PDC bits have the fixed blades with large, hard, abrasion-resistant cutters manufactured from a boned mass of microcrystalline synthetic diamonds and designed to excavate hole by shearing the formation while tricone insert bit works by crushing it.
Softer the formation and homogenous smaller the number of blades and bigger the size of the PDC cutters.
They drill very fast in soft to moderately hard formations as Sand, Shale, Clay and Siltstone.
PDC bits have the advantage that they last longer- you can usually drill shoe to shoe, eliminating tripping, and can often be rerun (with or without refurbishment) on subsequent wells, reducing the cost per foot if you have a number of wells to drill.
The PDC bit is now almost the default choice unless you can't use a PDC bit: really plastic, sticky formations or really hard formations (and PDC technology is encroaching into these areas with depth of cut limiters and the like).
In fact, as more and more drillers are only accustomed to using PDC bits, like the "top driver driller" phenomenon, we are loosing the "art" of roller cone drilling. PDC bits are essentially fire & forget: when they wear out the ROP drops and you pull them; it's very unusal for them to fall apart, whereas with roller cones, there is always the risk of dropping a cone if you push the bit a little too long!
PDC technology has come a long way in abrasive formation. The drillers doubling their footage drilled with a PDC bit (same area and sandstone formation) in only a couple of years. Such as East Texas area where the Travis Peak formation is over 2000' of consolidated sandstone. Ten years ago, a well would consist of over 20 insert bits just to get through, where now it takes 1, 2, or 3 PDC bits depending on exact location.
Overally rig time has been reduced from 90+ days on a well to 15.
Nowadays the casing equipment is PDC drillable and the cement will not ever be too hard for a PDC.
Why Steel Body Bit Versus Matrix Bit?
The use of steel as a material for the body of the PDC bit has numerous engineering benefits in that the high resilience of steel allows the designer to create bits with the highest possible blade off.
|Typical Yield Strength
|Typical Tensile Strength
|Low Carbon Steel
|Infiltrated tungsten carbide
Typical mechanical properities of low carbon steel and infiltrated tungsten carbide material used for fixed cutter drilling bits