The T2 Cutoff: BVI and FFI

Part 10, Chapter 10: NMR Logging

Learning objectives

Split the T2 distribution at the cutoff into BVI and FFI
Compute irreducible water Swi = BVI/phi from NMR alone
Recognize that NMR gives Swi without resistivity
Set the cutoff per lithology (sandstone vs carbonate)

One Line, Three Numbers

The T2 distribution becomes an answer the moment you draw one vertical line on it. Below the T2 cutoff lies fluid in pores too small to drain, the bound water held by capillarity; its area is the bulk-volume irreducible $\text{BVI}$ . Above the cutoff lies fluid in pores large enough to flow, the free-fluid index $\text{FFI}$ . Their sum is the NMR porosity, $\phi = \text{BVI} + \text{FFI}$ .

Irreducible Water Without Resistivity

That single split hands over something no other log gives directly: the irreducible water saturation,

S_{wi} = \frac{\text{BVI}}{\phi},

read straight from the pore sizes with no resistivity, no Archie, no $R_w$ . It is a completely independent estimate of the irreducible water, so it both feeds the Timur permeability and cross-checks the resistivity saturation of Chapter 7. Where the two agree, confidence soars; where they disagree, something, the $R_w$ , the cutoff, the shaliness, needs another look.

The Cutoff Is Rock-Specific

The one discipline NMR demands is the cutoff itself. Because surface relaxivity differs by mineral, so does the cutoff: near 33 ms in sandstone but 92 ms in carbonate. Switch the lithology in the widget and the cutoff jumps, sweeping more of the same spectrum into the bound side and raising $S_{wi}$ for the identical rock. Pick the wrong cutoff and the producible-water call goes wrong, which is why the cutoff is calibrated to the formation, ideally against capillary-pressure or core data.

References

Coates, G. R., Xiao, L., and Prammer, M. G. (1999). NMR Logging Principles and Applications. Halliburton Energy Services.
Straley, C. et al. (1997). Core analysis by low-field NMR. The Log Analyst, 38(2).