About the Author：Steve Devereux, C.Eng, FCIArb

Steve Devereux, C.Eng, FCIArb, is a drilling engineer and consultant with over 40 years of international experience in oil and gas operations. He specializes in well design, bit performance optimization, and drilling efficiency. Steve is the author of Practical Well Planning and Drilling Manual and Drilling Technology in Nontechnical Language, and continues to share his expertise with engineers and professionals across the globe through consulting, training, and technical writing.

In modern drilling engineering, the development of drill bit technology almost determines the upper limit of the entire drilling efficiency and cost. Among the many drill types, PDC Drill Bit has gradually become the mainstream choice in oil and gas drilling, geothermal drilling and unconventional resource exploitation due to its high efficiency, wear resistance and stability.

From the first experimental tools to today’s high-performance drilling tools widely used in complex formations, the technological evolution of PDC bits is a fusion of material science, mechanical design and field experience. It not only changed the way rock is broken, but also reshaped the economic model of drilling operations.

This paper will systematically introduce the structural composition, classification method, formation adaptability, parameter optimization method and performance evaluation system of PDC drill bit, and combine engineering practice, from the perspective of theory and practice, explain why PDC drill bit can stand out in modern drilling engineering and become the core equipment of efficient drilling.

Foundation and Structure of PDC Bit

What is a PDC bit?

PDC, full name is Polycrystalline Diamond Compact. In simple terms, it is not a traditional steel tooth or insert drill bit, but “shears” the rock through the diamond particles on the PDC composite sheet. The PDC compact is formed by sintering a layer of extremely hard polycrystalline diamond onto a tungsten carbide substrate. Its hardness is second only to natural diamond, but its toughness is better than natural diamond, and its cost is more controllable.

I personally think that the reason why PDC bit can become the mainstream, the biggest advantage is that it has high cutting efficiency, fast drilling speed and long life. The main difference between it and the traditional cone bit lies in the working principle of “cutting” and “crushing. Traditional drill is more impact crushing, while PDC is continuous cutting, the efficiency is naturally different.

Structure composition of PDC bit

A PDC drill bit looks quite complicated, but there are only a few core components.

1. Bit body: This is the “skeleton” of the drill bit “. There are two main types: steel drill bits and matrix drill bits.
   • Steel body drill: the main body is made of steel, and the PDC composite sheet is welded on the steel body. Its advantages are high strength and relatively good control of processing accuracy, but its disadvantage is that its wear resistance is relatively poor, especially in highly abrasive formations. We generally use it more in relatively stable formations or where higher dimensional accuracy is required.
   • Matrix drill: This is through the powder metallurgy process, the tungsten carbide powder sintering molding. The PDC compact is directly embedded in the matrix. The advantage of the matrix bit is excellent wear resistance and strong erosion resistance, especially suitable for drilling in hard rock or abrasive formation. The disadvantage is that the production process is complicated, the cost is high, and it is slightly more troublesome to repair. Which one to choose depends mainly on the specific situation of the underground formation and our budget.
2. Water eye design: Its main functions are three:
   • Cooling: PDC Cutter generates a lot of heat when cutting rock at high speed, and the drilling fluid sprayed from the water hole can effectively cool the PDC composite sheet and prevent overheating from causing failure.
   • Debris removal: the cutting debris must be taken away from the bottom of the well in time, otherwise it will be repeatedly ground, affecting the drilling efficiency and even sticking. The drilling fluid injected by the water hole can effectively wash the debris and carry it out of the wellbore.
   • Cleaning the bottom of the well: Keep the bottom of the well clean so that the PDC Cutter can better contact the rock and play a cutting role. The design of the water eye, including the number, size and injection angle, is precisely calculated to form the best flow field and improve the drilling efficiency.
3. Tread and blade: This part is in direct contact with the “working face” of the rock.
   • Tread: refers to the part of the drill body where the PDC Cutter is installed. Its geometric shape and the arrangement of the PDC Cutter directly affect the cutting efficiency and stability of the drill bit.
   • Knife wing: It is a raised structure on the tread, and PDC Cutter is also inlaid on it. The design of the blades, including their number, height and helix angle, is critical to the steering of the bit (controlling the direction of the bit in the wellbore), the ability to resist eccentric wear, and the path of debris removal. A good blade design can make the drill bit “walk more steadily and straighter” in the well, and at the same time, it can better discharge the cut cuttings upward.

Classification of PDC Bits

There are many types of PDC bits, and their classification is mainly based on several core elements: bit body material, cutting tooth type and layout, and blade number and shape.

According to the drill body material classification:

This is one of the most basic and critical classifications, which directly determines the strength, wear resistance and repairability of the drill bit.

Steel Body PDC Bits:

This kind of drill is what we see most in our daily work. Their drill body is made of high-strength alloy steel processing, and the structure is more flexible, water eye flow design can be very free. What I like most is that the steel body drill is very repairable. Once it is a little worn or damaged, we can repair it and reuse it. This is a great advantage in cost control.

I usually use them in medium-soft to medium-hard formations, especially in sections that require frequent tripping for repair. Of course, if the abrasiveness of the formation is too high, or if there are too many cracks, the life of the steel drill bit will be limited. At this time, I have to consider other options.

Matrix Body PDC Bits:

Its drill bit body is sintered by tungsten carbide powder at high temperature. Its hardness and wear resistance are simply incomparable to those of steel drill bits, and its erosion resistance is particularly strong.

Encountered those particularly hard, highly abrasive formations, or deep wells, complex wells need long time operation, high erosion requirements, my first thought is the matrix bit. Although the initial cost is indeed much higher than the steel drill bit, its long life can often bring higher comprehensive benefits.

Classified by PDC Cutter type and layout:

PDC Cutter is the “teeth” of PDC bit. Its type and layout directly affect the rock breaking efficiency and stability of the bit.

Common Plane PDC Cutter Drill:

This is probably the most common, PDC Cutter is flat, the advantage is simple and reliable, the ability to break the rock directly.

Concave/Convex PDC Cutter:

Now we use more and more of these shaped teeth. I have observed that the concave PDC Cutter can better “grasp” the rock during the drilling process, optimize the distribution of cutting force, and reduce the wear of PDC Cutter, especially in some formations with strong plasticity. And convex teeth, it can enhance the aggressiveness of the drill to a certain extent, help break more hard rock, but also improve the efficiency of chip removal. I feel that this small adjustment in geometry can often bring unexpected performance improvements.

Asymmetric PDC Cutter Layout Drill:

The drill bit with this layout gives me the feeling of “stability”. Through the irregular PDC Cutter arrangement, it can effectively reduce the vibration of the drill bit at the bottom of the well and prevent the premature failure of the cutting teeth. In my opinion, vibration is one of the biggest killers of drill life, so asymmetric layout plays an irreplaceable role in improving drill stability and prolonging life.

According to the number and shape of blade classification:

The number and shape of the blade affect the drilling speed, chip removal ability and guidance of the drill bit.

Multi-bladed PDC Bits:

More blades mean more PDC Cutters, so that in softer formations, the penetration rate will be very fast and the drill bit will be more stable. I usually use multi-blade drill bits in shallow wells or soft formations with good homogeneity, which can effectively improve engineering efficiency.

Few-bladed PDC Bits:

Relatively speaking, the less-blade bit has more advantages in clearing cuttings, especially in those viscous formations prone to mud bag effect, or the need to apply more drilling pressure to break the rock. It can ensure the timely discharge of rock cuttings and prevent the drill bit from “slipping” or “mud bag”.

Helical Blade PDC Bits:

The design of the spiral blade, in my opinion,a very clever optimization. It can provide better guidance, so that the drill bit runs more smoothly in the wellbore, greatly reducing vibration. I especially like to use this drill bit when drilling directional or horizontal wells. It can help me control the wellbore trajectory more accurately and reduce the risk of wellbore instability.

Application of PDC Bit in Drilling Engineering

Formation adaptability and type selection of PDC bit

• Soft formations: For soft, plastic formations, such as mudstone or unconsolidated sandstone, my preference is for PDC bits with larger PDC Cutters and more aggressive blade designs. As you can imagine, it’s like using a sharp shovel to dig soft soil, requiring a larger cutting area and more effective chip removal space. These drills are usually highly intrusive and have excellent chip removal capabilities, which can significantly increase the rate of penetration (ROP), which is what we value most in soft formations.
• Medium-hard formation: When it comes to medium-hard formations such as shale and tight sandstone, the selection strategy will become more balanced. We can not blindly pursue high drilling speed at the expense of the life of the drill bit, after all, frequent tripping to replace the drill bit is time-consuming and laborious. I would prefer a PDC Cutter with a moderate size and blade design that combines invasiveness and stability. This type of drill bit needs to be able to provide sufficient wear resistance while ensuring a certain drilling rate. To put it bluntly, it is to find the best “dessert” so that the drilling speed and life can reach a satisfactory level.
• Hard formations and abrasive formations: For hard granite, quartzite, and even formations containing a large number of abrasive minerals, this is the test of PDC bits. At this time, what I value most is the wear resistance and robustness of the drill. We will choose those bits with high wear resistance PDC Cutter, and the bit body design must also be extremely strong to withstand strong impact loads and abrasion.

Optimization of working parameters of PDC bit

Choosing the right PDC bit is only half the success. How to operate it and let it play its full potential is what our engineers really need to consider. Bit pressure, speed, torque and hydraulic parameters, any bad adjustment will affect the final effect.

• Weight on Bit (WOB): Reasonable weight on bit is a prerequisite for effective operation of PDC bits. If the WOB is too low, the PDC Cutter cannot be effectively embedded in the formation, resulting in increased skidding and wear, and the drilling speed cannot be increased naturally. If the weight on bit is too high, it may cause the PDC Cutter to overload and even damage to the bit body. It also increases the risk of drill string bending and hole irregularities. My experience is to find an optimal WOB range based on the formation hardness, bit type and size, and real-time monitoring of drilling speed, torque and other data. In practice, I will gradually increase the weight on bit and observe the lifting curve of the drilling speed. When the curve tends to be flat or abnormal vibration begins to appear, I will know that the limit is approaching.
• Rotation speed (RPM): Rotation speed is closely related to drilling speed, vibration and PDC Cutter wear. Generally speaking, increasing the rotation speed can increase the drilling speed, but too high a rotation speed will cause the drill bit to vibrate violently, which not only affects the quality of the borehole, but also accelerates the wear and even shedding of the PDC Cutter. In the soft formation, the speed can be appropriately increased to increase the drilling speed; while in the hard formation, it is necessary to reduce the speed and increase the weight on bit to obtain a stable cutting effect. I often use the vibration data provided by downhole tools to guide the adjustment of speed, as far as possible to avoid resonance.
• Torque: Torque is an important indicator for us to judge the working status of the drill bit. Normal torque fluctuations indicate that the drill is cutting effectively. If the torque suddenly increases, it may mean that the drill bit is locked, cuttings are accumulated at the bottom of the well, or the formation suddenly becomes hard; if the torque suddenly decreases, it may be that the drill bit jumps and the cutting efficiency decreases. I usually set an upper and lower limit of torque, and once it is out of range, I immediately adjust the weight on bit or speed. It is important to avoid locking of the drill bit, as that may lead to damage to the drill string or even a downhole accident.
• Hydraulic parameters: mud performance, pump capacity, and water hole size are critical to the cooling and chip removal efficiency of PDC bits. The mud needs to provide sufficient carrying capacity to bring out the cuttings at the bottom of the well in time. The pump capacity and water hole size determine the hydraulic power at the bottom of the well, which directly affects the cooling effect of PDC Cutter and the cleanliness of the bottom of the well. If the cooling is not good, the PDC Cutter will overheat and fail; if the chip removal is not smooth, the chip will be repeatedly ground at the bottom of the well, causing the drill bit to wear more and even “mud bag” the drill bit. I usually choose the appropriate water hole combination and pump capacity according to the water hole design and formation conditions of the drill bit to ensure that the bottom of the well is always clean and cool.
• The so-called “Parameter dessert” is the best combination of parameters such as bit pressure, speed, torque and pump capacity under specific formation conditions. This is not a fixed value and it adjusts dynamically as the formation changes. I will continue to pay attention to real-time drilling parameters, combined with geological logging data, based on experience to judge the formation changes, and then small, gradual adjustment of parameters. For example, when it is found that the formation becomes hard, I will appropriately increase the weight on bit and reduce the speed; when the formation becomes soft, I will increase the speed and slightly reduce the weight on bit. This dynamic adjustment capability is the key to efficient drilling.

Performance Evaluation and Failure Analysis of PDC Bit

After the drill bit goes into the well, how does it perform? How to judge and solve the problem? This requires us to have a set of rigorous evaluation system and failure analysis method.

• Key performance indicators: The rate of penetration (ROP) is undoubtedly the most intuitive indicator, which is directly related to the drilling cycle and cost. However, in addition to ROP, we should also pay attention to the life of the drill bit (total footage). After all, the drill is fast but the life is short, and frequent replacement is not cost-effective. Borehole quality is also an important indicator, and irregular boreholes can cause trouble for casing running. These data are our hard criteria for evaluating the performance of PDC bits.
• Wear mode:
  • Abrasion: This is the most common wear, and the working surface of the PDC Cutter gradually flattens. It is usually due to the high abrasive properties of the formation, or the mismatch of bit pressure and rotation speed that the cutting teeth repeatedly grind the formation without effective cutting.
  • Crashing: The edge of the PDC Cutter is chipped. This is often due to excessive impact loads (e. g., bit bounce, encountering interlayers or hard nodules), or the weight on bit is too high, which exceeds the impact resistance of the PDC Cutter.
  • Falling off: The entire PDC Cutter has fallen off the bit body. This may be due to the weak welding of the cutting teeth, or the deformation of the bit body under extreme loads, resulting in stress concentration at the root of the PDC Cutter and falling off.
  • Thermal damage: The performance of the PDC layer is degraded due to overheating, which is usually manifested as the color of the PDC layer becomes darker or even cracks. This is due to insufficient cooling, excessive drilling speed or excessive drilling pressure, resulting in frictional heat generation, and the heat cannot be dissipated in time.
• Troubleshooting: When the drill bit comes out, I always check it carefully for wear. If it is found that the cutting teeth are generally severely abraded but not chipped, it may mean that the wear resistance of the drill bit we use is insufficient, or the drilling parameters (especially bit and speed) need to be adjusted to improve cutting efficiency. If there are many collapses, it is likely that the impact load is too large in the hard formation, or serious vibration occurs, and it is necessary to adjust the parameters or replace the drill bit with stronger impact resistance. Through these “scars”, we can reverse the real situation downhole, so as to improve the next section of the drilling program.

Before drilling, I will carefully check the integrity of the PDC cutting teeth, the welding quality and whether there are cracks in the bit body. During the drilling process, real-time parameters are continuously monitored and compared with historical data. Once abnormal trends are found, adjustments are immediately intervened to nip minor problems in the bud. These seemingly trivial steps can effectively extend the life of the drill bit and avoid unnecessary losses.

Whether it is high-speed drilling in soft formations or long-term operation in highly abrasive hard rocks, PDC bits show strong adaptability and economic benefits. In the future, with the further development of material engineering, simulation analysis and intelligent drilling technology, the performance of PDC bits will continue to be optimized, moving towards higher efficiency, longer life and more intelligent adaptive direction.

In the system of modern drilling engineering, PDC bit is not only a tool, but also a key technology to drive efficiency and innovation.