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Optically Detected Earth-Field NMR for Oil Well Logging Applications





A Berkeley Lab team led by Vikram Bajaj and renowned Nuclear Magnetic Resonance (NMR) pioneer Alexander Pines has developed a new system for oilfield analysis that requires no magnets. It instead relies on optical magnetic sensors that detect the low frequency NMR signals of relevant analytes in the Earth’s magnetic field. The low RF frequency enables deeper penetration of metallic structures; relaxation-based measurements also provide greater chemical information at low magnetic fields.

An enormous boost in NMR signal detection sensitivity is made possible with the use of atomic vapor cell magnetometers. Laser-pumped alkali vapor cells are optical detectors of magnetic resonance that report on the magnetic field in which they are situated through a polarimetry measurement. Alternatively, solid state diamond magnetometers, which transduce magnetic fields through fluorescence measurements of optically-active defects in diamond, can also detect faint, low-frequency NMR signals, and could offer superior performance for Earth field systems in harsh environments.

This new system will provide valuable information to prospectors about the presence of oil, its viscosity, and how easy or difficult it will be to extract. It can deliver useful information about the oil-to-water ratio in these formations, whether comprised of rock, sand, or tar. It will not only provide data on porosity, but will reveal the degree to which pores are interconnected. By dispensing with magnets, the system not only lowers equipment costs, it enables NMR logging in metal-encased wells, which typically are required in offshore sites. It also will be useful in production projects that employ hydraulic fracturing. When combined with a related technology, Magnetic Resonance Microarray Imaging (MRMI), used in the design of a portable microfluidic device capable of analyzing the chemical constituents of unfiltered fluids, this new device could provide additional, real-time analytical information about the makeup of fluids encountered throughout the drilling process.

NMR, the same property harnessed to produce astonishing images of soft tissues inside the body, is widely used in oil well logging to analyze the nature of the rocks, sands, and fluids surrounding a borehole. However, the confined space and harsh engineering environment of the drill shaft limits the size of powerful magnets required for conventional NMR; high frequency NMR measurements also cannot be used in wells lined with metal casing.

Unlike conventional NMR logging tools, which can probe only a few inches from the instrument in the drill shaft, the Berkeley Lab system can gather structural information meters into the formation, in areas unaffected by lubricants or drilling induced artifacts. In this fashion, it overcomes what has been one of the most intractable problems in oil well logging since the introduction of NMR tools to the oilfield in the 1990s.

STATUS: Bench scale prototype


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