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PDC bit design
Principles for Polycrystalline Diamond Compact (PDC) bit design are discussed here.
Factors affecting bit design and performance
Four considerations primarily influence bit design and performance:
- Mechanical design parameters.
- Hydraulic conditions.
- Properties of the rock being drilled.
Geometric parameters of PDC bit design
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.
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.
Cutting structure characteristics
Cutting structures must provide adequate bottomhole coverage to address formation hardness, abrasiveness, and potential vibrations and to satisfy productive needs.
Early (1970s) PDC bits incorporated elementary designs without waterways or carefully engineered provisions for cleaning and cooling. By the late 1980s, PDC technology advanced rapidly as the result of new understanding of bit vibrations and their influence on productivity. Today, cutting structures are recognized as the principal determinant of force balancing for bits and for ROP during drilling.
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:
Formation failure can be depicted with stress-strain curves (Fig. 1). Stress, applied force per unit area, can be:
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.
PDC bits drill primarily by shearing. Vertical penetrating force from applied drill collar weight and horizontal force from the rotary table are transmitted into the cutters (Fig. 2). The resultant force defines a plane of thrust for the cutter. Cuttings are then sheared off at an initial angle relative to the plane of thrust, which is dependent on rock strength.
Formations that are drillable with PDC bits fail in shear rather than compressive stress typified by the crushing and gouging action of roller-cone bits. Thus, PDC bits are designed primarily to drill by shearing. In shear, the energy required to reach plastic limit for rupture is significantly less than by compressive stress. PDC bits require less weight on bit (WOB) than roller-cone bits.
Thermally stable PDC cutters are designed to plow or grind harder formations, because of their thermal stability and wear resistance. This grinding action breaks cementing materials bonding individual grains of rock.
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 bottomhole 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 (Fig. 3).
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.
Cutter density has been increased in the “outward” radial direction from the bit centerline for the bit depicted in Fig. 3 . Note that planar cutter strike pattern inscribes an image of bit profile.
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 (Fig 4). 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.
PDC cutter design
Components of PDC cutter
PDC cutters are made up of a working component, the diamond table, and a supporting component called the substrate (Fig. 5).
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 (Fig. 6). Geometries also attempt to control stresses at the bond to the lowest possible level.
The shape of a diamond table is governed by two design objectives. It must include the highest possible diamond volume and total diamond availability to its working features. It must also ensure the lowest possible stress level within the diamond table and at the substrate bond.
Geometric features of an interface between a diamond table and substrate can significantly improve the ability of the diamond table to withstand impact (Fig. 7).
Diamond table bonds
High stress concentrations in a diamond table can result in delamination failures between diamond tables and substrates, or in diamond table edge and corner chipping. Poor bonds between grains of diamond grit can lead to cracking in a diamond layer and, eventually, to diamond table and substrate failure.
Thermally stable PDC
As described earlier, the maximum safe operating temperature for PDC materials is 750°C [1382°F]. Higher temperature resistance can be achieved in a diamond table by removing residual cobalt catalyst from the manufacturing process. The resulting material is called thermally stable polycrystalline diamond (TSP). When cobalt is removed, problems related to differential thermal expansion between the binder and diamond are removed, making TSP stable to ≈1200°C [2192°F].
TSP is formed like PDC and, except for thermal properties, behaves like PDC with one important exception. Because cobalt contained in PDC plays a key role in bonding PDC diamond tables to tungsten carbide substrates, attachment of TSP cutters to a bit is relatively difficult. Therefore, TSP is generally used only in applications in which bit operating temperature cannot be reliably controlled.
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.
- A variety 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 (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.
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 and reduces requirement for WOB and torque, or beveled diamond tables that reduce effective cutter back rake and lower bit aggressiveness for specific applications.
Noteworthy papers in OnePetro
C.A. Bejarano, G. Muñoz, G. Perez, C. Cortina, and R. Palomo 2006. Case History - Application of a New PDC Bit Design in Deep Cretaceous and Jurassic Hard Formations in Southern Mexico, Case History - Application of a New PDC Bit Design in Deep Cretaceous and Jurassic Hard Formations in Southern Mexico, 102232-MS, http://dx.doi.org/10.2118/102232-MS
Mingguang Sun, Yunlian Zhang, Yukun Yang, Huangtao, and Jinhai Zhao 2000. New Cutting Structure Design Improves the Performance of The PDC Bit, SPE/AAPG Western Regional Meeting, 19-22 June. 62858-MS. http://dx.doi.org/10.2118/62858-MS.
Kerr, Callin Joe, 1988. PDC Drill Bit Design and Field Application Evolution, Journal of Petroleum Technology Volume 40, Number 3. 14075-PA. http://dx.doi.org/10.2118/14075-PA.
F. Dagrain, J.-P. Tshibangu, 2002, Use of the D3 Model for the Estimation of Forces Acting on a Cutter in Rock Cutting and Drilling, SPE/ISRM Rock Mechanics Conference, 20-23 October. 78242-MS. http://dx.doi.org/10.2118/78242-MS