Fixed and pulsed gradient diffusion methods in low-field core analysis

We review diffusion-weighted relaxation protocols for two-dimensional diffusion/relaxation time (D, T2) distributions and their application to fluid-saturated sedimentary rocks at low fields typical of oil-well logging tools (≤2 MHz for 1H). Fixed field gradient (FFG) protocols may be implemented in logging tools and in the laboratory; there, pulsed field gradient (PFG) protocols are also available. In either category, direct or stimulated echoes may be used for the diffusion evolution periods. We compare the results of several variant FFG and PFG protocols obtained on liquids and two contrasting sedimentary rocks. For liquids and rocks of negligible internal gradients (gint), results are comparable, as expected, for all the studied protocols. For rocks of strong gint, protocol-dependent artifacts are seen in the joint (D, T2) distributions, consistent with the effects of the internal fields. For laboratory petrophysics, the PFG methods offer several advantages: (a) significantly improved signal-to-noise ratio and acquisition times for repetitions over many samples; (b) freedom from heteronuclear contamination when fluorinated liquids are used in core holders; and (c) a palette of variants - one comparable with the FFG - for the study of rocks of significant gint. Given suitable hardware, both PFG and FFG methods can be implemented in the same bench-top apparatus, providing a versatile test bed for application in a petrophysical laboratory.