- 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
What does TSP-Bit stand for?
Thermally Stable Polycrystalline Bit
TSP-Bit (Description)
- Further development of PDC Bit (can tolerate way higher temperatures)
Which bits are the most common ones?
- roller cone bit
- pdc bit
Which are the 4 principle elements considered in the Roller Cone Bit Design?
- bearing assemblies
- cones
- cutting elements
- fluid circulation
Which types of bearings are used in Roller Cone Bits?
- Roller bearings
- Ball bearings
- Friction bearing
What is the task of the roller bearing?
- It forms the outer assembly → supports the radial loading (WOB)
What is the task of the ball bearings?
Resist longitudinal or thrust loads → help secure cones on journal
What is the most important factor in design of a bearing assembly ?
- Space availability → bearings should be large enough to support applied load but balanced against journal strength and cone shell
What are journal bearing bits?
- 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
What is the advantage of a journal bearing bit?
- larger contact area over which load is transmitted from cone to journal
What is one of the basic factors to be decided for the cone design?
journal or pin angle → since all three cones fit together, journal angle specifies the outside contour of the bit
What is the journal or pin angle ? (Graphik)
what is an important factor, which affects the journal angle of a bit?
The degree of meshing or interfit
Which aspects of the bit design are affected by the amount of interfit?
- 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
What is an advantage if the cone is designed so that it slips during rotation ? (cone slippage)
- allows rock bit to drill with normal grinding and crushing action + scraping action
What are the two ways on how cone slippage can be designed?
- 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
- offsetting axes of cones → journals have to be angled slightly away from centre → often in soft formations
What is the main factor on selecting milled teeth or inserts on a bit?
Mainly hardness of formation → main consideration then in design of cutting structure is height and spacing of teeth or inserts
What kind of teeth would you select for soft formations?
Deep penetration → long, thin and widely spaced to prevent bit balling
How does bit balling happen?
When soft formations are drilled and soft material accumulates on surface of bit → prevents the teeth from penetrating the rock
Why would you choose many short and stubby teeth when drilling a harder formation?
- teeth need enough strength to withstand the crushing and chipping action
- large number of teeth reduces unit load
Why is the outer edge of the cutting structure always protected by tungsten carbide hardfacing?
- Otherwise it could erode because of abrasive formation → decrease of hole diameter → undergauged section would have to be reamed out → waste of drilling time
- if gauge area is worn away → redistribution of thrust forces throughout bearing assembly → leading to possible bit failure
How is mud nowadays ejected on bit?
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
What material are jet nozzles made of and why ?
Of tungsten carbide → to prevent fluid erosion
What are the 7 major components of PDC bit design?
- Cutting material
- Bit body material
- cutter rake
- bit profile
- cutter density
- cutter exposure
- fluid circulation
What is the cutter material of a PDC-bit?
- Polycristalline Diamond (PDC) → 90-95% pure diamond, which is manufactured into compacts, which are set into body of bit
What is a problem with the PDC bits?
High friction temperatures generated with these bits leads to break up of the polycrystalline diamond
What does TSP-Bit stand for and why were they introduced?
- Thermally Stable Polycristalline Diamond → were introduced after seeing, that PDC-bits sometimes chipped during drilling
Why do the cutters of PDC-bits sometimes chip?
-internal stresses that are caused by differential expansion of the diamond and binder material (binder is mostly cobalt and expands faster than diamond)
Why do TSP-diamond bits not chip like PDC bits ?
- they don’t have a cobalt binder → no internal stress due to different expansion
What are the two types of bit body for PDC-bits?
- entirely steel body
- steel shell with tungsten carbide matrix on surface
What is an advantage of a steel body bit?
Can be removed and replaced if damaged
What is an advantage of a Matrix body bit?
is erosion and abrasion resistent + matrix pocket provides impact resistance for the cutter
Which angle determines the size of a cutting that is produced?
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
What is the task of the side rake angle?
To direct the cuttings towards the flank of the bit and into the annulus
- Three types of PDC bit crown profiles?
- flat or shallow cone
- tapered or double cone
- parabolic
What is the cutter density?
number of cutters per unit area on face of the bit
What is the cutter exposure?
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
How is fluid circulation supposed to be designed?
- 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
What is used to classify Roller cone drillbits?
comparison charts by the IADC (International Association of Drilling Contractors)
An IADC-Code consists of?
Three numbers and one character
What do the numbers of the IADC-code describe?
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
How are soft formations to be drilled?
- unconsolidated clays and sands
- low WOB
- high RPM
- large flow rates → to clean hole effectively , but excessive flow rates may cause washouts
How are medium hard formations to be drilled?
- 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
How are hard formations to be drilled?
- limeston, anhydrite, hard sandstone → rocks have high compressive strength and contain abrasive material
- high WOB
- slower rotary speeds
How are Fixed Cutter Bits classified?
Four character classification
” 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?
- 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
By which criteria is the bit performance judged? What is the goal?
- how much footage is drilled
- how fast (rpm)
- cost per foot of hole drilled
goal: lowest cost per foot drilled
Penetration rate of a roller cone bit is a function of ?
- WOB
- RPM
- mud properties
- hydraulic efficiency
If you increase the WOB, the penetration rate will increase linearly but there are certain limitations?
Name them!
- HHP at bit
- Type of formation
- Hole deviation
- bearing life
- tooth life
Why is HHP at bit a limitation factor for the penetration rate?
- 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
Formula to calculate the HHP?
(Pb x Q)/1714
Pb = Pressure drop across nozzles of bit (psi)
Q = flow rate through bit (gpm)
How can you increase the HHP?
higher Pb (smaller nozzles) or Q (faster pump speed or larger liners) → but could mean change to other drilling factors → may not be beneficial
Why is the type of formation a limitation factor for the penetration rate?
in soft formations, excessive WOB would bury teeth into rock → would increase the torque but not the ROP
Why is the hole deviation a limitation factor for the penetration rate?
- in some areas WOB will produce bending in the drillstring → crooked hole
Why is the bearing life a limitation factor for the penetration rate?
the greater the load on bearings, the shorter their operational life → optimising ROP will depend on compromise between WOB and bearing wear
Why is the tooth life a limitation factor for the penetration rate?
- In hard formations (high compressive strength) excessive WOB will cause teeth to break
The applied RPM is a function of?
- Type of bit → lower RPM for insert bits than milled tooth bits → give inserts more time to penetrate formation
- Type of formation → harder formations are less easily penetrated → require low RPM → high RPM may cause damage to bit or the drill string
Why do you have a filter cake at the bottom of the hole?
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
What happens when the tooth of a bit penetrates the surface of a rock?
- compressive strength of rock is exceeded and cracks develop, which loosen small fragments or chips from formation
What is the problem with a filter cake at bottom of Hole?
- 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
How can you reduce the chip hold down effect?
- reduction of positive differential pressure by lowering the mud weight
- reduction of solid content from mud → both, clay and drilled solids
Why does the dynamic chip hold down effect occur?
- 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
What is the general problem caused by static and dynamic hold down effects’?
They cause bit balling and bottom hole balling
With which settings should PDC-bits be used?
- low WOB and high RPM
- highest RPM possible should be used
What are reasons for improved performance of PDC-bits with oil based muds?
- increased lubricity
- decreased cutter wear temp.
- preferential oil wetting of bit body
