What Are the Different Types of Drill Bits that are Used in Oil & Gas Industry

This article explains the different types of drill bits used for oil and gas. It sorts them by how they cut the rock. It also groups them by their design and their job.

ENGINES OF ENERGY

The twin giants of Middle East and Africa are both blessed with enormous influence on the world energy landscape, oil deposits, but travelling vastly different ways. When you put them together, they generate a complicated web of geopolitical power, economic desire, and changing problems.

The Middle East, with Saudi Arabia, the UAE, and Qatar have been the center of the oil market for a long time. It is characterised by massive, low-cost production and immense proven reserves, granting it significant leverage within OPEC. These countries are not just exporters; they are the strategic players who are putting in heavy investments in the downstream refining and petrochemicals in order to gain more of the value in each barrel.

Moreover, the leaders of the Gulf are investing in diversification through tourism, finance, and technology in sovereign funds. On the other hand, the oil and gas situation in Africa is a potential and paradox narrative. The old players such as Nigeria and Angola are struggling with the ageing infrastructure, insecurity in the Niger Delta, and empty and full investment.

However, the continent is also the site of the world's most exciting new frontiers. Major discoveries in countries like Mozambique, Mauritania, and Senegal are positioning Africa as a key future supplier of LNG, particularly to European markets seeking to diversify away from Russian gas.

New initiatives in Uganda and Ghana might transform their economy, making them key actors. As globalization and modernization speed up, it makes sense that the number of energy suppliers in Africa will grow by many times over the next few years. Despite their differences, both regions face converging challenges.

The global push for decarbonisation and the transition to renewable energy threaten their long-term economic models. This pressure is catalysing a regional race to monetise resources quickly while also tentatively exploring green hydrogen, carbon capture, and solar energy projects.

ROLLING CUTTER VS. FIXED CUTTER BITS

The major division of bits is dividing them into Rolling Cutter Bits and Fixed Cutter Bits. This is the essential difference that determines the way this bit will react with and annihilate the rock.

• Rolling Cutter Bits: The cones on these bits are fixed on bearings that move when the bit is turned. The destruction mechanism is primarily crushing and pounding, as teeth or compacts on the cones crush the rock beneath them.
• Fixed Cutter Bits: There are no moving parts of these bits. The cutting parts are what hold the bits to the body. This destruction process is mainly shearing and scraping with the bit rotating and abrazing these cutters on the rock face. The change from rolling cutters to fixed cutter bits is one of the most important changes in cutting technology. It happened because people wanted to get faster, more efficient, and more durable tools.

ROLLING CUTTER DRILL BITS

Also known as "roller cone bits," these were the industry standard for much of the 20th century and are still widely used today, particularly in specific applications.

• Mill Tooth Bits: These bits are characterised by their large, wedge-shaped teeth. The cones are typically made of tough alloy steel. The long, sharp teeth were designed to cut, gouge, and break the rock. The teeth on the cones are often not lined up perfectly, which means that the cones don't roll in a perfect circle. This offset creates a grinding and scraping sound that works very well in soft rock.
• Application: These bits are mostly employed in clay, soft limestone, and unconsolidated sands. They penetrate non-abrasive rocks with high ROP due to their aggressive tooth design.
• Limitations: The biggest disadvantage of the mill tooth bits is that it can easily be worn and abrased. Harder and more abrasive formations may cause the steel teeth to wear off in a very short time and result in a sudden decrease in ROP.

TUNGSTEN CARBIDE INSERT (TCI) BITS

Also referred to as button bits, Tungsten Carbide Insert (TCI) bits were invented with the aim of addressing the wear restrictions of mill tooth bits in hard formations. TCI bits are provided with cones which instead of steel teeth have buttons or inserts of highly hard and abrasion-resistant tungsten carbide.

These inserts are forced into holes that have been drilled on the cones. Depending on the target formation the shape of the inserts can differ greatly:

• Chisel Shaped: To be used with soft to medium formations to offer crushing and fracturing effect. Conical Shaped: To provide medium-hard formations but with the benefit of durability and aggressiveness. Spherical Shaped: In the case of hardest and abrasive formations, the aim is to maximise durability and impact failure resistance.
• Applications: TCI bits are the workhorse for medium to extremely hard formations, including hard shale, dolomite, limestone, and abrasive sandstones. Their tungsten carbide inserts can withstand the punishing environment of these rocks far better than steel teeth.
• Limitations: TCI bits tend to be not that aggressive compared to mill tooth bits in soft formations and therefore the ROP is low. They consist also of complicated mechanical systems that bearings may fail under high loads or temperatures and may cause a cone loss downcutting, which is a serious drillage accident.

BEARING SYSTEMS IN ROLLER CONE BIT

A critical sub-component of roller cone bits is the bearing assembly, which allows the cones to rotate freely. The type of bearing system is a key differentiator.

• Friction Bearing (Journal Bearing): In this design, the cone rotates directly on a stationary journal or pin. The contacting surfaces are coated with special friction-reducing alloys like silver. Journal bearing bits can handle significantly higher weights-on-bit (WOB) than roller bearings, making them suitable for harder, slower-drilling formations where high load is necessary.
• Roller Bearing: The cone in this design is directly rotated on a stationary journal or pin. Special alloys that reduce friction such as silver are applied on the surfaces of contact. Journal bearing bits can be loaded with much heavier weights-on-bit (WOB) than roller bearings, and this renders them useful on harder, slower-drilling formations in which heavy loading is required.
• Sealed vs. Non-Sealed: The sealed bearings on modern roller cone bits are nearly universal. An elastic or metallic seal will keep drilling fluid and abrasive rock fragments out of the bearing assembly, which will increase the life of the bit many times over. Unsealed bits are mostly becoming obsolete.

FIXED CUTTER DRILL BITS

Fixed Cutter Bits (FCBs) have revolutionised the industry, especially with the advent of Polycrystalline Diamond Compact (PDC) technology. They now dominate the market in terms of performance and reliability.

POLYCRYSTALLINE DIAMOND COMPACT (PDC) BITS:

PDC bits are the most common and versatile type of drill bit used in the oil and gas industry today. Their design is deceptively simple but highly engineered. Design and Mechanism: A PDC bit is made up of a body made of a number of blades.

These blades have a great number of PDC cutters on their face and gauge. All of the PDC cutters are composite materials: a layer of synthetic polycrystalline diamond is attached to a tungsten carbide base. These cutters scrape away the rock as the bit spins in an endless, chip making process.

This shearing effect is much more effective than crushing effect of roller cone bits and hence ROP is much higher in suitable formations. Cutter Design and Orientation: The performance of a PDC bit is heavily influenced by the size, shape, orientation, and placement of the cutters. Engineers use complex modelling software to design the "cutter layout" to optimise cleaning, durability, and stability.

BODY TYPES

• Steel Body: Machined from a single piece of high-strength steel. They are tough and can be easily repaired and refitted with new cutters. They are, however, susceptible to erosion from abrasive drilling fluid.
• Matrix Body: Manufactured by powder metallurgy, forming a composite material of tungsten carbide powder infiltrated with a binder metal. Matrix bodies are extremely erosion and abrasion-resistant but are more brittle than steel bodies.
• Hydraulics and Blade Configuration: The shape and number of blades are crucial. The channels between the blades allow for the removal of drilled cuttings. Nozzles are installed at strategic positions to focus high streams of fluid energy in cutting the cutters and in removing cuttings. The designs of bits vary between aggressive and few blades on the soft, sticky clays and robust and many blades on the hard, abrasive formations.
• Applications: PDC bits excel in soft to medium-hard, non-abrasive, and ductile formations such as shales, mudstones, claystones, salts, and soft limestones. Their shearing action makes them vastly superior to roller cone bits in these rock types. Ongoing advancements in cutter technology are continually pushing their application into harder and more abrasive formations.

DIAMOND IMPREGNATED BITS

For the very hardest, most abrasive formations that would rapidly wear down even the toughest PDC cutters, the industry turns to Diamond Impregnated Bits.

• Design and Mechanism: These bits have no discrete, large cutters. Instead, the working surface of the bit is a "matrix" that is impregnated with thousands of small, natural or synthetic industrial diamonds. The bit works by a combination of micro-grinding and ploughing. As the diamond particles on the surface wear down, new, sharp diamonds are exposed from within the matrix, providing a self-sharpening effect.
• Applications: These are specialised bits used for drilling the hardest and most abrasive formations on Earth, such as quartzite, chert, and very hard, abrasive sandstones. They are also commonly used in geothermal drilling and mining.
• Limitations: They require very high weights-on-bit and are typically used with high-speed downhole motors, not rotary drilling. Their ROP is generally much lower than that of PDC bits, but their ability to consistently drill where other bits fail makes them invaluable in specific, challenging intervals.
• Natural Diamond Bits: Before the invention of PDC, natural diamond bits were the original fixed cutter bit. They feature a matrix body set with large, natural diamonds. While largely superseded by more cost-effective and durable PDC bits, they are still occasionally used in specialised coring applications or for drilling through hard, fractured formations where the single-point loading of a large diamond is advantageous.

HYBRID AND SPECIALISED DRILL BITS

The demarcation between the types of bits is not always definite. Engineers have been coming up with hybrid designs to address challenging lithologies or address a particular challenge. Hybrid Bits: The hybrid bit has features of the roller cone and fixed cutter technology. One common design was two or three little rolling cones in the centre of the bit with blades on the shoulder and gauge that were PDC equipped.

Mechanism and Application: The cones in the centre grind and slice the rock, and are in turn sliced by the PDC blades on the outer. Such a combination can address certain issues such as reducing excessive whirl vibrations, enhancing the steerability of directional drilling, or effectively drilling between hard and soft bedded formations which would be difficult with pure PDC or pure roller cone bit.

CORING BITS

When the objective is not just to drill a hole but to retrieve a continuous cylindrical sample of the rock for geological analysis, a coring bit is used. Coring bits are generally small diameter ring shaped bits which bore a hole through an annulus leaving a central column of rock which is retained in a core barrel in the drill string.

They may be PDC, diamond impregnated, or even TCI design, but the main aim of them is to ensure a large quality core sample, rather than a large ROP.

BI-CENTER BITS

A bi-centre bit is a clever solution for drilling a hole larger than the pass-through diameter of the wellbore. It has two distinct centres of rotation. It runs into the hole in a collapsed state and then, while drilling, its unique geometry allows it to ream out a larger diameter than the restriction above it. This is useful for drilling through casing shoes or in underbalanced drilling operations.

THE BIT SELECTION PROCESS

Different types of drill bits are available in the market made by different manufacturers and the selection of the correct drill bit is a complicated engineering choice which depends on a plethora of factors:

• Formation Properties: This is the most critical factor. Rock strength, abrasiveness, ductility, and the presence of interbedded layers are analysed from offset well logs and seismic data. Drilling System Parameters: The type of rig available, hydraulic horsepower, weight-on-bit capabilities, and desired RPM all influence bit choice.
• Drilling Objective: Is the goal maximum ROP to save time, maximum durability to ensure the section is drilled in one run, or precise directional control for a complex well path?
• Cost-Per-Foot Analysis: The ultimate metric. The valuation of the bit is approximated to the total value of the drilling period. A PDC bit, which is expensive to drill out the entire section in a day, is far more cost-effective than a less expensive roller cone bit, which requires three trips and four days to drill out the same section.

CONCLUSION

The lowly drill bit has been transformed to be an extremely complex engineered component which forms the cornerstone of efficiency and economics in oil and gas exploration. The technological environment is characterised by the conflict between the crushing effect of rolling cutter bits and shearing effect of fixed cutter bits.

The current technological revolution in cutter technology, hydraulic design, and hybrid systems will guarantee that different types of drill bits are offered to the customers as the industry is brought to levels where hydrocarbons will be tapped in increasingly difficult and more complex conditions.