(706b) Improving CO2-EOR in Shale Reservoirs Using Dilute Concentrations of Wettability-Altering Nonionic Surfactants during Lab-Scale Huff-and-Puff

The aim of this work is to determine if the efficacy of CO₂-based enhanced oil recovery (EOR) for unconventional reservoirs, which is based on numerous mechanisms (e.g. diffusion, oil swelling, oil viscosity reduction, pressure maintenance, etc.), can be improved with dilute concentrations of CO₂-soluble wettability-altering additives. Oil recovery during water-based hydraulic fracturing and water-based EOR in unconventional liquid reservoirs (ULRs) can improve when ionic and nonionic surfactants that greatly enhance the water-wetness of rock surfaces are added to the water. In this study, we assess whether surfactant-based wettability alteration can increase the amount of incremental oil recovered during the CO₂ huff and puff process. Although most ionic surfactants are essentially CO₂-insoluble, there are numerous inexpensive oil-soluble and water-soluble nonionic surfactants that are CO₂-soluble to ∼0.1 – 1.0% at formation conditions. This work focuses on altering the wettability of unconventional rock away from oil-wet using these non-ionic surfactants, and then assessing whether the additional amount of oil recovered during CO₂ huff and puff is significant considering the cost of the surfactant addition.

The solubility of various nonionic surfactants (Indorama SURFONIC® N-100, an ethoxylated nonylphenyl alcohol with 10 ethylene oxide (EO) groups; Indorama SURFONIC® TDA-9, -6 and -3, ethoxylated branched tridecyl alcohols with 9, 6 or 3 EO groups, respectively; Indorama SURFONIC® TDA-8PO, a tridecyl propoxylated alcohol with 8 propylene oxide groups; and Indorama SURFONIC® L12-6, an ethoxylated normal C10-C12 natural alcohol with 6 EO groups) in CO₂ has been measured between 25 – 100 °C. The surfactants exhibit a solubility of roughly 1 wt% at pressures of 2000 - 5000 psia, with lower pressures required for lower temperatures, lower surfactant concentrations, and increased oil solubility of the surfactant.

The rock samples were first aged by via immersion in dead Eagle Ford crude oil at high temperatures and high pressure for a prolonged period of time. Then, the oil-wetness was verified by measuring the contact angle of water droplets on the shale exposed to air. To determine CO₂-induced or CO₂-surfactant solution-induced changes in wettability, the oil-wet, oil-saturated shale disk was immersed in CO₂ or a CO₂-surfactant solution for 16h at 4000 psi and 80℃. No significant changes in contact angles were observed in samples immersed in CO₂, whereas immersion of the shale in CO₂-surfactant solutions showed dramatic changes in wettability toward water-wet for some of the surfactants. Small cylindrical shale samples were then used in huff and puff tests to assess oil recovery. Ultimate oil recovery after 8 huff and puff cycles increased substantially when some of the surfactants were added to the CO₂, especially during the early cycles. A comparison of the performance of these surfactants will be presented.