AC and AS Low-Interfacial-Tension Surfactants: Applications and Selection for Oil Displacement and Reservoir Cleaning
Compare eMAI™ AC and AS by form, active content, reference concentration and kerosene interfacial-tension specifications, with a four-step screening workflow and application boundaries for oil displacement and near-wellbore reservoir cleaning.
Full Article
In the middle and late stages of waterflood development, residual oil becomes trapped in reservoir pores by capillary forces and is difficult to mobilize. Low-interfacial-tension surfactants help improve oil recovery by reducing oil–water interfacial tension, modifying wettability and promoting oil-film detachment. eMAI™ AC and eMAI™ AS from eMAI New Materials belong to this product family. This article explains their actual differences, how to select between them and the boundaries of their applications.
01 Why does oil–water interfacial tension matter in oil displacement?
Crude-oil flow through reservoir pores is influenced by capillary forces, viscous forces, interfacial forces and rock wettability. After waterflooding, some crude remains trapped in small pores and pore throats as residual oil.
Under conventional waterflooding conditions, oil–water interfacial tension is generally in the range of 10–40 mN/m and varies substantially with crude-oil composition, including naturally surface-active resins, asphaltenes and naphthenic acids. The capillary number is then very low, and capillary forces pin residual oil within the pores, making it difficult to mobilize.
Once a surfactant enters the formation, it adsorbs at the oil–water interface and reduces interfacial free energy: lower interfacial tension → weaker capillary effects → higher capillary number → easier deformation, detachment and migration of residual oil. However, increasing the capillary number is only one of the necessary conditions for residual-oil mobilization. The ultimate result also depends on reservoir heterogeneity, sweep efficiency, wettability and the oil–water mobility ratio. Surfactant adsorption onto rock surfaces also reduces the effective concentration.
Evaluating a surfactant for oil displacement therefore requires more than asking how low it can reduce interfacial tension. The more important question is whether it can maintain interfacial activity and remain effective at the actual formation temperature and salinity and with the actual crude oil.
02 Introducing the two products
eMAI™ AC is a homogeneous, colorless to pale-yellow liquid with approximately 30% active content. Its interfacial-tension specification against kerosene is ≤0.1 mN/m at 25°C and a concentration of 0.3%. The liquid form is convenient for on-site solution preparation and metering.
eMAI™ AS is a pale-yellow paste with 68%–75% active content. Its interfacial-tension specification against kerosene is ≤1.0×10⁻² mN/m at 30°C and a concentration of 100–200 ppm.
| Parameter | eMAI™ AC | eMAI™ AS |
|---|---|---|
| Chemical system | Anionic/nonionic surfactant blend | Petroleum sulfonate, nonionic surfactants and specialty solubilizers |
| Appearance and form | Homogeneous, colorless to pale-yellow liquid | Pale-yellow paste |
| pH | 6.0–9.0 | 6.0–9.0 |
| Active content | 30%±2.0 | 68%–75% |
| Interfacial tension against kerosene | ≤0.1 mN/m (25°C, 0.3%) | ≤1.0×10⁻² mN/m (30°C, 100–200 ppm) |
| Reference concentration | Approximately 0.3% | Approximately 100–200 ppm |
| Key characteristics | Liquid form for convenient solution preparation and metering | Lower interfacial-tension specification and high active content |
| Packaging | 200 L drums / IBCs | 200 L drums / IBCs |
Table · Comparison of the principal specifications
Table notes: ① The interfacial-tension values are specification upper limits against kerosene under the stated test conditions. The products are tested at different temperatures and concentrations; ‘≤’ denotes an upper limit only and does not establish their actual performance gap. ② The 0.3% and 100–200 ppm values are specification-test concentrations based on the commercial product, not active-component concentrations. ③ Results with actual crude oil and formation water may differ from those obtained with kerosene.
03 What are the key differences between AC and AS?
1. Different reference concentrations and interfacial-tension specifications
AC's specification is ≤0.1 mN/m at 0.3% (approximately 3000 ppm), whereas AS's is ≤1.0×10⁻² mN/m at 100–200 ppm. AS's reference concentration is therefore approximately 1/15–1/30 of AC's, and its interfacial-tension specification is approximately one order of magnitude lower. However, the test conditions differ and both values are upper limits. A rigorous performance comparison requires measurements under identical conditions.
Table · Reference dosage ratio (approximately 1/15–1/30 of AC's)
| Reference dosage (commercial-product basis) | Ratio relative to AC |
|---|---|
| AC reference dosage (approximately 0.3% / 3000 ppm) | 1 |
| AS reference dosage (approximately 100–200 ppm) | Approximately 1/15–1/30 |
2. Active content affects storage, transport and solution preparation
AS's active content is more than twice that of AC, with more active material per unit volume, which is advantageous for transport and storage. Although AS is a paste, its low-temperature flowability, pump metering and on-site dissolution rate are also described as convenient.
| At a glance | eMAI™ AC | eMAI™ AS |
|---|---|---|
| Form | Liquid | Paste |
| Active content | Approximately 30% | 68%–75% |
| Reference concentration | Approximately 0.3% | 100–200 ppm |
| Key point | Convenient solution preparation and metering | Relatively rapid dissolution despite its paste form |
Table · The two products at a glance
04 How should the products be selected for practical use?
There is no universally optimal low-interfacial-tension surfactant independent of reservoir conditions. Screen candidates before selecting a final formulation, using the following four steps:
| Evaluation step | Tests and decision criteria |
|---|---|
| Step 1 · Compatibility and phase-behavior screening | Use actual formation water to evaluate solubility, phase behavior and salt tolerance, focusing on Ca²⁺ and Mg²⁺. Exclude systems that show cloudiness, precipitation or phase separation. |
| Step 2 · Interfacial-tension scan | Interfacial tension is a property of the surfactant–oil–water system. Use actual formation water, actual crude oil and the actual reservoir temperature to identify the concentration window giving the minimum interfacial tension, and compare AC and AS on that basis. |
| Step 3 · Stability and compatibility evaluation | Age the system for an extended period at reservoir temperature and check for interfacial-tension rebound or precipitation. If polymers or alkali are used, test for flocculation, precipitation, phase separation and performance deterioration. |
| Step 4 · Adsorption and coreflood validation | Quantify adsorption and retention on reservoir rock, which directly affect economics. Finally, verify oil recovery in a coreflood sequence of waterflooding → chemical flooding → subsequent waterflooding. Favorable static interfacial-tension results do not guarantee improved oil recovery in the field. |
| Application requirement | Evaluate first |
|---|---|
| On-site convenience of liquid handling, solution preparation and metering is a priority | eMAI™ AC |
| Low concentration, a lower interfacial-tension specification and reduced transport volume are priorities | eMAI™ AS |
| The process requirements can be met within a conventional concentration window | eMAI™ AC |
| Factory blending is available | eMAI™ AS |
Table · Quick-reference selection guide
05 Can the products be used for reservoir cleaning?
There is a mechanistic basis for using both products in reservoir cleaning. Here, ‘reservoir’ specifically means the underground oil-bearing formation, including near-wellbore blockage removal and cleaning. Cleaning surface tanks, pipelines and filter media falls under oily industrial cleaning. The mechanisms are related, but the evaluation criteria differ.
| Problems suited to surfactant treatment | Not suited / other technologies required |
|---|---|
| Retained crude oil, oil films and residual oil in the near-wellbore region | Carbonate, sulfate and silica scale |
| Oil-wetting damage and increased flow resistance caused by organic contamination | Inorganic particulate deposition and severe inorganic deposits |
| Reservoir cleaning: detachment of crude oil adhering to rock debris | (Acidizing, chelation-based scale removal or particulate-blockage removal) |
06 Important qualifications
- The specification data were obtained against kerosene under defined conditions. Results may change with actual crude oil and formation water; measurements for the target reservoir must determine suitability.
- Interfacial tension has a minimum-value concentration window; a higher concentration is not always better. Insufficient concentration gives inadequate adsorption, while excessive concentration may reach a plateau or even cause interfacial-tension rebound. Further dosing gives diminishing returns in interfacial tension, although it may still affect emulsification and wetting.
- Different applications depend on different mechanisms. Oil recovery by imbibition relies on capillary forces, so excessively low interfacial tension does not necessarily benefit imbibition. Heavy-oil viscosity reduction through emulsification depends on emulsion type and oil/water ratio; low interfacial tension does not imply effective viscosity reduction.
- Salt and temperature tolerance both have limits. Conditions outside those limits require prior experimental validation. Blends with polymers or alkali require compatibility testing of the complete formulation; unverified on-site blending is not recommended.
Conclusion
AC is a liquid blend that is convenient for solution preparation and metering. AS is a highly concentrated paste with an interfacial-tension specification of ≤10⁻² mN/m against kerosene at a reference concentration of just 100–200 ppm. The actual value of either product must be assessed with the specific crude oil, formation water, temperature and reservoir conditions.
For an actual project, use the following workflow: screen AC and AS in parallel → evaluate compatibility and phase behavior → scan interfacial tension → evaluate stability and adsorption → validate by coreflood → select the product and concentration. Suitability, product choice, concentration and whether to blend with other chemicals must all be determined from experimental data under actual reservoir conditions.
Note | The data are taken from eMAI New Materials product specifications—upper limits against kerosene under defined test conditions—and are provided only as a selection reference. For samples or evaluation of oil and water samples, contact the eMAI New Materials application technology team.
| Product quick reference | eMAI™ AC | eMAI™ AS |
|---|---|---|
| Appearance / active content | Liquid / 30%±2.0 | Paste / 68%–75% |
| Interfacial tension | ≤0.1 mN/m | ≤1.0×10⁻² mN/m |
| Storage | Cool, dry, ventilated location, away from ignition sources; 24 months | Cool, dry, ventilated location, away from ignition sources; 24 months |