New Surfactants and Cosolvents Increase Oil Recovery and Reduce Cost Karasinghe A. Nadeeka Upamali, Pathma Jith Liyanage, Sung Hyun Jang, Erin Shook, Upali P. Weerasooriya, and Gary A. Pope, University of Texas at Austin Summary Scientific understanding of how the molecular structures of surfactants and cosolvents affect microemulsion properties greatly speeds up the process of arriving at optimal chemical formulations for enhanced recovery of a specific crude oil. With the main emphasis on reducing the chemical cost of the formulations, novel surfactants and cosolvents have been developed and shown to have superior performance. We have synthesized and tested surfactants with different hydrophobe sizes and structures varying from ultrashort to large to satisfy a variety of crude- oil requirements over a wide range of reservoir conditions. The cosolvents and surfactants with ultrashort hydrophobes offer advantages such as short equilibration time for the microemulsion formation and lower microemulsion viscosity. Chemical formulations developed using these chemicals have shown excellent performance with very low cosolvent and surfactant retention in cores. Low retention means less chemicals can be used to recover each barrel of oil from the reservoir. These chemicals can be made commercially at low cost. Through use of these new developments, the chemical cost per barrel of oil is low enough to be economically viable, even at low crude-oil prices. Introduction Chemical enhanced oil recovery (EOR) (CEOR) offers the potential for recovering a large fraction of the remaining oil after waterflood- ing. Significant breakthroughs in the recent past have made it possible to use CEOR over a wider range of reservoir conditions. These advances have also led to higher efficiencies, lower surfactant retention, and lower chemical costs to recover incremental oil. Pandey et al. (2016) reports the results of a recently completed and very successful alkaline/surfactant/polymer (ASP) pilot with new surfactants after a polymer flood (Pandey et al. 2012; Prasad et al. 2014). Many of the recent advances in CEOR technology have come about because of the development of new or improved surfactants and cosolvents (Iglauer et al. 2009; Adkins et al. 2010, 2012; Barnes et al. 2010, 2012; Yang et al. 2010; Puerto et al. 2012; Ge and Wang 2015; Ju¨ rgenson et al. 2015; Liyanage et al. 2015a, b). New chemical structures are beneficial in several ways, but especially in terms of increasing robustness and reducing surfactant retention and chemical cost. The emphasis in this paper is on new hydrophobes with varying carbon-chain lengths and degrees of branching in both surfactants and cosolvents. Different crude oils prefer different surfactant-hydrophobe types, so various classes of surfactants with different hydrophobe sizes and structures, varying from very large to medium to very short, are needed (Arf et al. 1987; Puerto et al. 2012). Large-hydrophobe Guerbet alkoxy sulfate and carboxylate surfactants have been shown to yield ultralow interfacial tension (IFT) and high oil recovery, even for crude oils with a very high equivalent alkane carbon number (Adkins et al. 2010, 2012; Lu et al. 2014). The midpoint bent oleyl hydrophobe of medium chain length (C 18 ) has been found to be highly effective for many crude oils when an appropriate number of pro- poxy (PO) and ethoxy (EO) units are incorporated in either the sulfate or the carboxylate surfactant molecule (Liyanage et al. 2015b). By varying the number of POs and EOs for different hydrophobe structures, the surfactants can be fine-tuned for enhanced interac- tion at the oil/water interface (Gale et al. 1978; Aoudia et al. 1995; Min˜ana-Perez et al. 1996; Barnes et al. 2010; Solairaj et al. 2012; Song et al. 2015). Light crude oils are more compatible with short hydrophobes such as tridecyl alcohol (C 12–13 ) rather than large- or medium-sized hydrophobes. Furthermore, the type and structure of the surfactant molecule as well as that of the hydrophobe is very im- portant for microemulsion formation (not macroemulsion), hardness tolerance, thermal stability, and favorable microemulsion rheology. In this context, we have also revisited the anionic Gemini surfactant as a new class of large-hydrophobe EOR surfactant (Menger and Littau 1993; Gao and Sharma 2013; Kamal 2016). Many conventional cosolvents (e.g., simple alcohols and their low-numbered ethoxymers) improve aqueous stability, facilitate faster equilibration of the microemulsion phase, lower the microemulsion viscosity, mitigate the formation of ordered/condensed structures, and broaden the low-IFT salinity window, among other benefits (Salter 1977; Abe et al. 1986; Sanz and Pope 1995; Frank et al. 2007; Levitt et al. 2009; Sahni et al. 2010; Fortenberry et al. 2015). However, cosolvents also tend to increase the IFT, although some cosol- vents with lipophilic and hydrophilic linkers can decrease the IFT (Acosta et al. 2004; Graciaa et al. 1993a, b). We show that 2-ethylhexanol-xPO sulfate (2-EHS), a surfactant with an ultrashort hydrophobe, has both surfactant and cosolvent properties. Our results indicate that branching in the 2-EHS structure not only minimizes the formation of viscous phases and gels, but also enhances the rapid equilibration of microemulsion and minimizes or eliminates the need for cosolvent. We have also studied the effect of the incorporation of PO (more branching) followed by EO groups into previously developed cosolvents such as iso-butanol (IBA) and phenol ethoxylates. Highly branched di-isopropyl amine ethoxylate could be another attractive cosolvent candidate that has not been tested before in CEOR. In the following sections, we demonstrate how novel cosolvents in combination with surfactants can impart superior CEOR perform- ance such as reduced chemical usage while maintaining high surface activity. This does not mean to suggest that cosolvents are always needed or should always be used. There are exceptional cases where all the stringent performance requirements such as aqueous stabil- ity can be met without using cosolvent. In such cases, the decision whether to use cosolvent in the chemical slug and/or the polymer drive should be dependent on whether it lowers the chemical cost per barrel of incremental oil production by reducing the surfactant use, or from its other performance benefits for the particular reservoir conditions. Experiment Materials and Procedure Materials. Anionic Surfactants. Oleyl, tridecyl alcohol, and 2-ethylhexanol alkoxylates were supplied by an industry manufacturer and subsequent sulfation and carboxymethylation steps were performed in the laboratories of the University of Texas at Austin. Alcohol Copyright VC 2018 Society of Petroleum Engineers This paper (SPE 179702) was accepted for presentation at the SPE Improved Oil Recovery Conference, Tulsa, 11–13 April 2016, and revised for publication. Original manuscript received for review 23 June 2017. Revised manuscript received for review 1 February 2018. Paper peer approved 5 February 2018. 2018 SPE Journal 1