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Knowing the right rock bit for the right job cuts the cost of drilling.
There are hundreds of bits out there to choose from. Choosing the right bit is very important to getting
the best drilling performance and to reduce the cost of drilling.
The best source of data for an accurate analysis is the drilling records of the other wells in the vicinity.
The performance from other bits that have drilled through similar formations can show what bit features are important and which
features do not perform well and should be avoided. Gradings of dull bits are also very important to get a full understanding.
Being able to analyze the drilling performance (rate of  penetration) and the drilling parameters (WOB (weight on bit), RPM and flow rate)
for each foot drilled will tell a lot about a bit too. Doing all this takes a good amount of time, depending on the amount of wells to
analyze, but this work pays off through optimized performance.
Different kinds of electric logging data can also contribute to bit selection.
Sonic logs, that measure the speed of sound through formations, can be interpreted to give the
compressive strengths of the rock. Clay content can be analyzed with gamma ray logs, and may help indicate the best
size of PDC cutters to use if PDC bits can be economically ran.
In directional drilling bits that resist a change of wellbore direction
should be used in the straight portions. The bits that do give some side cutting action can be
used over the 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's 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 larger bit diameters.
In conclusion, 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?
Works Cited:
Devereux, Steve. Drilling Technology in nontechnical language. Tulsa, OK: Penn Well, 1999
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