To the non-wireline logging engineer, every logging tool looks like a piece of shiny metal pipe. In reality, these tools have some of the most sophisticated electronic, hydraulic and mechanical systems embedded within.

Wireline logging tools measure a variety of petrophysical properties from basic radioactive gamma-ray from the rocks to porosity and density. There are much more complex measurements being developed as technology evolves.

Each logging tool has a measurement sensor positioned within the tool, the rest of the tool consists of either the electronic or hydraulic components. Logging service companies have acquisition software which acquires and processes the data from the logging tools. Typically several tools are connected together to acquire several petrophysical properties in one descent in the oil/gas well. It is the function of the acquisition software to collect all this data and tie all the measurements to a given depth. This is where it is important that the correct tool measure point is used by the software. Typically this is done automatically by the software when the tools are selected. The engineer has the option to change the measure points in special cases, however this is rarely required.

There have been incidents where the engineer erroneously changes the measure points and also cases where a software bug results in wrong measure points being used. This has resulted in tool measurements assigned to wrong depths. This will cause serious problems when interpreting the log data and assigning rock properties versus depth. This resulting depth shift in the measurement can sometime not be detected immediately especially if they are <5ft.

The easiest way to detect this problem is to view the measure points on the tool diagram generated by the acquisition software prior to acquiring the data. Also this problem can be detected by the engineer or petrophysicist while quality checking the logs, preferably real-time while logging.

Most oil companies have a wellsite representative or witness at the wellsite during logging operations. It is his/her job to detect these kinds of problem while logging is ongoing. It is always recommended to have two engineers viewing a log so if one misses a problem, hopefully the other engineer will not.

Independent wellsite log witnesses are very frequently hired specifically to monitor and QC real-time logging operations. They are usually ex-logging engineers with many years of experience as field engineers with a service company.