Tuesday, 14 November 2017


How To Perform A Successful PDC Bit Design


What is The Main Steps Of PDC Bit Design

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

Introduction To PDC Bit Design

PDC bits have 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. A thorough review of the economics of running a PDC bit needs to be performed prior to selection due to its increased cost.

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 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.
PDC Bit  Diamond Design
Fig.1: NL Hycalog's DS-23 diamond bit (Courtesy NL Hycalog) 
PDC bit design 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 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.

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

Befor Going Into PDC Bit Design What Is The Major Applications OF PDC 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 back-up 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 enable optimum fluid flow across the cutter to efficiently reduce the friction temperatures induced. This efficient cooling will help minimize 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 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 Bit Design

1) PDC Bit Body Design

The bit body design may be: 1- Steel Body 2- Matrix Body
PDC Bit | 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 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 preferable as they can be easily repaired but suffer from erosion.

PDC Bit | 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 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.

2) Get PDC Bit Cutters Material To Your Advantage: 

PDC Cutters are made from carbide substrate and diamond grit. 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 to the sintering process. The cobalt helps bond the carbide and diamond.

3) PDC Bit Cutters Geometry Is So Important In PDC Bit Design:

Number of PDC Bit 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.

PDC Bit 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 any one bit.

PDC Bit Cutter Rake Design

PDC Bit Design Cutter Rack angle

Cutter Rack Design Orientation is described by back rake and side rake angles.
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 subjected to accelerated wear and greater risk of impact damage.

Back rack and side rack pdc bits design cutters
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 bits design cutters

4) The Effect Of Cutter Shape On PDC Bit Design

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.

PDC 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 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 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.

    How To Control Bit Cleaning While Performing Your PDC Bit Design

    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.

    5) The Factors Affecting PDC Bit Geometry Design

    Number of PDC Bit Blades & Rock Hardness

    Using the same analogy for roller cone bits, a PDC bit designed for soft rocks has a 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

    Soft hard PDC Bit Design blades
    Soft & Hard Formations PDC Bits Design

    Blade Height Relation With Rock Hardness

    A soft formation PDC bit will have a lager blade height in its design than a hard PDC bit with a consequent increase in 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 matrix.

    Yes, Blade Geometry Design Affects Bit Stability

    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 a greater stability to the bit especially when the bit first contacts the rock.

    Bit Profile Design & PDC Bit Stability

    Bit profile affects both 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 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.

    The Direct Effect Of Bit Length On The Steerability 

    This is important for steerability. Shorter bits are more steerable. The two bits on the left of below Figure are sidetrack bits, with a short, flat profile.
    The ‘Steering Wheel’ bit on the right is designed for general directional work.
    Different PDC Bit Length Designs

    The PDC Bit Design Feature : 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 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 table against gauge. Tungsten carbide inserts, some with natural or synthetic diamonds embedded in them, may be placed on the flank of the 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.


    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

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