Emulsification Mechanisms in Alkali-Surfactant-Polymer (ASP) Flooding Enhanced Oil Recovery

Abstract

Alkali-surfactant-polymer flooding (ASP) is one of most attractive chemical EOR techniques in view of incremental recovery upon waterflooding. Emulsification mechanism in ASP flooding is very important but not well understood. Effects of emulsification in ASP flooding is reviewed based on reported field tests as well as laboratory studies, especially on progress and understanding of ASP flooding in China, where the only commercial ASP flooding has been conducted. The main mechanism of ASP flooding can be summarized to the displacement efficiency improvement due to the ultra-low interfacial tension (IFT) between oil and water and the sweep efficiency increase due to mobility control technique by polymer viscosifying and emulsification effect. Emulsification is crucial in ASP flooding since all ASP flooding pilots were seen emulsification with different extent. Oil emulsifying and emulsion profile controlling was regarded as important in ASP flooding mechanisms. Laboratory tests showed that emulsification increase the oil recovery by 5%-6% when emulsified compared with not emulsified. Experience from Daqing oilfield in China reported that contribution of emulsifying ability of ASP system to oil displacement efficiency can be as high as 30%. Factors affecting emulsification included the properties of oil and water, type and concentration of chemical, water cut, external force applied and permeability. Effects of alkali to ASP system and oil emulsification was carefully studied. IFT was an important but not crucial factor to emulsification. Lower IFT at higher alkali concentration promoted easier emulsification, while too low IFT was detrimental to emulsion stability due to the competitive adsorption of in-situ surfactants and added surfactant in oil/water interface. Addition of polymer was beneficial to the stability of emulsion and the effect of associate polymer recent was obvious. ASP field tests in Daqing oilfield verified the emulsify ability of NaOH was almost the same as Na2CO3, which was quite different from laboratory studies. In all development stages of ASP flooding, emulsification was seen. Injection pattern and water cut affected emulsification. Separate injection of alkali and surfactant as one system, while polymer as the other make higher degree of O/W type emulsion. Emulsification in main slug and vice slug showed difference characteristics, which was attributed to the relative content of surfactants in different water cut stage.