Borehole and Environmental Effects
Learning objectives
- Explain how the borehole degrades the log readings
- Distinguish pad tools from mandrel tools by their borehole sensitivity
- Describe environmental corrections and what they use
- Apply data-QC judgment in a washed-out hole
The Hole Bites Back
The borehole records itself in the caliper, but it also leaves its mark on every other curve. How much depends on how the tool reads. A pad tool presses a sensor against the wall and reads a thin slab right at the surface; a mandrel tool hangs in the hole and senses the whole formation around it. The same washout that the caliper flags will quietly bias the pad tools while the mandrel tools shrug it off.
Pad Tools Suffer Most
In a clean gauge hole everything reads true. Enlarge the hole and the density and neutron pads, lifted off the rock by washout, standoff, or thick mudcake, begin to read the mud instead: their error climbs steeply. The resistivity (an induction mandrel) barely notices. This is why the density curve in a bad hole is the first thing a careful analyst distrusts, and why the caliper from the first section is read alongside it.
Environmental Corrections and QC
Every measurement comes with environmental corrections that use the caliper, the mud weight and properties, the temperature and pressure, and the tool standoff to undo most of the bias. They recover a great deal, but not everything: a density reading from a badly washed-out interval is best flagged and down-weighted rather than trusted after correction. Good petrophysics begins here, in the discipline of reading the caliper, applying the corrections, and knowing when a curve has simply been ruined by the hole.
References
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
- Theys, P. (1999). Log Data Acquisition and Quality Control, 2nd ed. Editions Technip.