Glenn Asks PERM:
When doing a downhole NMR log, how do you calibrate with an irregular borehole if you used a centralized tool where it is in the center of the borehole? Or is that data no good and you must use a sidewall NMR log?
Dr. Jonathan Bryan from PERM Answers:
Irregular boreholes are a big problem, because the NMR has a shallow depth of penetration. You generally want the NMR to be on the sidewall, but that assumes a good borehole i.e. good sidewall contact. If you don’t have this, you will know because the NMR porosity will way overshoot the density porosity (you will only be measuring fluid in the borehole, not porous medium). So you will know if that happens but unfortunately that just tells you not to trust that data, it doesn’t help you with whatever reason you were running the NMR for in the first place.
Dr. Jonathan Bryan