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Low magnetic fields for flow propagators in permeable rocks.

Philip M Singer¹, Gabriela Leu, Edmund J Fordham

¹Schlumberger-Doll Research, 36 Old Quarry Road, Ridgefield, CT 06877, USA. psinger@slb.com

Journal of Magnetic Resonance (San Diego, Calif. : 1997)

|September 12, 2006

Summary

Low-field pulsed field gradient nuclear magnetic resonance (PFG-NMR) effectively measures water flow in sandstone. Signal-to-noise ratio is not a limitation, and results match high-field measurements.

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Area of Science:

Geophysics
Physical Chemistry
Petrophysics

Background:

Nuclear magnetic resonance (NMR) is a powerful tool for studying fluid dynamics in porous media.
Pulsed field gradient NMR (PFG-NMR) specifically probes molecular diffusion and flow.
Understanding water flow in rocks like sandstone is crucial for various applications, including hydrology and petroleum engineering.

Purpose of the Study:

To investigate the feasibility of using low-field PFG-NMR for water flow characterization in Bentheimer sandstone.
To compare low-field (2 MHz) PFG-NMR results with high-field (85 MHz) measurements.
To assess the impact of low signal-to-noise ratio and internal magnetic field gradients at low fields.

Main Methods:

Measurements of water flow propagators in Bentheimer sandstone using low-field (2 MHz) PFG-NMR.
Comparison of propagator data obtained at low fields with those acquired at high fields (85 MHz).
Utilized both unipolar and bipolar pulsed gradient sequences and analyzed the effects of internal gradients.

Main Results:

Flow propagators measured at low fields were found to be equivalent to those obtained at high fields for Bentheimer sandstone.
The lower signal-to-noise ratio at low fields did not significantly limit the accuracy of the measurements.
Internal magnetic field gradients were observed to affect propagator measurements at 2 MHz, a phenomenon that may persist in certain rock types even at low fields.

Conclusions:

Low-field PFG-NMR is a viable technique for studying water flow in porous rocks like sandstone.
The technique offers comparable results to high-field NMR despite potential challenges with signal-to-noise and internal gradients.
Further investigation into the influence of internal gradients at low fields is warranted for specific rock characterizations.