Page 211 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 187

2.4.3
Drill Bit Selection, Parameters, and Hydraulics [ ]

Rotary speed
Rotary torque
Pump output
Pump pressure
Mud density in
Mud density out
Mud temperature in
Mud temperature out
Gas readings

In addition, the mud loggers take samples from the shakers and
describe lithology, cuttings appearance, and cavings percentage and
appearance.

2.4.3. Drilling Hydraulics

Much of the power produced by the mud pumps is lost in the cir-
culating system through the surface lines, drillstring, and annulus.
These are parasitic losses that produce no direct benefit to the drill bit
performance. The power that is left can be used in different ways to
help clean the bit/hole bottom, aid ROP by the direct effect at the bit
face, and drive downhole motors or turbines.
The calculations involved in drilling hydraulics are best done with
computer programs and these are available free from various bit com-
panies (e.g., Reed or Smith). Using a hydraulics program allows differ-
ent scenarios to be compared quickly and easily; the entered data can
usually be saved and modified later as required by different mud prop-
erties or pump capabilities. Calculating these pressures by calculator is
tedious since they have to be repeated for each change in the flowpath
size, consuming valuable time if you are playing “what if” with differ-
ent mud properties or flow rates.
There are two current theories for optimum hydraulics. One gives the
total nozzle area to maximize hydraulic horsepower. The other calculates
for maximum hydraulic impact force on bottom. Of the two methods,
maximizing HHP gives greater pump pressure and lesser flow rate.
To maximize bit hydraulic horsepower, the pressure drop across
the bit should be 65% of the total pressure loss in the system. If the

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