Thickening supercritical CO2 at high temperatures with rod-like reverse micelles

Earlier studies demonstrated the ability of some fluorinated surfactants to form rod-like reverse micelles with the ability to thicken water/supercritical CO2 (scCO2) mixtures at temperatures below 45 ºC [Langmuir 26 (2010) 83–88. Soft Matter 8 (2012) 7044–7055. Colloids and Surfaces B, 168 (2018), 201–210.]. Such viscosity enhancement of scCO2 is known to increase sweep efficiency for oil recovery with CO2 flooding. However, temperatures of up to ∼100 ºC in conventional reservoirs are much higher than those employed in laboratory studies, and tend to weaken inter- and intra-molecular interactions between surfactant molecules, discouraging rod-like reverse micelle formation. With the aim of designing surfactants which form rod-like reverse micelles and thicken CO2 at high temperatures, this study examined phase behavior, nanostructures of reverse micelles and thickening ability of double ω-hydroperfluorocarbon-tail anionic surfactants in W/scCO2 mixtures at temperatures of 35 - 75 ºC and pressure of 80 - 400 bar with different water-to-surfactant molar ratios (W0). The measured CO2 viscosity increased by 1.9–2.2 × for double-chain surfactants M(di-HCF6)x (counterion Mx+ = Ni2+ and Co2+) at 40 mM, over the experimental temperature range. On the other hand, the shorter chain H(CF2)4CH2 twin-tail surfactants M(di-HCF4)x and Na(di-HCF6) gave only 1.1–1.5 × viscosity enhancements. The maximum thickening ability of M(di-HCF6)2 was at W0 = 10 in the W0 range of 5–20 75 ºC and 350 bar. High pressure and high temperature small-angle neutron scattering (SANS) was used determine the micellar structure in these systems, and rod micelles of aspect ratios of 4.5–6.5 were found. The results clearly suggest that ω-hydroperfluorohexyl-tails and divalent counterions induce the formation of rod-like reverse micelles in W/CO2 mixtures, even at high temperatures commensurate with in-reservoir conditions.