Dual-Energy CT-Scanning Applications in Rock Characterization

Abstract

Abstract The use of dual-energy CT-scanning for evaluating reservoir rocks is not a new technique. It involves scanning the same location within the rock twice, using a different X-ray energy each time. Wellington and Vinegar, in a 1987 paper stated that one image proportional only to bulk density and another proportional to atomic number can be obtained by scanning the sample at high (above 100 kV, Compton scattering dominant) and low (well below 100 kV, Photoelectric absorption dominant) X-ray energies. They generated CT slices showing density and atomic number variations. Although dual-energy scanning has been used quite successfully in fluid flow visualization studies, especially in the case of three-phase flow, its use has been limited in the field of petrophysical core characterization. Part of the problem is due to the unavailability of appropriate software and procedures to generate the bulk density and atomic number images from the high and low-energy CT data and part is due to the lack of necessary steps to utilize the density and atomic number data to characterize the cores. Commercially available software can now allow generating the density and atomic number images from the dual-energy data. This paper describes some techniques to utilize the density and atomic number data to better characterize the reservoir rocks. This technique was first applied to small plug samples of known petrophysical properties for validation purposes. After that, it was applied to a 60 ft-long section of preserved core from an Upper Jurassic carbonate reservoir in Saudi Arabia, which was CT-scanned at two different X-ray energies at 5 cm (2 inch) intervals. Interesting qualitative results were obtained that can improve our understanding of the carbonates. This paper also shows step-by-step instructions for calculating porosity values from the approximate mineral compositions. Introduction Computerized Tomography (CT) is a non-destructive imaging technique that utilizes X-ray technology and mathematical reconstruction algorithms to view cross-sectional slices of an object. Although CT-scanners are medical diagnostic tools they have been used extensively by the petroleum industry for studying reservoir cores for more than 20 years. Excellent reviews of the various applications of CT in the petroleum industry can be found in the literature1–8. Basically, the petroleum industry applications of CT can be divided into two major areas with some overlaps: core characterization and fluid flow visualization. CT applications for core characterization involve scanning whole cores and plugs, mostly in preserved conditions although unpreserved cores are also scanned. The preservedcondition scanning is particularly useful for the unconsolidated cores or cores that need to be tested without compromising their wettability states. Qualitatively CT provides information on heterogeneities, lithology changes, fractures, vugs, and mud invasion damage. Quantitative CT data are used to measure bulk density and porosity; to quantify heterogeneity9; to make core-to-log comparisons for depth matching and log calibrations. Fluid flow visualization constitutes the more prevalent use of CT in the petroleum industry, which almost invariably involves the use of radiopaque tracers (dopants) to monitor fluid movement inside the cores during coreflooding. The dopants provide sufficient contrast between the various fluid phases to view and quantify saturation distribution and changes, gravity and viscous effects, trapping and bypassing, effects of heterogeneity on flow. CT is an excellent tool to view the effect of a treatment fluid such as acid, gel, polymer, stream, and foam, with core kept at reservoir conditions inside a CT-transparent coreholder. When conducting three-phase flow experiments CT provides a very effective way to monitor individual saturations in-situ1,10,11. Withjack et al.8 provide a comprehensive list of some of the applications of CT. In this paper, we focus mainly on the core characterization aspects using the dual-energy X-ray technique which has been around for almost as long as X-ray CT-scanning.