Journal of Petroleum Science and Engineering 190 (2020) 107106 Available online 20 February 2020 0920-4105/© 2020 Elsevier B.V. All rights reserved. Partitioning of non-ionic surfactants between CO 2 and brine Albert Barrabino a, * , Torleif Holt a , Erik Lindeberg b a Petroleum Department, SINTEF Industry, NO-7465 Trondheim, Norway b CO 2 Technology, NO-7030 Trondheim, Norway A R T I C L E INFO Keywords: Mobility control of CO 2 Foam Enhanced oil recovery Aquifer storage Surfactant partitioning ABSTRACT The partitioning of non-ionic surfactants in a CO 2 /synthetic brine system was studied for a selection of surfac- tants at reservoir conditions for CO 2 enhanced oil recovery and aquifer storage. Alkyl and alkylphenol ethox- ylates with different degrees of branching in their hydrophobic moiety were chosen. Generally, higher temperature and pressure promoted increased solubility in CO 2 . Branching of the hydrophobic moiety tends to favour CO 2 solubility (higher partition coeffcient). Highly branched moieties were found to hinder solubility probably due to a decrease of their conformational entropy. The addition of an aromatic ring connecting the ethoxylate moiety and the hydrophobic moiety seemed to have an adverse effect at lower temperatures. For two surfactants, the effect of concentration on partitioning was also studied. The partition coeffcient decreased for increasing concentrations until a plateau was reached above the corresponding surfactant critical micelle con- centration (CMC). This may indicate micelle formation both in the CO 2 and in the aqueous phase. 1. Introduction CO 2 is becoming a prominent solvent for different applications. Its foams and emulsions with water are of interest due to its potential for various applications (Johnston and Rocha, 2009). Two large scale ap- plications are enhanced oil recovery (EOR) and subsurface sequestra- tion. However, these methods encounter various technical challenges. The relatively low density and low viscosity of CO 2 can lead to gravity segregation and viscous fngering giving early CO 2 breakthrough and poor volumetric sweep effciency. The result of this may be low oil re- covery (in EOR) and reduced utilisation of the storage capacity (in CO 2 sequestration) (Solbakken et al., 2013; Tsau; Grigg, 1997). Early breakthrough can be counteracted by decreasing the mobility of the CO 2 . This can be achieved by increasing the viscosity using ad- ditives to the CO 2 or by dispersing the CO 2 into another fuid (brine). It is not easy to increase CO 2 viscosity. The additives (direct thickeners) must solubilize in CO 2 and provide self-interactions that can give the desired viscosity enhancement. During decades efforts have been made trying to fnd suitable additives (Enick et al., 2012). However, the best CO 2 ad- ditives found are not practical due to their high costs and detrimental environmental impact. The second method for decreasing CO 2 mobility is through creating dispersed systems. CO 2 -in-brine dispersions (hereafter called foams) may have high apparent viscosities depending on the surfactant used. Foams can also be formed and stabilized by nanoparticles. Even though nanoparticles adsorb more strongly at interfaces, larger energy input is required to form foam. This criterion is not met at reservoir fow ve- locities which typically do not exceed few feet/day (except close to in- jection wells) (Binks, 2002; Espinosa et al., 2010; San et al., 2016; Yu et al., 2012). Surfactant-stabilized foam is so far the most promising CO 2 mobility reduction method. However, this also faces several challenges. One potential problem is the presence of oil which may destabilize foam through different mechanisms including spreading and entering phe- nomena (Manlowe and Radke, 1990; Schramm and Novosad, 1990; Wasan et al., 1994). However, the sensitivity of foam to oil may have both advantageous and disadvantageous consequences. During miscible CO 2 fooding, the foam may be more stable in pores where the oil has already been displaced helping to divert CO 2 within the reservoir to places where the oil is not displaced. There, the lamella will collapse and release CO 2 in the CO 2 /oil front. On the other hand, if the stability of the foam is sensitive to minute oil residues it can have an adverse impact on foam propagation (Vassenden et al., 2000). Another challenge to face during surfactant-stabilized foam fooding is surfactant depletion due to adsorption on the pore walls of the reservoir rock. The adsorption will be determined by the pore wall mineralogy and charge, type of surfactant, pH, temperature, ionic strength, electrolyte concentration (Bera et al., 2013; Curbelo et al., * Corresponding author. E-mail address: albert.barrabino@sintef.no (A. Barrabino). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol https://doi.org/10.1016/j.petrol.2020.107106 Received 1 November 2019; Received in revised form 28 January 2020; Accepted 18 February 2020