PDC ( Polycrystalline diamond compact ) bit – Drilling Equipment – Readyzone

PDC bit ( Polycrystalline diamond compact bit)

A polycrystalline diamond compact bit ( PDC bit ) is a new generation of the old drag or fishtail bit (Fig. 1) and employs no moving parts (e.g. there are no bearings). The new bit is designed to break the rock in shear and not in compression, as is the case with roller cone bits, or by a ploughing/grinding action, as is the case with diamond bits. Breakage of rock in shear requires significantly less energy than in compression. Thus, less weight on bit can be used, resulting in less wear and tear on the rig and drill string. Figure 2 shows two different types of PDC bit. PDC bits are also known as ‘stratapax’ bits.

The mechanics of rock breakage by a diamond compact (or stratapax) bit, a three-cone bit and a conventional diamond bit is depicted in Fig. 3. The fact that a PDC bit fails the rock in shear limits its application to the drilling of rocks of soft and medium hardness. Shear failure also requires that the bit be self-sharpening for efficient rock cutting.

A PDC bit employs a large number of cutting elements, each called a drill blank. The drill blank is made by bonding a layer of polycrystalline man-made diamond to a cemented tungsten carbide substrate in a high pressure, high temperature process to produce an integral blank. This process produces a blank (or compact) having the hardness and wear resistance of diamond complemented by the impact resistance of the cemented tungsten carbide layer. The diamond layer is composed of many tiny diamonds which are grown together at random orientation for maximum strength and wear resistance. The blanks are bonded to specially shaped tungsten carbide studs and are then attached to the bit body using a low-temperature brazing method or by interference fitting. Cutting efficiency is maximised by precise positioning and angling of the cutters. Figure 4 shows different types of cutter.

During drilling the compact provides a continuous sharp cutting edge, owing to continuous micro chipping of the diamond surface resulting from wear. This feature is necessary for efficient rock cutting.

PDC bit design is influenced by nine variables: (1) bit body material; (2) bit profile; (3) gauge protection; (4) cutter shape; (5) number of concentration of cutters; (6) locations of cutters; (7) cutter exposure; (8) cutter orientation; and (9) hydraulics.

Bit body material: Two types of body materials are in use: (1) heat-treated alloy steel, as used in roller cone bits; and (2) a tungsten carbide matrix, as used in natural diamond bits.

Steel body bits are less durable and less resistant to erosion by the drilling fluid than the matrix body bits. Steel body bits use stud-type cutters ( Fig. 4 ) which are attached to the body by interference of shrink fitting. The steel body is also provided three or more nozzles for fluid passage.

Tungsten carbide matrix body bits are manufactured (or cast) in a mould similar to the process of manufacturing diamond bits. This allows more complex profiles to be obtained. Owing to the high temperature required when casting a matrix body, it is not possible to insert compact blanks until after the furnacing of the body, as the compact diamond will be destroyed. The bonding between the small diamond crystals is destroyed at about 750 0C.

Bit profile affects cleaning and stability of the hole and gauge protection. Two bit profiles are in common use: double cone and shallow-cone (see Figure 5). The double-cone profile allows more cutters to be placed near the gauge and will control hole deviation. The shallow-cone profile affords less area for cleaning but the bit drills faster than the bit of the double-cone profile owing to the more direct loading of the cutters on the bit face by the weight applied by the drill collars.

Gauge protection in steel body bits is provided by tungsten carbide inserts placed near the edges, while the matrix body bit utilises natural diamonds for gauge protection.

Cutter shape: Polycrystalline diamond blanks are produced in three basic shapes: (1) the standard cylindrical shape; (2) the chisel (or parabolic) shape; and (3) the convex shape.

Concentration of cutters: Longer bit life is generally obtained with greater concentration of cutters. However, the penetration rate decreases with increasing concentration. Owing to the difficulty of cleaning the areas between the cutters.

Location of cutters: Field experience and fracture mechanics models are used to locate cutters for maximum cutting and minimum wear and torque.

Cutter exposure: Penetration rate increases with increased cutter exposure; however, greater exposure makes the cutter more vulnerable to breakage ( Fig. 6 ).

Cutter orientation is described by back and side rake angles, as shown in Fig. 7. The back rake angle varies between 00 and 250, and its magnitude directly affects the rate of penetration. As the rake angle increases, the penetration rate decreases, but the resistance to cutting edge damage increases, as the load is now spread over a larger area ( Fig. 3 and 7 ). The side rake angle assists hole cleaning by mechanically directing cuttings towards the annulus.

Hydraulics : PDC bits require optimum hydraulics for efficient hole cleaning, and, in turn, efficient hole making. Also, since the bit nozzles are close to hole bottom, maximum jetting speed will result in improved cleaning and high penetration rates.

It should be noted that PDC bits may have more than three nozzles and, in addition, the nozzles may not be round, as with the roller cone bit. Hydraulic calculations are, therefore, based on total flow area (TFA), and manufacturers charts should be used to determine the size of nozzles corresponding to the calculated TFA.

PDC bits are also used in coring applications.

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Reference : Mc Cray & Cole, Oil Well drilling Technology, New India Publication.

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