Wim Looyestijn

Nuclear Magnetic Resonance in a nutshell


Most nuclei have a magnetic moment. When placed in a magnetic field, they tend to align, just like a compass needle. Alignment is not instantaneous, but takes some time. This is called relaxation, with a time constant T1 or T2, depending on the type of measurement. When not (yet) aligned, they rotate about the magnetic field at a specific frequency ("resonate"), which gives an electic signal that can be observed. The strongest signal by far is that from hydrogen, and that is what NMR logging measures.

An NMR signal has two features: amplitude and relaxation.
The relaxation is the decay of the signal to equilibrium and reflects the interaction of the nuclear moment with its environment. The dominant component is the interaction with the solid surface, and as a result, the relaxation of a fluid in a pore is determined by the ratio of the pore volume and the pore surface. This ratio is proportional to the pore diameter. This means that the relaxation time is a direct measure of the pore size. In practice, one measures a distribution of relaxation times, and thus a pore size distribution.

The amplitude is directly proportional to the amount of hydrogen in the sample. By definition, the signal in a bucket of water corresponds to 100% porosity. Relaxation of hydrogen in minerals is far too short to observe. Therefore, the signal reflects only fluids, independent of the type of mineral. Water and most oils have approximately the same hydrogen concentration, so that the NMR porosity is virtually without any correction.

NMR logging tools use the strongest magnetic materials, but depth of investigation is still limited to 5 to 10 cm away from the sonde. The resonance frequency is in the 0.5 to 2 MHz range, which is extrmely low compared to other NMR technologies.

Petrophysical applications

As mentioned above, NMR porosity is lithology-independent. Only a small correction may be required if the hydrogen concentration of the fluids differ from that of pure water. This is obviously the case with gas, where the effect is the same as on neutron logs, but without the disturbing shale effect.

The relaxation time distribution gives a poresize distribution. This is a unique feature that can only be probed by NMR logging. Apart from qualitative information, such as fining upward trend, a range of quantitative applications have been to be very successful.

- Bound water : the fraction of small pores.

- Permeability: at a given porosity, small pores give low permeability, etc.

- Capillary pressure: pore necks usually correlate with pore size.

Advanced applications

The apparent relaxation time can be made sensitive to the mobility of the molecules, which is related to the viscosity. This feature has been exploited to discriminate oil from water.

If the rock is not fully waterwet, a fraction of the surface is in contact with oil. In many cases it is possible to interpret the compounded signal of water and oil in terms of a quantitative wettability index.

Other NMR technology

The NMR method used in logging has some resemblance with the medical MRI. MRI is more complicated as it has to vary the magnetic field in three directions to obtain a 3D scan.

NMR is a powerful spectroscopy tool in (bio-)chemical labs. When used at very high and homogeneous magnetic fields with pure substances, one can observe extremely small differences in resonance frequency that depend on the chemical structure ("chemical shift"). This is not possible in NMR logging where oil and water signals fully overlap.

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