2D spin-echo and 3D chemical-shift-imaging techniques for analysis of oil-water replacement in limestone

Abstract

NMR imaging for biological studies and medical diagnostics is now a well-established technique and most of the commercially available NMR imaging devices have been developed with this type of application in mind. Efforts to apply NMR imaging to material science are relatively recent: applications to wood, polymers, composites, and ceramics have been reported. Imaging of fluid-filled porous rocks appears to be a scientifically and technically important potential application of MRI. Pioneering work in this field began in 1979, when Gummerson et al. ( 1) and Rothwell et al. (2) reported images which depicted the distribution of fluids within porous samples. Since then several papers on the subject have appeared in the literature and some of them are referenced in this paper (3-l 1). When several fluid phases are present at the same time or when displacement of a fluid by another is studied, it is necessary to have an imaging technique which can distinguish readily between fluids of different chemical compositions. Organic fluids (oil ) and water are very often distinguished by doping one component with paramagnetic ions so that by a shortening of the transverse relaxation time, T2, the corresponding component is eliminated and only one signal is observed. Differences in the longitudinal spin-relaxation time, T, , of the oil and water phases have also been used by Hall et al. (5). In this technique an inversion-recovery step is incorporated in the imaging sequence. Selective images of a particular species can also be obtained either by selective excitation of the fluid under investigation or by suppression of the unwanted species by presaturation (11). These methods are currently used in biomedical application but have not been applied, so far, for the study of fluid-filled porous media. Global methods of chemical-shift imaging( CSI ), producing essentially a chemical-shift spectrum for each localized region or volume element, have been developed for biomedical research.