Introduction to Drilling Bits

Introduction

The drill bit is an important component in the drill string. The bit drills the rock in many mechanisms. The drilling bit is selected according to the formation to be drilled. The bit performance is related to several operating parameters like: weight on bit, revolution per minute, mud properties and hydraulic efficiency (Click Here). When the bit is pulled out of the hole, the level of damage on the bit must be carefully recorded. The system followed to evaluate the bit is called the IADC dull grading system which is designed to facilitate the damage grading. An accurate grading can contribute effectively in bit selection in future operations. There are two main types of drilling bits: Roller cone bits, fixed cutter bit.

I- Roller Cone Drilling Bits:

They are also called tri-cone bits. The cutting structures are mounted on three rolling cones and all these cones are attached to main bit body. There are two main types of tri-cone bits:

Mill tooth bits:

These types of bits have steel teeth which are milled on the cones. The size and shape of teeth vary according to the formation to be drilled. In soft formation, the teeth are long and slender, where in hard formation the teeth are short and broad.

Fig 01- Mill Tooth Drilling Bit 
Insert bits:

In these types of bits, the teeth are not milled into the cones, instead of that, tungsten carbide inserts are pressed into the cones. These technics make the bits much harder and can last longer when drilling through hard formation. The size and shape of teeth also depend on the formation to be drilled, the teeth can be long and chisel shapes in soft formation, and for hard formation can be short and round shapes.

Fig 02- Insert Drilling Bit 

The tri-cones bits have many features:

- Fluid courses:

These are paths for drilling fluid to pass through the drilling bits to carry the cuttings away from the bottom. The earliest bits were fitted with a hole in the center of the body, as wells become deeper; more hydraulic force was needed to carry the cuttings, jet nozzles were used to increase the hydraulic force. Nozzles can be in various sizes which can be changed to adjust the pressure and flow requirements of the operations. The size of jet nozzles is defined in thirty-seconds of an inch (ex: 13/32 In in diameter).

- Gauge protection:

The drilling bit has to be reinforced at its outer parts to resist against the impacts caused by the direct contacts with hole walls. Many methods can be used to protect the bit, one method is hard facing the bit with a tungsten carbide layer at the outer parts, also a series of tungsten carbide inserts can be pressed at the same parts concerned by the impacts and wears.

- Journals:

It is a shaft where the cones are mounted. The bearings are inserted on these shafts in order to allow the cones to rotate. The angle of inclination of the shaft from the horizontal depends on the formation to be drilled.

- Bearings:

There are two types of bearings which are used in tri-cone bits. Roller ball bearing can be used if the bit life is not a problem; friction bearings with a specialized metal rings can last longer and consequently enhance the drilling performances. The bearings have to be lubricated to withstand the pressure and temperature while drilling. Two types of lubrication can be used:

- Non sealed bearings: in this case the drilling fluids are used to cool the bearings, but the solid content of drilling fluids can affect the bearing's life and will not last longer;
- Sealed bearings: in this type of bearings a system of lubrication is used, graphite-type lubricant is sealed in a container to cool the bearing. Pressure fluctuations cause the lubricant to flow around the bearings.

- Offset: 

As the cones rotate the teeth scrape and gauge the formation, and this is made by the off-center alignment of the cones. The level of scrapping depends on the offset. For soft formation a 1/4 in to 3/8 in offset is used. For hard formations the offset decreases till the level when no offset is applied.

II- FIXED CUTTER DRILLING BITS:

Fixed cutter bit has no moving parts; the cutting structures and bit body rotate as one part. The main types of these bits are: PDC (polycrystalline diamond compact), TSP (thermally stable PDC) and diamond bits.

TSP Drilling Bits:

It was noticed that the bonding materials are the weakest part of the cutter. Under the high temperature at the bottom while drilling, the bonding materials lose their strength. The TSP bits are composed with same artificial diamond without using the bonding materials. 

Fig 03- TSP Drilling Bit

The PDC Drilling Bits: 

This type of bits can be used for different formations from soft to hard. The PDC bit has cutters which consist of a layer of artificial diamond (polycrystalline diamond). Using a high pressure high temperature technique the polycrystalline diamond is bonded on layer of cementer tungsten carbide.
The cutter is self-sharpening because the sharp crystals are exposed continuously as each layer wears or disappears.

The polycrystalline diamond is bonded to studs of tungsten carbide, which are then pressed into holes on the head of the bit. The cutters are placed in a helical pattern on the face of the bit. This combination results to a cutter which has wear resistance and hardness of diamond, and also impact resistance and strength of tungsten carbide.

The bit body is forged with same high steel used to make the cones of the tri-cone bits, and in order to reinforce the bit against fluid erosion, the face of the bit is coated by a layer of tungsten carbide.

Fig 04- PDC Drilling Bits 

PDC bit design:

The PDC bit has an important advantage if it is compared with the tri-cone bit; it has no moving parts like bearings or cones which they have to be fished in case of failure. 

The PDC bit shears the formation rather than crushing or gauging the formation as do the tri-cone bit 
The PDC bit has longer life and extended gauge with tungsten carbide wears pads which help to maintain gauge. 

The concave shape of bit's face permits the cutters to drill the rock simultaneously, and also increasing bit stabilization and decreasing the potential for deviation.

The cleaning action of the bottom of the hole and the cutters are performed by the jet nozzles which vary in number and size. They are located in such manner to increase the quality of cleaning.
Fig 05- PDC Bit Design 

Diamond Drilling Bits: (Click Here)

The natural diamond bits are used to drill through very hard abrasive formation. This type of bits was developed from mining industry. The main advantages of this type of bits are their longevity, reduced number of run bits and tripping time to get to the next casing point.

There are also some drawbacks of using a natural diamonds bits. The slow rate of penetration is one of the main down sides of using this kind of bits. The diamonds are set into a steel body; roughly two thirds of the cutter length is embedded into the bit head which can cause easy balled bits in soft formation, for this reason the natural diamonds bits are used for hard, abrasive formations.

The natural diamond bits require relatively high RPM and moderate WOB. High hydraulic horsepower is not a major requirement, so the jet nozzles are not used. The drilling fluids flow through orifices then by flow channels cut into the bit head to promote cutter cooling and cleaning.
Fig 06- Diamond Bit 

IADC Dull Grading:

The bit grading code was developed by the international association of drilling contractors. It is a standard methodology to describe the used drilling bits. The information delivered in this grading are essential for future operations which can be optimized.

The grading is consisted of an 8 character code that describes the bit after running in the hole.


Column1: Inner cutting structure: Describe the condition of the cutting structures on the inner of the bit.

Column2: outer cutting structure: Describe the condition of cutting structure on the outer side of the bit.

Column 1&2 are scaled from 0 to 8, o for no wear and 8 for total wear.

Column 3: dull characteristics: Describe the major dull characteristics of the cutting structure; this is represented by two letter code.

Column 4: location: in this column, a letter is used to locate where the major wear has appeared. ( ex: A for all area)

Column 5: bearings & seal: A scale from 0 to 8 is used to describe the bearings, o for no wear, and 8 for total wear on the bearing.

About sealing, a letter code is used to describe the level of sealing. E indicates that sealing mechanism is still effective, while F indicates that the sealing mechanism has failed.

Column 6: Gauge: This is used for bit gauge. I for no gauge wear, that means that the bit is in gauge. If the bit is under-gauge, it is described in sixteenths of an inch.

Column7: other dulling characteristics: This indicates any other type of wear which can appear on the bit. It is indicated by two letter code.

Column8: reason pulled This is used to mension the reason behind pulling the bit (ex: low penetration rate, changing BHA, Casing point)

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