There was a time when drillers selected a bit they believed would give the greatest R.O.P. Or, they would base their
selection on achieving the longest possible run because their choice was the most durable.
However a high Rate of Penetration is normally of little value if it can be sustained for only a short time, while a durable
bit is of little use if it drills painfully slowly.
The two goals are often in conflict, so it is more practical to seek a balance between them, rather than trying to
maximize one or another. (There are exceptions: for example, if only a short, straightforward interval is left to drill then
bit selection should be based on drilling it as rapidly as possible – durability is not an issue.)
Normally, then, we wish to select a bit that will deliver the optimum R.O.P. and the optimum durability.
Optimum selection will deliver the lowest cost per foot - this is the primary aim of bit selection:
To choose a bit, which, when properly run, will deliver the lowest cost per foot.