Page 438 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 414

[ ] Practical Wellsite Operations
3.6.5

Leave the bit in longer if offset information indicates that the slow-
down is mainly due to decreasing formation drillability. Sometimes a
bit is pulled under these circumstances and the next bit in does not
drill any faster. Clearly in this case it is better to extend the bit run if
there are no concerns as to bit condition. The hole section summary
showing offset bit runs at the same place may indicate this.
There are different theories that aim to make a bit pull decision
easy, such as by hours on bit or number of revolutions. However,
these will lead to below optimum drilling performance and should
only be used when bit bearing condition cannot be monitored.
Drilling with a downhole motor or in small, deep, or deviated holes
where the off-bottom torque is high would qualify for pulling on
hours. It is possible to consistently pull bits at the end of their eco-
nomic lives, maximizing the overall performance without seriously
risking leaving cones in the hole. This requires close and skilled
supervision of the run.

3.6.5. When to Pull the Bit

“Cost per foot” calculations can help to decide when to pull the
bit. If this is done consistently, the chance of having to fish for cones
is small and the overall cost per foot will be minimized.
Set up a spreadsheet. By entering how many minutes per foot or
meter drilled (which the mud loggers record) against depth, distance
drilled, and overall cost per foot can be easily calculated. You can also
download the Excel 5.0 workbook “wellcalc.xls” from the web site at
http://www.drillers.com, which contains a cost per foot sheet.
The point where the cost per foot is consistently increasing is the
point that the bit should be pulled. If the indications are to pull the
bit, do not waste time drilling the kelly down. This may be modified
by other factors.

Pull the bit early if there are any signs of bearing failure.
Leave the bit in the hole if you expect a more drillable formation
ahead. Monitor carefully for bearing failure.

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