A holistic approach to low-field NMR and MICP data integration for the accurate determination of the absolute pore size distribution in siliciclastic rocks

Low-field nuclear magnetic resonance (LF-NMR) has emerged as a critical tool in geophysical prospecting, particularly for reservoir rocks exploration and their pore space characterization. However, accurate assessment of pore size distribution (PSD) in unconventional reservoirs, such as tight sandstone and shale formations, remains challenging due to their complex pore structures, high clay mineral content, low porosity, and nanometer-scale pore sizes. Additionally, these challenges can be further amplified by the presence of internal gradients (G), induced by differences in magnetic susceptibility of the rock matrix and saturating fluid, which can distort PSD estimates obtained under traditional assumptions of their negligibility.

In the fast diffusion regime, as demonstrated by Brownstein and Tarr, NMR transverse relaxation time (T₂) is proportional to pore size, with the proportionality factor governed solely by surface relaxivity (ρ₂). However, conventional approaches assume negligible internal gradients which often lead to unrealistic PSD results, especially in nanometer-scale pores where the induced gradient effect can be significant. Internal gradient is inversely proportional to the pore size and can cause substantial distortions even when applying low field and CPMG sequence with low echo times (TE) in rocks of minimal paramagnetic mineral content.

This work presents a novel methodology integrating differential LF-NMR with mercury injection capillary pressure (MICP) for simultaneous estimation of PSD, ρ₂, and internal gradients in siliciclastic reservoir rocks. This approach enables comprehensive evaluation of pore space, accounting for both ρ₂ and internal gradients without requiring additional magnetic susceptibility measurements. LF-NMR relaxometry was conducted on rock core samples saturated with kerosene having a bulk self-diffusion coefficient almost three times lower compared to water to minimize the effects of diffusion in nanopores and stay within detection limits for this pore size range, as well as to preserve samples from dissolving. ρ₂ and G were estimated based on the specific conversion framework established using physical characteristics of the kerosene molecule, percolation theory and non-linear PSD–T₂ transformation.

The methodology was applied to core samples from various lithologies, including sandstones, heteroliths, and mudstones of wide pore size ranges, offering insights into the interplay of ρ₂ and internal gradient influence over PSD across diverse siliciclastic rock matrices.

Preliminary findings demonstrate that the differential LF-NMR protocol effectively identifies open-pore space systems while mitigating the influence of clay minerals and organic matter on PSD estimates. Furthermore, by incorporating ρ₂ and internal gradient effects on T₂ relaxation, our approach provides realistic PSD values, covering the open-pore size range down to 0.6 nm, the smallest particle diameter of kerosene. Validation of obtained PSD was additionally conducted through nitrogen (N₂) adsorption measurements. Importantly, it is planned to develop empirical LF-NMR PSD models that can overcome the limitations of traditional destructive MICP and nitrogen adsorption methods in detecting pores in siliciclastic rocks with diameters below 3 and 1.78 nm, respectively.