Porosity Evaluation of Shales Using NMR Secular Relaxation

Determining the porosity associated with organic and inorganic components of shales is an important but difficult part of formation evaluation in unconventional resources. Nuclear magnetic resonance (NMR) measurements offer a means of quantifying organic and inorganic porosity by separating the inorganic porosity, where proton relaxation occurs by paramagnetic interactions, from the organic porosity, where proton relaxation occurs by intermolecular dipole interactions. We performed laboratory measurements on preserved Bakken and Eagle Ford samples with a 2.2 MHz nuclear magnetic resonance (NMR) core analysis system. We constructed two-dimensional maps of T1 and T2 with different echo spacings for the T2 measurement and computed distributions of T1/T2 ratio and the secular relaxation rate, which is the difference between the transverse and longitudinal relaxation rates. Based on the distribution of T1/T2 ratios and the change in secular relaxation rate with echo spacing, we were able to differentiate organic porosity, inorganic porosity, and the relaxation signal from the organic material itself. The differentiation is based on theoretical consideration of relaxation times due to paramagnetic and dipole interactions.