STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 799
Figure 4-152. PDC bit designed for soft formations [43A]. (Courtesy Hughes
Christensen.)
Cutter Exposure. Figure 4-154 shows the types of cutter exposure on PDC bits
[43]. Cutter exposure is the distance between the cutting edge and the bit face.
Stud bits typically have full exposure that proves very aggressive in soft
formation. In harder formations, less than full exposure may be preferred for
added cutter durability and enhanced cleaning. Matrix body bits are designed
with full or partial exposure depending on formation and operating parameters.
Cutter Orientation. Figure 4-155 shows the cutter orientation for typical PDC
bits. The displacement of cuttings can be affected by side and back rake
orientation of the cutters. Back rake angle typically varies from 0 to -25". The
greater the degree of back rake, generally the lower the rate of penetration,
but the greater the resistance to cutting edge damage when encountering a hard
section. Side rake has been found to be effective in assisting bit cleaning in some
formations by mechanically directing cuttings toward the annulus. Matrix body
bits allow greater flexibility in adjusting cutter orientation for best drilling
performance in each formation.