(599a) Solutions of Nonionic Surfactants in CO2 to Improve Unconventional CO2-EOR Via Wettability Alteration

CO₂ huff n’ puff is a promising method for enhanced oil recovery (EOR) in unconventional shale formations. In this work, we postulate that CO₂-EOR may be improved by the dissolution of surfactants into CO₂ if both oil-rich and CO₂-rich phases are present. A complete pressure-composition phase behavior diagram of CO₂-dead Eagle Ford crude mixtures indicates that at 80 ^oC and pressures as high as 62 MPa, two such phases co-exist for CO₂-rich mixtures. Therefore the dissolution of a surfactant in CO₂ has the potential to improve oil recovery via wettability alteration from oil-wet to CO₂-wet and/or CO₂-oil interfacial tension (IFT) reduction. Three nonionic surfactants (linear dodecyl ethoxylate Indorama SURFONIC^® L12-6, branched nonyl ethoxylate Indorama SURFONIC^®TDA-9, and branched nonylphenol ethoxylate Indorama SURFONIC^® N-100) were evaluated for CO₂ solubility, shale wettability alteration, ability to increase oil recovery during laboratory huff n’ puff experiments, and effect on CO₂-oil interfacial tension (IFT). Each surfactant exhibited CO₂-solubility at temperatures up to 100 ^oC of roughly 1 wt% at pressures commensurate with CO₂-EOR. The presence of surfactant in CO₂ did not have a significant effect on CO₂-oil IFT. High-pressure, high-temperature contact angle experiments demonstrated that the (CO₂+surfactant) solutions can shift the wettability of oil-wet shale toward CO₂-wet and water-wet. CO₂ and (CO₂+surfactant) huff n’ puff experiments for oil-saturated Eagle Ford, Mancos, and Bakken shale cores immersed in CO₂ showed that CO₂ solutions containing the branched nonyl ethoxylated alcohol gave highest cumulative oil recoveries of 72% and 75% (at 0.01 wt% and 0.1 wt% of TDA-9, respectively) compared to 71% recovery for the core immersed in pure CO₂. Analysis of oil produced during confined-core huff n’ puff experiments indicated that heavier oil components were produced when the surfactant was added to CO₂. These results suggest that CO₂-soluble surfactants may increase oil recovery by wettability alteration rather than IFT reduction.