Surfactant-based EOR has gained prominence, especially in recent years for improving recovery from unconventional reservoirs. Recent studies have indicated that surfactants increase production through a wettability alteration mechanism. In addition, surfactant driven IFT reduction causes relative permeability changes and improves hydrocarbon mobility. There is, however, still no clear understanding of how surfactant molecular structure influences the performance of the spontaneous imbibition process. There is also a lack of clarity on how surfactant interaction with the rock mineralogy and oil composition impacts the overall hydrocarbon recovery. This study presents a novel x-ray diffraction (XRD), oil composition-based correlation that could be used to predict the impact of surfactant molecular structure on the imbibition performance.
This study investigated cores samples from the Eagle Ford, Bakken, Scioto sandstone, Indiana Limestone, and Bonne Terre Dolomite. The unconventional rocks (Eagle Ford and Bakken) were saturated in oils from their respective reservoirs while the conventional samples were saturated in Bakken, Wolfcamp, and hexadecane. Subsequently, contact angle and IFT measurements were performed with all rock-fluid systems to characterize wettability alteration and interfacial tension reduction. This was followed by spontaneous imbibition experiments to validate surfactant performance and quantify oil recovery. Seven surfactants were employed in this study (4 non-ionic and 3 cationic) and their performance was compared against a benchmark of experiments conducted with deionized water.
The imbibition experiments demonstrated clear surfactant-assisted improvements to oil recovery compared to the base case of deionized water. They also highlighted key performance trends with different rock-oil-surfactant systems. Cationic surfactants had the highest recovery with carbonate-rich rocks but performed poorly with silica-rich rock samples. Nonionic surfactants performed well irrespective of the mineralogy or oil composition and featured greater oil recovery with fewer ethylene oxide (EO) molecules. Analysis of the crude oil composition indicated that the saturate component played a significant role in determining the performance of the imbibition process, particularly in quartz-rich rocks.
This work presents a guide towards optimizing surfactant selection for chemical EOR applications in lower 48 shale reservoirs. We investigate the interaction of surfactant molecular structure with the rock/oil system and its overall impact on the hydrocarbon recovery process. We believe that in addition to proving the viability of surfactant based EOR in unconventional reservoirs, this study would allow us to design specialized surfactant molecules for the highest recovery based on mineralogy and oil composition.
