Page 212 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 188

[ ] Well Programming
2.4.3

nozzles were sized smaller than this, less HHP would result on bottom
because the flow rate would have to be reduced to keep the same sur-
face pressure. If the nozzles were larger, the same flow rate with less bit
pressure drop would also reduce the HHP expended at the bit.
To maximize hydraulic impact force on the bottom, the pressure
drop across the bit should be 48% of the total pressure loss in the system.
The actual percentage loss across the bit will decrease as drilling
progresses due to increased pressure loss in the system (adding more
drillpipe). The calculations can be made for the middle of the expect-
ed bit run interval, unless the expected bit run is very long and the
pumps will be running at or near to maximum pressure; in this case,
calculate for the expected end of the bit run.
Hydraulic horsepower expended at the bit is often expressed as
horsepower per square inch or HSI. Using HSI above 5 is likely to lead
to significant bit erosion except on very short bit runs. Erosion on the
bit is acceptable as long as it does not lead to premature bit failure
(such as PDC cutters dropping out of the body).
Following are some advantages to optimizing for impact force
rather than hydraulic horsepower:

1. Larger nozzles will reduce nozzle plugging and will be better for
pumping LCM.
2. Lower pressures give less pump wear, reducing downtime due to
pump failure.
3. Higher AVs give better hole cleaning.
4. Erosion of the wellbore due to annular velocity, except in very
unconsolidated formations or fractured formations with a nonseal-
ing mud system, probably does not occur. It is a commonly held
opinion that high AVs lead to washouts. What is more likely is that
the high hydraulic energies at the bit lead to hole enlargement or
washouts and optimizing impact force may help reduce this effect.

Maximizing bit HHP is not guaranteed to lead to improved ROP
through better bottom hole cleaning (especially in harder formations)
at normal pressures. Some success has been reported with experiments
using ultra high pressures (up to 30,000 psi) in hard rocks, however,
this is not in widespread use (see references in Section 2.4.9,
“References for Drill Bit Selection”).
If maximizing HHP does not improve ROP, then either maximize
impact force or choose nozzles for the flow rate required at a pressure

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