4. Drill Bits And Bit Design

DRILL BITS

Drill Bits A drilling bit is the cutting or boring tool which is made up on the end of the drillstring The bit drills through the rock by scraping, chipping, gouging or grinding the rock at the bottom of the hole Drilling fluid is circulated through passageways in the bit to remove the drilled cuttings There are however many variations in the design of drill bits and the bit selected for a particular application will depend on the type of formation to be drilled

Drill Bits Drillers want bits that drill holes as fast as possible They want bits that give a good rate of penetration (ROP) They also want bits that last a long time A bit should be able to drill a full-size or fullgauge hole during the entire time it is on bottom and drilling If the sides of the bit wear down, it will drill an undersize or under-gauge hole

Drill Bits This presentation will present the different types of drill bit used in drilling operations and the way in which these bits have been designed to cope with the conditions which they will be exposed to An understanding of the design features of these bits will be essential when selecting a drill bit for a particular operation Since there are a massive range of individual bit designs the drill bit manufacturers have collaborated in the classification of all of the available bits into a Bit Comparison Chart

Drill Bits When a section of hole has been drilled and the bit is pulled from the wellbore the nature and degree of damage to the bit must be carefully recorded A system, known as the Dull Bit Grading System, has been devised It was devised by the Association of Drilling Contractors - IADC to facilitate this grading process This system will also be described in detail

Drill Bits The choice of which bit to use depends on several things One is the type of formation to be drilled Is the formation: Hard? Soft? Or is it medium-hard or medium-soft? Another item that bears on which bit to use is its cost

Drill Bits There are basically three types of drilling bit Drag Bits (no longer in use) Roller Cone Bits (Rock Bits) Diamond Bits

Roller Cone Bits (Rock Bits) Roller cone bits (or rock bits) are still the most common type of bit used world wide The cutting action is provided by cones which have either steel teeth or tungsten carbide inserts These cones rotate on the bottom of the hole and drill hole predominantly with a grinding and chipping action Rock bits are classified as milled tooth bits or insert bits depending on the cutting surface on the cones

Roller Cone Bits (Rock Bits) The roller-cone bit has conical cutters or cones that have spiked teeth around them As the drillstring is rotated, the bit cones roll along the bottom of the hole in a circle As they roll, new teeth come in contact with the bottom of the hole, crushing the rock immediately below and around the bit tooth A high-velocity fluid jet strikes the crushed rock chips to remove them from the bottom of the hole and up the annulus As this occurs, another tooth makes contact with the bottom of the hole and creates new rock chips

Roller Cone Bits (Rock Bits)

Roller Cone Bits (Rock Bits) The cones of the 3 cone bit are mounted on bearing pins, or arm journals, which extend from the bit body The bearings allow each cone to turn about its own axis as the bit is rotated The use of 3 cones allows an even distribution of weight It also gives a balanced cutting structure And drills a better gauge hole than the 2 cone design

Roller Cone Bits (Rock Bits) Milled Tooth Bit

Roller Cone Bits (Rock Bits) Soft Formation Milled Tooth Bit Soft-formation rock bits have their cones offset The teeth scrape and gouge the formation as the cone rolls on the bottom of the hole The amount of scraping depending on the amount of cone offset

Roller Cone Bits (Rock Bits) Hard Formation Milled Tooth Bit

More closely spaced, shorter, stronger teeth are used for harder rocks

Roller Cone Bits (Rock Bits) Insert Bit Run at high rotary speeds, up to 180 revolutions per minute (rpm) or more Compared to the 45 rpm used with older ones New high-speed carbide insert bits also run with much higher weight on bit (WOB)

Roller Cone Bits (Rock Bits) Insert Bit

Roller Cone Bits (Rock Bits) The advantages of the carbide insert bit are: Great durability Good insert burial into the formation - up to 80 percent of the insert per revolution in soft formations The ability to drill different types of formations with the same bit

Roller Cone Bits (Rock Bits) The disadvantages of the carbide insert bit are: Erosion around the base of inserts can result in their loss The possibility that with complete insert burial an area of the cone shell can come into contact with the formation and transmit shock loads from the drill string directly to the bearing

Roller Cone Bits (Rock Bits)

Roller Cone Bits (Rock Bits) The major advances in rock bit design since the introduction of the Hughes rock bit include: Improved cleaning action by using jet nozzles Using tungsten carbide for hard-facing and gauge protection Introduction of sealed bearings to prevent the mud causing premature failure due to abrasion and corrosion of the bearings

Roller Cone Bits (Rock Bits) Many general types of milled tooth and insert bits are now available:

Manufacture of Roller Cone Bits

Care and Maintenance of Roller Cone Bits Pointers for making up the bit include the following: If the box containing the bits has been opened previously, check the threads on the bit shank, and clean them if necessary Dope the threads with a good-quality, clean thread lubricant of the type recommended for tool joint threads Be sure to use the correct breaker plate for the size and type of bit being run (see next slide)

Care and Maintenance of Roller Cone Bits Making up the bit:

Care and Maintenance of Roller Cone Bits Pointers for making up the bit include the following: Cover the hole, place the bit breaker in the locked rotary table, place the bit in the breaker, and lower the collar over the shank then rotate the collar by hand until it shoulders onto the bit Place the makeup tongs on the collar just above the bit and make up to the proper torque, never use a sledgehammer on a bit or on the tongs in an effort to tighten the bit

Care and Maintenance of Roller Cone Bits Running the bit to bottom: If there are ledges or boulders encroaching on the hole, work the bit past them If the hole is under-gauge, especially near bottom, ream down to bottom with due care If there is likely to be a fill of cuttings settled on bottom while the pipe is out of the hole, run the bit in carefully, with full circulation to clean out

ROLLER CONE BIT DESIGN

Roller Cone Bit Design The design of roller cone bits can be described in terms of the four principle elements of their design We will be looking at the following aspects of the design: Bearing assemblies Cones Cutting elements Fluid circulation

Roller Cone Bit Design - Bearing Assemblies The cones of a roller cone bit are mounted on journals as shown below

Roller Cone Bit Design - Bearing Assemblies There are three types of bearings used in these bits: Roller bearings Which form the outer assembly and help to support the radial loading (or WOB) Ball bearings Which resist longitudinal or thrust loads and also help to secure the cones on the journals

Roller Cone Bit Design - Bearing Assemblies There are three types of bearings used in these bits: A friction bearing In the nose assembly which helps to support the radial loading The friction bearing consists of a special bushing pressed into the nose of the cone This combines with the pilot pin on the journal to produce a low coefficient of friction to resist seizure and wear

Roller Cone Bit Design - Bearing Assemblies All bearing materials must be made of toughened steel The most important factor in the design of the bearing assembly is the space availability The bearings should be large enough to support the applied loading The bearings should not wear out before the cutting structure i.e. all bit components should wear out evenly

Roller Cone Bit Design - Bearing Assemblies There have been a number of developments in bearing technology used in rock bits The bearing assemblies of the first roller cone bits were open to the drilling fluid Two new bits were developed in the late 1950’s to combat this: Sealed bearing bits Journal bearing bits

Roller Cone Bit Design - Bearing Assemblies Sealed bearing bits: Were introduced in the late 1950s, to extend the bearing life of insert bits The sealing mechanism prevents abrasive solids in the mud from entering and causing excess frictional resistance in the bearings The bearings are lubricated by grease which is fed in from a reservoir as required Some manufacturers claim a 25% increase in bearing life by using this arrangement

Roller Cone Bit Design - Bearing Assemblies Sealed bearing bits:

Roller Cone Bit Design - Bearing Assemblies Journal bearing bits: The cones are mounted directly onto the journal This offers the advantage of a larger contact area over which the load is transmitted from the cone to the journal The contact area is specially treated and inlaid with alloys to increase wear resistance Only a small amount of lubrication is required as part of the sealing system

Roller Cone Bit Design - Bearing Assemblies Journal bearing bits:

Roller Cone Bit Design – Cone Design All three cones have the same shape except that the No. 1 cone has a spear point

Roller Cone Bit Design – Cone Design

One important factor which affects journal angle is the degree of meshing or interfit i.e. the distance that the crests of the teeth of one cone extend into the grooves of the other The amount of interfit affects several aspects of bit design

Roller Cone Bit Design – Cone Design Interfit affects several aspects of bit design: It allows increased space for tooth depth, more space for bearings and greater cone thickness It allows mechanical cleaning of the grooves, thus helping to prevent bit balling It provides space for one cone to extend across the centre of the hole to prevent coring effects It helps the cutting action of the cones by increasing cone slippage

Roller Cone Bit Design – Cone Design Cone slippage: In some formations, it is advantageous to design the cones so that they do not rotate evenly but that they slip during rotation This allows a rock bit to drill using a scraping action, as well as the normal grinding or crushing action

Roller Cone Bit Design – Cone Design Cone slippage: Cone slippage can also be attained by offsetting the axes of the cones This is often used in soft formation bits

Roller Cone Bit Design – Cutting Structure The selection of a milled tooth or insert bit is largely based on the hardness of the formation to be drilled The design of the cutting structure will therefore be based on the hardness of the formation for which it will be used The main considerations in the design of the cutting structure is the height and spacing of teeth or inserts

Roller Cone Bit Design – Cutting Structure Soft formation bits: These require deep penetration into the rock so the teeth are long, thin and widely spaced to prevent bit balling Bit balling occurs when soft formations are drilled and the soft material accumulates on the surface of the bit preventing the teeth from penetrating the rock The long teeth take up space, so the bearing size must be reduced

Roller Cone Bit Design – Cutting Structure Moderately hard formation bits: These are required to withstand heavier loads so tooth height is decreased Tooth width increased Such bits rely on scraping/gouging action with only limited penetration The spacing of teeth must still be sufficient to allow good cleaning

Roller Cone Bit Design – Cutting Structure Hard formation bits: These rely on a chipping action and not on tooth penetration to drill So the teeth are short and stubbier than those used for softer formations The teeth must be strong enough to withstand the crushing/chipping action and sufficient numbers of teeth should be used to reduce the unit load

Roller Cone Bit Design – Fluid Circulation The original design for rock bits only allowed the drilling mud to be ejected from the middle of the bit This was not very efficient and led to a build up of cuttings on the face of the bit (bit balling) and cone erosion The fluid is now generally ejected through three jet nozzles around the outside of the bit body The turbulence created by the jet streams is enough to clean the cutters and allow efficient drilling to continue

Roller Cone Bit Design – Fluid Circulation Fluid circulating through jet nozzles

Roller Cone Bit Design – Fluid Circulation The original design for rock bits only allowed the drilling mud to be ejected from the middle of the bit This was not very efficient and led to a build up of cuttings on the face of the bit (bit balling) and cone erosion The fluid is now generally ejected through three jet nozzles around the outside of the bit body The turbulence created by the jet streams is enough to clean the cutters and allow efficient drilling to continue

Roller Cone Bit Design – Fluid Circulation Jet nozzles Available in many sizes The size of the nozzle refers to the inner diameter of the ring The nozzles are easily replaced Extended nozzles may also be used to improve the cleaning action The nozzles are made of tungsten carbide to prevent fluid erosion

Roller Cone Bit Design – Fluid Circulation Jet nozzles

DIAMOND BITS

Diamond Bits Diamond has been used as a material for cutting rock for many years Since it was first used however, the type of diamond and the way in which it is set in the drill bit have changed There are three types: Natural diamond bits PDC bits TSP bits

Natural Diamond Bits The diamond bit has no moving cones and operates as a single unit The cutting action of a diamond bit is achieved by scraping or plowing away the rock The diamonds are set in a specially designed pattern and bonded into a matrix material set on a steel body Diamond is sensitive to shock and vibration and therefore great care must be taken when running a diamond bit

Natural Diamond Bits Natural-diamond bits drill slowly by plowing which requires high bit weight and torque

Natural Diamond Bits Natural diamond drill medium-hard and extremely hard formations that are moderately to extremely abrasive Diamond bits and diamond sizes are matched to formation hardness Bits for softer formations use large diamonds to produce a plowing action Small diamonds produce more of a grinding action and are used to drill hard formations

Diamond Impregnated Bits Diamond impregnated bits grind formations like a grinding wheel or sandpaper

PDC (Polycrystalline Diamond Compact) Bits Uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion These cutters are synthetic diamond disks about 1/8-in. thick and about 1/2 to 1 in. in diameter The small discs may be manufactured in any size and shape and are not sensitive to failure along cleavage planes as with natural diamonds PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds

PDC (Polycrystalline Diamond Compact) Bits

PDC (Polycrystalline Diamond Compact) Bits PDC bits drill fast by shearing formations much like a lathe As a general rule, it is easier to remove rock by shear

PDC (Polycrystalline Diamond Compact) Bits Back-rake and side-rake angles and cutter exposure define how aggressively PDC cutters will contact formations

Thermally Stable Polycrystalline (TSP) Diamond Bits A further development of the PDC bit concept was the introduction in the later 1980’s of Thermally Stable Polycrystalline (TSP) diamond bits These bits are manufactured in a similar fashion to PDC bits but are tolerant of much higher temperatures than PDC bits

Natural Diamond Bits Disadvantages of a diamond bit: The major disadvantage of diamond bits is their cost Sometimes 10 times more expensive than a similar sized rock bit There is also no guarantee that these bits will achieve a higher ROP than a correctly selected roller cone bit in the same formation

Natural Diamond Bits Advantages of a diamond bit: They are cost effective when drilling formations where long rotating hours (200300 hours per bit) are required They have no moving parts and so, tend to last longer than roller cone bits and can be used for extremely long bit runs In addition, the diamonds of a diamond bit can be extracted, so that a used bit does have some salvage value

PDC BIT DESIGN

PDC Bit Design Cutter Material The material used to manufacture the cutting surface on PDC bits is called Polycrystalline Diamond – PCD This synthetic material is 90-95% pure diamond It is manufactured into compacts which are set into the body of the bit Hence the name of these bits

PDC Bit Design The cutters can be manufactured as disc shaped cutters or as stud cutters

PDC Bit Design Steel Body Bit The cutters on a steel body bit are studs Tungsten carbide button inserts can also be set into the gauge of the bit to provide gauge protection An advantage of using a stud is that it may be removed and replaced if the cutter is damaged and the body of the bit is not damaged Steel body bits tend to suffer from broken cutters

PDC Bit Design Matrix Body Bit The cutters on a matrix body bit are cylindrical cutters The advantage of this type of bit is that it is both erosion and abrasion resistant and the matrix pocket provides impact resistance for the cutter Matrix body bits have an economic disadvantage because the raw materials used in their manufacture are more expensive

PDC Bit Design Setting of Cutters

PDC Bit Design Cutter Rake The PDC cutters can be set at various rake angles These rake angles include back rake and side rake The back rake angle determines the size of cutting that is produced Side rake is used to direct the formation cuttings towards the flank of the bit and into the annulus

PDC Bit Design Profile: There are three basic types of PDC bit crown profile: Flat or Shallow Cone Tapered or Double cone Parabolic There are variations on these themes but most bits can be classified into these categories

PDC Bit Design Cutter Density The cutter density is the number of cutters per unit area on the face of the bit The cutter density can be increased or decreased to control the amount of load per cutter If a high density is used the cutters must be small enough to allow efficient cleaning of the face of the bit

PDC Bit Design Cutter Exposure Cutter exposure is the amount by which the cutters protrude from the bit body High exposure of the cutter provides more space between the bit body and the formation face Low exposure provides good backup and therefore support to the cutters

PDC Bit Design Fluid Circulation The fluid circulation across the bit face must remove the cuttings efficiently This may be satisfied by increasing the fluid flowrate and/or the design of the water courses that run across the face of the bit This increased fluid flow can cause erosion of the face and bit failure More than three jets are generally used on a PDC bit

Discussion and Feedback