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PDC Bit In Oilfield Drilling Rigs Complete Design Guide

PDC Drilling bit has been used extensively and successfully over a wide range of formation types. The lack of rotating parts leads to greater life expectancy and as such long bit runs are achievable with resultant time and cost savings (check also drilling cost per foot). A thorough review of the economics of running a PDC bit needs to be performed prior to bit selection due to its increased cost.

PDC Bit Design Features

PDC bits have several significant design features that enhance their ability to drill:

  1. The lack of internal moving parts reduces bit failure potential.
  2. Since it fails the rock by shearing, less drilling effort is required than the cracking, grinding principles used in roller cone bits.
  3. High bit weights are not required. (This feature allows these bits to be used in deviation control in some cases.)
  4. The combination of low weight requirements and no internal moving parts makes them well suited for turbine drilling.

As more experience is gained with the bit, enhanced design features will probably further improve a product that currently is innovative and well-proven.

Bit manufacturers integrate the PDC blank into their respective bit designs (Fig.1). Variations in designs include the number and placement of the blanks, jet structure, and watercourse development. In some applications, PDC bits will drill 3-4 times the footage of a conventional roller bit at 2-3 times the drill rate if sticky formations do not pose problems. For example, 18,000-20,000 ft wells in South Texas are now typically completed in 70-80 days using PDC bits vs 120-130 days with conventional roller bits.

NL Hycalog's DS-23 diamond bit (Courtesy NL Hycalog)
Fig.1: NL Hycalog’s DS-23 diamond bit (Courtesy NL Hycalog) 

PDC drilling bit designs are generally based on high or low RPM applications, i.e., turbine vs rotary drilling. Turbine applications use more blanks to compensate for friction-related wear considerations. The PDC bit is tapered to allow placement of the cutters. PDC bits for rotary applications have fewer cutters and a somewhat flat design.

Rotary-designed PDC bits often use nozzles to allow fluid circulation for cuttings removal and cooling of the bit. Manufacturers may vary the number and distribution of the nozzles. A tendency for applications in Water-Based Mud is to use small jets to achieve high fluid velocities. This feature does not appear to be as significant in Oil Based Mud applications.

PDC bits are manufactured with a machined, steel body or a matrix body process. The matrix process is similar to the manufacturing of diamond bits. The cutters are attached to the bit by proprietary techniques. Matrix bodies appear to be more erosion-resistant.

The shape of the PDC cutters is becoming an important consideration. Most manufacturers use the original circular design. However, the effort is being given to the research and development of alternate shapes to enhance the design and improve drilling performance.

The Major Applications OF PDC Drilling Bits:

  1. PDC bits are typically useful for drilling long, soft to medium shale sequences which have a low abrasivity. In such formations, they typically exhibit high ROP and extended life enabling entire sections to be drilled on one run.
  2. PDC bits are not usually appropriate for highly abrasive well-cemented sand sequences. When drilling tight siliceous formations the incidence of PDC chipping and breaking is dramatically increased resulting in less than expected ROP and bit life.
  3. When drilling heterogeneous formations containing alternating shales and or shale limestone sequences the use of hybrid PDC bits is encouraged. This bit incorporates the use of backup diamond studs behind the PDC cutter.
  4. When drilling harder abrasive strings, the diamond stud absorbs the increased weight required to drill the stringer and prevents premature damage and wear to the PDC cutter.
  5. The use of bladed hybrid PDC bits is recommended for drilling hard formations. The deep watercourse on these bits enables optimum fluid flow across the cutter to efficiently reduce the friction temperatures induced. This efficient cooling will help minimize the fracture of the PDC cutters.
  6. When drilling mobile, plastic formations such as salt sections the use of eccentric PDC bits should be considered. These bits have proved successful in preventing the incidence of stuck pipe in many areas where salt flow problems are experienced.
  7. When planning the use of mud motors or turbines, the use of long tapered profile bits should be considered. In addition, radial jetting bits reduce the potential for friction-induced high cutter temperatures when run on a motor or turbine which reduces temperature degradation of the cutter.s

PDC Drilling Bit Design & Components

What are The Main Steps Of PDC Bit Design are:

  1. Bit Body Material Design
  2. PDC Cutters Material 
  3. PDC Cutter Geometry
  4. Cutter Shape
  5. Bit Profile Geometry

Body Design

The bit body design may be:

  • Steel Body
  • Matrix Body

Steel Body :

The bit body is forged or milled from steel (steel-bodied bits). The cutters on a steel body bit are manufactured as studs. The stud can be set with a fixed back rake and/or side rake (will be discussed later). Field experience with the steel body bit indicates that face erosion is a problem, but this has been overcome to some extent by the application of a hard-facing compound. Steel body bits also tend to suffer from broken cutters as a result of limited impact resistance. This limited impact resistance is because there is no support to the stud cutter. They are preferred as they can be easily repaired but suffer from erosion.

Matrix Body :

Matrix Body Bits are constructed in a cast from tungsten carbide (matrix bit). They are more resistant to erosion but are prone to bit balling in soft clay formations due to their low blade height compared with steel-bodied bits. Matrix body bits have an economic disadvantage because the raw materials used in their manufacture are more expensive.

PDC Bit Design Steel Body & Matrix Body

Fortunately, both steels and matrix are rapidly evolving, and their limitations are diminishing. As hard-facing materials improve, steel bits are becoming extremely well protected with materials that are highly resistant to abrasion and erosion. At the same time, the structural and wear-resisting properties of matrix materials are also rapidly improving, and the range of economic applications suitable for both types is growing.

Today’s PDC drilling Bit Design as a matrix has little resemblance to that of even a few years ago. Tensile strengths and impact resistance have increased by at least 33%, and cutter braze strength has increased by ≈80%. At the same time, geometries and the technology of supporting structures have improved, resulting in strong, productive matrix products.

PDC Drilling Bit Cutters Material: 

PDC Cutters are made from carbide substrate and diamond grit. The high heat of around 2800 degrees and high pressure of approximately 1,000,000 psi forms the compact. A cobalt alloy is also present and acts as a catalyst for the sintering process. The cobalt helps bond the carbide and diamond.

Please visit PDC Bit Cutters Material article for more detailed information.

PDC Drilling Bit Cutters Geometry

This feature includes a number of cutters, its size & its rake design.

Number of Cutters

Soft rocks can be penetrated easily and hence fewer PDC cutters are used on soft PDC bits as each cutter removes a greater depth of cut. More cutters must be added to hard PDC bits for harder formation to compensate for the smaller depth of cut.

Cutters Size

Large cutters are used on softer formation bits and smaller cutters on the harder formation bits. Usually, a range of sizes is used, from 8 mm to 19 mm is used on anyone bit.

Bit Cutter Rake Design

 Cutter Rack angle

The cutter Rack Design Orientation is described by back rake and side rake angles.

Cutter Back Rack

The back rake is the angle presented by the face of the cutter to the formation and is measured from the vertical. Back rake angles vary between, typically, 15° to 45°. They are not constant across the bit, nor from bit to bit. 

The magnitude of PDC cutter rake angle affects Penetration Rate (ROP) and cutter resistance to wear. As the rake angle increase, ROP decreases but the resistance to wear increases as the applied load is now spread over a much larger area. PDC cutters with small back rakes take large depths of cut and are therefore more aggressive, generate high Torque, and are subject to accelerated wear and greater risk of impact damage.

Back rack and side rack
Cutter Side Rack

Side rake is an equivalent measure of the orientation of the cutter from left to right. Side rake angles are usually small. The side rake angle assists hole cleaning by mechanically directing cuttings toward the annulus.

 side rack pdc drilling bits design

Cutter Shape

The most common PDC shape is the cylinder, partly because cylindrical cutters can be easily arranged within the constraint of a given bit profile to achieve large cutter densities. Electron wire discharge machines can precisely cut and shape PDC diamond tables (Fig. 8). Nonplanar interface between the diamond table and substrate reduces residual stresses. These features improve resistance to chipping, spalling, and diamond table delamination. Other interface designs maximize impact resistance by minimizing residual stress levels.

Cutters Design Shape
PDC Cutters Shapes

Certain cutter designs incorporate more than one diamond table. The interface for the primary diamond table is engineered to reduce stress. A secondary diamond table is located in the high-abrasion area on the ground-engaging side of the cutter. This two-tier arrangement protects the substrate from abrasion without compromising the structural capability to support the diamond table.

Highly specialized cutters are designed to increase penetration in tough materials such as carbonate formations. Others include engineered relief in the tungsten carbide substrate that increases Penetration Rate (ROP) and reduces the requirement for Weight On Bit (WOB) and Torque, or beveled diamond tables that reduce effective cutter back rake and lower bit aggressiveness for specific applications.

Cutter Exposure

The Cutter exposure is the amount by which the cutters protrude from the bit body. It is important to ensure that the exposure is high enough to allow good cleaning of the bit face but not so high as to reduce the mechanical strength of the cutter.

High exposure of the cutter provides more space between the bit body and the formation face, whilst low exposure provides good backup and therefore support to the cutters.

PDC Drilling Bit Geometry Design

Number of Blades

Using the same analogy for roller cone bits, a PDC bit designed for soft rocks has fewer blades (and cutters) than one designed for hard rocks.

  • The soft formation PDC bit will therefore have a large junk slot area to remove the large volume of cut rock and to reduce bit balling in clay formations
  • A hard formation PDC bit with many blades requires many small cutters, each cutter removing a small amount of rock
PDC Bit Design blades
Soft & Hard Formations PDC Bits Design

Blade Height

A soft formation PDC bit will have a larger blade height in its design than a hard PDC bit with a consequent increase in the junk slot area. Higher blades can be made in steel-bodied- bits than matrix bits, because of the greater strength of steel over that of the matrix.

Blade Geometry Design

PDC bits can be manufactured with a variety of blade shapes ranging from straight to complex curve shapes. Experience has shown that curved blades provide greater stability to the bit especially when the bit first contacts the rock.

Bit Profile Design

Bit profile affects both the cleaning and stability of the PDC bit. The two most widely used profiles are:

A – Double cone: The double cone profile allows more cutters to be placed near the gauge giving better gauge protection and allowing better directional control.

B- shallow cone: The shallow cone profile gives a faster Penetration Rate (ROP) but has less area for cleaning.

In general, a bit with a deep cone will tend to be more stable than a shallow cone.

PDC Bit Profile Design

Bit Length

This is important for steerability. Shorter bits are more steerable. The two bits on the left of the below Figure are sidetrack bits (check also: sidetrack Drilling), with a short, flat profile.

The ‘Steering Wheel’ bit on the right is designed for general directional work.

Different Bit Length
Different PDC Bit Length Designs

Gauge Protection

As discussed before, the greatest amount of work is done on the heel and gauge of the drilling bit. A PDC bit that wears more on the gauge area will leave an Under gauge Hole which will require reaming from the next bit. Reaming is time-consuming and costly and in some cases can actually destroy an entire bit without a single foot being drilled.

Hence maintaining a gauge is very important. One or more PDC cutters may be positioned at the gauge area. Pre-flatted cutters are used to place more diamond tables against the gauge. Tungsten carbide inserts, some with natural or synthetic diamonds embedded in them, may be placed on the flank of bit 1.

A major advantage with fixed cutter bits over roller cone bits is that the gauge on fixed cutter bits may be extended to a larger length of the drill bit.

Conclusion

When all of the above PDC Bit Design features are put together you will find :

The PDC bit on the extreme left of the below figure is a light set bit with a few, high blades and a few but large cutters with small back rake angles. Thus light set bits typically have a few, high blades, with few large cutters, probably with low back

For hard rocks, PDC bits will have more blades, with smaller and more numerous cutters, and this trend continues to the heavy set bits on the extreme right of the below figure

Drilling Bit conclusion

IADC Classification System For PDC

Bit Dull Grading For PDC

PDC Bit Performance

PDC Drilling & Operating Parameters

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