Improving CO2 Enhanced Oil Recovery in Unconventional Formations via the Dissolution of Wettability Altering CO2-Soluble Nonionic Surfactants

Primary oil recovery from fractured, unconventional deposits such as shale or tight sands is typically below 10%. Developing an enhanced oil recovery (EOR) approach that is affordable and applicable to unconventional liquid reservoirs (ULRs) could result in enormous increases in domestic oil output. Injecting CO2 into unconventional shale reservoirs is a potential technique for improved oil recovery (EOR). In this study, we propose that the dissolution of non-ionic surfactants into CO2 may enhance CO2 EOR. Although CO2 is a good oil solvent, we establish that mixtures of Eagle Ford oil and CO2 form two equilibrium fluid phases at concentrations more than 70 wt% CO2, even at pressures as high as 62 MPa. The presence of a CO2-oil interface under reservoir conditions shows that the addition of a surfactant has the potential to influence oil recovery, either by changing the wettability from oil-wet to CO2-wet, by reducing the CO2-oil interfacial tension (IFT) or causing CO2-in-oil foams to form. Several nonionic surfactants (e.g., a branched tridecyl ethoxylate with nine ethylene oxide groups, a branched nonylphenol ethoxylate with ten ethylene oxide groups) were evaluated for CO2-solubility. Every surfactant was slightly soluble (0.1 wt%) in CO2 at pressures and temperatures suitable for CO2 EOR. CO2-dissolved surfactants did not significantly alter CO2-oil IFT or promote CO2-in-oil foam formation. However, at 80 °C and 27 MPa, the surfactants did induce a substantial shift in the contact angle of an oil droplet on an oil-aged shale chip in CO2 from highly oil-wet (11° contact angle) to inter�mediate CO2-oil wettability (82°contact angle). CO2 huff ’n puff studies were performed, and the branched tridecyl ethoxylated surfactant with nine ethylene oxide groups provided the best cumulative oil recovery of 75%, compared to 71% recovery with pure CO2. Little change in oil recovery was observed with some of the other surfactants. These findings show that surfactants dissolved in CO2 may slightly boost oil recovery from shale by changing the wettability from oil-wet to CO2-wet