This article is based on
Chapter 2 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell Books 1986 Updated 2004.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
CONTROL IS JOB ONE
Log quality control and assessment is usually part
of a petrophysicists job description. Modern logs
are run and calibrated under control of a computer
program, monitored by the logging engineer. Most
problems will be related to poor borehole condition
and mistakes in recording the log as set out in the
logging program found in the well prognosis. Tool
failures and missing curves may cause difficulties
later during the analysis phase.
On current drilling wells, the first items to
tabulate are which service company ran the log, the
engineer's name, who the log was run for, and who
was the witness. In time, these names will become
familiar and the particular failings or good points
of the individuals involved will be helpful in
solving future problems.
Older logs can have more serious calibration
problems , with lower precision and vertical
resolution issues in addition to the recording
On projects, keep track of the service company, tool
type, age of the logs, and mud system variations.
These factors create differences in log response
that may need to be accounted for.
Examine headings for any notes concerning tool
problems or scale changes. Monitor log scales over
the interval in question to ensure they are
reasonable for the type of log being reviewed.
Verify that calibrations have been run and are
attached to the bottom of the log. In addition,
check that the repeat section is present and that
the log does repeat. On older logs, some of these
features may be missing.
To improve consistency, establish a realistic policy
for wellsite and office QC. Few logs are perfect but
few are completely useless. You want the best
consistent with rig time and logging cost
CRAIN'S QUALITY CONTROL
1. If the problem (e.g. wrong scale, sonde error,
off depth) can be fixed by re-play on a computerized
truck, re-play the log, and label the heading
2. If the problem can be fixed by a re-play in the
service company's computer center, label field
prints accordingly and arrange for the re-play in
3. If the problem can be overcome by use of another
(redundant) log curve (e.g. GR, caliper) arrange to
re-play log with this curve. Label the heading
4. If the problem is a function of hole size or
condition, and sufficient repeat sections indicate
that no improvement can be made, do not re-run
further. Label the heading accordingly.
5. If log does not repeat, shifts, does not compare
with offsets, or contains unexplainable anomalies
(e.g. conductive spikes, very high density), or
cannot be replayed to be corrected, re-run with a
different set of tools (all components should be
6. If a log cannot be re-run when requested (due to
lack of tools, hole condition, client request), note
this on the log heading and in your report.
In older versions of this
Handbook, various forms were offered to assist in QC at the well
site. These are pretty obsolete. You should obtain copies of the
current service company tool catalogs that can explain calibration
and accuracy for each logging tool.
faults (tool failures and log problems) should be noted in your
report, even if they do not cause lost rig time or invalidate
the log. This information is used to point out potential areas
of concern, and provide historical information to track service
company and logging engineer performance. Reporting forms to
keep track of problems, rig activity, and log quality can be
found in Appendix One of this handbook.
While it is your duty and desire to obtain the best logs
possible for your clients, this objective may create a conflict
with the service company doing the logging. You are not in a
position to insist on unreasonable or impossible demands, but
you are expected to mediate diplomatically in such a way as to
ensure that a reasonable effort is made to achieve useable,
valid logs. Bear in mind that "the client" is the oil company
(your boss) and not the service company.
do not have the full authority of the client at your disposal.
All significant decisions which may involve the safety of the
well, the time and cost of the job, and the need to continue
logging in the face of bad hole conditions, must be discussed
with the client. No attempt should be made to usurp the
authority of the drilling supervisor or wellsite geologist, but
you are expected to make well reasoned presentations of the
current situation, the possible alternatives, and the expected
outcome of each choice to these people.