Virtual Petrophysics

VPPS Byte 4: NMR Echoes – The Good, The Bad and The Ugly


Have you ever questioned the quality of the NMR log data in your well?  There are all those LQC checks you may make to ensure the CPMG echo train is of good quality, i.e. noise, noise flag, gain, chi, delta_B0, SNR….

Why not look at the echo train instead.  All of the above checks are analogous to checking your blood pressure, or having an ECG, but the final validation is an Angiogram to look at the artery health.

Below are a few examples of good, bad and ugly echo trains.  I will let you be the judge of the T2 distribution created from the inversion of these CPMG trains.

Track 1: Reconstructed echo amplitude decay
Track 2: T2 Distribution resulting from CPMG echo train inversion
Track 3: CPMG echo train, phase corrected signal channel, inversion input
Track 4: CPMG echo train, noise channel

Figure 1 Good CPMG echo train example.  Low average noise 2.2 p.u..  Typical for good quality acquisition.

Figure 2 The just plain BAD.  No hint of noise problems on the T2 distribution.  Drilled with oil-based mud.  Good NMR porosity match to PHIT.  Would you trust the T2 distribution?  Average echo noise 3.5 p.u.

Figure 3 – Stick n Slip clearly visible in track 1 of the reconstructed echo amplitude decay.  Note blocky appearance caused by sticking tool. No visible issue with the T2 distributions or echo train.  Average echo noise 2.5 p.u.

Figure 4 Don’t forget to install the flowline ditch magnets to clean up the metallic material entrained in the mud.  Ask your friendly driller to install the ditch magnets at the start of the well.  Definitely not just before logging.

Go ahead and look at your echoes, it is not that hard, but you will not find it in the friendly LAS file.  Ask your service company to provide the DLIS containing the raw unstacked PAPS.

Contact me if you would like to have a chat about your data.

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