Favorable Attributes of Alkali-Surfactant-Polymer Flooding
- Danhua Zhang | Shunhua Liu | Wei Yan | Maura Puerto | George J. Hirasaki (Rice University) | Clarence A. Miller (Rice University)
- Document ID
- Society of Petroleum Engineers
- SPE/DOE Symposium on Improved Oil Recovery, 22-26 April, Tulsa, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 5.4.1 Waterflooding, 4.1.5 Processing Equipment, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.4 Scale, 5.5.2 Core Analysis, 2.4.3 Sand/Solids Control, 5.3.2 Multiphase Flow, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.1 Phase Behavior and PVT Measurements, 1.8 Formation Damage, 4.1.2 Separation and Treating
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A laboratory study of the alkali-surfactant-polymer (ASP) process was conducted. It was found from phase behavior studies that for a given synthetic surfactant and crude oil containing naphthenic acids, optimal salinity depends only on the ratio of the moles of soap formed from the acids to the moles of synthetic surfactant present. Adsorption of anionic surfactants on carbonate surfaces is reduced substantially by sodium carbonate but not by sodium hydroxide. The magnitude of the reduction with sodium carbonate decreases with increasing salinity.
Particular attention was given to a surfactant blend of a propoxylated sulfate having a slightly branched C16-17 hydrocarbon chain and an internal olefin sulfonate. In contrast to alkyl/aryl sulfonates previously considered for EOR, alkaline solutions of this blend containing neither alcohol nor oil were single-phase micellar solutions at all salinities up to and even exceeding optimal salinity with representative oils. Phase behavior with a West Texas crude oil at ambient temperature in the absence of alcohol was unusual in that colloidal material, perhaps another microemulsion having a higher soap content, was dispersed in the lower phase microemulsion. Low interfacial tensions existed with the excess oil phase only when this material was present in sufficient amount in the spinning drop device. Overall solubilization of oil and brine for this system was high, leading to low interfacial tensions over a wide salinity range and to excellent oil recovery in both dolomite and silica sand packs. These experiments were performed with surfactant concentrations as low as 0.2 wt%. It was necessary that sufficient polymer be present to provide adequate mobility control and that salinity be below the value at which phase separation occurred in the polymer/surfactant solution.
A one-dimensional simulator was developed to model the process. By calculating transport of soap formed from the crude oil and injected surfactant separately, it showed that a gradient in local soap-to surfactant ratio develops during the process. This gradient increases robustness of the process in a manner similar to that of a salinity gradient in a conventional surfactant process. Predictions of the simulator were in excellent agreement with the sand pack results.
Although both injection of surfactants and injection of alkaline solutions to convert naturally occurring naphthenic acids in crude oils to soaps have long been suggested as methods to increase oil recovery, key concepts such as the need to achieve ultralow interfacial tensions and the means for doing so using microemulsions were not clarifed until a period of intensive research between about 1960 and 1985.[1-3] Most of the work during that period was directed toward developing micellar-polymer processes to recover residual oil from sandstone formations using anionic surfactants. However, Nelson et al. recognized that in most cases the soaps formed by injecting alkali would not be at the "optimal" conditions needed to achieve low tensions. They proposed that a relatively small amount of a suitable surfactant be injected with the alkali so that the surfactant/soap mixture would be optimal at reservoir conditions. With polymer added for mobility control the process would be an alkali-surfactant-polymer (ASP) flood. The use of alkali also reduces adsorption of anionic surfactants on sandstones because the high pH reverses the charge of the positively charged clay sites where adsorption occurs.
The initial portion of a Shell field test, which did not use polymer, demonstated that residual oil could be displaced by an alkaline/surfactant process. Several ASP field projects have been conducted with some success in recent years in the US.[6,7] Pilot ASP tests in China have recovered more than 20% OOIP in some cases, but the process has not yet been applied there on a large scale.
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