Page 162 - Drilling Technology in Nontechnical Language

Chapter 6 – DRILL BITS 153

Drillbit Economics

If a bit is used past the end of its economic life, the rate of wear
accelerates, and eventually parts of roller cone bits might drop off in
the hole. This is a problem that will cost a lot of money to solve because
special tools have to be used to recover the bits of junk from the hole before

drilling can resume. This is called fi shing, which can be defined as “a set
of activities to remove unwanted material from the wellbore before normal
operations may resume.” Fishing is discussed in more detail in chapter
13, “Drilling Problems and Solutions.” The economic life of a drill bit is
measured by calculating how many dollars are spent to drill the distance
drilled with that bit. This calculation is repeated frequently as drilling
continues. Within the economic life, the cost per foot (or cost per meter)
decreases. Eventually the cost per foot starts to increase. This indicates the
end of the economic life of that drill bit.
The cost per foot is calculated by adding together the cost of the
bit and the cost of the time spent so far during that bit run (dollars per

hourly rig operating cost × hours), and this figure is divided by the
distance drilled:

Bit cost $ + (tripping hours × $1500) + (drilling hours × $1500)
Cost per Foot = —————————————————————————————
Feet drilled
The time starts when the new bit is screwed onto the BHA, and it
includes the time taken to run it in the hole. The estimated time to pull out
of the hole at the end of the bit run is also added.
Assuming the hourly operating cost is $1,500, with a low-cost rig (e.g.,
a land rig) and a high-cost drill bit (e.g., large PDC bit), the bit would have
to drill extremely fast and for a long time if the economics were to compare
favorably with a roller cone bit. The dominant factor in this case is the bit
cost. However, with a high-cost rig (latest generation semisubmersible in a

high-cost area like the North Sea), a high-cost bit is justified if it drills fast
and stays drilling for a long time.
Sometimes the bit is pulled out before the minimum cost per foot is
seen. This happens if conditions indicate that the drill bit is damaged, if
casing point is reached, or if logging is required before the next casing
depth. There may be many reasons why the bit run might terminate early.

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