Laboratory-Scale CO2 Huff ‘n Puff EOR using Single Phase Solutions of CO2 and CO2 Soluble, Nonionic, Wettability Altering Additives

Abstract This study attempts to determine if the efficacy of CO2-based enhanced oil recovery (EOR) techniques for unconventional liquid reservoirs (ULRs) can be increased through wettability alteration by adding a dilute non-ionic surfactant to CO2. The use of surfactants to increase the water-wetness of rock surfaces has previously been shown to improve oil recovery during water-based hydraulic fracturing and waterbased EOR in ULRs. In this study, nonionic surfactants are dissolved in CO2 to attain analogous significant shifts in wettability toward CO2-philic and oil-phobic. This could provide another EOR mechanism for the CO2-based recovery of oil from unconventional formations. The solubility of a nonionic, water-soluble, surfactant (Indorama SURFONIC® TDA-9, an ethoxylated alcohol with a branched tridecyl, oil-philic tail and nine ethylene oxide groups in the hydrophilic head group) in CO2 has been measured between 25 – 100 °C. This surfactant exhibits a solubility of roughly 1 wt% at pressures of 2000 – 5000 psia, with lower pressures required for lower temperatures. Eagle Ford outcrop samples were first aged in dead Eagle Ford crude oil at high temperature to attain oilwet characteristics. The oil wetness of shale samples was verified by measuring contact angles of water droplets in air. Then the samples were immersed in CO2 or CO2-surfactant solution for 16 h at 4000 psi and 80 °C. Contact angle measurements were then performed to identify shifts in wettability. No substantial change in wettability were observed for samples exposed to CO2, however the samples exposed to CO2-surfactant solution revealed a significant shift toward water-wet. Two CO2 huff ‘n puff experiments were then conducted for small, oil-saturated shale cores at 80 °C and 4000 psi. Faster oil recovery (i.e. more oil recovery in the first cycle) and cumulative oil recovery of 73% (after 7 huff and puff cycles) were achieved for the core immersed in CO2-surfactant solution (0.7 wt% surfactant) compared to 78% recovery for the core immersed in pure CO2. Continuing tests will examine the effects of rock type, oil properties, temperature, pressure, surfactant type (oil-soluble vs. water-soluble ethoxylated alcohols), surfactant concentration, and the presence of brine. CO2-soluble propoxylated alcohols will also be assessed because the polypropylene oxide oligomer is more CO2-philic than the conventional polyethylene oxide oligomer.