Page 161 - Drilling Technology in Nontechnical Language

152 Drilling Technology in Nontechnical Language Second Edition

The most important source of data for analysis is the drilling records
of other wells in the vicinity. The performance of other bits that have
drilled through the same formations in other wells shows what particular
bit features are important and which should be avoided or are not needed.
Dull bit gradings are especially important, but to see the full story, it is
also important to be able to analyze the drilling performance (rate of

penetration) and the drilling parameters (WOB, RPM, and flow rate)
for each foot drilled. This takes some time to complete, especially if
there are a lot of wells to analyze, but the work will be amply repaid in
optimized performance.

Electric log data can also contribute to bit selection. Sonic logs (which
measure the speed of sound through the formations) can be interpreted
to give rock compressive strengths, which clearly help in bit selection.
Gamma ray logs analyze clay content and may indicate the best size of
PDC cutters to use if PDC bits can be economically run.
If a directional well is drilled, bits that resist a change of wellbore
direction should be used in the straight sections. Bits that do give some
side cutting action can be run over the hole sections where a change of
direction is required.
On exploration wells where pore pressures are poorly known, it is
better to avoid PDC bits because it is important to recognize changes in
pore pressure while drilling. As PDC bits are less sensitive to pore pressure
changes, predictions are better with tricone bits.
In deeper, small-diameter holes, PDC bits start to give some significant

advantages as they have no moving parts. Small roller cone bits have small
bearings and the bearing condition, usually monitored by watching torque
while drilling, cannot be monitored properly due to the high torque from
the long hole. The cost difference between small PDC bits and small roller
cone bits is also relatively small, certainly when compared to the larger
bit diameters.

In the end, bit selection is an economic decision: Which bits are most
likely to drill to the next casing, logging, or coring point for the least
overall cost?

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