Some Useful Guidelines for Whole Core CT-Scanning for Petrophysical Applications

Abstract

From the time CT-scanning was introduced in the oil and gas industry, its use could be divided into SCAL and routine type applications. The first involves flowing fluids through the cores, whereas the second involves scanning, mainly the whole cores, in their native (preserved or unpreserved) state. For whole core CT-scanning, there has been a constant shift from qualitative to quantitative analysis. The size of whole cores is ideal for scanning them with a ‘converted’ medical CT-scanner but obtaining meaningful data requires more than just artifact-free images. In this paper, useful guidelines are offered for extracting meaningful quantitative data from whole core CT-scanning. Today’s fast, multi-slice medical CT-scanners generally provide sufficient image resolution for most of the whole core related applications. However, they usually require some modifications to the equipment and the calibration procedures. Special solid phantoms matching the size and density should be used for calibrating the scanner at different X-ray energies. A set of calibration ‘standards’ is necessary for converting the CT numbers into density and Zeff (Effective Atomic numbers), which should be scanned under the similar ‘environment’ as the whole cores. Using pilot scans is highly recommended for every core tube to reduce uncertainties. For dual energy scanning, it is important to take scans at the exact same locations, selecting the right energy pair and corresponding calibration tables. For image processing, important guidelines include aligning the first and last slices for selecting the largest region-of-interest, quality controlling each slice and assigning depths to each slice before converting from CT numbers to bulk density and Zeff. Additional information is provided for scanning with industrial CT-scanners which have both advantages and disadvantages. Guidelines such as the ones mentioned above allow the data from whole core CT imaging useful for a number of applications. Such data were successfully used for bulk density and porosity determination, heterogeneity quantification, lithology determination, dual-energy based mineralogy detection, density-based micro-imaging, core-log correlations and depth matching, fracture characterization, formation damage evaluation, and many others. Several examples are included in this paper. Adherence to strict guidelines helps extracting artifact-free, meaningful data on whole cores that can improve our understanding of rocks. This paper demonstrates at least 10 different applications of CT-scanners along with the proper guidelines that can be useful for engineers and geoscientists.