X-Ray CT and NMR Imaging of Rocks

Abstract

Introduction In little more than a decade, X-ray computerized tomography (CT) and nuclear magnetic resonance (NMR) imaging have become the premier modalities of medical radiology. Both of these imaging techniques also promise to be useful tools in petrophysics and reservoir engineering, because CT and NMR can nondestructively image a host of physical and chemical properties of porous rocks and multiple fluid phases contained within their pores. The images are taken within seconds to minutes, at reservoir temperatures and pressures, with spatial resolution on the millimeter and submillimeter level. The physical properties imaged by the two techniques are complementary. CT images bulk density and effective atomic number. NMR images the nuclide concentration, Mo, of a variety of nuclei ( H, 19-F, 23-Na, 31-P, etc.), their longitudinal and transverse relaxation-time curves (t1 and t2 ), and their chemical shift spectra. In rocks, CT images both rock matrix and pore fluids, while NMR images only mobile fluids and the interactions of these mobile fluids with the confining surfaces of the pores. Principles CT uses an X-ray source that rotates around the sample to obtain one-dimensional projections of X-ray attenuation at different angles. From these projections, a crosssectional slice through the sample is reconstructed by computer. Finally. a three-dimensional image is reconstructed from sequential cross-sectional slices taken as the sample is moved through the scanner. To obtain both density and effective atomic number, CT images are taken at two X-ray energies. One energy is high enough for the X-rays to be predominantly Compton-scattered and one is low enough for them to be mostly photoelectrically absorbed. A combination of the two images in the computer. on a pixel-by-pixel basis, generates separate images of bulk density and effective atomic number. In NMR imaging, the sample is placed inside an intense, homogeneous, magnetic field, one preferably generated by a superconducting magnet. Radio frequency (RF) coils apply RF magnetic fields at the precise Larmor frequency to cause the particular nuclear species to precess. The RF coils then detect the signals generated by the precessing nuclei. Spatial localization within the sample is obtained by x. y, and z magnetic-field gradient coils that cause nuclei in different parts of the sample to precess at slightly different frequencies. The resulting frequency-modulated signal is Fourier-transformed to obtain the spatial distribution of nuclear-spin density. Unlike CT, an image is obtained of the entire sample volume within the RF coil. JPT P. 257^