Computed Tomography as a Core Analysis Tool: Applications, Instrument Evaluation, and Image Improvement Techniques

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Summary In recent years, the use of computerized tomography (CT) to characterize two-phase fluid flow through porous media has become increasingly popular. This paper describes a different application of CT: its use as a core analysis tool. The advantages and disadvantages of the different technological generations of commercial medical CT scanners available as core analysis instruments are also discussed. Additionally, methods are presented for improving images and reducing CT-number errors inherent in the scanning of high-density rock samples on instruments whose software was designed for the scanning of low-density human patients. Introduction CT, or computerized axial tomography or computer-assisted tomography (CAT) scanning, is a nondestructive X-ray technology that produces an image of the internal structure of a cross-sectional slice through an object by the reconstruction of a matrix of X-ray attenuation coefficients.1,2 The method is fast - 50 msec to 7 minutes per image, depending on the technological generation of the instrument - and requires little or no sample preparation. An imaged slice can be divided into an n×n matrix of voxels (volume elements). The attenuation of No X-ray photons passing through any single voxel having a linear attenuation coefficient µ reduces the number of transmitted photos to N according to Beer's law:N=No exp(-µx), Where x is the dimension of the voxel in the direction of the X-rays. Material parameters that determine the linear attenuation coefficient of a voxel include its density ? and mass attenuation coefficient µm:µ=µm?. Mass attenuation coefficient, in turn, depends on the atomic number of the material and the photon energy of the beam. For multicomponent voxels - i.e., mineral grains and porosity - the atomic-number dependence is weighted by the volume fraction of each component (partial volume effect). Thus the composition and density of the material in a voxel will determine its linear attenuation coefficient. A convention in medical imaging is to normalize the measured linear absorption coefficient to that of water:Equation By definition, air and water have CT numbers, NCT's, of -1000 and zero, respectively. In addition to conventional medical applications of CT, its use to characterize flow through porous media has become common in recent years.3–8 Other applications include coal,9 soil,10 and core11–13 analyses; core-sample/borehole-position correlation14; engineered-material quality-control analysis15,16; and geotomography, an extension of CT principles.17 Experimental All CT data were obtained on conventional medical scanners. Unless otherwise stated, the instrument was an unmodified, second-generation, dual-slice Technicare Deltascan 100 head scanner with a tungsten source, seven bismuth germanate detectors, and a DEC PDP 11/04 computer system. This unit operates at a tube voltage and current of 120 kV and 25 mA, respectively. For display, each voxel is assigned one of 64 gray levels on the basis of its CT number, with dark corresponding to a low CT number and bright corresponding to a high CT number. Quoted voxel size is 1×1×8.3 mm [0.4×0.04×0.3 in.]. To reduce beam-hardening effects, samples were surrounded with either sand or an aqueous potassium iodide solution, as noted in the applications. Beam hardening is a phenomenon that occurs at the air/sample interface and results in edge-brightening image artifacts that have falsely high CT numbers. In cases where a liquid medium was used to surround a porous sample, the sample was first coated with a low-attenuation, waterproof silicon gel. These media are not visible in the images because they have lower attenuation factors than the samples and do not appear at settings optimal for viewing the samples. All images and CT-number data were obtained directly from the system computer. Core Analysis Applications Applications presented and discussed here includevisualization of the extent of mud invasion;detection of fractures;lithologic characterization of cores encased in conventional opaque preservation material, rubber-sleeve core barrels, and stainless-steel pressure vessels;core screening before laboratory flow tests; andcorrelation of CT data to porosity, permeability, and mineralogy. Visualization of Mud Invasion. Visualization of the extent of mud invasion is illustrated in Fig. 1, a CT image through a slabbed, 8.9-cm [3.5-in.] -diameter homogeneous sandstone core. The sample was embedded in sand to reduce beam-hardening artifacts. Drilling mud has a high attenuation factor because of its high barite content. Consequently, its CT number is higher than that of sandstone. It can be seen that mud invasion (bright areas) occurred both at the core perimeter (bottom and left) and along a fracture near the top edge. In contrast, the dark areas in the interior represent rock not invaded by drilling mud. Knowledge that drilling mud is present aids in the calculation of accurate native oil and water saturations. Visualization of Mud Invasion. Visualization of the extent of mud invasion is illustrated in Fig. 1, a CT image through a slabbed, 8.9-cm [3.5-in.] -diameter homogeneous sandstone core. The sample was embedded in sand to reduce beam-hardening artifacts. Drilling mud has a high attenuation factor because of its high barite content. Consequently, its CT number is higher than that of sandstone. It can be seen that mud invasion (bright areas) occurred both at the core perimeter (bottom and left) and along a fracture near the top edge. In contrast, the dark areas in the interior represent rock not invaded by drilling mud. Knowledge that drilling mud is present aids in the calculation of accurate native oil and water saturations.

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