Gas Water Oil
volumes. Although physicists were aware that much more information about the fluids could be coaxed from NMR data, downhole tools capable of providing more advanced acquisition and processing techniques were needed to extract fluid properties in a continuous log. This article discusses developments in mea - surement techniques that provide NMR-based in situ formation-fluid properties. A recently intro - duced downhole tool capable of making these measurements in both continuous and stationary modes is described, along with NMR logging theory applicable to these new measurements.1
With
these new NMR capabilities, fluid-characterization measurements resolve interpretation ambiguities as demonstrated by case studies from South America, the North Sea, the Middle East and west Africa.
A Bit of Science
All NMR tools share some common features. They have strong perma nent magnets that are used to polarize the spins of hydrogen nuclei found in reservoir fluids. The tools generate radio frequency pulses to mani pulate the magnetization of hydrogen nuclei and then use the same antennas that generate those pulses to receive the extremely small RF echoes originating from the resonant hydrogen nuclei. Because of their magnetic moments, hydrogen nuclei behave like microscopic bar magnets. Upon exposure to the static magnetic field, B0, of the NMR tool’s permanent magnets, the hydrogen’s magnetic moments tend to align in the direction of B0. Exposure time is referred to as the wait time, WT, and the time required for polarization to occur is influenced by various
formation and fluid properties. The buildup in the resulting magnetization is represented by a multicomponent exponential curve, each component of which is characterized by a T1 relaxation time.
1. For more on NMR theory and logging:
Kenyon B, Kleinberg R, Straley C, Gubelin G and Morriss C: “Nuclear Magnetic Resonance Imaging— Technology for the 21st Century,” Oilfield Review7, no. 3 (Autumn 1995): 19–33.
Allen D, Crary S, Freedman B, Andreani M, Klopf W, Badry R, Flaum C, Kenyon B, Kleinberg R, Gossenberg P, Horkowitz J, Logan D, Singer J and White J: “How to Use Borehole Nuclear Magnetic Resonance,” Oilfield Review9, no. 2 (Summer 1997): 34–57.
Allen D, Flaum C, Ramakrishnan TS, Bedford J, Castelijns K, Fairhurst D, Gubelin G, Heaton N, Minh CC, Norville MA, Seim MR, Pritchard T and Ramamoorthy R: “Trends in NMR Logging,” Oilfield Review12, no. 3 (Autumn 2000): 2–19.
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