CO2 injection is a promising method for enhanced oil recovery (EOR) in unconventional liquid reservoirs. Addition of surfactants, which have been shown to improve oil recovery during water-based EOR, could also improve CO2 EOR. Because oil and CO2 are not fully miscible, even at pressures up to 9000 psi, oil extraction by CO2 can be improved through addition of surfactants. The three surfactants evaluated in this study included linear alkyl ethoxylate Indorama SURFONIC® L12-6, branched alkyl ethoxylate Indorama SURFONIC® TDA-9, and branched phenol ethoxylate Indorama SURFONIC® N-100. Surfactants were evaluated for CO2 solubility, shale wettability alteration, ability to increase oil recovery during core soaking, and effect on CO2-oil interfacial tension (IFT). All three surfactants exhibited good solubility in CO2 (roughly 1 wt% at pressures of 2000-5000 psi). Contact angle experiments demonstrated that the CO2+surfactant solutions can shift the wettability of shale toward water-wet. CO2 and CO2+surfactant huff 'n puff experiments using oil-saturated Eagle Ford and Mancos shale cores showed that CO2 solutions of SURFONIC® TDA-9 gave highest cumulative oil recoveries of 72% and 75% (0.01 wt.% and 0.1 wt.% of TDA-9 respectively) compared to 71% recovery for the core immersed in pure CO2. The addition of surfactant to the CO2 phase did not have a significant effect on CO2-oil IFT, suggesting that CO2-soluble surfactants can increase oil recovery by wettability alteration rather than IFT reduction.
Oil from unconventional reservoirs has dominated U.S. domestic energy production in the past decade. However, oil extraction from shale reservoirs typically remains below 10%. The petroleum industry has had to rapidly adapt technology developed for porous conventional reservoirs to extremely tight unconventional reservoirs. CO2 EOR is one oil extraction technique that works well for conventional reservoirs but is still being optimized for unconventional reservoirs. CO2 is a relatively good solvent for oil and, in conventional reservoirs, its extraction ability is increased even further as it moves through the porous matrix collecting hydrocarbons-a process referred to as multiple contact miscibility (MCM). During CO2 EOR in unconventional reservoirs, however, CO2 is injected through high-permeability fractures and then allowed to soak into the shale matrix. MCM cannot be achieved because CO2 does not flow through the matrix.