The Factors Affecting Bit Geometry
1- Number of Blades
- Using the same analogy for roller cone bits, a PDC bit designed for soft rocks has a fewer blades (and cutters) than one designed for hard rocks.
- The soft formation PDC bit will therefore have a large junk slot area to remove the large volume of cut rock and to reduce bit balling in clay formations
- A hard PDC bit with many blades requires many small cutters, each cutter removing a small amount of rock
2- Blade Height
- A soft formation PDC bit will have a lager blade height than a hard PDC bit with a consequent increase in 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 matrix.
3- Blade Geometry
- PDC bits can be manufactured with a variety of blade shapes ranging from straight to complex curve shapes.
- Experience has shown that curved blades provide a greater stability to the bit especially when the bit first contacts the rock.
4- Bit Profile
- Bit profile affects both cleaning and stability of the bit. The two most widely used profiles are:
- Double cone
The double cone profile allows more cutters to be placed near the gauge giving better gauge protection and allowing better directional control.
- shallow cone
The shallow cone profile gives faster penetration but has less area for cleaning.
- In general a bit with a deep cone will tend to be more stable than a shallow cone.
5- Bit Length
- This is important for steerability. Shorter bits are more steerable. The two bits on the left of below Figure are sidetrack bits, with a short, flat profile.
- The ‘Steering Wheel’ bit on the right is designed for general directional work.
6- Gauge Protection
- As discussed before, the greatest amount of work is done on the heel and gauge of the drilling bit. A PDC bit that wears more on the gauge area will leave an undergauge hole which will require reaming from the next bit. Reaming is time consuming and costly and in some cases can actually destroy an entire bit without a single foot being drilled.
- Hence maintaining gauge is very important. One or more PDC cutters may be positioned at the gauge area. Pre-flatted cutters are used to place more diamond table against gauge. Tungsten carbide inserts, some with natural or synthetic diamonds embedded in them, may be placed on the flank of the bit 1.
- A major advantage with fixed cutter bits over roller cone bits, is that the gauge on fixed cutter bits may be extended to a larger length of the drill bit.
ConclusionWhen all of the above features are put together you will find :
- The bit on the extreme left of the below figure is a light set bit with a few, high blades and a few but large cutters with small back rake angles. Thus light set bits typically have a few, high blades, with few large cutters, probably with low back
- For hard rocks, PDC bits will have more blades, with smaller and more numerous cutters, and this trend continues to the heavy set bits on the extreme right of the below figure