62nd Annual Report on Research 2017

Over this reporting period, we have performed experiments on several different rock samples, including the sandstones Berea 500, Fontainebleau, Benthimer, and Austin chalk using ²³Na NMR. In particular we implemented specific aims 1+2 of the original proposal. These experiments were performed in order to provide new tools for characterizing wettability of different rock pore surfaces.

Within specific aim 1, we have optimized methodology for the use of ²³Na NMR in inhomogeneous media at high magnetic fields. Such studies have never been done on rock samples before, because susceptibility differences lead to large inhomogeneous broadenings. In particular, we have performed nutation NMR spectra, and a variety of T₂ and T₁ measurements and analyzed multiexponential relaxation data using regularized inverse Laplace transforms. The relaxation studies allowed us to identify transitions between three different diffusion regimes, diffusion length lD, the distance traveled during a characteristic experimental delay time τ, the pore size lS, and the dephasing length lg, which is the distance of travel in order for a spin to dephase by 2¹ radians under the influence of IGFs, three main regimes can be distinguished, which can be used to classify the measurement outcomes:

When lD≪lS,lg, the measured spins are in the free diffusion regime. In this case, the diffusion term TD-1 is proportional to τ2. When lS≪lD,lg, the spins are in the motional averaging regime. In this case, (they experience a motionally averaged IGF), and the diffusion term TD-1 does not depend on τ, and is inversely proportional to the diffusion coefficient. When lg≪lD,lS, the measured spins are in the so-called localization regime.

Four saturated rock core samples, a Berea 500 sandstone, a Fontainebleau 1 sandstone, a Bentheimer sandstone, and an Austin chalk were obtained from Schlumberger-Doll Research. Each core sample was cleaned and cut into a cylindrical plug with a 22.35 mm diameter and 42.21 mm of height. The samples were saturated with 0.55 M of NaCl solution for over 48 hours to reach full equilibrium.

We were able to determine that the investigation of ²³Na in the pores of Berea 500, Fontainebleau 1, Bentheimer sandstones and Austin chalk are possible in situations with strong internal field gradients. The comparison of the experiments with different values of rf-fields was na ecessary condition for obtaining correct qualitative and quantitative interpretation of the experimental data.

We were able to detect multiple quantum filtered signals from the rock samples, which can be attributed either to immobilized sodium spins, or to sodium pools located within electric field gradients, both of which could be induced by the surface.