3D Virtual Core Plug Acquisition Using Whole-Core CT Scanning-Efficacy of the Technique for Porosity Prediction in Sandstone Cores
Authors: Ugurlu Ibrahim Olgun , Usdun Hasan Caglar 
Published: November 2024

Core data has traditionally served as the ground truth for formation evaluation, providing direct measurements of reservoir petrophysical properties. After the core is plugged, special core analysis (SCAL) plugs are preserved, while routine core analysis plugs must be thoroughly cleaned of oil and salts, usually through the Soxhlet extraction method, which involves extraction with hot toluene and methanol. This cleaning process can sometimes take several weeks, depending on factors such as rock permeability, pore structure heterogeneity, and the type of hydrocarbon present. Emerging digital imaging technologies, such as whole-core computed tomography (CT)scanning, have become particularly useful in these circumstances, as they provide a quick tool for detailed core characterization and representative sample selection.

Whole-core CT imaging is a robust, nondestructive technique utilized in the oil and gas industry to evaluate the internal structures of cores. The resulting images not only provide a quick preview of core tubes for sample selection but also reveal critical physical and geological details, such as fractures, key facies changes, and variations in porosity along the scanned core tube. One of the most powerful aspects of X-ray CT imaging is its ability to produce quantitative data for analyzing scanned core samples. During the research conducted for this paper, standard whole-core CT scanning was performed using a single high-energy setting (140 kV), primarily reflecting the density variations along the core interval, and the extracted data were used for porosity prediction. Obtaining such high-resolution, continuous images and quantitative data along the core length is extremely valuable in the initial stages of the core analysis program to characterize reservoir heterogeneity and optimize sample selection for SCAL studies. Within this context, this study aims to achieve expedited porosity estimation by integrating whole-core CT scanning, 3D virtual core plug acquisition, and quantitative CT data analysis, while also addressing the CT response to different Winland r35 rock types. Additionally, we investigate how fluid content within the plugs impacts porosity calculations by comparing the CT numbers before and after core cleaning and drying.

The 3D virtual core plugging technique is a relatively new technology that offers innovative solutions in core analysis. The 3D virtual core plug acquisition process involves creating a digital version of a core plug using commercial software to generate cylinders of any desired diameter. In this research, we acquired approximately thirty virtual plug samples (1-inch diameter) from 15-meter sandstone cores. Porosity of the virtual plugs was calculated through the correlation between bulk density and the CT numbers, and these values were compared with laboratory-measured helium porosity. The findings revealed a strong correlation between helium porosity and CT-derived virtual plug porosity, with a coefficient of determination (R2) of 0.9474.

This integrated approach not only expedites reservoir porosity estimation during the initial stages of core analysis but also has great potential for optimizing SCAL sampling strategies in thinly laminated, fractured, and heterogeneous cores, where the acquisition of physical core plugs presents significant challenges.