Summary
A laboratory study of the alkaline-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 approximately
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. Some birefringence was observed
near and above optimal conditions. While this phase behavior is somewhat
different from the conventional Winsor phase sequence, 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 sandpacks. The sandpack experiments were performed with surfactant
concentrations as low as 0.2 wt% and at a salinity well below optimal for the
injected surfactant. 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 1D simulator was developed to model the process. By calculating transport
of soap formed from the crude oil and injected surfactant separately, it showed
that injection below optimal salinity was successful because a gradient in
local soap-to-surfactant ratio developed 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 sandpack results.
Background
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 clarified until a period of intensive research
between approximately 1960 and 1985 (Reed and Healy 1977; Miller and Qutubuddin
1987; Lake 1989). 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. (1984) 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
alkaline-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 (Falls et al. 1994). Several ASP field projects have been conducted
with some success in recent years in the US (Vargo et al. 2000; Wyatt et al.
2002). 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 (Chang
et al. 2006).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
20 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
2 May 2007
- Manuscript approved:
16 May 2007
- Version of record:
20 March 2008
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