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PDC Drill Bits

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Overview

Diamond is the hardest material known. This hardness gives it superior properties for cutting any other material. Polycrystalline Diamond Compact (PDC) is extremely important to drilling, because it aggregates tiny, inexpensive, manmade diamonds into relatively large, intergrown masses of randomly oriented crystals that can be formed into useful shapes called diamond tables. Diamond tables are the part of a cutter that contacts a formation. Besides their hardness, PDC diamond tables have an essential characteristic for drill-bit cutters: They efficiently bond with tungsten carbide materials that can be brazed (attached) to bit bodies. Diamonds, by themselves, will not bond together, nor can they be attached by brazing.

Steel Body Advantage

Steel is capable of withstanding high impact loads, but is relatively soft and, without protective features, would quickly fail by abrasion and erosion. Quality steels are essentially homogeneous with structural limits that rarely surprise their users. This makes it possible for steel-body PDC bits to be relatively larger than their counterparts the matrix bits and to incorporate greater height into features such as blades.

The strength and ductility of steel give steel-bit bodies’ high resistance to impact loading. Steel bodies are considerably stronger than matrix bodies. Because of steel material capabilities, complex bit profiles and hydraulic designs are possible and relatively easy to construct on a multi-axis, computer-numerically-controlled milling machine. A beneficial feature of steel bits is that they can easily be rebuilt a number of times because worn or damaged cutters can be replaced rather easily. This is a particular advantage for operators in low-cost drilling environments.

 

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Nozzle Series 55 | 60

Synthetic Construction

To achieve cutter durability and reliable bonds between diamond tables and substrates, design engineers use a variety of application-specific cutter options. These include:

  • Cutter diameter options between ≈6 and 22 mm.
  • Optimized total diamond volumes in diamond table designs.
  • Special diamond table blends.
  • Varieties of nonplanar interface shapes that increase bond area and reduce internal stresses between the diamond table and substrate.
  • A variety of external cutter geometries designed to improve performance in particular drilling environments.

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. Nonplanar interface between the diamond table and substrate reduces residual stresses. These features improve resistance to chipping, spalling, and diamond table delamination.

Substrates are a composite material made up of tungsten carbide grains bonded by metallic binder. This material bonds efficiently with diamond tables, but is very hard and capable of impeding erosive damage to a working cutter.

Cutter geometry, at the interface between diamond table and substrate, seeks to enhance bonding between the two. Generally, geometries that increase interface surface area improve bonding. Geometries also attempt to control stresses at the bond to the lowest possible level.

Why PDC

Diamond grit is commonly used to describe tiny grains (≈0.00006 in.) of synthetic diamond used as the key raw material for PDC cutters. In terms of chemicals and properties, manmade diamond is identical to natural diamond. Making diamond grit involves a chemically simple process: ordinary carbon is heated under extremely high pressure and temperature. In practice, however, making diamond is far from easy.

Individual diamond crystals contained in diamond grit are diversely oriented. This makes the material strong, sharp, and, because of the hardness of the contained diamond, extremely wear resistant. In fact, the random structure found in bonded synthetic diamond performs better in shear than natural diamonds, because natural diamonds are cubic crystals that fracture easily along their orderly, crystalline boundaries.

Diamond grit is less stable at high temperatures than natural diamond, however. Because metallic catalyst trapped in the grit structure has a higher rate of thermal expansion than diamond, differential expansion places diamond-to-diamond bonds under shear and, if loads are high enough, causes failure. If bonds fail, diamonds are quickly lost, so PDC loses its hardness and sharpness and becomes ineffective. To prevent such failure, PDC cutters must be adequately cooled during drilling.

Design & Mechanics

 

Forming PDC into useful shapes for cutters involves placing diamond grit, together with its substrate, in a pressure vessel and then sintering at high heat and pressure.

PDC cutters cannot be allowed to exceed temperatures of 1,382°F [750°C]. Excessive heat produces rapid wear, because differential thermal expansion between binder and diamond tends to break the intergrown diamond grit crystals in the diamond table. Bond strengths between the diamond table and tungsten carbide substrate are also jeopardized by differential thermal expansion.

Geometric considerations include bit shape or profile, which is predicated based on:

  • Cutter geometry.
  • Cutter placements.
  • Cutter density.
  • Hydraulic requirements.
  • The abrasiveness and strength of the formations to be drilled and well geometry.

Design & Mechanics

Each of these factors must be considered on an application-to-application basis to ensure achievement of rate of penetration (ROP) goals during cooling, cleaning the bit, and removing cuttings efficiently. During design, all factors are considered simultaneously.

The method in which rock fails is important in bit design and selection. Formation failure occurs in two modes:

  • Brittle failure
  • Plastic failure

The mode in which a formation fails depends on rock strength, which is a function of composition and such downhole conditions as:

  • Depth
  • Pressure
  • Temperature

Formation failure can be depicted with stress-strain curves. Stress, applied force per unit area, can be:

  • Tensile
  • Compressive
  • Torsional
  • Shear

Strain is the deformation caused by the applied force. Under brittle failure, the formation fails with very little or no deformation. For plastic failure, the formation deforms elastically until it yields, followed by plastic deformation until rupture.

Cutters are expected to endure throughout the life of a bit. To perform well, they must receive both structural support and efficient orientation from bit body features. Their orientation must be such that they are loaded only by compressive forces during operation. To prevent loss, cutters must be retained by braze material that has adequate structural capabilities and has been properly deposited during manufacturing.

Cutters are strategically placed on a bit face to ensure complete bottom hole coverage. “Cutter density” refers to the number of cutters used in a particular bit design. PDC bit cutter density is a function of profile shape and length and of cutter size, type, and quantity. If there is a redundancy of cutters, it generally increases from the center of the bit to the outer radii because of increasing requirements for work as radial distance from the bit centerline increases. Cutters nearer to the gauge must travel farther and faster and remove more rock than cutters near the centerline. Regional cutter density can be examined by rotating each cutter’s placement onto a single radial plane.

Reducing the number of cutters on a bit faces yields the following results:

  • The depth of cut increases.
  • ROP increases.
  • Torque increases.
  • Bit life is shortened.

Increasing cutter density yields:

  • Decrease in ROP.
  • Decrease in cutting structure cleaning efficiency.
  • Increase in bit life.

PDC cutters are set into bits to achieve specific rake (attack) angles relative to the formation. Back rake angle has a major effect on the way in which a bit interacts with a formation. Back rake is the angle between a cutter’s face and a line perpendicular to the formation being drilled. This angle contributes to bit performance by:

  • Influencing cleaning efficiency.
  • Increasing bit aggressiveness.
  • Prolonging cutter life.

Back rake causes the cuttings to curl away from the cutting element, and as the back rake angle is increased, the tendency for cuttings to stick to the bit face is reduced. Back rake is the amount that a cutter in a bid is tilted in the direction of bit rotation. It is a key factor in defining the aggressiveness or depth of cut by a cutter. Aggressiveness is increased by decreasing back-rake angle. This increases depth of cut and results in increased ROP. Smaller back-rake angles are thus used to maximize ROP when softer formations are drilled. Increased back-rake angles reduce depth of cut and, thus, ROP and bit vibration. It increases cutter life. An increase in angle also reduces cutter breakage from impact loading when harder formations are encountered. Harder formations require greater back rake angles to give durability to the cutting structure and reduce “chatter” or vibration. Individual cutters normally have different back-rake angles that vary with their position between the bit center and gauge.