New Insights into the Understanding of Capillary Trapping of Residual Oil in Carbonate Rocks Using Capillary Desaturation Curves

Most of the oil remain trapped in the reservoir after both primary and secondary recovery stages. Enhanced oil recovery (EOR) techniques are usually implemented in the tertiary stage to recover the trapped oil. Accordingly, the inaccurate determination of residual oil saturation after waterflooding (S_orw) in the secondary stage affects the success and economics of the EOR processes in the tertiary stage. Thus, the capillary desaturation curve (CDC) is usually introduced as guidance to estimate the mobilized residual oil. The objectives of this study include determining the true S_orw for carbonate Indiana limestone outcrops under harsh conditions, then investigating the effect of trapping number, permeability, and initial oil saturation on S_orw, and finally characterizing the CDC for carbonate rocks by further reducing the S_orw using surfactant flooding. For this purpose, six carbonate Indiana limestone outcrop samples with different permeabilities (4-69 mD) and fluid samples i.e., field-representative oil, formation water, seawater, and surfactant solutions were utilized. The drainage process was performed systematically using a coreflooding system to establish initial water saturation by injecting heavy oil followed by crude oil and aging for two weeks. Afterward, all six cores were subjected to spontaneous imbibition using Amott cell. This was further followed by forced imbibition using both ultra-centrifuge and coreflooding systems for comparison purposes and achieving S_orw condition. Finally, forced imbibition was performed on all cores using coreflood to generate CDC using three different surfactants with varying IFT values. The results showed that all rock samples achieved initial water saturation (S_wi) in the range of 18-32% with no correlation between S_wi and rock permeability. In addition, spontaneous imbibition tests showed slight oil production which reflect the oil-wetness of these cores used. It was noted that this slight production varied among cores with the same rock permeability range, which indirectly indicating the existence of heterogeneity within each permeability range. Furthermore, S_orw of 20-30% was reached using ultra-centrifuge and coreflooding method, indicating no correlation of permeability with S_orw. Based on the CDC studies, the critical trapping number was in the range between 10⁻⁵ and 10⁻⁴ for the tested cores, which is higher than the reported values in literature (10⁻⁸ to 10⁻⁶). This work provides a new insight into the understanding of capillary trapping effect on residual oil using CDC in carbonates. The complications in carbonate rocks, including the complex nature of high heterogeneity, mixed-to-oil wettability, high temperature, and high salinity, render accurate determination of true S_orw is a challenge at lab-scale. S_orw determination and CDC characterization aid in EOR screening to find the effective and economically viable methods for production enhancement.