Effects of clays on spin-spin relaxation: a route for non-invasive total clay content quantification

TL;DR

This paper presents a non-invasive NMR-based method to quantify total clay content in sandstones by analyzing the effects of internal gradients on transverse relaxation, enabling improved petrophysical characterization.

Contribution

It introduces a novel approach using internal gradient effects on T2 relaxation to estimate total clay content in rocks, linking NMR measurements to geochemical properties.

Findings

Linear correlation between clay content and T2 peak displacement.

Method accurately estimates clay content across diverse mineralogies.

Proposes a new geochemical rock typing based on NMR-derived parameters.

Abstract

Clay minerals are important components of sandstone rocks, due to their significant role in petrophysical properties like porosity and permeability. These minerals have a particular impact on Nuclear Magnetic Resonance measurements since the iron inside clays generates internal gradients that impact the transverse relaxation directly. Here, we apply a methodology recently developed to a set of 20 sandstones with diverse clay content and mineralogy to estimate the total clay content. This estimation relies on the effect of internal gradients and restricted diffusion on transverse relaxation. Our analysis revealed a linear correlation between the total clay content and the displacement of the peak of the T$_2$ distribution as a function of $\tau$, which is half the echo time in the CPMG sequence. Based on these measurements, we propose a geochemical rock typing from quantities determined by our measurements, namely total clay content and porosity.