Permeability from NMR
Learning objectives
- Read a T2 distribution as a pore-size spectrum
- Split it at the T2 cutoff into bound (BVI) and free (FFI) fluid
- Apply the Timur-Coates and SDR permeability transforms
- Recognize NMR as the closest log to the pore geometry
A Picture of the Pores
Every estimator so far has guessed the throat size from something else, porosity, irreducible water, a crossplot. NMR logging comes closest to measuring it. The tool records a T2 relaxation distribution, and because small pores relax fast and large pores relax slowly, the curve is a direct picture of the pore-size spectrum, short T2 on the left for tight pores, long T2 on the right for big ones.
Cutoff, BVI, and FFI
A single threshold turns that spectrum into permeability information. The T2 cutoff, about 33 ms in sandstone, splits the distribution into the bound fluid below it, the water capillarity and clay hold in the small pores, and the free fluid above it, the movable fluid in the large pores. The bound part is the bulk-volume irreducible ; the free part is the free-fluid index . A rock with a fat long-T2 peak is coarse and permeable; one with everything below the cutoff is tight.
Two Transforms
Two equations turn the split into a number:
Coates rewards a high free-to-bound ratio; SDR rewards a long average relaxation time , that is, large average pores. They read the same data two ways and, like every estimator, must be calibrated to core, but they get closest to the throats logs otherwise miss. The full NMR story, and why the cutoff and these transforms behave as they do, is the next chapter.
References
- Coates, G. R., Xiao, L., and Prammer, M. G. (1999). NMR Logging Principles and Applications. Halliburton Energy Services.
- Kenyon, W. E. (1997). Petrophysical principles of applications of NMR logging. The Log Analyst, 38(2).