A Systematic and Comprehensive Approach for Characterizing Capillary Desaturation Curves in Low-Permeability Carbonate Rocks

It is widely recognized that the determination of true residual oil saturation to water (S_orw) is a critical factor in predicting waterflooding performance and implementing enhanced oil recovery (EOR) methods. A particular EOR method will provide a high prospective result if there is an overestimation of S_orw. The concept of capillary desaturation curve (CDC) is used to determine how much amount of oil can be recovered, when implementing a certain EOR technique. The objective of this study is to determine the true residual oil saturation to water (S_orw) for low permeability Indiana limestone outcrops using the centrifuge technique under reservoir conditions and further generate their CDCs. In this work, three carbonate Indiana limestone outcrops with low permeability range (4-8 mD) and representative fluid samples i.e., field oil, formation water, and seawater, were utilized. The CDC was then characterized for carbonate rocks by further reducing S_orw using surfactant flooding where three anionic surfactant formulations with different IFT values were selected. A systematic approach was followed starting with conventional core analysis followed by special core analysis. For the CDC generation via surfactant flooding, three surfactant formulations having different IFT values were selected through a preliminary screening. This study showed that there is no correlation between initial water saturation (S_wi) and absolute permeability for the cores tested. In addition, variations in spontaneous oil recovery was noted among cores within the same range of rock permeability, which indirectly indicates the existence of heterogeneity within each rock. Furthermore, a true S_orw of 20-29% was achieved using the centrifuge method, which was confirmed during the surfactant flooding stage. Additionally, CDC studies indicated that a critical trapping number of 10^-4 was achieved for the tested cores, which is higher than most of the reported values of 10⁻⁸ to 10⁻⁶ in the literature. Accordingly, the complete desaturation of mixed-to-oil wet carbonate rocks is quite challenging since it requires a further increase in trapping number, which could possibly be achieved using ultra-low IFT surfactants. This work presents a systematic and comprehensive approach for determining true S_orw and understanding microscopically trapped oil in carbonate rocks based on CDC. The produced results would be useful in EOR screening for future surfactant flooding pilots in carbonate rocks with low permeability.