Terrestrial NMR: Extra

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  • Published: Sep 1, 2014
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Terrestrial NMR: Extra

Oil exploratory

Paul Ganssle is the corresponding author of a paper in Angewandte Chemie describing ultra-low-field NMR using an optical magnetometer. (Photo by Roy Kaltschmidt)

The Earth's magnetic field is strong enough for experiments in low-field NMR spectroscopy according to a research team from Lawrence Berkeley National Laboratory, which is developing the technique for analysis of fluids in situ in their native environments.

Although the Earth's magnetic field has been studied for many decades in geology and archaeology only recently have spectroscopists considered it a viable tool for generic NMR although it has been used in NMR well-logging. The recent resurgence in interest in Earth’s field NMR has been at least in part due to the development of high-sensitivity low-frequency NMR detectors like alkali-vapor-cell magnetometers. Alexander Pines and colleagues working in a long-standing collaboration with Dmitry Budker of the University of California, Berkeley, and a team at the US National Institute of Standards and Technology (NIST) have demonstrated a proof of principle in analyzing a mixture of hydrocarbons and water using a high-sensitivity magnetometer and a magnetic field comparable to that of the Earth.

“This fundamental research program seeks to answer a broad question: how can we sense the interior chemical and physical attributes of an object at a distance, without sampling it or encapsulating it?” explains principal investigator Vikram Bajaj of the Pines group. “A particularly beautiful aspect of magnetic resonance is its ability to gently peer within intact objects, but it’s tough to do that from far away.”

SpectroscopyNOW has reported periodically on the increasing strength and sensitivity of NMR machines with more and more powerful magnets in chemical analysis and medical applications. Of course, such high-field machines are incredibly important to modern research and medicine, but they are very expensive, large and heavy and require superconducting conditions in which the magnets can function. They are not tenable as portable devices for exploratory analysis in the field.

Of course, ex situ NMR spectroscopy for the chemical characterization of samples that cannot be placed within a high-field magnet has been a focus of research for years. Indeed, by inverting the geometry of a typical high-field experiment it is possible to probe the surface of a sample as the magnetic field is projected into the object. Unfortunately, generating an adequate homogeneous magnetic field over a sufficiently large sample area is a challenge and as such it is currently not possible to get field strengths suitable for conventional high-resolution NMR measurements. However, given a magnetometer sensitive enough at 2 kHz, it should be possible theoretically to carry out low-field NMR measurements that use a weak magnetic field such as the Earth’s magnetic field.

Gentle Earth

“One nice thing about the Earth’s magnetic field is that it’s very homogeneous,” explains principal author on the paper Paul Ganssle. “The problem with its use in inductively detected magnetic resonance imaging is that you need a magnetic field that’s both strong and homogeneous, so you need to surround the whole subject with superconducting coils, which is not something that’s possible in an application like oil-well logging.” Bajaj adds that, "Sensitivity of magnetic resonance depends profoundly on the magnetic field, because the field causes the detected spins to align slightly. The stronger the applied field, the stronger the signal, and the higher its frequency, which also contributes to the detection sensitivity.”

Optical magnetometers can serve as detectors for ultra-low-field NMR measurements, the team suggests. Although such an ex situ approach has much lower chemical sensitivity due to the much lower field strength, it has advantages. Moreover, while it is not possible to perform chemical shift spectroscopy at low fields, these portable, robust low-field devices can still determine the bulk properties of the sample using relaxation and diffusion spectroscopies - which are sufficiently sensitive as to chemically distinguish between hydrocarbons and water.

“The approach at low-field, which you can achieve using permanent magnets or Earth’s magnetic field, is to measure spin relaxation,” explains Ganssle. A key difference between the new approach and conventional experiments is that the relaxation and diffusion properties are resolved through optically detected NMR, which operates sensitively even in low magnetic fields.

“A previous achievement of our collaboration has been the development of magnetometers for the detection of NMR,” says Bajaj. “This experiment represents the first time that magnetometers have been used to make combined relaxation and diffusion measurements of multicomponent mixtures.”

...and relax

The oil industry commonly uses relaxation and/or diffusion measurements for exploratory underground NMR measurements. Geochemists use conventional probes with a permanent magnet to increase the local magnetic field and attempts at oil well logging using the Earth's magnetic field date back to the 1950s but they never achieved adequate sensitivity for the task, hence the permanent magnet devices.

“What’s novel here is that using magnetometers, we finally have technology that might be sensitive enough for efficient detection in the Earth’s field, perhaps ultimately enabling detection at longer distances,” explains team member Scott Seltzer. The researchers have carried out tests on their design by measuring relaxation coefficients first for various hydrocarbons and water by themselves, then for a heterogeneous mixture, as well as in two-dimensional correlation experiments.

“This proof of concept might be productively applied in the oil industry,” says Ganssle. “We mixed hydrocarbons and water, pre-polarized them with a magnet, and applied a magnetic field the same as the Earth’s. Then we made measurements with our magnetometer and determined that we had easily enough sensitivity to separate components of oil and water based on their relaxation spectra.”

This technology could help the oil industry to characterize fluids in rocks. The team's next step will involve trying to understand to what depth in a geological formation they might be able to image with this technology. They hope to be able to image to a depth of at least a metre into a formation and elucidate the chemistry within. Eventually, probes could be used to characterize entire borehole environments in this way, while current devices can only image inches deep. The combination of terrestrial magnetism and versatile sensing technology again offers an elegant solution. The same technology might also be used to determine the content of water and oil flowing in a pipeline, in food quality analysis or for assessing curing processes in polymer or cement processing and drying, for instance.

"The work presented in our Angewandte Chemie article was mostly concerning the use of alkali vapour-cell magnetometers to detect 1- and 2-dimensional relaxation and diffusion spectra and demonstrating the sensor’s use as an alternative detector for these types of measurements (which are already in common use at slightly higher fields, around 100 kHz to 1.5 MHz), and with this successfully demonstrated, in my opinion the next step in the proof-of-concept vein is to build something of a mock up to use these sensors for “depth logging”,  i.e. showing that we can measure signals deep into the rock formation using this technique," Ganssle told SpectroscopyNOW. "Because the fringe field of a permanent magnet dies off very quickly the further away from the magnet you get, it’s very difficult to build NMR tools that look deep into the rock (usually no more than say 3 to 5 inches into the formation), but there are some significant advantages to looking deeper into the formation, so one of the big hopes for a revived Earth’s field logging would be its application towards looking feet or meters into the rock, rather than inches." 

Related Links

Angew Chem, Int Edn, 2014, online: "Ultra-Low-Field NMR Relaxation and Diffusion Measurements Using  an Optical Magnetometer "

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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