PDC BITS
Body Material & Hardfacing
There are two types of body material used for PDC bit manufacture:
1. Tungsten carbide matrix (referred to as ‘Matrix’) 2. Steel
In very basic terms, the steel bodied PDC bit is turned, milled, and drilled out of a forged piece of steel. The matrix bodied design is created by infilling a pressed or cast moulding of the bit with matrix powder and alloy and furnacing to form a solid body.
The steel bodied design possesses higher mechanical strength and thus can be used to give higher blade standoff and reduced blade width to optimise both junk slot volume and area for effective cleaning of the bit, particularly useful for drilling clays / shales with WBM. It is also cheaper and relatively easy to modify as there is no requirement to update or create a new pattern as with a matrix bodied design.
The primary downside of the steel bodied design is that it is potentially subject to erosion and abrasive wear. This usually occurs around the cutter pockets, and if severe, can lead to lost cutters. It is common practice now to apply a layer of erosion resistant hardfacing to the blades, the extent and grade of which, is dependant on the individual bit manufacturer.
The matrix bodied design has excellent surface properties with an exceptional ability to resist fluid erosion, however, the mechanical properties are weaker than that of steel and thus the blades are generally lower and wider, with resultant lower junk slot area and volume.
As a general rule, you will observe that steel bodied designs are generally light set, open faced bits for drilling soft formations at high penetration rates. The matrix bodied bits are generally heavier set and are used to drill for longer hours and in abrasive / erosive environments. However, the diversity of applications, bit manufacturers ranges, and operator preferences, all combine to make the rule above very general indeed.
Schlumberger Private
Durability Factors
PDC BITS
Gauge Protection
The protection used at gauge differs for the two body types. Steel bodied bits typically rely on inserts, which are pressed into drilled holes in the gauge pads. The inserts may be any combination of TSP, tungsten carbide, diamond, or PDC. In addition, there will also be coverage of gauge PDC cutters / trimmers.
Matrix bodied bits are dominated by surface set diamonds or TSP (thermally stable polycrystalline) tiles of various sizes and shapes. These are placed in the pattern prior to infill and are thus embedded in the matrix body after furnacing. It is also possible to have inserts (as for steel body designs) placed in the pads, though these would be brazed instead of pressed due to the reduced size tolerance of matrix to generate the interference fit required. Pre flatted gauge trimmers would also be used.
The key importance of gauge cutters is to maintain gauge diameter, thus avoiding
undersize hole, high torque, and the potential need to ream. Selection of gauge protection should be based on prior offset dulls and knowledge of the formation abrasiveness.
Schlumberger Private
Durability Factors
PDC BITS
Backreaming Cutters
This is a common option from most bit suppliers, also known as upreamers. Basically, PDC cutters are positioned along the back angle of the gauge pad with exposure on the pin side of the bit. Thus if encountering material when pulling out of hole, the bit has cutting elements which can aid removal when backreaming.
Schlumberger Private
Durability Factors
ROLLER CONE BITS
Insert Metallurgy
The metallurgical content of the tungsten carbide insert components will vary depending on the application. Hard formation inserts are durable in terms of both shape and length thus the key material factor is to be resistant to abrasion, thus a low cobalt percentage and small grain size is used. The reverse is seen for soft formation bits where a high percent of cobalt is used with larger grain size to provide mechanical strength as the inserts are relatively long and run at high speeds.
Schlumberger Private
Durability Factors
ROLLER CONE BITS
Diamond Protection
Diamond inserts and tungsten carbide protection can be utilized in varied locations on the bit design in order primarily to protect gauge which is prone to high wear:
• Heel row – Abrasive formations. Use tungsten carbide inserts which may also be diamond coated. Reduced insert length for durability.
• Gauge – Use where prior offset dulls have shown gauge wear / damage to be the limiting factor in bit life. Ideal in soft to medium formations where high abrasivity is encountered. Various gauge trimmers are available from different bit suppliers.
They supplement the action of the gauge inserts but additionally cut formation as well.
• Inner row / Nose – Some or all of the standard tungsten carbide inserts on the bit design may be replaced with diamond coated inserts for greater durability in the face as well as gauge. Application dependant.
Schlumberger Private
Durability Factors
ROLLER CONE BITS
Shirttail Protection
A number of features can be applied to the shirttail to reduce wear to this component.
Shirttail wear may lead to exposed seals and premature bearing failure.
1) Application of hard metal on the top leading edge for abrasion resistance.
2) Pressed inserts into the actual shirttail itself. These may be a combination of both tungsten carbide and diamond coated inserts. Inserts not only add extra wear resistance but will also improve bit stabilization. Use of shirttail inserts is particularly good for directional / horizontal / abrasive formation applications.
3) Lug Pads. Used to reduce gauge breakage and bearing damage by absorbing impact between bit and bore. Particularly useful in applications where you expect wash out or if drilling on bent housing and thus eccentric hole. Lugs pads also help to minimize shirttail wear and prolong seal life.
Stabilization
Protection
Lug Pad
Schlumberger Private
Durability Factors
ROLLER CONE BITS
Hardfacing
Application of abrasion resistant material to the actual steel teeth in order to extend the durability of the teeth. This is packaged by the bit manufacturers in terms of both coverage and grade of applied material. For example, hardfacing may be applied on one side only in order to obtain a ‘self-sharpening’ wear effect. Two sides may be coated in order to maintain maximum tool height, though commonly full coverage is used.
The importance of hardfacing has developed due to the increased technology (and thus life) of the bearings, resulting in the bits staying downhole longer and requiring more abrasion resistance.
Schlumberger Private
Durability Factors
IMPREG / NATURAL DIAMOND BITS
Cone Wear
One of the most common failure modes of diamond bits is to ringout, or core, in the cone area. This is due to the fact that the hydraulics waterways converge in the cone thus creating areas of low diamond coverage. In order to avoid this, look for designs that have waterways that converge into the cone at different radii – Basically an asymmetric waterway design that does not concentrate areas of no diamonds into one zone.
Schlumberger Private
Durability Factors
IMPREG / NATURAL DIAMOND BITS
Gauge Protection
It is usual to supply these bits mounted on turbine sleeves, which are commonly a steel sleeve mounted onto the bit with various diamond / tungsten carbide inserts for gauge protection (similar to that for steel bodied PDC bits). Gauge protection is very important in these applications due to the hard and abrasive nature of the formation, and the high rotational speeds required to drill efficiently.
Schlumberger Private