QUALITY 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 problems.


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 considerations.


  CRAIN'S QUALITY CONTROL POLICY 

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 accordingly.

 

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 the office.


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 accordingly.


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 changed).


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.

All 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.

You 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.
 

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