Abstract. The capability of nuclear magnetic resonance (NMR)
relaxometry to characterise hydraulic properties of iron-oxide-coated sand
and gravel was evaluated in a laboratory study. Past studies have shown that
the presence of paramagnetic iron oxides and large pores in
coarse sand and gravel disturbs the otherwise linear relationship between
relaxation time and pore size. Consequently, the commonly applied empirical
approaches fail when deriving hydraulic quantities from NMR parameters.
Recent research demonstrates that higher relaxation modes must be taken into
account to relate the size of a large pore to its NMR relaxation behaviour
in the presence of significant paramagnetic impurities at its pore wall. We
performed NMR relaxation experiments with water-saturated natural and
reworked sands and gravels, coated with natural and synthetic ferric oxides
(goethite, ferrihydrite), and show that the impact of the higher relaxation
modes increases significantly with increasing iron content. Since the
investigated materials exhibit narrow pore size distributions, and can thus
be described by a virtual bundle of capillaries with identical apparent pore
radius, recently presented inversion approaches allow for estimation of a
unique solution yielding the apparent capillary radius from the NMR data. We
found the NMR-based apparent radii to correspond well to the effective
hydraulic radii estimated from the grain size distributions of the samples
for the entire range of observed iron contents. Consequently, they can be
used to estimate the hydraulic conductivity using the well-known
Kozeny–Carman equation without any calibration that is otherwise necessary
when predicting hydraulic conductivities from NMR data. Our future research
will focus on the development of relaxation time models that consider pore size distributions. Furthermore, we plan to establish a
measurement system based on borehole NMR for localising iron clogging and
controlling its remediation in the gravel pack of groundwater wells.
Jointly deriving NMR surface relaxivity and pore size distributions by NMR relaxation experiments on partially desaturated rocks