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Polarized Laser Light Helps Reveal Porous Structures

Photonics Spectra
Feb 2000
Laurel M. Sheppard

CAMBRIDGE, Mass. -- As a rule, nuclear magnetic resonance (NMR) imaging has relied on liquid probes because gases are too low in density to return a strong signal. Scientists at the Harvard-Smithsonian Center for Astrophysics have challenged that standard by obtaining NMR images with probes based on noble gas atoms that had been hyperpolarized with a laser.

The original work returned images from biological structures, such as the lungs and brain tissue. Recently, in collaboration with Schlumberger-Doll Research, an oil exploration company based in Ridgefield, Conn., Harvard-Smithsonian demonstrated that the technique can be applied to nonbiological porous media, such as oil-bearing sandstone and carbonate rocks.

More than actual images, laser-polarized gas-phase NMR provides information about the internal structure of porous rock, such as the ratio of surface area to volume and tortuosity -- a measure of how the pores restrict the flow of gases or liquids through the material.

Gas-phase nuclear magnetic resonance, which has proved useful in imaging biological structures, can provide information about the internal structure of oil-bearing sandstone, carbonate rocks and other porous materials. Courtesy of Schlumberger-Doll Research.

The technique uses the angular momentum of circularly polarized 795-nm light from a diode laser array to orient the electron spins of rubidium vapor atoms. These intermediary atoms collide with the xenon gas atoms, polarizing the spin of their nuclei. Injected into a sample, the xenon atoms rapidly diffuse into the pores of the medium without relaxing from their hyperpolarized state.

The atoms are essentially moving targets as they travel through the pores, so the spatial resolution is limited. Instead, the measurements of diffusion itself can be exploited to give information about the pore structure. The most novel aspect of the noble gas diffusion technique is its ability to probe distances well beyond the pore diameter. This can also provide information on the material's fluid transport properties.

The researchers demonstrated gas-diffusion NMR on two types of rocks that commonly hold oil, natural gas or underground water: a clay-free sandstone, which has a narrow range of pore sizes; and Indiana limestone, a carbonate rock with a complex pore structure. Such complex structures are difficult to analyze with any other single technique.

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