STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Weight on Bit and Rotary Speed for Core Bits
Drilling Bits and Downhole Tools 795
Weight on Bit. Figure 4-147 shows the drilling weights for diamond core bits
in various formations. These are average values determined in field tests [50].
The proper weight on the bit for each core run can be determined by increasing
the bit weight in steps of 1,000 to 2,000 lb, with an average speed of 100 rpm.
Coring should be continued at each interval while carefully observing the
penetration rate. Optimum weight on the bit has been reached when additional
weight does not provide any further increase in penetration rate or require
excessive torque to rotate the bit. Using too much weight can cause the diamonds
to penetrate too deeply into a soft formation with an insufficient amount of
mud flow able to pass between the diamonds and the formation, resulting in
poor removal of the cuttings. The core bit could clog or even burn, and
penetration rate and bit life will be reduced. In harder formations, excessive
weight will cause burning on the tips of the diamonds or shearing with a
resulting loss in salvage.
Rotary Speed. The best rotational speed for coring is usually established by
the limitations of the borehole and drill string. The size and number of drill
collars in the string and the formation being cored must be considered when
establishing the rotational speed. Figure 4-148 shows the recommended rotating
speed range for optimal core recovery in different formations [49]. Concern
should also be given to the harmonic vibrations of the drill string. Figure 4-149
gives critical rotary speeds [51] which generate harmonic vibrations.
Polycrystalline Diamond Compacts (PDC) Bits
PDC bits get their name from the polycrystalline diamond compacts used
for their cutting structure. The technology that led to the production of
STRATAPAX drill blanks grew from the General Electric Co. work with polycrystalline
manufactured diamond materials for abrasives and metal working
tools. General Electric Co. researched and developed the STRATAPAX (trade
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Bit size (inches)
Figure 4-147. Bit weight for core bits [50]. (Courtesy Hughes Christensen.)