Fundamentals of Fluid Flow in Porous Media

Chapter 3

Diffusion Coefficient: Measurement Techniques

NMR Method: Principles of NMR and Processing

T2 Transverse Relaxation Time

During T₂ relaxation, no energy is exchanged from the nuclei to lattice. Exchange of energy happens among nuclei. Therefore T₂ also called spin-spin relaxation. Transfer relaxation corresponds to the loss of phase coherence or randomization of spins in transverse direction (x-y) direction, which causes the loss of transfer magnetization. T₂ refers to the time required for the transvers component M_xy to decay to 37% of initial value. A 90° pulse B₁ gives energy to the protons and M rotates entirely into the x-y plane (Figure 3‑14). Coherence now exists in this transverse plane at the end of the pulse. The protons are all synchronized and precess at the same frequency. A transfer of energy can occur between these protons. Spin-spin relaxation refers to this energy transfer from an exited proton to a nearby unexcited proton. This energy exchange produces a gradual loss of phase coherence to the spins. As the coherence gradually disappears, the value of M_xy decreases toward zero (Figure 3‑14). This loss of coherence is a consequence of T₂. T₂ relaxation is more efficient in large molecules since they reorientate more slowly than small molecules.

Figure 3-14: De-Phasing (Loss of Phase Coherence) During T₂

When a wetting fluid fills a porous medium, such as a rock, both T₂ and T₁ are dramatically decreased, and the relaxation mechanisms are different from those of the protons in either the solid or the fluid.

There are many different properties of the fluid and porous media that could be measured or explained by using the relaxation process and (T₁, T₂) values.