CO₂-Soluble Surfactants for Enhanced Oil Recovery from Shale

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.