Improved oil recovery by adsorption–desorption in chemical flooding Q. Liu a , M. Dong a, * , W. Zhou a , M. Ayub a , Y.P. Zhang b , S. Huang b a Faculty of Engineering, University of Regina, Regina, SK, Canada S4S 0A2 b Saskatchewan Research Council, 6 Research Drive, Regina, SK, Canada S4S 3Y9 Received 10 April 2003; accepted 18 December 2003 Abstract Surfactant loss is one of the major concerns with chemical oil recovery processes in which surfactant is one of the components of the chemical formula. Surfactant loss due to adsorption on the reservoir rocks weakens the effectiveness of the injected chemical slug in reducing oil – water interfacial tension (IFT) and makes the process uneconomical. Adsorption-related interfacial tension behavior and its effect on oil recovery has not been understood completely. This paper reports the investigation of oil – water IFT behavior when the chromatographic separation of the surfactant mixture occurs during surfactant/alkaline corefloods. In this work, surfactant and alkaline concentrations in the effluent of corefloods and oil – water interfacial tension were determined under different injection strategies. It was found that, in an extended waterflood following an alkaline-surfactant slug injection, surfactant desorbed into the water phase. This desorption of surfactant lasted for a long period of the waterflood. Although the concentration of the desorbed surfactant in the extended waterflood was very low, an ultra-low oil– water IFT was obtained by using a suitable alkaline concentration. Coreflood results showed that an additional 13% of the initial oil in place (IOIP) was recovered after the alkaline – surfactant injection by the synergism of the desorbed surfactant and alkaline. This result indicates that the efficiency and economics of a chemical flood could be improved by utilizing the desorbed surfactant during extended waterflood processes. D 2004 Published by Elsevier B.V. Keywords: Adsorption; Desorption; Surfactant; Interfacial tension; Chemical flood; Enhanced oil recovery; Coreflood 1. Introduction In order to mobilize residual oil trapped by capil- lary forces in oil reservoirs, many enhanced oil recovery (EOR) methods rely on reducing the oil– water interfacial tension (IFT) to extremely low val- ues, often to 10  2 dyn/cm or less (Morgan et al., 1979). Examples of improved oil recovery processes utilizing surfactants include micellar polymer flood- ing, alkaline/surfactant/polymer (ASP) flooding, and alkaline surfactant foam flooding (Berger and Lee, 2002). Surfactant reduces the IFT between the brine and residual oil and therefore increases the capillary number. The capillary number, N c , is used to express the ratio of viscous forces to capillary forces acting on entrapped oil drops within porous media. A capillary number of about 10  6 is found after a typical water- flood, and this number can be increased by two or 0920-4105/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.petrol.2003.12.017 * Corresponding author. Tel.: +1-306-337-2269; fax: +1-306- 585-4485. E-mail address: mingzhe.dong@uregina.ca (M. Dong). www.elsevier.com/locate/petrol Journal of Petroleum Science and Engineering 43 (2004) 75 – 86