T1, T2, and Fluid Typing

Two Relaxations

So far we have used only $T_2$, the transverse decay. NMR also measures $T_1$, the longitudinal recovery, the time the tipped protons take to re-align with the field. They are different physics, and $T_1 \geq T_2$ always, but their ratio turns out to be a fingerprint of which fluid is relaxing.

The Ratio Is a Fingerprint

On a crossplot of $T_1$ against $T_2$, each fluid claims its own ground. Water wetting the pore walls keeps $T_1/T_2$ near 1 to 2 and sits on the diagonal, sliding along it with pore size. Oil also hugs the diagonal, but its position carries viscosity: light oil relaxes slowly and plots far right at long $T_2$, heavy oil quickly and plots left at short $T_2$. So even among the liquids, the map separates a moveable light oil from a sluggish heavy one.

Why Gas Stands Out

Gas is the dramatic case. Its protons recover slowly, giving a very long $T_1$, yet they dephase quickly because gas molecules diffuse fast through the field gradient, giving a short $T_2$. The result is a $T_1/T_2$ of tens, far above the diagonal where no liquid can sit. That makes NMR an independent gas detector, confirming the density-neutron crossover of Chapter 4 with a completely different physics. Reading T1 and T2 together, NMR does not just measure porosity and pore size, it tells you what is in the pores.

References

• Akkurt, R. et al. (1996). NMR logging of natural gas reservoirs. The Log Analyst, 37(6).
• Dunn, K.-J., Bergman, D. J., and LaTorraca, G. A. (2002). Nuclear Magnetic Resonance: Petrophysical and Logging Applications. Pergamon.