Page 435 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 411

3.6.2
Drill Bits [ ]

Good stabilization probably decreases bit whirl and many bits are
already advertised to be an “anti-whirl” design. Whirl is often initiated
when the bit just starts drilling, such as after making a connection.
Research has indicated that using the following procedure after
making a connection will minimize the chances of bit whirl starting:

While still off bottom, bring the mud pumps and the rotary
table up to speed. Slowly slack off until the bit starts to take
weight. Increase the WOB in small increments (say about 20%
of planned total WOB) and allow the rotary table to stabilize in
between increments for 10-30 seconds (longer for deeper hole).

Hydraulics. 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.
To maximize bit hydraulic horsepower, the pressure drop across
the bit should be 65% of the total pressure loss in the system. To max-
imize hydraulic impact force on the bottom, the pressure drop across
the bit should be 48%.
Of the two methods, maximizing HHP gives greater pump pressure
and lesser flow. Erosion is more likely to be a problem, though erosion
on the bit is acceptable as long as it does not lead to premature bit fail-
ure. Maximizing bit HHP is not guaranteed to lead to improved ROP
through better bottom hole cleaning (especially in harder formations).
There are some advantages to optimizing for impact force. These
include:

1. Larger nozzles will reduce nozzle plugging and will be better for
pumping LCM
2. Lower pressures cause less pump wear, reducing downtime due to
pump failure
3. Higher AVs lead to better hole cleaning

If maximizing HHP does not improve ROP, then either maximize
impact force or choose nozzles for the flow rate required at a pressure
below maximum at section TD. Calculating exact nozzles and flow rates
for optimum hole cleaning under the actual conditions at the time cannot
be done due to the large number of variables, some of which are unknown.

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