Imam from Indonesia Asks PERM:
Is it necessary to determine T2 cut off to obtain permeability with NMR logging? If we don’t have any NMR lab measurement data, how to adjust T2 cut off and justification for that? I am worried when I used default T2 cut off (let’s say 33 ms for clastic). I got low correlation while compared with core permeability.
Please, advance give me suggestion
Dr. Jonathan Bryan from PERM Answers:
Yes, having knowledge of the proper T2cutoff for your formation is extremely crucial to being able to estimate permeability off the NMR logs. All of the normal NMR models are based somehow on the ratio of bound to free fluid in the core, which requires knowledge of the T2cutoff to separate out these components. And especially when going to tighter rock systems, T2cutoff values can be much lower than 33 ms: we have often observed values on the order of 10 ms and sometimes even lower in rocks that are on the order of 1 mD.
If you have core analysis data (i.e. Dean-Stark water saturations) then maybe you can try to compare this to the NMR spectra and match the NMR and Dean-Stark values and get a T2cutoff that way. But if you have the ability to do Dean-Starks, then you probably also have the core. In that case, we recommend running some core T2cutoff calibration tests. They are not very expensive but they allow you to get a lot more value out of your NMR log data.
At PERM Inc we have considerable experience in running NMR core analysis tests and also conducting petrophysics interpretations of NMR log data. Please feel free to contact us with any questions or for further discussion.
Dr. Jonathan Bryan