Development of an NMR workflow for determining nano-petrophysical properties of marine and lacustrine mudrocks

Abstract

Nuclear Magnetic Resonance (NMR) is a powerful method of pore structure and fluids characterization for a wide range of porous media due to its fast and non-destructive capabilities, but its utilities of nano-petrophysics (the properties of rocks and fluids, as well as their interactions, in the presence of predominantly nm-sized pores) for tight oil formations warrant more studies. In this work, a total of four cylindrically-shaped tight reservoir samples at 2.54 cm in diameter with four different lithologies in USA and China were selected for systematic NMR analyses with T2 relaxation time, T1-T2 mapping, NMR cryoporometry (NMRc), and magnetic resonance imaging (MRI) techniques, along with other supplemental tests of X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and vacuum-assisted water immersion porosimetry (WIP), to characterize their petrophysical properties. The results show that the effective porosity obtained from NMR was highly consistent with WIP. Three samples characterized by MRI showed that T2 spectra had a good response to the fluid distribution in the samples. In this study, T2 time was converted into the pore size by combining corrected NMR T2 spectrum and MIP pore-throat size distribution. Finally, The Katz-Thompson model was applied for NMR data to calculate the permeability at different inflection points (connected pore networks) to quantitatively evaluate their individual permeabilities and porosities. Furthermore, except for sample N9 (carbonate-rich Wolfcamp A mudrock), a satisfactory agreement was achieved between the results of K2C (cut-off time at the second inflection point) and helium method for permeability analyses.