Page 208 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 184

[ ] Well Programming
2.4.2

BITREX is a database application, created using Microsoft Access
that runs under Windows. It can be downloaded free from the web site
at http://www.drillers.com. In addition to recording data from the bit
run, it can calculate a performance index value to allow comparisons
of offset bit runs.
The Normalized Bit Performance Index (NBPI) takes the total dis-
tance drilled and divides it by the total time for the bit run (including
an assumed trip time based on depths with bit cost expressed in rig
hours). In that respect, it is a type of cost per foot calculation. However,
it also takes into account the time taken by the next bit in to ream to
bottom, where this is caused by the bit coming out undergauge. This is
important because if the next bit in has to ream undergauge hole for
(say) eight hours to bottom, the penalty for this should go against the
undergauge bit when comparing performance. A judgment may have to
be made to decide what proportion of next bit reaming time to use, for
instance, if the bit came out undergauge but a stiffer bottom hole
assembly was used on the next run. Fishing time could also be put
against a bit if it came out in more than one piece.
A direct comparison of the NBPI for several bits run in the same
interval in different wells is only completely valid if both the following
conditions are met:

The bits under comparison were pulled at the end of their eco-
nomic life. This would normally be on ROP (e.g., cost per foot cal-
culations) or on indications of bearing failure.
The bits under comparison were run using parameters for best per-
formance, normally on fully stabilized rotary assemblies where no
constraints on WOB or RPM are applied, except the manufacturer’s
recommended maximums.

However, even if both these conditions are not met, the NBPI still
provides guidance on how successful or otherwise the bit run was.
The NBPI is calculated as follows:

Normalized Bit Performance Index = Bit footage
Drilling Hours + Normalized Tripping Hours +   bit cost   + Next Bit Ream hours
 rig hourly cost

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