Types of drilling bit based on performance
Different types of drilling bit in oil and gas industry can be classified base on performance
1- Roller Cutter Bits or Roller cone Bits
- Milled Tooth Bits
- Tungsten Carbide Insert tooth(TCI)
2- Fixed Cutter Bits
Steel Blade
- Drag Bits
- Steel Cutter or Fish Tail
Diamond Bits
- Natural Diamond Bits
- Polycrystalline Diamond compact Bits
- Thermally Stable Polycrystalline Bits
3- Special-purpose Bits
- Eccentrics bits
- Sidetracking
- Antiwhirl Bits
Related Article: Introduction to drilling methods
Roller Cutter Bits
Roller Cutter drilling Bit or Roller Cone Bits have cutting components arranged on cones and these cones rotate around their axis when the drill body rotates. The number of drill cones can be one, two, three, four, five or even six. But the most common rotary cone bits made with three cons which are called tri-cone bits.
About 95% of oil and gas drillings are done by Roller Cutter Bits, especially tri-cone bits. In soft formations, the length of the teeth and their distance from each other is high. For harder rocks the size of the teeth and their distance is reduced.
The function of Roller Cutter Bit is based on the penetration of teeth and cutting of rocks. The teeth on the cone are made of steel or tungsten carbide, which is located on the drill and penetrates the stone by moving the drill.
One cone bit: This type of drilling bit rotates around its axis and is mostly used in directional drilling. These bits are used in rotary drilling system for cracked stones, stones with scraping properties, medium hard stones and also for sloping surfaces.
One cone bit minimizes the possibility of well deflection. In this case, the shape of the drill is spherical and the teeth on it are hemispherical or V-shaped. These drills have holes around them to transfer the cuttings. Also, to transfer the drilling mud to the lower part of the well and to cool and lubricate the drill. Holes have been installed on the drill, which are called Nozzle or jet type bit.
Two cone bit: Mostly used in soft formations and for directional drilling. New types of Two cone bit that are used to speed up drilling operations and to clean wells are called extended nozzles.
Tri cone bit: Tri cone bit are most common type of bit and the most wells in the world are drilled by this type.
Milled Tooth Drilling Bits (MT)
Milled Tooth Bits with older models had two cones with no collision between the teeth and the grooves when rotating. Collision of adjacent cone teeth causes automatic cleaning of the drill, increases the drilling speed and facilitates the process. Steel toothed drills are used for soft to semi-hard formations.
Drills for soft formations have the following specifications:
- Long teeth with a long distance from each other
- Pines with high offset
- With Interruption to pass fluids
- Teeth with Self-Sharpening
- The circular rotation of the cons causes Collision of teeth
Milled Tooth Bits for hard formations have the following specifications:
- The teeth are short and close to each other
- Performance through crushing and chipping
Tungsten Carbide Insert Tooth (TCI)
Tungsten Carbide Insert Tooth (TCI) or Button Bits, like the MI type, have rotating cones. Unlike MI bits, instead of teeth cut from the pine itself, tungsten carbide buttons are installed on the pine body at very high temperatures. These drilling bits are used for operations in the hard and corrosive layers of the earth at low depths. Milled Tooth drills perform very poor at great depths and replacing it was time consuming.
Advantages of TCI drills
- Durability and long life
- Ability to operate in different formations
- Complete penetration in the bottom of the well (About 80% in soft formations)
- Cost reduction due to the long life of the drill
Disadvantages of TCI drills
- Cone shall erosion around the buttom, which causes separation of tungsten carbide buttoms
- High penetration of the buttons in the formation causes the cons to crack and wear and the buttons to fall.
Fixed Cutter Bits
Fixed Cutter drilling bits are one piece and have no moving parts.
In fact, instead of having cones that rotate independently, these Types of bits have a fixed body and a fixed head which rotates integrated with the drilling pipe and string. The body of this type of bits is made of steel or tungsten carbide. Steel body bits have high resistance to impact and force applied to the blades but the resistance of steel to erosion caused by drilling fluids is low. Conversely, drills with tungsten Carbide body are highly resistant to erosion but weaker to impact.
Steel Blade
CNC machines are used to produce steel blade drills. First, a piece of steel is formed into the desired drill shape by forging operations. It is then placed in a CNC machine and the drill body is cut to the desired shape according to a pre-determined computer program.
These drills have two types of steel cutter or fish tail and drag bits. Drag bits are used for drilling soft formations in the oil and gas industry. Drag bits were the first bit type used in rotary drilling, but due to their low efficiency, they were practically replaced by cone bits. These drills had steel blades that were mostly used for drilling in soft formations.
The use of this type of drill has decreased for the following reasons;
These bits are not very efficient in hard formations.
When a large load is applied to the drill, the steel blades sink into the formation, and if the drill torque is increased, it causes the drill pipes to be cut and fall into the well.
It is difficult to control the drilling path with this drill and often the well deviates from the main path.
Diamond Bits
In this drilling bit types, diamond particles are placed on the drill body. Diamond is the hardest material ever known and is made of pure carbon. Due to the hardness of diamond, these bits are the most suitable selection for drilling hard formations with erosion properties. The lower shear strength of the rocks causes the drilling with Diamond bits to be done with one third of the force required to crush the rocks, therefore less number of pipes and weight is required for drilling. Using this bit type increases the drilling speed and life of the drill and reduces round trips.
Compared to roller cutter and steel blade bits, diamond bits are less sensitive to drilling mud. Diluted drilling mud usually increases the efficiency of the drill and is more economical. Although diamond drills are several times more expensive than roller cutter bits, they increase drilling speed significantly. However, reducing the frequency of bite replacement and downtime, has made it cost-effective to use this drill. The smaller the diameter of the diamond drill, regardless of the low initial cost of the drill, the higher its efficiency will be compared to the thick drill.
Advantages of diamond drills
- Increasing the drilling speed in different formations and different ground layers
- Reduce Trip time
- Reduce bit Under Gauge
- No need for spare parts
- Possibility of use in drilling with special conditions such as high pressure wells
- Due to the reduction of trip time and increase of drilling area, the cost of drilling in oil and gas wells will be reduced
- These drills can be used for other wells after drilling operations
Diamond Drill Bits are divided into the following categories;
- Natural Diamond Bits
- Polycrystalline Diamond compact Bits
- Thermally Stable Polycrystalline Bits
Polycrystalline Diamond compact Bits (PDC)
Polycrystalline Diamond compact Bits (PDC) have a prefabricated body (usually made of tungsten carbide) and blades are mounted on body surface. The design of these type of drilling bit may be made with or without jet. Two important features of a PDC cutter include Impact Resistance and Abrasion Resistance, which depend on the size of the diamond particles (Diamond Graine). The larger the diamond particle size, the higher the impact resistance and the lower the abrasion resistance of the manufactured PDC cutter.
Synthetic diamond is produced from a combination of carbon and cobalt as a catalyst. The presence of cobalt in PDC makes these cutters have lower thermal resistance than natural diamond. Because when the PDC cutter heats up, cobalt increases in volume and this increase in volume causes the diamond to break.
Thermally Stable Polycrystalline Bits (TSP)
Thermally Stable Polycrystalline Bits (TSP) were made due to the thermal limitations of PDC cutters. In the process of producing these diamonds to increase thermal resistance of final products, refining is done by acid and cobalt is removed from the compound. Another method of making TSP is the use of silicon carbide, which is used due to its low coefficient of thermal expansion compared to cobalt. Therefore TSP Bits have better performance than PDCs in very hard formations.
Related Article: Effects of drilling fluid waste on soil properties
Drilling bit selection for oil and gas rigs
Drilling bit selection oil and gas is a very important factor that has a significant impact on the time and cost of process. While the cost of drilling bits is less than 10% of the total cost of drilling, choosing the right drill can reduce well completion time by less than half.
The following factors are effective in drill bit selection for oil and gas industry:
- Formation specifications
- Drill specifications
- Drilling rig specifications
- Drilling Cost per Meter
Although Specifications of formation are not changeable, play an essential role in drilling speed. By examining the properties of the formation before drilling, suitable program for drilling wells can be designed.
Formation properties that are effective in drill bit selection are:
- Compressive strength
- Compressive strength
- Elasticity
- Abrasiveness
- Overburden Pressure
- Porosity and Permeability
- Pore Pressure
- Stickiness
By determining the formation properties, the formations can be classified as follows and the drilling bit can be selected accordingly. Formations are generally divided into four categories: Soft, Medium Soft, Medium Hard and Hard. In general, short tooth drills are used for hard formations. In very hard formations such as Chert rocks, the drilling speed of toothed drills is very low and is not economically affordable.
Mechanical parameters of drilling
Bit rotation speed (RPM) and Weight On bit are two mechanical parameters of drilling oil and gas wells, which are determined according to the type of drill and the specifications of the formation. Although these two mechanical parameters are determined by the drill manufacturer, but the effect of the formation specifications and well cleaning should also be taken into the account. In hard formations, increasing the weight on the drill, increases the drilling speed accordingly. But in soft formations, applying too much weight on the drill causes the bit to become balled up and deviate from the drilling path.
Drill specifications
Fixe Cutter and Roller Cone drills are designed and used for drilling a wide range of formations. Generally in Top Hole formations, which are soft and unstable formations, drills with long teeth are used. If the use of these drills is not economical, PDC drills are used instead. The use of drilling reports in Offset well is also a good experimental guide for choosing a drilling bit.
Rig Capacity
The drilling rig must be able to provide the required flow, proper weight on the drill and proper rotating table speed.
