Page 436 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 412

[ ] Practical Wellsite Operations
3.6.2

Where LCM is anticipated, use nozzles of at least /32 in size, even
16
if the optimization calculations suggest smaller nozzles.
Bit parameter optimization and drill-off tests. Perform regular
drill-off tests to optimize ROP and bit life. If applying more WOB or
RPM does not increase ROP by a roughly linear amount, then bit wear
will be accelerated faster than the increase in ROP. When a drill-off test
is performed, you are not necessarily looking for the greatest possible
ROP, but a good ROP that gives a reasonable bit life. This will lead to
the lowest cost per foot.
Unless the drilling parameters are constrained by factors such as
deviation performance, the drillers should be told to optimize parame-
ters for best ROP within a range. Telling a driller to “drill with 35,000
WOB and 90 RPM” will not optimize performance.
Read the manufacturer’s recommended WOB and RPM. Look at
offset bit runs; see what parameters were used on the best runs and the
bit grading. Check the design parameters of the BHA. If a vibration
analysis was done on the BHA, note any peak stress WOB/RPM combi-
nations that should be avoided.
Decide on a maximum WOB and associated RPM, and a maxi-
mum RPM and associated WOB. Tell the driller to work within those
parameters, but to conduct regular drill-off tests to determine the
best combination.
Experience will sometimes show that a manufacturer’s recommen-
dations for maximum parameters are too conservative. The following
bit record illustrates this.

An 8 /2 in Smith F47OD drilled 271 m in 82 rotating hours.
1
Average WOB 55,000 lbs, RPM 80. This bit graded 3/3/CT/M/E-E-
F/3/NO/TQ. Even after this severe run, the bit grading looks good.

To do a drill-off test, set a fixed weight and vary the RPM up to the
maximum. Note the lowest RPM that ROP peaked. (You may see that
above a certain RPM, ROP does not increase.) Then set that RPM and
vary the weight. At the lowest WOB that gave the best ROP, hold the
WOB and again try to vary the RPM. This will work towards the opti-
mum parameters.
Field experience in variable lithology should allow the drillers to
spot a change in lithology. The drill-off test should then be repeated.

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