CO2-Soluble Nonionic Surfactant Candidates for Huff-and-Puff in Unconventional Formations

Abstract

The dissolution of dilute concentrations of surfactants in CO2 is emerging as a promising method for improving huff-and-puff oil recovery from shale or low permeability rock. The benefit of surfactant addition is primarily attributable to wettability alteration away from oil-wet toward CO2-wet. Field-scale implementation of this method requires cost-effective, pumpable surfactants that are sufficiently CO2-soluble at injection conditions and reservoir conditions during the huff portion of the cycle. Therefore, the CO2-solubility of many nonionic surfactants was measured at 23 and 80 °C in the form of isothermal cloud point curves in the 0.1–1.0 wt % surfactant concentration range. The nonionics included ethoxylated alcohols, propoxylated alcohols, a propoxylated-ethoxylated alcohol, sorbitan-based surfactants, and triblock surfactants composed of poly(ethylene oxide) (EOx) and poly(propylene oxide) (POy) segments. The sorbitan-based surfactants were either CO2-insoluble even at pressures to 69 MPa or were sparingly soluble at extremely high pressure. The most promising CO2-soluble nonionic surfactants identified for future testing in CO2 huff-and-puff oil recovery studies include branched tridecyl ethoxylates (TDA-EO3, TDA-EO6, TDA-EO9), branched tridecyl propoxylates (TDA-PO8, TDA-PO12), and n-hexyl ethoxylate (C6-EO6). A branched propoxylated-ethoxylated 2-ethylhexanol (2EH-PO5-EO10) and a triblock surfactant (PO14-EO12.5-PO14) exhibited lower CO2-solubility but both could also be considered as viable candidates.