The Analysis and Specification of Manufacturing Processes - Mechanical Engineering
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For drilling purpose, the main tools used for drilling are roller cone and drag bits. Drag bits with PDC cutters have been a huge success over last few decades as they have shown double efficiency compared to the roller bits. Technology advancements are always made to cut the costs, increase efficiency and save time. PDC bits are one of such advancements for oil and gas industry (WP, 2006, p.2).
The purpose of this paper is to discuss manufacturing of PDC bits. The paper emphasizes on the making of each component, materials used for making and the whole assembly of matrix-body PDC bits. A general idea of different properties of the bit shall also be presented.
PDC bit is a fixed head that rotates as one piece. It does not contain any separable moving parts that could be changed or replaced. PDC cutters are used for drilling since many decades now after the popularity of diamond technology. PDC bits could drill twice as efficiently as roller bits in hard geological formations. They can cut longer and faster than roller bits. There are two structurally dissimilar types of PDC bits (Mian, 1992, p.41).
- Matrix-body bit
- Steel-body bit
Parts of PDC Bit
A typical bit is a fixed part itself but it has different sub-parts according to its typical design. Such parts include apex, nozzle, cutters, nose, shoulder, blade, gauge, gauge protection, water way, breaker slot and pin. A nomenclature of PDC bit is shown in the Figure 1.
Figure 1: (Source: Smith bits. A Schlumberger Company)
Matrix-body bit is very hard material. The composite is combination of tungsten carbide bonded with metallic binder. The matrix-body bit is considered highly desirable as it is strongly resistant to any kind of erosion or abrasion. It has the capability to withstand high compressive loads as compared to the steel-body bits, but it has low resistance to impact loading while steel-body bits have high resistance (IADC, 2005, p.52).
The physical properties of matrix-body bit are less predictable due to the composite material it is made from. There are variations in size and placement of tungsten carbide particles due to the circumstances or design, so the matrix is heterogeneous.
Matrix-body bits are preferred over steel-body bits where the environment is considered to cause body erosion of the equipment. Matrix is brittle and robust. Matrix-body has diamond impregnated bits, which gives the strength to PDC. These bits can only be used in matrix-body construction (Bellin, et al, 2010, p.231).
Making of PDC Bit
A PDC bit is made from a wafer of sintered diamond that is 0.025 to 0.030 inches thick. It is infused with a catalyst under high pressure and temperature. It is formed onto 0.115 to 0.140 inches substrate of tungsten carbide. Each wafer is brazed into tungsten carbide stud and then pressed into tungsten carbide matrix body. During the manufacturing process, the diamond grit is reacted with cobalt in high temperature and pressure to produce a cylinder of polycrystalline diamond. It has no cleavage planes, making the PDC cutter extremely hard and brittle. It is resistant to any wear and tear. The substrate is responsible for the structural support to diamond. It makes bond with diamond and provides medium to resist brazing. The stress on the bit is reduced by the non-planar interface between substrate (tungsten carbide) and diamond table as such a non-planar bond strengthens the structure (Bruton, et al, 2014, p.48)
Structure of PDC bit:
Geometric surface of a bit is defined by the bit profile. It contacts the formation. There are various profile types such as full round, stepper, short tapper, long tapper, concave, convex, and parabolic. There are different numbers of cutters on different bits. The cutter consists mainly of two parts: polycrystalline diamond table and its substrate. During the formation process, table comes in contact with the formation. The number of cutters per unit area of bit diameter is called cutter density. Each cutter is placed on separate diameter according to the designated cutter placement. It provides the redundancy such that 100% of cutters work for formation in single revolution. The distance to the body of PDC bit from the edge of stud is called cutter exposure.
Hydraulics characteristics of a bit are extremely important in order to clean the debris sheared by bit. It is necessary to flush out the debris to maintain full effectiveness of bit. Side of the bit body have junk slots or gauge reliefs. They are cut into the body in order to aid the flushing of debris. Nozzles provide the turbulent jet flow to clean the cutting area (Jones, et al, 2008, p.21).
The most prominent features of PDC bits are its blades and cutters. Before assembling the whole bit, designers are required to consider the placement and number of cutters. It is important to know the specifications off depth of the cut, in order to design accordingly. Lesser the number of cutters, deeper the cut is but it also increases the wear and tear of the cutter. When a bit rotates, the cutters near the gauge or sides travel the higher distance than the cutters present near the centre. In some sophisticated designs, the spacing between the cutters on sides, are reduced to minimum, to extend the life of the bit. On some designs, two rows of cutters are brazed to each blade. These blades could be backup or secondary blades. They work behind the primary sets of cutters and can be placed deeper into the bit body. The purpose of secondary cutters is to engage into formations if the primary cutters start to wear off (Yahiaoui, et al, 2013, p.32).
An angle in maintained while placing the cutters along the leading edge of the blades. The angle controls how the cutter will engage to rocks and how aggressively. The angle is also known as bake rack angle. Soft geological formations angles require less back rake angle but the harder formations need greater back rake angle. The position of the back rake angle also depends upon the position of the cutters along the blade line. Back rake can influence the rate of penetration and depth of the cut. It also helps reduce the vibration of the bit and associated wear and tear.
The cutters are designed in such a way that they endure throughout the life of a bit. To maintain such endurance, cutters are placed strategically with effective orientation and strong structural support. The orientation of the cutters during the assembly of the bit must be such that they are only stacked by compressive forces during the whole operation time. Bottomhole of the bit is completely covered by the cutters (Yahiaoui, et al, 2013, p.33).
Properties of Bit
There are varying types of cutters present it is difficult to infuse all the types in one cutter. Different tasks require different cutters, so the operator decides which PDC bit to use, according to the drilling site. The geological formations and drilling conditions lead to the selection of bit (Barnard, 2000, p.26).
PDC cutters can be categorized by three properties: impact resistance, abrasion resistance, and thermal abrasion resistance. All three defines the selection criteria as per requirements of the drilling site. It is very difficult to merge all these properties into one single cutter. It is the job of an application engineer to select the best cutter, to ensure the maximum efficiency and eliminating the risk of loss of any kind.
A cutter that is highly abrasion resistant is made by fine diamond grain that is well-sintered. Multiple lab tests are done by cutting granite with a coolant and analyzing the rate of wear. These types of cutters are significantly used for sharp sandstones of low to moderate compressive strengths. Drilling stability is managed by the bit design and drilling interval to minimize the impact damage (Barnard, 2000, p.27).
A cutter with high impact resistance is made through coarse grain size in a well-sintered part. The diamond table interface is especially designed to endure the impacts and avoid getting cracked from high impacts. Impact resistant cutters are tested by drop tests or other diamond strength checking tests. Interbedded formations require high impact resistance cutters. This category is also used in bit designs that have lesser drilling stability. They are also use in the bit designs, where chances of impacts are higher such as in secondary row cutters, trimmers or gauge cutters (Barnard, 2000, p.28)
Apart from the PDC properties, substrate properties of the PDC cutter are also important as tungsten carbide is an integral component of the cutter and it contributes as a key compound to the performance and overall application of the PDC. The tungsten carbide acts as a transition layer between the bit body that is less brittle and the diamond body which is highly brittle. The transition layer makes the diamond layer absorb all the load impacts by drilling process. It also provides the growth platform for the PDC layer during the HPHT press and also gives the required surface for mounting the PDC on head of the bit (Lyons & Plisga, 2011, p.51).
Substrate is a mixture of three elements; tungsten, carbide and cobalt. Cobalt is used to add the toughness to the substrate. The substrate is considered to be brittle with more cobalt content and more abrasion resistant when the cobalt content is low. PDC cutters are made with fixed cobalt content to make it equally brittle and abrasion resistant at the same time.
Diamond is the hardest material present on earth. Thermal conductivity, robustness and wear resistance make diamond ideal for material for rough conditions. It is considered best for bearings. Diamond crystals cleave together easily in parallel planes. The process of sintering overcomes the weak planes by bonding small diamond structures into one giant unified structure. Sintered diamond has greater strength than a regular diamond structure as individual crystals are arranged randomly. This sintered diamond is hard to crack from any hard impact while the traditional diamond will cleave easily. They are also uniform in thermal conductivity and hardness. All these properties make sintered diamond ideal for all kinds of bearings (Poletto & Miranda, 2004, p. 16).
Other than the drilling bits, PDC can be used in various applications of technology. It can be used as:
Most of the thermal conductors that are thermally conductive are also electrically conductive; while the polycrystalline diamond has negligible electrical conductivity but high thermal conductivity. It makes it suitable for the heat sinks in electrical devices. High power electrical devices can work longer with efficient heat dissipation system. Sintered diamond can save systems by preventing overheating to silicon and other semiconductor systems (Poletto & Miranda, 2004, p. 18).
Sintered diamond can be used in radiation detection devices. Diamond lacks the stable oxide unlike other semiconductors. It creates potential for ultra violet radiation to access active semiconductor without getting absorbed into the surface layer.
Another application of sintered diamond is to use it as electrode for DNA tests. There are many petrochemical methods of linking DNA to polycrystalline diamond, covalently. This process can detect different bimolecular (Bruton, et al, 2014, p.50)
Sintered diamond is also used for transmitting microwave and infrared radiation. As diamond has high thermal conductivity, is chemically inert and has low coefficient of thermal expansion, it has replaced zinc selenide in application of gyrotrons and CO2lasers (Gabolde & Nguyen, 2006, p.22)
PDC bit is the most advanced form of technology for the drilling process at well sites. There are more advance forms of PDC bits available now than a conventional drag bits. Different applications with different form of PDC cutters are satisfied, according to the need of operation. Blades and cutters are the main components of a bit, and normally a bit is selected according to these parts comparing to the work. This paper has discussed the details of PDC bit, from its components to the assembly. This paper also gives an idea about the other uses of PDC with reference to the technology.