X-Ray Computerized Tomography

Abstract

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. Computerized tomography (CT) is a new radiological imagingtechnique that measures density and atomic composition inside opaque objects. Arevolutionary advance in medical radiology since 1972, CT has only recentlybeen applied in petrophysics and reservoir engineering. This paper discussesseveral petrophysical applications, including three-dimensional (3D)measurement of density and porosity; rock mechanics studies; correlation ofcore logs with well logs; characterization of mud invasion, fractures, anddisturbed core; and quantification of complex mineralogies and sand/shaleratios. Reservoir engineering applications presented include fundamentalstudies of CO2 displacement in cores, focusing on viscous fingering, gravitysegregation, miscibility, and mobility control. Introduction X-ray CT is a radiological imaging technique first developed in GreatBritain in 1972 by Hounsfield. CT revolutionized medical radiology by producinganatomical images of extraordinary accuracy and clinical detail. Hounsfield wasawarded the Nobel prize in medicine in 1979 for his contributions. Tounderstand the advantages of CT, first consider conventional X-ray radiography, e.g., chest X-rays. Conventional radiographs view an object from only one angleso that shadows from all irradiated matter along a ray path are superimposed onone another (Fig. 1). The attenuation information along the ray path isaveraged together so that localized regions with small attenuation contrast areobscured. By comparison, CT scanners generate cross-sectional image slicesthrough the object by revolving an X-ray tube around the object and obtainingprojections at many different angles (Fig. 2). From a set of these projections, a cross-sectional image is reconstructed by a back-projection algorithm in thescanner's computer. The cross-sectional image of attenuation coefficients isdisplayed on a cathode-ray-tube (CRT) monitor. The beauty of CT is thatattenuation differences as small as 0.1% can be measured accurately within aninterior region of 2 mm(2) or less. With a fourth-generation medical scanner, the entire imaging process is completed in seconds. CT images (3D) can bereconstructed from sequential cross-sectional slices taken as the sample ismoved through the scanner (Fig. 3). Once this 3D data set has been acquired, any plane through the object can be viewed. For example, Figs. 4A and 4B showhow vertical and horizontal slices can be used to separate gravity and viscouseffects during tertiary gas injection. Instrumentation CT scanners have undergone considerable development since 1972. First-generation scanners used a single pencil-beam source and detectorarrangement. Second-generation scanners improved image quality by use ofmultiple detectors in a translate/rotate configuration. A large improvement inspeed occurred in the third-generation scanners, which used a rotate-onlyfan-beam geometry with source and detectors rotated together around the object. Finally, the fourth-generation scanners use a fan-beam geometry with sourcerotating within a fixed ring of high-efficiency detectors. The second- throughfourth-generation medical CT scanners are satisfactory for petroleumengineering applications because they have adequate X-ray energy and dose forscanning core material. Used medical CT scanners are readily available at asmall fraction of the original cost. JPT P. 885^