(270d) Improving CO2-EOR in Shale Using Dilute Amounts of CO2-Soluble, Wettability-Altering Surfactants

The objective of this work is to determine if the efficacy of CO₂-based EOR techniques for unconventional reservoirs can be enhanced with dilute concentrations of wettability-altering additives. Addition of ionic or nonionic wettability-altering surfactants to brine to improve oil recovery from unconventional formations has been previously studied for water-based hydraulic fracturing operations and subsequent water-based EOR projects. In this work, nonionic surfactants are dissolved in CO₂ (which is a reasonably strong solvent for several types of oil-soluble or water-soluble nonionic surfactants) to attain similar favorable shifts in wettability toward increased water wetness. In essence, this project aims to add favorable shifts in wettability to the suite of mechanisms (e.g. miscibility with oil, oil swelling, oil viscosity reduction, extraction of oil into CO₂, CO₂ diffusion into oil, solution pressure drive upon depressurization) that have been previously attributed to the success of CO₂ recovering oil from shale samples.

The solubility of several nonionic surfactant (which also exhibit water-solubility) was determined at concentrations between 0.1 – 1.0 wt% in CO₂. The cloud point curves for several surfactants were determined at temperatures representative of unconventional formations.

Contact angle measurements were performed on Bakken and Eagle Ford rock samples using dead oil from both formations and synthetic brine. Shale samples were first aged in oil at high temperature and pressure to restore oil-wet characteristics to the rock. The contact angles of water and oil droplets on these samples were determined. Then, shale samples were immersed in water (control test) or an aqueous nonionic surfactant solution containing 0.1 – 1.0 wt% surfactant to verify that the previously reported shifts toward water-wetness occur. Oil-wet shale samples were also immersed in high pressure, single phase CO₂-nonionic surfactant solutions containing 0.1 – 1.0 wt% surfactant, after which contact angles of oil and water droplets on the rocks were determined.

After assessing the change in wettability attributable to the addition of surfactant, lab-scale CO₂ huff ‘n puff EOR experiments were conducted on cylindrical, oil-saturated, oil-wet cores immersed in either pure CO₂ or CO₂-surfactant solution. The amount of additional oil recovery attributable to the introduction of dilute amounts of CO₂-soluble surfactants was assessed.

To the best of the authors’ knowledge, this is the first attempt to dissolve dilute concentrations of wettability-altering chemicals in CO₂ for EOR in unconventional liquid reservoirs. Although dilute amounts of water-soluble/CO₂-soluble surfactants have been previously dissolved in CO₂ for improved mobility control in conventional formations via the in situ generation of CO₂-in-water foams as the surfactant partitions into the brine, this study focuses on the addition of nonionic surfactants to CO₂ with the sole objective of having the surfactant adsorb onto the rock and favorably alter wettability.