eMAI™ AS/AC Ultra-Low-Interfacial-Tension Surfactants: Residual-Oil Mobilization Mechanisms and Product Selection
Explain the capillary trapping of residual oil, the boundaries of ultra-low interfacial tension, wettability control and emulsified transport, and compare eMAI™ AS, AC330 and AC430P for field selection.
Full Article
Oilfields after years of waterflooding face a familiar situation: overall water cut rises above 80% or 90%, most produced fluid is water, yet large amounts of crude oil remain underground. Recovery in China's main waterflooded oilfields is often only 30%~40%, leaving more than half of the original oil in place unrecovered by water injection.
Why is this oil so difficult to wash out, and what can surfactants do? Combining the fundamentals of chemical flooding with the technical data for eMAI™ AS, AC330 and AC430P, this article explains when ultra-low interfacial tension is needed, why it helps and how to select a product.
Why oil left after waterflooding is difficult to "wash out"
Where is the oil trapped after waterflooding?
Crude oil is not stored in underground "cavities". It is distributed through interconnected pores and pore throats of different scales. After long-term water injection, water preferentially follows low-resistance channels, while two types of oil remain in the swept pores: isolated oil droplets trapped at pore throats and oil films attached to rock surfaces.
The root cause is capillary resistance. An oil droplet must overcome capillary force to pass through a narrow pore throat. The force is proportional to oil–water interfacial tension σ and inversely proportional to pore-throat radius r: Pc = 2σ·cosθ / r.
Pore-throat radius is a physical constraint and contact angle is governed by wettability. The variable that chemicals can change over a wide range is σ. In ordinary oil–water systems, interfacial tension is usually 20~30 mN/m. If σ does not change, injected water simply passes through the center of the pore and cannot dislodge the oil film on the pore wall.
First distinguish displacement efficiency from sweep efficiency
Displacement efficiency describes how much oil can be removed from pores already contacted by water; sweep efficiency describes how much of the reservoir is actually swept by injected water. Surfactants mainly address the former. In strongly heterogeneous reservoirs with severe water channeling, polymers or other components are also needed for mobility control.
This explains a basic fact: surfactants are rarely used alone. They are combined with polymers in binary or ternary flooding systems, with each component serving a different role—one washes oil off the rock and the other controls the water's mobility.
What is ultra-low interfacial tension, and which conditions truly need it?
How low must interfacial tension be to qualify as ultra-low?
A surfactant molecule has a hydrophilic end and an oil-loving end. Above a certain concentration it spontaneously adsorbs at the oil–water interface: the hydrophobic chain holds the oil phase while the hydrophilic head holds the water phase, turning a sharp interface into a gradual transition layer and greatly reducing interfacial free energy.
| Interfacial-tension level | Engineering meaning |
|---|---|
| 1~10 mN/m | Generally considered a conventional reduction |
| Around 10⁻¹ mN/m | Low interfacial tension |
| 10⁻² mN/m or below | Ultra-low interfacial tension, about three orders of magnitude below ordinary oil–water systems |
| Around 10⁻³ mN/m | The ideal range pursued in chemical flooding |
A system with low surface tension but high oil–water interfacial tension may be an excellent wetting agent, but it is not a qualified oil-washing agent. The two metrics must be kept separate.
One common misunderstanding must be clarified: surface tension and interfacial tension are different. The former is tension between a liquid and air (water is about 72 mN/m) and reflects wetting, spreading and penetration. The latter is tension between oil and water and determines whether an oil film can be detached.
A rough industry classification is: 1~10 mN/m is a conventional reduction; around 10⁻¹ mN/m is low interfacial tension; 10⁻² mN/m or below is ultra-low interfacial tension, about three orders of magnitude below the original value; and the 10⁻³ mN/m range is the ideal target in chemical flooding.
Why does this order of magnitude matter? Consider the capillary number, Nc = v·μ / σ, the ratio of viscous driving force to capillary force. Reservoir velocity v and viscosity μ offer limited room for adjustment; σ in the denominator is the variable that can change substantially. When σ falls by three orders of magnitude, the capillary number rises by three orders of magnitude, largely neutralizing capillary resistance and allowing droplets trapped in pore throats and oil films on pore walls to loosen.
| Concept | Meaning and common mistake |
|---|---|
| Surface tension | Tension between a liquid and air; water is about 72 mN/m. It reflects wetting, spreading and penetration |
| Interfacial tension | Tension between oil and water; it determines whether an oil film can be detached |
| Keep them separate | A system with low surface tension but high oil–water interfacial tension may be an excellent wetting agent, but not a qualified oil-washing agent |
Six conditions where it can genuinely make a difference
| Condition | Why it is needed / boundary of use |
|---|---|
| Medium-to-high water cut reservoirs where waterflooding can no longer wash out oil | In blocks with high water cut, a flattened waterflood curve and low marginal benefit from more water injection, ultra-low interfacial tension is useful when capillary trapping is the main issue. It cannot solve oil in unswept zones or severe channeling by itself |
| Binary / ternary combination flooding | Polymers increase viscosity, control mobility and expand sweep, while surfactants reduce interfacial tension and improve displacement. Alkali in a ternary system can generate petroleum soaps in situ. Compatibility with polymer, formation water, minerals and crude oil is essential |
| Spontaneous imbibition in low-permeability reservoirs | Small pore throats and strong capillarity dominate low-permeability formations. Adjusting wettability and sharply reducing interfacial tension can turn capillary force from resistance into a driving force. The value is often in imbibition rather than pushing |
| Enhanced waterflooding and pressure-drive treatments | Surfactant carried with injection water can improve displacement and reduce injection pressure. If pressure rises because of blockage or formation damage, however, it is not a universal remedy |
| Heavy-oil reservoirs | High oil viscosity and waterflood fingering can be addressed by lowering interfacial tension and using suitable emulsification to disperse the continuous oil phase into transportable droplets. This series is for heavy-oil displacement, not for reducing bulk oil viscosity; a dedicated viscosity-reduction system should be selected for that goal |
| High-temperature, high-salinity reservoirs | Salinity and calcium/magnesium ions can change solubility, adsorption and interfacial activity. eMAI™ AC is the salt-tolerant series, while AS has relatively limited salt tolerance. Salt-tolerant grades and laboratory formulation evaluation should be prioritized for high-salinity, high-calcium/magnesium water |
How does it wash out oil, and what field difficulties remain?
Four mechanisms of action
| Mechanism | Action and evaluation focus |
|---|---|
| Interfacial adsorption and tension reduction | Surfactants align at the oil–water interface and greatly reduce interfacial free energy. Within a suitable concentration, salinity and temperature window, interfacial tension can enter the ultra-low range. Formulation design is fundamentally about finding and holding this window |
| Wettability control | Oil-wet or mixed-wet rock pins oil films to pore walls. Adsorbed surfactant can shift wettability toward water-wet conditions, allowing water between the wall and film and detaching the film as a whole. In many reservoirs this is more important than tension reduction alone |
| Emulsification and transport | Once interfacial tension falls, flow shear can disperse oil into small droplets and carry an emulsion with the water phase. An overly stable emulsion increases separation difficulty and may cause emulsion blockage, so emulsification and demulsification must be evaluated together |
| Detachment and coalescence | Released droplets meet at pore throats and coalesce into larger oil segments that form a continuously flowing oil bank toward the production well. Interfacial-tension reduction makes oil detachable; wettability and mobility control make it transportable |
Four field issues that cannot be avoided
| Issue | Field implication |
|---|---|
| Adsorption loss | Surfactants adsorb onto rock minerals after entering the formation, with especially high loss on high-specific-surface-area clay minerals. Effective concentration may be insufficient at the target zone, so adsorption tests and corefloods are necessary |
| Window sensitivity | Ultra-low interfacial tension usually exists only within a narrow concentration, salinity and temperature window. Dilution, mixing with formation water and temperature changes can push the system out of that range |
| Compatibility and formation damage | Compatibility with polymers, alkali, injection water and other chemicals must be checked, along with precipitation and scale. Solid-particle blockage is especially important in low-permeability reservoirs |
| Economics | Dosage, retention loss, incremental oil and produced-fluid treatment costs must be assessed together. Low interfacial tension does not guarantee incremental oil or project profitability |
Three eMAI™ products: specifications, positioning and extended applications
Technical specifications of the three products
| Item | eMAI™ AS | AC330 | AC430P |
|---|---|---|---|
| Composition | Petroleum sulfonate, nonionic surfactant and specialty solubilizer | Anionic and nonionic surfactants | Anionic and nonionic surfactants |
| Appearance | Pale-yellow paste | Colorless to pale-yellow uniform liquid | Colorless to pale-yellow uniform liquid |
| Active content | 68.0%~75.0% | 30%±2.0% | 30%±2.0% |
| pH | 6.0~9.0 | 6.0~9.0 (1% solution) | 6.0~9.0 (1% solution) |
| Surface tension | — | ≤28.0 mN/m (25℃, 0.3%) | ≤30.0 mN/m (25℃, 0.3%) |
| Interfacial tension (kerosene) | ≤1.0×10⁻² mN/m (30℃, 100~200 ppm) | 6.0~9.0 mN/m (25℃, 0.3%) | 6.0~9.0 mN/m (25℃, 0.3%) |
How to interpret these three sets of data
| Product positioning | Technical interpretation and boundary |
|---|---|
| AS: primary interfacial-tension reducer | At a low concentration of 100~200 ppm, interfacial tension can reach the 10⁻² mN/m range. Its high active content makes it suitable for the core task of mobilizing residual oil, but its salt tolerance is relatively limited and it is better suited to reservoirs with moderate salinity |
| AC series: salt tolerance and wetting | At 0.3%, interfacial tension is 6.0~9.0 mN/m, a low-interfacial-tension level. Its surface tension can fall below 28~30 mN/m, with strong wetting and imbibition performance and good salt tolerance, making it more suitable for wettability control, imbibition, high-salinity reservoirs and cleaning |
| Common selection boundary | The three products are tested at different concentrations and temperatures, so their flooding ability cannot be ranked directly from these data. "Ultra-low" is a result under specified conditions, not an inherent product attribute. Final selection must follow the target crude oil, formation water, temperature and core-test results |
Where else can they be used besides oil displacement?
The value of these surfactants comes from the general property of interfacial activity. The two formulated systems can also extend to the following areas, subject to sample testing for specific suitability and dosage:
| Area | Application value |
|---|---|
| Specialty industrial cleaning | Removes heavy oil, grease, wax deposits and metalworking residues by lowering the interfacial tension between contaminant and substrate so the cleaning fluid can penetrate, detach, emulsify and disperse them |
| Metalworking and surface treatment | Emulsifying and wetting component in emulsified cutting fluids and rust-preventive cleaners |
| Textiles, leather, paper, pesticides and coatings | Wetting, penetrating, emulsifying and dispersing component to improve processing performance and emulsion stability |
| Other oil-production operations | Petroleum sulfonate surfactants have an application basis in crude-oil demulsification, oil-well wax control, near-wellbore blockage removal, oil–water separation and emulsifying lubrication for drilling fluids |
What matters most during selection?
| Selection principle | Practical point |
|---|---|
| Read metrics together with test conditions | Interfacial tension must be evaluated with concentration, temperature, oil phase (kerosene or target crude), and aqueous-phase salinity. A number without its conditions has no practical meaning |
| Put salt tolerance first | For high-salinity and high-calcium/magnesium blocks, prioritize grades with verified salt tolerance and evaluate compatibility with polymers and alkali together |
| Test before designing the scheme | Interfacial tension, wettability, imbibition recovery, adsorption loss, emulsification and demulsification difficulty can only be established by laboratory and pilot tests |
| Calculate input and output | Only a combination of low dosage and high interfacial activity can maintain economic performance through oil-price fluctuations |
Conclusion
Ultra-low-interfacial-tension surfactants are not "magic chemicals." Their role can be summarized in one sentence: by orienting molecules at the interface, they reduce the capillary resistance that blocks crude-oil flow to a negligible level and make oil that waterflooding could not wash out mobile again.
A truly effective oilfield chemical is not defined by attractive numbers on a test report. It is a product that maintains performance under real formation conditions and ultimately delivers verifiable recovery improvement and a reasonable economic return.