AOS in Enhanced Oil Recovery and Foam Flooding

Sodium alpha-olefin sulfonate (AOS, CAS 68439-57-6) is one of the surfactants used in chemical enhanced oil recovery (EOR), where the fluid has to survive harsh conditions: high salinity, high hardness and elevated reservoir temperature. AOS is chosen because it is a sulfonate – stable, hard-water tolerant and a good foamer. This article covers why AOS is used in EOR, how it works, and what to specify.

1. Where chemical EOR fits

After primary and secondary recovery, a large share of the oil can remain in the reservoir. Chemical EOR – surfactant flooding, polymer flooding and alkaline-surfactant-polymer (ASP) flooding – aims to mobilise that remaining oil by lowering the interfacial tension between oil and water, improving sweep, and changing wettability.

Method	Main chemical	Role
Polymer flooding	Polyacrylamide (PAM)	Improves mobility ratio and sweep
Surfactant flooding	AOS and other surfactants	Lowers oil-water interfacial tension so trapped oil mobilises
ASP flooding	Alkali + surfactant + polymer	Combined mechanisms; alkali generates in-situ soap
Foam flooding	AOS as foamer + gas	Diverts flow from swept zones to unswept oil

2. Why AOS is used in EOR

Requirement in the reservoir	Why AOS fits
High salinity / high hardness brine	A sulfonate; calcium and magnesium tolerance is far better than for many sulfates and carboxylates
Elevated temperature	The carbon-sulfur bond is thermally stable – no sulfate-ester hydrolysis
Good foaming	Rich, stable foam for foam flooding
Low interfacial tension	Effective at reducing oil-water IFT when matched to the crude and brine
Adsorption and cost	Availability at scale from China supports large flood volumes

3. How it works

Interfacial tension (IFT). The surfactant adsorbs at the oil-water interface and drives IFT down – orders of magnitude below that of plain water – so the oil can be mobilised as an emulsion rather than trapped by capillary forces.
Winsor type behaviour. At the right salinity and surfactant concentration, a microemulsion (Winsor III) forms with ultralow IFT – the target of a surfactant flood.
Wettability. Surfactants can shift a rock surface toward water-wet, improving oil displacement.
Foam. For foam flooding, the surfactant stabilises a gas-liquid foam that blocks high-permeability swept channels and diverts flow into unswept oil.

4. Typical conditions and dosage

Parameter	Typical practice (project-specific)
Surfactant concentration in the flood	About 0.2 – 0.5 wt% active
Brine	Hard, saline; the formulation is tuned to the reservoir salinity and hardness
Temperature	Set by the reservoir (sulfonate stability is a key advantage)
Formulation	Often a blend: AOS with other surfactants (for example betaines or nonionics) and a co-solvent
Foam quality	Set by gas-liquid ratio, foam quality and foam stability testing

EOR formulations are always developed in the laboratory against the actual crude and brine (phase behaviour and core-flood tests) – the figures above are starting points, not a recipe.

5. AOS vs other EOR surfactants

Surfactant	Strengths	Notes
AOS	Hard-water and thermal stability; good foaming; available at scale	Cost sits above commodity petroleum sulfonates
Petroleum sulfonates	Low cost; widely used historically	Variable composition; hard-water tolerance can be limited
Alkylbenzene sulfonates	Cost-effective	Hard-water tolerance varies with structure
Betaines / amphoterics	Very hard-water tolerant; often blended with sulfonates	Higher cost; used as co-surfactants

The practical answer is usually a blend: a sulfonate (such as AOS) for stability and foam, with a co-surfactant to fine-tune the phase behaviour to the reservoir.

6. What to specify when buying AOS for EOR

Active matter – the liquid grade (34-36%) is usually preferred for large floods, because it is easy to dilute and dose; check the exact level.
Brine compatibility – confirm the surfactant tolerates your salinity and hardness (phase-behaviour tests).
Thermal stability – confirm performance at your reservoir temperature.
Form – liquid in IBCs, ISO tanks or (for field-scale floods) bulk; powder where logistics favour it.
Documents – per-batch COA and SDS; third-party inspection on request.

Frequently Asked Questions

Why is AOS used in enhanced oil recovery?

Because it is a sulfonate: it tolerates hard, saline brine and high reservoir temperatures better than sulfate or carboxylate surfactants, foams well, and is available at the scale a flood requires.

What is the dosage in a surfactant flood?

Typically about 0.2-0.5 wt% active, but the concentration is set by the phase-behaviour and core-flood testing on the actual crude and brine.

Is AOS used in foam flooding?

Yes – its rich, stable foam makes it a common foaming agent for foam flooding and for drilling or workover foams.

Can AOS be used in high-salinity reservoirs?

It has much better hard-water tolerance than sulfate and carboxylate surfactants, which is why it is chosen for hard, saline reservoirs – but confirm against your specific brine by test.