The effects of increased hydraulic horsepower at the bit are similar to their effect on roller cone bits. However manufacturers will often recommend a minimum flowrate in an attempt to ensure that the bit face is kept clean and cutter temperature is kept to a minimum. This requirement for flowrate may adversely affect optimisation of HHP.
In order to prevent an influx of formation fluids into the wellbore the hydrostatic mud pressure must be slightly greater than the formation (pore) pressure. This overbalance, or positive pressure differential, forces the liquid portion of the mud (filtrate) into the formation, leaving the solids to form a filter cake on the wall of the borehole. In porous formations this filter cake prevents any further entry of mud into the formation. This overbalance and filter cake also exists at the bottom of the hole where it affects the removal of cuttings. When a tooth penetrates the surface of the rock the compressive strength of the rock is exceeded and cracks develop, which loosen small fragments or chips from the formation (Figure 30). Between successive teeth the filter cake covers up the cracks and prevents mud pressure being exerted below the chip. The differential pressure on the chip tends to keep the chip against the formation. This is known as the static chip hold down effect, and leads to lower penetration rates. The amount of plastering which occurs depends on mud properties. To reduce the hold down effect:
Reduce the positive differential pressure by lowering the mud weight (i.e. reduce the overbalance to the minimum acceptable level to prevent a kick).
Reduce the solids content of the mud (both clay and drilled solids). Solids removal is essential to increase drilling efficiency.
In less porous formations the effect is not so significant since the filter cake is much thinner. However dynamic chip hold down may occur (Figure 30). This occurs because, when cracks form around the chip mud enters the cracks to equalise the pressure. In doing so, however, a pressure drop is created which tends to fix the chip against the bottom of the hole. The longer the tooth penetration, the greater the hold down pressure. Both static and dynamic hold down effects cause bit balling and bottom hole balling. This can be prevented by ensuring correct mud properties (e.g. mud weight and solids content).
The ROP will also be affected by the rotary speed of the bit and an optimum speed must be determined. The RPM influences the ROP because the teeth must have time to penetrate and sweep the cuttings into the hole. Figure 28 shows how ROP varies with RPM for different formations. The non-linearity in hard formations is due to the time required to break down rocks of higher compressive strength. Experience plays a large part in selecting the correct rotary speed in any given situation.
The RPM applied to a bit will be a function of :
a. Type of bit
In general lower RPMs are used for insert bits than for milled tooth bits. This is to allow the inserts more time to penetrate the formation. The insert crushes a wedge of rock and then forms a crack which loosens the fragment of rock.
b. Type of formation
Harder formations are less easily penetrated and so require low RPM. A high RPM may cause damage to the bit or the drill string.
Institute of Petroleum Engineering, Heriot-Watt University
The fluid circulation across the bit face must be designed to remove the cuttings efficiently and also to cool the bit face. These requirements may be satisfied by increasing the fluid flowrate and/or the design of the water courses that run across the face of the bit. This increased fluid flow may however cause excessive erosion of the face and premature bit failure. More than three jets are generally used on a PDC bit.
There are three basic types of PDC bit crown profile: flat or shallow cone; tapered or double cone; and parabolic. There are variations on these themes but most bits can be classified into these categories.
The flat or shallow cone profile evenly distributes the WOB among each of the cutters on the bit (Figure 21). Two disadvantages of this profile are limited rotational stability and uneven wear. Rocking can occur at high RPM, because of the flat profile. This can cause high instantaneous loading, high temperatures and loss of cooling to the PDC cutters.
The taper or double cone profile (Figure 22) allows increased distribution of the cutters toward the O.D. of the bit and therefore greater rotational and directional stability and even wear is achieved.
The parabolic profile (Figure 23) provides a smooth loading over the bit profile and the largest surface contact area. This bit profile therefore provides even greater rotational and directional stability and even wear. This profile is typically used for motor or turbine drilling.
The PDC cutters can be set at various rake angles. These rake angles include back rake and side rake. The back rake angle determines the size of cutting that is produced. The smaller the rake angle the larger the cutting and the greater the ROP for a given WOB. The smaller the rake angle , however, the more vulnerable the cutter is to breakage should hard formations be encountered. Conversely the larger the rake angle the smaller the cutting but the greater resistance to cutter damage. Back rake also assists cleaning as it urges the cuttings to curl away from the bit body thereby assisting efficient cleaning of the bit face. Side rake is used to direct the formation cuttings towards the flank of the bit and into the annulus.
The cutters of a PDC bit are mounted on a bit body. There are two types of bit body used for PDC bits. One of these is an entirely steel body and the other is a steel shell with a Tungsten Carbide matrix surface on the body of the shell.
The cutters on a steel body bit are manufactured as studs (Figure 19). These are interference fitted into a receptacle on the bit body. Tungsten carbide button inserts can also be set into the gauge of the bit to provide gauge protection. The stud can be set with a fixed backrake and/or siderake (see below). An advantage of using a stud is that it may be removed and replaced if the cutter is damaged and the body of the bit is not damaged. The use of a stud also eliminates the need for a braze between the bit body and the cutter. Field experience with the steel body bit indicates that face erosion is a problem, but this has been overcome to some extent by application of a hardfacing compound. Steel body bits also tend to suffer from broken cutters as a result of limited impact resistance (Figure 20). This limited impact resistance is because there is no support to the stud cutter.
Matrix body bits use the cylindrical cutter (Figure 18) that is brazed into a pocket after the bit body has been furnaced by conventional diamond bit techniques. The advantage of this type of bit is that it is both erosion and abrasion resistant and the matrix pocket provides impact resistance for the cutter. Matrix body bits have an economic disadvantage because the raw materials used in their manufacture are more expensive.
Institute of Petroleum Engineering, Heriot-Watt University
Drilling fluid passes from the drillstring and out through nozzles in the bit. As it passes across the face of the bit it carries the drilled cutting from the cones and into the annulus. The original design for rock bits only allowed the drilling mud to be ejected from the middle of the bit (Figure 12). This was not very efficient and led to a build up of cuttings on the face of the bit (bit balling) and cone erosion. A more efficient method of cleaning the face of the bit was therefore introduced. The fluid is now generally ejected through three jet nozzles around the outside of the bit body (Figure 13). The turbulence created by the jet streams is enough to clean the cutters and allow efficient drilling to continue.
Jet nozzles (Figure 15) are small rings of tungsten carbide and are available in many sizes. The outside diameter of the ring is standard so that the nozzle can fit into any bit size. The size of the nozzle refers to the inner diameter of the ring. Nozzles are available in many sizes although diameters of less than 7/32″ are not recommended, since they are easily plugged. The nozzles are easily replaced and are fitted with an “O” ring seal (Figures 17). Extended nozzles (Figure 16) may also be used to improve the cleaning action . The nozzles are made of tungsten carbide to prevent fluid erosion.
The teeth of a milled tooth bit and the inserts of an insert bit for the cutting structure of the bit. The selection of a milled tooth or insert bit is largely based on the hardness of the formation to be drilled. The design of the cutting structure will therefore be based on the hardness of the formation for which it will be used. The main considerations in the design of the cutting structure is the height and spacing of teeth or inserts.
Soft formation bits require deep penetration into the rock so the teeth are long, thin and widely spaced to prevent bit balling. Bit balling occurs when soft formations are drilled and the soft material accumulates on the surface of the bit preventing the teeth from penetrating the rock. The long teeth take up space, so the bearing size must be reduced. This is acceptable since the loading should not be excessive in soft formations.
Moderately hard formation bits are required to withstand heavier loads so tooth height is decreased, and tooth width increased. Such bits rely on scraping/gouging action with only limited penetration. The spacing of teeth must still be sufficient to allow good cleaning.
Hard formation bits rely on a chipping action and not on tooth penetration to drill, so the teeth are short and stubbier than those used for softer formations. The teeth must be strong enough to withstand the crushing/chipping action and sufficient numbers of teeth should be used to reduce the unit load. Spacing of teeth is less critical since ROP is reduced and the cuttings tend to be smaller.
The cutting structure for insert bits follows the same pattern as for milled tooth bits. Long chisel shaped inserts are required for soft formations, while short ovide shaped inserts are used in hard formation bits.
Tungsten carbide hardfacing is applied to the teeth of soft formation bits to increase resistance to the scraping and gouging action. Hard formation bits have little or no hardfacing on the teeth, but hardfacing is applied to the outer surface (gauge) of the bit. If the outer edge of the cutting structure is not protected by tungsten carbide hardfacing two problems may occur.
1. The outer surface of the bit will be eroded by the abrasive formation so that the hole diameter will decrease. This undergauge section of the hole will have to be reamed out by the next bit, thus wasting valuable drilling time
2. If the gauge area is worn away it causes a redistribution of thrust forces throughout the bearing assembly, leading to possible bit failure and leaving junk in the hole (e.g. lost cones)
All three cones have the same shape except that the No. 1 cone has a spear point. One of the basic factors to be decided, in the design of the cones, is the journal or pin angle (Figure 9). The journal angle is formed between the axis of the journal and the horizontal. Since all three cones fit together, the journal angle specifies the outside contour of the bit. The use of an oversize angle increases the diameter of the cone and is most suitable for soft formation bits. Although this increases cone size, the gauge tip must be brought inwards to ensure the bit drills a gauge hole.
One important factor which affects journal angle is the degree of meshing or interfit (i.e. the distance that the crests of the teeth of one cone extend into the grooves of the other). The amount of interfit affects several aspects of bit design.
1. It allows increased space for tooth depth, more space for bearings and greater cone thickness
2. It allows mechanical cleaning of the grooves, thus helping to prevent bit balling
3. It provides space for one cone to extend across the centre of the hole to prevent coring effects
4. It helps the cutting action of the cones by increasing cone slippage.
In some formations, it is advantageous to design the cones and their configuration so that they do not rotate evenly but that they slip during rotation. This Cone slippage, as it is called, allows a rock bit to drill using a scraping action, as well as the normal grinding or crushing action.
Cone slippage can be designed into the bit in two ways. Since cones have two profiles: the inner and the outer cone profile, a cone removed from the bit and placed on a horizontal surface can take up two positions (Figure 10). It may either roll about the heel cone or the nose cone. When the cone is mounted on a journal it is forced to rotate around the centre of the bit. This “unnatural” turning motion forces the inner cone to scrape and the outer cone to gouge. Gouging and scraping help to break up the rock in a soft formation but are not so effective in harder formations, where teeth wear is excessive.
Cone slippage can also be attained by offsetting the axes of the cones. This is often used in soft formation bits (Figure 11). To achieve an offset the journals must be angled slightly away from the centre. Hard formation bits have little or no offset to minimise slippage and rely on grinding and crushing action alone.