ASP Processes: Wide Range of Conditions for Good Recovery

摘要

Design of an alkaline-surfactant-polymer (ASP) process requires knowledge of the amount of soap formed under alkaline conditions from naphthenic acids in the crude oil. We show here for several crude oils that the acid number determined by nonaqueous phase titration is approximately twice that found by hyamine titration of a highly alkaline aqueous phase used to extract soaps from the crude oil. This acid number by soap extraction should provide a better estimate than nonaqueous phase titration. This soap interacts with the injected surfactant to form surfactant films and microemulsion droplets during an ASP process. In a previous paper (Liu et al., 2006), an unusually wide range of salinities of ultra-low oil-water interfacial tensions (IFTs) was found for one alcohol-free crude oil/anionic surfactant system under alkaline conditions where naphthenic soaps were present. This surprising and favorable behavior has now been found to exist with the same surfactant blend and another crude oil. In the same paper, a one-dimensional simulator for the ASP process was presented. Here this ASP simulator has been used for various acid contents, injected surfactant concentrations, slug sizes and salinities to show that high recoveries of waterflood residual oil (>90%) can be expected for a wide range of near-optimal (Winsor III) and under-optimum (Winsor I) conditions for a constant salinity process, even with relatively small slug sizes. A key factor leading to this good performance is development of a gradient in soap-to-surfactant ratio, which assures that a displacement front with ultralow IFT forms and propagates through the formation. Similar high recoveries can be attained for certain over-optimum (Winsor II) conditions but only for much larger slug sizes owing to the tendency for surfactant to partition into the oil phase and become retarded. Large dispersion such as might be expected for field conditions can significantly reduce recovery for small surfactant slugs even for near- optimal and under-optimum conditions. However, this problem can be overcome by injecting the slug and/or drive at salinities below reservoir salinity, thereby creating a salinity gradient.