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Mud Hydraulics Optimization 85

Between the maximum bit hydraulic horsepower and the maximum bit
hydraulic impact force criteria, neither criterion has been proved better in
all cases because there is little difference in the application of the two
procedures. If the jet impact force is a maximum, the hydraulic horse-
power will be within 90% of the maximum and vice versa. Another
argument is that in many cases bits provide higher than required hydrau-
lics, so the effects of design using different criteria are masked.
The concept of bit hydraulic horsepower was introduced as a design
criterion in the early 1950s. It is a measure of the work required to
squeeze mud through the bit nozzles. This work is related to the removal
of cuttings from below the bit. Bit hydraulic horsepower is the most
common design procedure, probably because it was used first.
The concept of hydraulic impact force as a design criterion was intro-
duced in the mid-1950s. Hydraulic impact force is a measure of the force
exerted by the fluid at the exits of the bit nozzles. This fluid force cleans
the bottomhole by direct erosion and by cross flow beneath the bit.
Hydraulic impact force below the bit seems more logical than the bit
hydraulic horsepower when considering design procedures for bottom-
hole cleaning. Rock bits with jet nozzles extended closer to the bottom
of the hole are widely used. Both laboratory and field tests have shown
better bottomhole cleaning with extended bit nozzles (Sutko, 1970).
Since extending the nozzles does not change the bit hydraulic horse-
power but does change the hydraulic impact force on the bottom of the
hole, it is believed that the latter relates more directly to hole cleaning.

4.2.5 Economical Bit Hydraulics
Both bit hydraulic horsepower and hydraulic impact force criteria are
widely used in designing mud hydraulics programs. The argument about
which design criterion to use may be moot because either can be utilized
to optimize bottomhole cleaning requirements. Drilling tests in actual
drilling operations determine the optimum cleaning requirements. There-
fore, if the bottomhole cleaning requirements are determined using bit
hydraulic horsepower, then bit hydraulic horsepower should be the
design base. The same holds true for hydraulic impact force.
In formations of normal hardness where no specific breaking point is
present, the amount of bottomhole cleaning necessary may be determined
directly in field operations. It may be difficult to determine the hole
cleaning required for maximum penetration rates in very soft formations.
In these formations, maximum penetration rates are achieved with