Rapid measurements of heterogeneity in sandstones using low-field nuclear magnetic resonance

Sandstone rocks can contain microscopic variations in composition that complicate interpretation of nuclear magnetic resonance (NMR) relaxation time measurements. In this work, methods for assessing the degree of sample heterogeneity are demonstrated in three sandstones. A two-dimensional T-1-Delta chi(app) correlation (where Delta chi(app) is the apparent solid/liquid magnetic susceptibility contrast) reveals the microscopic heterogeneity in composition, whilst a spatially resolved T-1 profile reveals the macroscopic structural heterogeneity. To perform these measurements efficiently, a rapid measure of longitudinal T-1 relaxation time has been implemented on a low-field NMR spectrometer with a magnetic field strength B-o=0.3 T. The "double-shot" T-1 pulse sequence is appropriate for analysis of porous materials in general. Example relaxation time distributions are presented for doped water phantoms to validate the method. The acquisition time of the double-shot T-1 sequence is equivalent to the single-shot Carr-Purcell Meiboom-Gill (CPMG) sequence used routinely in petrophysics to measure transverse T-2 relaxation. Rapid T-1 measurements enable practical studies of core plugs at magnetic field strengths previously considered inappropriate, as T-1 is independent of molecular diffusion through pore-scale (internal) magnetic field gradients. (C) 2014 Elsevier Inc. All rights reserved.