Bits and coreheads cut rock by crushing, shearing, grinding or jetting. Crushing bits are the well known roller cone bits and hammer bits.
1 Crushing bits - roller cone rock bits
Crushing bits work by crushing the rock to make an indentation with a tooth and then scouring the rock away with the subsequent tooth movement. The harder the rock the smaller the indentations and the slower the drill rate per revolution. Rocks have a certain threshold pressure which is required to initiate the indentation of each tooth. Thus a tri-cone bit needs a certain minimum WOB to start drilling and increasing WOB and RPM result in increasing penetration rates, providing the bit is adequately cleaned. Nozzles are some distance off the rock face to be scoured and extended nozzles would not appear to be feasible in small deviated holes. The penetration rate is dramatically increased as the overbalance of fluid in the well relative to pore pressure is reduced.
Traditionally roller cone bits have not been considered reliable in sizes much below 8 1/2"-7 5/8" because the bearings were too small and drillstring vibrations made it difficult to see a cone locking with the poor rig instrumentation. Pressure on the rock bit industry from impregnated diamond and PDC bits has resulted in improved bearing and tooth materials and therefore more footage. Sealed bearing 5 7/8" and 4 3/4" roller cone bits for hard and soft formations are now well established and also available in smaller sizes. The majority of these small bits are run in horizontal drain holes on Moineau motors where their low aggressiveness makes bit steering easier. The aggressiveness of a bit is defined as the bit torque divided by the product of bit diameter and WOB. The low aggressiveness means that they require higher bit weights than PDC bits.
Relatively few roller cone bits are run in deep holes in the smaller sizes but they are cheap compared with PDC and diamond bits and are useful for drilling through chert, junk, and other difficult formations which respond best to cutting by crushing, or are too abrasive, or in-homogeneous to be cut by shear bits. For bits of 4 3/4" and smaller the complex geometry and tight clearances of the closely interlocking teeth in a small tri-cone bit mean that superficially trivial changes in OD result in a full redesign of the complete bit. It also means that the bit designer has fewer options to change the "hardness" of the bit, i.e. to adapt the design to cope with subtle variations in formation hardness.
2 Crushing bits - hammer bits
Hammer bits are also crushing bits. The available energy is concentrated in blows which cause the solid button bits to crush the formation. The exhaust from the hammer is used to flush the formation face to avoid re-drilling the cuttings. Hammer bits are at the moment exclusively used on air hammers and this means that they operate under high drawdown, which undoubtedly aids bit cleaning, and reduces the confining stress on the rock, thus reducing the tooth load required to penetrate the rock.
For normal hole sizes substantial air compressors are required and the high mobilisation costs and short-term rental rates have inhibited the development of this technique. Air drilling works well in dry rocks, but as the hole waters up, the air stream is unable to lift the water out, especially after a connection, and so hammering has to stop. If the hole size is reduced then a given set of compressors can cope with more water. When drilling through gas bearing formations care must be taken to avoid downhole fires. Field experience has shown that air hammers wear out quickly when used with water or mud.
Oil field air hammers are available for 7 7/8" and larger holes. Care should be taken when evaluating smaller hammers to ensure that they have been strengthened for operations at deeper depth. At the moment no steerable hammers are available.
The development of reliable, long life percussion bits presents a challenge to both the bit designer and his materials scientist, as most materials used in bits today are rather brittle.
Air hammer drilling, especially under drawdown, therefore offers a promising line of development for hard rock slim wells. The fluidic mud hammer has to prove successful in larger hole sizes before it can be considered for slim wells.
3 Drag or fixed cutter bits
Drag bits and core heads are superficially simple and therefore feature prominently in slim wells. They comprise shear bits and grinding bits.
Drag bits and coreheads have no moving parts to fall off downhole, generate far fewer longitudinal vibrations than crushing bits and can be quickly, simply and relatively cheaply made to special sizes and special designs. They are, however generators of lateral vibrations, because of their whirling. Providing vibrations are minimised drag bits can handle very high mechanical power densities in small sizes and therefore, if the cutting or grinding surfaces are kept clean they can drill fast and long. The reduction in mechanical damage due to vibrations means that boreholes are drilled more to gauge, thereby enhancing borehole stability. Once a hole has been drilled to gauge it is easier to control drillstring vibrations so as to avoid damaging the hole and creating zones of minor crush damage which can develop into washouts. It is speculated that this reduction in mechanical damage will, in some circumstances, result in less sand influx during production.
4 Drag bits - shear bits
Shear bits and core heads are those utilising PDC and Thermally Stable Diamond (TSD) cutting elements. They cut the rock by shearing and are the most energy efficient method of removing the rock in many circumstances.
Depth of cut per cutter, which is kept clean by jetting, in soft shales is a maximum of about 10 mm when the cutter is sharp. For a Thermally Stable Diamond (TSD) bit depths of cut tend to be slightly lower. When a bit is whirling the cutters are instantaneously exposed to backward motion which can cause the diamond layer on the cutting face of the bit to flake away from the tungsten carbide backing. The diamond layer on the front of a PDC blank is between 0.5 mm and 0.7 mm. thick. They are in general strong in compression, but brittle. Thus PDC bits are sensitive to shock loads. There is continuous progress being made in improving the shock resistance of PDC bits and designing them to avoid whirling, the so-called anti-whirl bits.
Shear PDC bits are among the fastest drilling bits and are capable of giving extraordinarily long runs and of drilling in harder formations, if vibrations are minimised. Thus minimising shock loads on PDC bits has been a key technical objective in the development of the retrofit slim hole drilling system.
PDC/Thermally Stable Diamond (TSD) bits are aggressive in that a small change in bit weight gives a big increase in torque. This means that precise control of bit weight is essential for drilling at maximum efficiency. When drilling oriented with a Moineau motor and PDC bit small changes in WOB caused by stick-slip friction in the sliding mode can cause the toolface to oscillate, making holding a toolface, and hence steering difficult.
5 Drag bits - grinding bits
Diamond bits and coreheads cut by grinding the formation away. If a PDC/Thermally Stable Diamond (TSD) bit is compared to a wood plane, then a diamond bit is like sand paper. Obviously removing a given volume of rock by grinding requires more energy than removing it by planing. However, for hard abrasive rocks, diamond bits are more suited to slim holes than roller cone bits.
Diamond bits and core heads come in two types, surface set and impregnated. Surface set have only one layer of diamonds set in a matrix. They are often used for coreheads when only limited life is required. Impregnated bits have several layers of diamond in the matrix so that when one layer wears away the next can carry on drilling. Most slim hole diamond bits for deep hard formations, where long, fast runs are required, are impregnated.
Like roller cone bits, diamond bits have a low aggressiveness. They thus require higher bit weights than PDC/Thermally Stable Diamond (TSD) bits. This lower aggressiveness also means that the coupling between torsional and longitudinal vibrations is weak. However, they are often used in abrasive formations. If inadequate gauge protection is used and the bit and near bit stabiliser become undergauge severe stick-slip torsional oscillations can result. These torsional vibrations often cause severe damage to drilling equipment such as MWD tools and the stretching of the pin on single shouldered connections can often contribute to twistoffs. Thus in abrasive formations gauge protection on the bit or core head, and the subsequent stabilisers, should be considered.
Diamonds are weak in shear, but strong in compression. Therefore in impregnated bits they have to be embedded in a matrix to give them the maximum amount of support. Just sufficient diamond has to be left exposed so that each diamond can grind away the rock, and the mud can flow past the diamond to cool it and carry away the cuttings. These cuttings are very fine and are often described as "rock flour". Larger cuttings are typically 300-500 microns in diameter. During this grinding process the diamond also wears away. In order to ensure that the bit lasts as long as possible the hardness of the matrix can be varied so that it wears away at such a rate that amount of diamond exposed remains constant. Finding the optimum hardness for the matrix for a given formation requires a number of bit runs in successive wells in the same formation, using bits with a range of matrix hardnesses.
6 Drag bits - core ejector bits
Core ejector bits are a form of drag bit. They have a hole in the centre of the bit face which cuts a small core. This is ejected out of the side of the bit in small lengths about an inch long. It is then hopefully carried up the annulus without too much degradation where it is recovered over the shaker. Obviously "cores" cut from harder more cemented formations are more likely to survive the journey. These formations are more likely to be drilled with diamond bits, where cuttings are smaller and such "cores" become more valuable. This is especially true in slim holes where the volume of cuttings produced is small, and few of them are separated by the shaker.
Early versions of these bits were prone to the core jamming in the centre of the bit due to friction or wear on the throat of the "core bit". Improved understanding of the detailed mechanisms of rock cutting/breaking, post yield failure mechanisms of rock under complex stress fields, vibration control techniques, rock/steel contact mechanics, geometrical/mechanical jamming criteria, and improved diamond coating techniques, would suggest that it may be worth revisiting core cutter bits to see if they could now be made reliable.
7 Core heads
Core heads in smaller sizes are always drag bits. Bit weights are reduced relative to drilling bits as there are fewer PDC cutters or diamonds to be pushed into the formation. The smaller number of cutters/diamonds, and the fact that they are all near the outside of the head, changes the coupling between variations in WOB and torsional vibrations. The coupling is greater for PDC/Thermally Stable Diamond (TSD) cutters than for diamonds. Thus a core barrel and drive system which has been perfected for diamond coring in hard igneous rock could be subject to unexpected torsional vibrations when used with a core head dressed with sharp new PDC elements. Torsional and other vibrations are critical on core barrels as the resultant forces translate directly to stresses on the core, core damage and premature jamming of the core in the barrel, resulting in low recovery. The subject is covered in more detail below.
The drilling fluid circulation through a core head may be face discharge or throat discharge. In a throat discharge core head the drilling fluid passes between the inner and outer barrels, through the throat of the core head, and then radially across the head to cool and clean the cutters/diamonds. In a face discharge core head a rotating seal is placed between the two barrels and the flow is diverted through flow channels or nozzles in the bottom face of the core head, and then radially outwards into the annulus. In the face discharge corehead the resultant minimisation of downward flow through the core head throat minimises eroding the core as it passes through the throat, thus giving better recovery in soft formations. Invasion of the core by drilling fluid is also reduced. Viscous-dynamic seals can be used between the inner and outer barrels to minimise the friction torque.
Using this understanding a low vibration coring system has been developed and tested.