FAVORABLE ATTRIBUTES OF ALKALI-SURFACTANT-POLYMER FLOODING George J. Hirasaki, Rice University with Clarence Miller, Danhua Leslie Zhang, Shunhua Liu, Wei Yan, Maura Puerto, Brent Biseda SPE ATW on Chemical Flooding September 25-29, 2005 Daqing, China ABSTRACT Alkali-surfactant-polymer (ASP) flooding for EOR offers prospects of lower surfactant requirements and a more robust process. However, it is more complex than conventional micellar-polymer flooding because phase behavior and interfacial tension are dictated by relative amounts of injected surfactant and naphthenic soaps formed from naphthenic acids in the crude. The most significant benefit of alkali, especially sodium carbonate, is lower surfactant adsorption. In sandstones high pH reverses the charge of positively charged clay sites on which anionic surfactants adsorb. In carbonate formations carbonate is a potential determining ion and can reverse the initial positive charge. We present data showing the resulting decrease in anionic surfactant adsorption. Phase behavior and interfacial tensions (IFT) for a system containing two branched-chain anionic surfactants, Na2CO3/NaCl brine and a crude oil were investigated. Optimal salinity, where the middle phase microemulsion has equal amounts of oil and water, was determined for different surfactant concentrations and water-to-oil ratios and found to depend only on the soap-to-surfactant ratio. Below optimal salinity, sometimes a thin layer accumulated between the lower-phase microemulsion and excess oil. This layer might be an emulsion, microemulsion or liquid crystal. The IFT of excess oil with the equilibrated lower-phase microemulsion was high. However, ultra-low IFT was observed when material from the thin layer was dispersed in the lower-phase or added to it. Our hypothesis is that this layer has a higher soap-tosurfactant ratio than the lower phase and is more lipophilic, enabling it to reduce tension between the lower-phase and oil. If this behavior is widespread, ASP offers promise for a more robust system with ultra-low IFT over a wide salinity range. One-dimensional simulations of the ASP process showed development of a gradient in soap-to-surfactant ratio, which produced a shift from over-optimum to under-optimum conditions. With adequate mobility control predicted oil recovery was high. This soap-to-surfactant ratio gradient provides a design strategy for maintaining a region of low tension similar to the salinity gradient in conventional surfactant flooding.