STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 793
Figure 4-1 46.
Christensen.)
4 5 6 7 8 9 1 0 1 1 1 2
Bit Size
Pump discharge for diamond bits [50]. (Courtesy Hughes
The hydraulic pumpoff force FJlb) can be approximated by [50]
Fpo = 1.29(APb)(dh - 1) (4-83)
for the radial flow watercourse design bits, and
F, = 0.32(APb)(dh - I) (4-84)
for cross flow watercourse system (refer to IADC classification of fixed-cutter bits).
Diamond Bit Weight on Bit and Rotary Speed
Weight on Bit. Drilling weight should be increased in increments of 2,000 lb
as the penetration rate increases. As long as no problems are encountered with
the hydraulics and torque, weight can be added. However, when additional
weight is added and the penetration rate does not increase, the bit may be
balling up, and the weight on the bit should be decreased.
Rotary Speed. Diamond bits can usually be rotated at up to 150 rpm without
any problem when hole conditions and drill string design permit. Rotary speeds
of 200 and 300 rpm can be used with stabilized drill strings in selected areas.
Diamond bits have also operated very successfully with downhole motors at 600
to 900 rpm. The actual rotary speed limits are usually imposed by safety.
Core Bits
Most core barrels utilize diamonds as the rock cutting tool. There are three
types of core barrels.