Permeability estimation from NMR diffusion measurements in reservoir rocks

Area of Science:

• Geophysics
• Petrophysics
• Materials Science

Background:

• Fluid diffusion in porous media is time-dependent.
• Apparent diffusion coefficients (D) reveal pore space characteristics.
• These characteristics are vital for oil industry applications.

Purpose of the Study:

• To characterize porous media using time-dependent diffusion.
• To relate diffusion measurements to petrophysical parameters like permeability.
• To validate nuclear magnetic resonance (NMR) techniques against conventional methods.

Main Methods:

• Utilized stimulated spin-echo pulse sequences with pulsed magnetic field gradients.
• Measured diffusion coefficients (D) on homogeneous and heterogeneous sandstone samples.
• Analyzed the time dependence of D(t) to estimate surface-to-volume ratio and connectivity.

Main Results:

• Petrophysical parameters derived from NMR measurements showed good agreement with conventional gas permeability and electrical conductivity.
• Short-time behavior of D(t) effectively estimated surface-to-volume ratio (S/V).
• Long-time behavior of D(t) probed pore space connectivity.

Conclusions:

• NMR diffusion measurements provide reliable and time-efficient characterization of porous media.
• The technique is valuable for estimating key petrophysical parameters relevant to the oil industry.
• Despite limitations in observing true asymptotic behavior and differences in probed length scales compared to BET, the method remains robust.