Fixed and pulsed gradient diffusion methods in low-field core analysis Gabriela Leu, Edmund J. Fordham T , Martin D. Hqrlimann, Phil Frulla Schlumberger-Doll Research, Ridgefield, CT 06877, USA Abstract We review diffusion-weighted relaxation protocols for two-dimensional diffusion/relaxation time (D, T 2 ) distributions and their application to fluid-saturated sedimentary rocks at low fields typical of oil-well logging tools ( V 2 MHz for 1 H). 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 ( g int ), results are comparable, as expected, for all the studied protocols. For rocks of strong g int , protocol-dependent artifacts are seen in the joint (D, T 2 ) 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 g int . 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. D 2005 Elsevier Inc. All rights reserved. Keywords: Fixed gradients; Pulsed gradients; Rock cores; Internal gradients 1. Introduction Recent work by Hqrlimann and Venkataramanan [1] and Hqrlimann [2] have demonstrated the quantitative applica- tion of diffusion-weighted protocols in grossly inhomoge- neous B 0 and B 1 fields characteristic of oil-well logging tools [3,4]. Understanding of their spin physics is now sufficiently complete that these bdiffusion editingQ protocols [5,6] may be considered a quantitative measurement class for fixed (uniform) field gradients (FFGs), ready for application in the borehole environment, for petrophysical interpretations. The principal output is a joint distribution (two-dimensional correlation map) of diffusivity D vs. T 2 , generated by the analysis of Venkataramanan et al. [7]. First applications have been to the identification of different types of fluid in the rock (water, hydrocarbons and drilling fluid filtrates) because of the typically strong diffusivity contrast. However, structural information from restricted diffusion effects [1,8] are also obtainable and of high practical value in the oil field. Laboratory development of the diffusion editing proto- cols has taken place in the fringe field of a large superconducting magnet, offering a uniform (and very stable) FFG. This mimics a fixed-gradient logging tool directly but is restricted in signal-to-noise ratio (SNR) because of the slice selection of finite bandwidth RF pulses. Resonant volumes within a standard test core are much smaller than the resonant shell volumes interrogated by a logging tool. Laboratory acquisition times can be long, especially for the thin slices selected at the higher gradient strengths (up to 50 G cm 1 ). Another problem of FFG methods is apparent from considerations of experi- ments at elevated pressure and temperature ( p , T). The standard approach is to confine the rock sample in a nonconducting core holder, with confining pressure applied via a nonhydrogenic liquid. Deuterated liquids (e.g., D 2 O) are usually deprecated on grounds of (a) cost and (b) safety (for operation above the fluid’s atmospheric pressure boiling point). The usual solution is a perfluorinated hydrocarbon; but in a FFG, these will exhibit a strong 19 F resonance from selected slices offset only a few 0730-725X/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2004.11.065 T Corresponding author, present address. Schlumberger Cambridge Research, Cambridge CB3 0EL, UK. Tel.: +44 1223 325 263; fax: +44 1223 467 004. E-mail address: fordham1@slb.com (E.J. Fordham). Magnetic Resonance Imaging 23 (2005) 305 – 309