Helium-3 NMR as a valid probe to study pore distribution of clay-based rocks

Two probes for nuclear magnetic resonance (NMR) technique are compared and discussed in evaluation terms for basic properties of porous media. The first and conventional one is ¹H NMR of water, the second one is based on ³He NMR. Both approaches allow to estimate the porosity and pore size distribution (PSD) by measurements of the transverse and longitudinal magnetization relaxation times of the probe nuclei, as well as a $T_1-T_2$ correlation map between relaxation times. The validity of ³He approach is shown by the comparison of both probes applied to sandstones with a relatively small clay abundance. The advantages of ³He NMR technique are demonstrated on the clay-rich samples that get destroyed by water. The obtained PSD-relevant results show that the ³He method is suitable for characterization of various clay-rich formations and can be used to study unconventional extra-heavy oil reservoirs, tight shales, etc., where a conventional water NMR is inapplicable.