Perm National Research Polytechnic University, Perm, Russia:

Ya. V. Savitsky, Engineer, yansavitsky@pstu.ru
S. V. Galkin, Professor, Doctor of Geological and Mineralogical Sciences

The use of X-ray tomography in the studies of reservoir rocks is usually limited to visualizing the distribution of X-ray density in order to identify large cavities and fractures. The quantitative characterization of poro-perm properties of core samples faces some limitations because of the instrumentation resolution in exposure of standard size samples. Based on the analysis of the X-ray density distribution histograms, the quantitative evaluation method is proposed to distinguish between the solid mineral skeleton and the storage space filled with liquid or gaseous phases. The thresholding process, which determines the boundaries between grains and pores based on density, is a very complicated and fuzzy problem in tomographic studies. The size effect of core samples on the resolving power of X-ray tomography in 3D image reconstruction is analyzed. The porosity coefficients from the tomography data are always lower than the values found by the standard gas-volumetric method, which is explained by the limited resolution of tomographic studies. The differences in the ranges of visible pores in the tomography of cores of different sizes are explained by the higher physical resolution of volumetric images for smaller samples. Namely, the apparent porosity is 0.02 mm on a 5 mm core and 0.03 mm on a 30 mm core. Moreover, in the lower range of pore sizes, tomography allows imaging not all pores but only some fraction of the actual volume of pores. The analysis of average diameter distribution in the binarized images of pores shows that the largest pores, which are mostly involved in fluid flow, stand out well both in the high-resolution images of small size samples and in the images of samples with standard diameter of 30 mm. This result proves the applicability of X-ray tomography in the study of large pores on standard core samples with the possibility of quantitative assessment of their poro-perm properties.
The study was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Basic Research Contract No. FSNM-2020-0027 in 2020 and in the panning periods in 2021 and 2022.
The study was carried out using equipment provided by the Share Center for Poro-Perm Properties of Rocks, as well as using unique scientific facility—Rock Storage Capacity Structure Analysis Equipment.

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