Determination of surface relaxivity from NMR diffusion measurements

Area of Science:

• Geophysics
• Petrophysics
• Materials Science

Background:

• Nuclear Magnetic Resonance (NMR) T2-decay measurements are crucial for interpreting pore-size distributions in porous media.
• Accurate surface relaxivity values are essential for quantitative NMR applications, including NMR-derived capillary pressure curves.
• Existing methods often assume uniform pore systems, limiting their applicability to complex rock formations.

Purpose of the Study:

• To demonstrate a novel method for directly determining surface relaxivity from NMR measurements.
• To extend the application of NMR T2-decay analysis to realistic rock pore-size distributions.
• To enable new applications of NMR logging in oil and gas reservoir characterization.

Main Methods:

• Simultaneous measurement of T2 relaxation and restricted diffusion using NMR in a static magnetic field gradient.
• Combining restricted diffusion, which is independent of surface relaxivity, with T2 relaxation to isolate surface relaxivity.
• Laboratory NMR diffusion measurements performed on sandstone core plugs.

Main Results:

• Successfully determined surface relaxivity directly from NMR T2-decay and diffusion data.
• Validated the method by comparing results with those obtained from other established techniques.
• Demonstrated the method's capability to handle full pore-size distributions in realistic rock samples.

Conclusions:

• The developed NMR method provides a direct and accurate way to measure surface relaxivity.
• This technique overcomes limitations of previous methods, enabling analysis of complex pore structures.
• The approach has significant potential for in-situ reservoir characterization using NMR logging data.