STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

796 Drilling and Well Completions
4oy I I I I 1 I
Figure 4-148. Recommended rotary speed for core bits [49]. (Courtesy
Hughes Christensen.)
name) drill blank in 1973 and Christensen, Inc. used these in PDC bit field tests.
The bits were successfully applied in offshore drilling in the North Sea area in
the late 1970s and in on-shore areas in the United States in the early 1980s. In
some areas, the PDC bits have out-drilling roller rock bits, reducing overall cost
per foot by 30 to 50% and achieving four times the footage per bit at higher
penetration rates [52,53].
Figure 4-150 shows the major components and design of the PDC bit. The
polycrystalline diamond compacts, shown in Figure 4-15 1. The polycrystalline
diamond compacts (of which General Electric's) consist of a thin layer of
synthetic diamonds on a tungsten carbide disk. These compacts are produced
as an integral blank by a high-pressure, high-temperature process. The diamond
layer consists of many tiny crystals grown together at random orientations for
maximum strength and wear resistance.
The tungsten carbide backing provides mechanical strength and further
reenforces the diamond compact wear-resistant properties. During drilling, the
polycrystalline diamond cutter wears down slowly with a self-sharpening effect.
This helps maintain sharp cutters for high penetration-rate drilling throughout
the life of the bit.
PDC Bit Design. Figures 4-152 and 4-153 show typical PDC bits. Figure 4-152
is for soft formation. Figure 4-153 is for hard and abrasive formation [43A].
Bit Body Material (Matrix). There are two common body materials for PDC
bits, steel and tungsten carbide. Heat-treated steel body bits are normally a "stud"
bit design, incorporating diamond compacts on tungsten carbide posts. These
stud cutters are typically secured to the bit body by interference fitting and
shrink fitting. Steel body bits also generally incorporate three or more carbide
nozzles (often interchangeable) and carbide buttons on gauge. Steel body bits
have limitations of erosion of the bit face by the drilling mud and wear of the
gauge section. Some steel body bits are offered with wear-resistant coatings
applied to the bit face to limit mud erosion.