Z Charts

For evenly spooled wire/cable, maintaining a Z Chart while running in hole is a good practice. The hope would be that it is not needed, however, it is simple to complete & can be very helpful in some undesirable circumstance, e.g., loss of normal depth control due to mechanical or electrical malfunction or wire break downhole.

This chart is fictional for demonstration purposes. After setting the toolstring zero the wire is spooling from the centre of the reel, as the wire is run in hole the spooling moves across to the right side of the drum, (if the wire is spooling to the left, simple start the records from the second line in this spreadsheet), when the wire reaches the right side of the drum the operator logs the depth, pick up & hanging weights. This continues from right to left down the well.

If normal depth control has been lost or the wire has broken downhole, the Z Chart can be used to estimate where the bottom of the toolstring is or if wire is broken, what length of wire has been left in hole, how much wire will be recovered until the broken end is above the downhole safety valve, when it is likely to exit the stuffing box, etc.

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