Modelling effective diffusion for accurate NMR pore size analysis in nano- and microporous rocks

Area of Science:

• Geophysics
• Petrophysics
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Low-field NMR (LF-NMR) is crucial for pore size distribution (PSD) analysis in porous materials.
• Conventional LF-NMR methods often yield systematic errors in nano- and microporous systems due to diffusion assumptions.
• Accurate pore characterization is vital for understanding fluid flow and storage in tight rocks.

Purpose of the Study:

• To introduce the Effective Diffusion Cubic (EDC) model for LF-NMR-based PSD estimation in tight rocks.
• To address and correct for diffusion-induced distortions in LF-NMR measurements.
• To enhance the accuracy and physical consistency of petrophysical characterization in tight formations.

Main Methods:

• Development of the Effective Diffusion Cubic (EDC) model framework.
• Incorporation of pore-size dependent effective diffusion coefficient and internal magnetic field gradients.
• Parameterization of the effective diffusion coefficient using a logistic function to approximate the Padé form for T2 relaxation analysis.

Main Results:

• The EDC model successfully corrects for diffusion-induced distortions in LF-NMR PSD estimations.
• Application to siliciclastic core samples yielded PSDs in closer agreement with reference data than conventional models.
• The model demonstrates improved quantification of diffusion effects on T2 relaxation.

Conclusions:

• The EDC methodology offers a physically consistent and more accurate approach to PSD quantification in tight rocks.
• This advancement enhances the reliability of NMR-based petrophysical characterization.
• The EDC model is a valuable tool for analyzing complex pore systems in geological formations.