NPI
One of the primary revolutions within PDC cutter technology in the recent years is the development of non-planar interface (NPI) cutters and the rapid variation of NPI geometries by the bit manufacturers.
The interface between the polycrystalline diamond and the tungsten carbide substrate is one of high stress due to the fact that in order to form the diamond layer, both are
subjected to 1400 oC. The tungsten carbide will shrink more than the diamond on cooling, as it has a lower expansion coefficient and will thus set up a regime in which the tungsten carbide support is in tension whereas the diamond is in compression at the interface.
Thus, in a planar interface, this stress is dissipated over a very narrow band.
Planar Cutter
An NPI cutter basically has an irregular interface, where numerous ridges or circles exist in the tungsten carbide substrate, so that when adhered, the diamond has a non-planar interface with the substrate and a large surface area contact. This results in the stress being dissipated across a wider area at the interface, reducing peak stress, and allowing higher loads to be applied prior to failure. This provides improved impact resistance.
NPI Substrate: a) Side View b) Face View
An integral part of the NPI cutter is a recessed rim around the cutter diameter that provides further load support, particularly with the residual stresses around the edge. It also further increases surface area and aids dissipation of heat away from the cutting tip.
The design and geometry of both the ridges and circles that define the interface, and the rim, are the key aspects relating to the marketing of cutters by the manufacturers.
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Smith: The premium cutter offering from Smith is grouped as the GeoMax range. This includes SonicMax (NPI consisting of multiple circular rings of varying amplitude), GridMax (rippled grid interface with sloped rim), and TecMax (consists of both a primary and secondary diamond table).
Security DBS: The latest cutter technology from DBS falls under the ‘Elite Series’. DBS prior offerings have included grooved interface geometries with thick diamond edges such as Ring Claw and Deep Ring Claw. The Elite Series designates that these thick diamond table designs have been improved via a new process methodology to offer greater impact resistance.
Reed-Hycalog: The ‘standard’ NPI design utilized by Hycalog is the Iris geometry. Their range of products, though, also include Star, Nodule, and fan geometries.
a) Iris b) Fan
c) Star d) Nodule
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Hughes Christensen: The latest cutter technology from Hughes is grouped into a family called the ‘Genesis Cutters’. These have been marketed as either applicable for abrasive applications (‘A’ prefix) or fracture orientated applications (‘D’ prefix). The current offering includes:
• A1 – AXSYM
• A2 – Manhattan
• A3 – Alba
• D2 – BXD
• D3 – Niagara
• D4 - Modesto
Abrasion & Impact Resistance
These two properties of the PDC are a primary function of grain size of the diamond;
Large grain sizes give good mechanical locking and are thus more resistant to impact.
Smaller grain sizes improve abrasion resistance, as there is higher surface area to wear against the formation. The selection of grain size is an intermediate between the two.
The goal has been to continually develop cutter technology so that both impact resistance and abrasive resistance can be maximized without detriment to the other. Multimodal diamond selection, where grains of various sizes are used, led to improved abrasion resistance, whereas NPI substrates provided greater impact resistance due to its interlocking nature. However, recently a technological breakthrough has led to the development of a cutter that uses a thin layer of ultra resistant diamond at the cutting edge of a multimodal NPI cutter to radically improve abrasion resistance without any compromise to impact.
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This was developed by Reed-Hycalog and commercially released as the TReX cuter.
Numerous offsets have proven that this cutter is extremely successful in enduring longer sections but also providing higher penetration rates, as the cutter remains sharp for longer.
Side View - TReX Cutter
Diamond Thickness
The general rule is that the thicker the diamond table, the lower the impact resistance, resultant from the sintering process. Normally the cobalt used to sinter the diamond is drawn from the tungsten carbide substrate. With a thicker diamond layer, more cobalt is drawn and the cobalt concentration at the interface is reduced making it brittle and weak under tensile loads.
Two methods can be followed in order to reduce this effect: One is to add cobalt directly to the diamond grit; the second is to use coarser diamond grit that requires less cobalt.
Unfortunately, both methods result in significantly reduced abrasion resistance.
The issues with thick diamond tables may be overcome using a suitably designed rim, which will provide thick diamond at the cutting edge and increased strength, particularly when formulated with the interlocking strength of a non-planar interface. Problems may arise though in manufacturing such geometries due to cracking of the diamond or carbide and thus great importance is placed on the manufacturing process in place.
Cutter Shape
The vast majority of cutters are cylindrical in shape. However, there are two other shapes that are relatively common in the oilfield; Oval and pointed.
Oval cutters are actively promoted by one specific bit manufacturer (BBL). From their geometry it can be observed that there are two key differences between oval and cylinder cutter wear. The first is that a smaller horizontal wear flat will be generated (less PDC contact with formation and less friction / heat), the second being that there is a greater
PDC
NPI Substrate Ultra abrasion resistant layer
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vertical quantity of diamond volume. However, due to this greater vertical component, you may have some cutter placement constraints on tight profiles resulting in less PDC coverage. It is also claimed by BBL that the use of oval cutters will provide optimum point loading for high depths of cut and thus ROP.
Oval Cutters
Pointed cutters are basically cylindrical in shape with the cutting edge shaped to form a point. Their primary application is for hard chalk / limestone formations where a point load is applied in order to fracture the formation as opposed to a shear failure mechanism normally associated with cylindrical cutters.
Pointed Cutters
Several bit manufacturers have used these cutters, notable Smith (Arrow cutter), DBS, and Hycalog (Scribe cutter). The bit designs may use a mixture of both cylindrical and points in order to obtain both fracture and shear. The downside of the pointed cutter is that it is formation specific (not good for heterogeneous) and may dull the point rapidly under high weights.
Miscellaneous geometry
Several other cutter or substrate geometries exist which may be encountered. Some of these include:
• Modified Substrates: This is where the actual substrate has had a relief groove cut into it on the cutting edge side. The idea is to enhance depth of cut and reduce substrate / formation interaction when the PDC starts to generate a wear flat. Both Smith (Quick cutter) and Hycalog (Hibernia substrate) promote this substrate geometry in varied cutter diameter sizes.
Chamfered Substrate
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• Chamfered Cutters: Conventionally the angle between the front face of the PDC and the circumference of the carbide support is 90 degrees. This angle is
chamfered in order to improve the carbide support of the diamond cutting edge so as to reduce incidence of PDC breakage that is often resulting from hard
formation drilling. One such product is the Tuffedge cutter from Hycalog.
Tuff Edge Cutter
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