BASIC Log Editing
Sounds easy! But most of us underestimate the severity of the problem. Bad data can be easily recognized in cases of obvious noise, such as cycle skips on the sonic log or hole washouts. It may be difficult in subtle hole condition changes, different lithologies, borehole weathering, and undetected or unrecorded log calibration problems.
logs were perfect, editing would not be required. However, logs
can suffer from a number of problems, such as:
Good judgment, interpretation, and background data from offset wells are needed in order to substitute better data.
If a log cannot be repaired, note this fact and consider your task complete - don't use the data if it isn't any good.
Logical use of other log curves in the well, or in offset wells, plus regional trend data prepared in advance by the analyst, will be the basis for most edits.
On older sonic logs, the worst cases are caused by cycle skipping, which results from a large rough borehole, a poor logging tool, a sleepy logging engineer, gas in the borehole, or gas in the formation. On uncompensated logs, spikes caused by hole size changes must be removed. On modern array or full wave sonic logs, missing data due to low amplitude signals must be interpolated.
Rock alteration due to drilling affects both the sonic and density logs. If regional trends for sonic and density data are known for each major lithology (shale, sand, carbonates), these can be used to draw a more reasonable log.
On density logs, the worst cases are caused by large or rough borehole, which often occurs in shale sections, in stress relieved carbonates, and in gas bearing formations. An example of a reconstructed density log, corrected for bad hole and rock alteration is shown above.
It is sometimes difficult to discriminate coal and salt beds from rough hole effects (they often go together), so recourse must be made to other logs or sample descriptions. Needless to say, no two analysts will do exactly the same job of editing. An example of salt interbedded in carbonates and evaporites is shown below. Although, the logs show great activity and the caliper shows a large hole, the log readings are valid and consistent with the lithology descriptions. No edits are needed.
Contrast this example with the log shown below, in which the density log is badly affected by large and rugged hole conditions. An edit is definitely needed here. Although the sonic log is a bit noisy, it really doesn't need any editing.
Even resistivity logs may need edits. The illustration below shows a noisy induction log, run in a salt mud by mistake, compared to one from a nearby well in fresh mud. Since resistivity logs are used to edit sonic logs, it pays to be sure that they are valid before using them for this purpose.
When in doubt, we feel that the more severe editing should be done first, and adjustments towards leniency be made after the first few response computations have been reviewed. Integrated time discrepancies are the most obvious clues to over edited or under edited data, and usually the offending zone can be identified readily, when compared to seismic section character, check shot data, or VSP data.
It is not unethical to edit, correct, repair, or otherwise modify a log, if corrections are needed and made properly. Some people are horrified by the concept of modifying logs arbitrarily, preferring to believe either the service company can never be wrong or that bad data should not be used. This attitude results in interpretation errors or wasted data.
watchword in editing is CAUTION ! Try to edit the garbage, but
leave in all legitimate anomalies.
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