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82 Part I Liquid Drilling Systems

• Poor cleaning below the bit hinders the detection of changes in
formation properties that otherwise can be identified from the rate of
penetration.
Achieving an adequate level of hole cleaning requires maximum use of
the power from the mud pumps on the bit hydraulics. This means the
maximal use of pump pressure and flow rate.
For given pumps with fixed horsepower, the available pump pressure
and flow rate are determined by the size of the liner (piston) used.
Therefore, the optimum selection of pressure and flow rate is not
straightforward. This section describes the criteria and procedure used in
the drilling industry for optimizing mud hydraulics to help achieve the
maximum rate of penetration.
There are different theories regarding the mechanism of hole cleaning.
Different design criteria have been used to optimize fluid hydraulics for
maximizing hole cleaning and thus the rate of penetration. These criteria
include the maximum bit hydraulic horsepower, the maximum bit
hydraulic impact force, and the maximum bit jet velocity.

4.2.1 The Maximum Bit Hydraulic Horsepower Criterion
Horsepower is defined as the rate of doing work. One horsepower is
equivalent to 33,000 foot-pounds of work done in one minute. This
definition is universal, and, other than changes in units, it applies all over
the world. The maximum bit hydraulic horsepower criterion may be sta-
ted as follows: Within the maximum available pump pressure, mud flow
rate and nozzle size should be chosen so the bit will gain the maximum
possible horsepower to clean the bottom hole.
Speer (1958) pointed out that the effectiveness of jet bits could be
improved by increasing the hydraulic power of the pump. He reasoned
that the penetration rate would increase with hydraulic horsepower until
the cuttings were removed as fast as they were generated. After this
“perfect cleaning” level was achieved, there should be no further increase
in the penetration rate with hydraulic power. Shortly after Speer published
his findings, several authors pointed out that due to the frictional pressure
loss in the drill string and annulus, the hydraulic power developed at the
bottom of the hole is different from the hydraulic power developed by the
pump. They concluded that bit horsepower rather than pump horsepower
is the important parameter. Furthermore, it was concluded that bit
horsepower is not necessarily maximized by operating the pump at the