PDC Drill bits have been used extensively and successfully over various formation types. The lack of rotating parts leads to greater life expectancy, as long bit runs are achievable with resultant time and cost savings (check also drilling cost per foot). Due to its increased cost, drilling engineers shall thoroughly review the economics of running a PDC drill bit before bit selection.
Drilling Manual | Underbalanced dri...
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Drilling Manual | Underbalanced drillingPDC bits have several significant design features that enhance their ability to drill:
Gaining more experience with the bit will enhance design features.
Bit manufacturers integrate the PDC blank into their respective bit designs (Fig.1). Variations in designs include the number and placement of the blanks, jet structure, and watercourse development. In some applications, PDC bits will drill 3-4 times the footage of a conventional roller bit at 2-3 times the drilling rate if sticky formations do not pose problems. For example, 18,000-20,000 ft wells in South Texas typically complete in 70-80 days using PDC bits vs. 120-130 days with conventional roller bits.
Fig.1: NL Hycalog’s DS-23 diamond bit (Courtesy NL Hycalog)The manufacturing of PDC bits is either with a machined steel or a matrix body process. The matrix process is similar to the manufacturing of diamond bits. The cutters are attached to the bit by proprietary techniques. Matrix bodies appear to be more erosion-resistant.
The shape of the PDC cutters is becoming an important consideration. Most manufacturers use the original circular design. However, there are many research and development efforts to enhance the design and improve drilling performance.
What are The Main Steps Of PDC Bit Design are:
The bit body design may be:
Fortunately, steel and matrix PDC drill bits rapidly evolve, and their limitations are diminishing. As hard-facing materials improve, steel bits are becoming significantly well-protected with materials that are highly resistant to abrasion and erosion. At the same time, matrix materials’ structural and wear-resisting properties are also rapidly improving, and the range of economic applications suitable for both types is growing.
Today’s PDC drill Bits Design as a matrix has little resemblance to that of even a few years ago. Tensile strengths and impact resistance have increased by at least 33%, and cutter brazes strength has increased by ≈80%. At the same time, geometries and the technology of supporting structures have improved, resulting in robust and productive matrix products.
PDC Cutters are made from carbide substrate and diamond grit. The high heat of around 2800 degrees and high pressure of approximately 1,000,000 psi forms the compact. A cobalt alloy also acts as a catalyst for the sintering process. The cobalt helps bond the carbide and diamond.
Please visit the PDC Bit Cutters Material article for more detailed information.
We usually use fewer cutters on soft PDC bits as each cutter removes a greater depth of cut. For harder formations, it is essential to use more cutters to compensate for the smaller depth of cut.
For softer formations, we typically choose larger cutters than in harder formations. Usually, the standard range of sizes is from 8 mm to 19 mm on anyone bit.
We generally describe the cutter rack design orientation by back rake and side rake angles.
The most common PDC Drill Bits shape is the cylinder, partly because it is easier to arrange cylindrical cutters within the constraint of a given bit profile to achieve large cutter densities. Electron wire discharge machines can precisely cut and shape PDC diamond tables (Fig. 8). Nonplanar interface between the diamond table and substrate reduces residual stresses. These features improve resistance to chipping, spalling, and diamond table delamination. Other interface designs maximize impact resistance by minimizing residual stress levels.
PDC Drill Bits Cutter ShapesSpecific cutter designs incorporate more than one diamond table. The interface for the primary diamond table is engineered to reduce stress. A secondary diamond table is located in the high-abrasion area on the ground-engaging side of the cutter. This two-tier arrangement protects the substrate from abrasion without compromising the structural capability to support the diamond table.
Highly specialized cutters are designed to increase penetration in tough materials such as carbonate formations. Others include engineered relief in the tungsten carbide substrate that increases Penetration Rate (ROP) and reduces the requirement for Weight On Bit (WOB) and Torque or beveled diamond tables that reduce effective cutter back rake and lower bit aggressiveness for specific applications.
The Cutter exposure is the amount the cutters protrude from the bit body. It is essential to ensure that the exposure is high enough to allow good cleaning of the bit face but not so high as to reduce the mechanical strength of the cutter.
High exposure of the cutter provides more space between the bit body and the formation face, while low exposure provides good backup and support to the cutters.
Using the same analogy for roller cone bits, a PDC bit designed for soft rocks has fewer blades (and cutters) than one designed for hard rocks.
A soft formation PDC bit will have a larger blade height in its design than a hard PDC bit, with a consequent increase in the junk slot area. Higher blades can be made in steel-bodied- bits than matrix bits because of the greater strength of steel over that of the matrix.
Bit blades range from straight to complex curve shapes. Experience has shown that curved blades provide excellent stability to the bit, especially when the bit first contacts the rock.
Bit profile affects both the cleaning and stability of the PDC bit. The two most widely used profiles are:
A – Double cone: The double cone profile allows placing more cutters near the gauge giving better gauge protection and allowing better directional control.
B- shallow cone: The shallow cone profile gives a faster Penetration Rate (ROP) but has less area for cleaning.
Generally, a bit with a deep cone will tend to be more stable than a shallow cone.
The bit length selection is important for steerability. Shorter bits are more steerable. The two bits on the left below Figure are sidetrack bits (also check: sidetrack drilling) with a short, flat profile.
The ‘Steering Wheel’ bit on the right is designed for general directional work.
Different PDC Bit Length DesignsAs discussed, the most significant amount of work is done on the heel and gauge of the drilling bit. PDC drill bits that wear more on the gauge area will leave an Under gauge Hole, which will require reaming from the next bit. Reaming is time-consuming and costly and, in some cases, can destroy an entire bit without drilling a single foot.
Hence maintaining a gauge is very important. We can position one or more PDC cutters in the gauge area. Pre-flatted cutters are used to place more diamond tables against the gauge. Tungsten carbide inserts, some with natural or synthetic diamonds embedded in them, may be placed on the flank of bit 1.
A significant advantage of fixed cutter bits over roller cone bits is that there may be an extension for the gauge on fixed cutter bits to a more considerable length of the drill bit.
When we put all of the above PDC Bit Design features together, you will find :
If you have any questions on Matrix Body PDC Drill Bit. We will give the professional answers to your questions.