Dual-Energy CT-Based Lithology and Porosity Prediction for Petrophysical Rock Typing: Insights into SCAL Optimization in Heterogeneous Carbonate Reservoirs

Abstract Whole-core CT imaging is a powerful, nondestructive technique widely utilized in the oil and gas industry to assess the internal structure of core samples. This method enables rapid visualization of core tubes for sample selection while providing critical geological and petrophysical insights, including fractures, facies transitions, and variations in porosity and mineralogy along the scanned cores. One of the key advantages of X-ray CT imaging is its ability to generate quantitative data for in-depth core sample analysis. The acquisition of high-resolution, continuous images and quantitative data along the core length is particularly valuable in the early stages of core analysis, facilitating the characterization of reservoir heterogeneity. Additionally, CT scanning plays a crucial role in detecting anomalous materials, assessing sample damage, and identifying heterogeneity within core plug samples. The quantitative data acquired from X-ray CT imaging is essential for selecting samples for special core analysis (SCAL) and has significant implications for the development of accurate reservoir engineering models. Within this context, this study examines approximately 400 core plug samples subjected to dual-energy CT (DECT) scanning at two energy levels (80 kV and 140 kV) to capture variations in mineralogy and density, respectively. The plug samples were selected from various formations, including limestone, dolomitic limestone, and dolomite. Their porosity and permeability ranged from 0.009% to 40.7% and from 0.0005 to 9876 millidarcies, respectively. The extracted DECT data from these diverse samples were subsequently used for porosity and lithology prediction as well as heterogeneity quantification. By integrating DECT scanning with quantitative CT data analysis, the study demonstrated a strong correlation between laboratory-measured porosity and CT-derived porosity (R2=0.98), as well as between CT-derived lithology and rock mineralogy confirmed with X-ray diffraction (XRD) analysis. Visual inspection of CT images, combined with 3D heterogeneity quantification parameters derived from DECT, such as the Coefficient of Variation (CV) and Heterogeneity Index (HI), enabled effective grading of core plug samples based on their suitability for SCAL testing. Additionally, this study investigates how CT textural parameters—including fractures, separate and touching vugs, and the uniformity of porosity distribution—impact permeability variations in plug samples. Furthermore, this paper presents a new SCAL sample selection workflow that integrates visual inspection of CT textures with heterogeneity quantification parameters (CV and HI). The overall procedure not only facilitates accurate lithology and porosity prediction in the early stages of core analysis program but also has significant potential for optimizing SCAL sampling strategies.