Page 156 - Drilling Technology in Nontechnical Language

Chapter 6 – DRILL BITS 147

Optimizing Drilling Parameters

As the bit drills, it starts to wear out. The teeth that cut into the formation
in order to drill will wear. The bearings that allow the cutting cones to
turn will wear. The more weight put on the bit, and the faster it turns, the
greater the rate of wear. Also (up to a point), the faster it drills. As the bit

wears out, it will not drill as efficiently, and the rate of penetration (ROP)
will decrease. (The rate of penetration is the speed at which the bit drills,
usually measured in feet per hour or meters per hour.) Eventually the bit
slows down so much that it is not economic to leave it drilling any longer,
so it is pulled out of the hole and changed for another one.
The faster a well can be drilled, the cheaper it is. Time-related costs on
a drilling operation are high (up to $950,000 a day on a modern generation

floating rig offshore; a land operation probably costs more like $80,000
a day). In general, to drill faster, the weight on bit (WOB) and the rotary
speed (revolutions per minute, abbreviated to RPM) can be increased.
Increasing either or both of these parameters also increases the rate of
wear. So it is very important that the driller can find the optimum set of

drilling parameters to achieve a good rate of penetration and a moderate
rate of wear.
If the WOB is increased, the teeth penetrate deeper into the formation
and create larger cuttings. However, there is a point beyond which increasing
the WOB does not increase the ROP. This can be because the teeth fully
penetrate the formation, and the formation then touches parts of the bit that
do not drill. It could also be that at the RPM used, the teeth do not have
time to penetrate further before they get pulled out again as the bit rotates.
So the driller can keep a constant RPM and increase the WOB a little at a
time, each time measuring the ROP. Eventually he can recognize the point
at which increasing WOB does not significantly improve performance.

If the RPM increases, a tooth penetrates the formation more times in a
minute. However, there is a point beyond which increasing the RPM does
not increase ROP. This is because the teeth do not have time to penetrate
as much as the WOB would otherwise allow. So by first establishing the

optimum WOB, and then holding that constant while increasing the RPM,
the driller can find the best combination of WOB and RPM with which

to drill. In general, drilling economics are improved by drilling as fast as
possible (but within the constraints that other factors might place on ROP)

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