Surface Active Agents: Unleashing Their Potential in Enhanced Oil Recovery Processes

1. Introduction

The global demand for oil remains high, and with the depletion of easily accessible oil reserves, enhanced oil recovery (EOR) techniques have become increasingly crucial. Among these techniques, the use of surface active agents has emerged as a promising approach to improve oil recovery from reservoirs. Surface active agents, also known as surfactants, have the unique ability to reduce the interfacial tension between oil, water, and rock surfaces in reservoirs, thereby enhancing the displacement of oil. This article delves into the role, types, properties, and applications of surface active agents in EOR processes, along with challenges and future perspectives.

2. Fundamentals of Surface Active Agents

2.1 Chemical Structure

Surfactants are amphiphilic molecules, meaning they have both a hydrophilic (water – loving) and a hydrophobic (water – hating) part. The hydrophilic part can be anionic (such as sulfates, sulfonates), cationic (quaternary ammonium salts), non – ionic (polyethylene glycol ethers), or zwitterionic (betaines). The hydrophobic part is usually a long – chain hydrocarbon. Figure 1 shows a general representation of the chemical structure of a surfactant.

[Insert Figure 1: General Chemical Structure of a Surfactant]

2.2 Key Properties

| Surfactant Type | IFT (mN/m) with Crude Oil and Water |

|—|—|

| Anionic Surfactant (Sodium Dodecyl Sulfate) | 1 – 5 |

| Non – ionic Surfactant (Triton X – 100) | 2 – 6 |

| Cationic Surfactant (Cetyltrimethylammonium Bromide) | 3 – 8 |

| Surfactant | CMC (mmol/L) |

|—|—|

| Sodium Dodecyl Sulfate | 8.2 |

| Triton X – 100 | 0.24 |

| Cetyltrimethylammonium Bromide | 0.92 |

3. Mechanisms of Surfactants in Enhanced Oil Recovery

3.1 Interfacial Tension Reduction

The primary mechanism by which surfactants enhance oil recovery is through IFT reduction. When the IFT between oil and water is high, the capillary forces in the porous rock prevent efficient oil displacement. By reducing the IFT, surfactants enable the oil to be mobilized more easily. The relationship between IFT and capillary pressure (\(P_c\)) is given by the Young – Laplace equation:\( P_c=\frac{2\gamma\cos\theta}{r} \)

where \(\gamma\) is the IFT, \(\theta\) is the contact angle, and \(r\) is the radius of the capillary. A decrease in \(\gamma\) leads to a decrease in \(P_c\), allowing oil to flow more freely.

3.2 Wettability Alteration

Surfactants can adsorb onto the rock surface and change its wettability. In a water – wet reservoir, the oil tends to be trapped in the pores. By making the rock surface more oil – wet or intermediate – wet, surfactants can release the trapped oil and improve its mobility. Figure 2 shows the change in wettability before and after surfactant addition.

[Insert Figure 2: Wettability Alteration by Surfactants]

3.3 Emulsification

Surfactants can form emulsions with oil and water. In EOR, the formation of oil – in – water emulsions can be beneficial as it can improve the sweep efficiency of the displacing fluid. The emulsified oil droplets are more easily transported through the reservoir pores, reducing the tendency for fingering and channeling.

4. Types of Surfactants Used in EOR

4.1 Anionic Surfactants

Anionic surfactants are widely used in EOR due to their relatively low cost and high IFT – reducing ability. Sodium dodecyl sulfate (SDS) and alkylbenzene sulfonates are common examples. They are effective in reducing the IFT between oil and water, but their performance can be affected by the presence of divalent cations (\(Ca^{2 +}\), \(Mg^{2+}\)) in the reservoir brine, which can cause precipitation.

4.2 Non – ionic Surfactants

Non – ionic surfactants, such as polyethylene glycol ethers, are less sensitive to divalent cations. They have good solubility in both water and oil, which makes them suitable for EOR applications. Their CMC values are generally lower than those of anionic surfactants, which means they can form micelles at lower concentrations. However, their high cost compared to anionic surfactants can limit their widespread use.

4.3 Cationic Surfactants

Cationic surfactants, like cetyltrimethylammonium bromide (CTAB), have a positive charge on the hydrophilic head. They can strongly adsorb onto negatively charged rock surfaces, which can be useful for wettability alteration. However, their use in EOR is limited due to their high cost and potential for strong adsorption, which can lead to surfactant loss in the reservoir.

4.4 Zwitterionic Surfactants

Zwitterionic surfactants, such as betaines, have both positive and negative charges within the same molecule. They exhibit good stability in high – salinity and high – temperature environments, making them suitable for harsh reservoir conditions. However, their relatively complex synthesis and high cost restrict their large – scale application. Table 3 compares the four types of surfactants in terms of their properties and EOR performance.

Surfactant Type	Cost	IFT – reducing Ability	Sensitivity to Divalent Cations	Wettability Alteration	Stability in Harsh Conditions
Anionic	Low	High	High	Moderate	Moderate
Non – ionic	High	High	Low	Good	Good
Cationic	High	Moderate	Low	High	Low
Zwitterionic	High	High	Low	High	High

5. Factors Affecting Surfactant Performance in EOR

5.1 Reservoir Temperature

Higher reservoir temperatures can affect the stability and performance of surfactants. Some surfactants may degrade or lose their effectiveness at high temperatures. Table 4 shows the maximum recommended temperatures for different types of surfactants.

Surfactant Type	Maximum Recommended Temperature (°C)
Anionic	80 – 100
Non – ionic	100 – 120
Cationic	60 – 80
Zwitterionic	120 – 150

5.2 Reservoir Salinity

The salinity of the reservoir brine can also impact surfactant performance. High salinity can cause surfactant precipitation, especially for anionic surfactants. On the other hand, some surfactants are designed to be salt – tolerant and can maintain their effectiveness in high – salinity environments.

5.3 Rock – Surfactant Interaction

The interaction between the surfactant and the rock surface is crucial. Strong adsorption of the surfactant onto the rock can lead to surfactant loss and reduced effectiveness. The type of rock (sandstone, limestone, etc.) and its surface properties can influence this interaction.

6. Field Applications and Case Studies

6.1 The North Sea Oil Fields

In the North Sea oil fields, surfactants have been used in EOR projects. A study by Shell (2018) showed that the use of a blend of anionic and non – ionic surfactants in a water – flooding process increased the oil recovery by 15 – 20%. The surfactants were able to reduce the IFT and improve the wettability of the reservoir rocks, leading to better oil displacement.

6.2 The Daqing Oilfield in China

In the Daqing oilfield, China’s largest oilfield, polymer – surfactant flooding has been successfully implemented. A combination of anionic surfactants and partially hydrolyzed polyacrylamide polymers has been used. The surfactants helped in reducing the IFT, while the polymers improved the sweep efficiency. According to a report by PetroChina (2020), this EOR method increased the oil recovery by more than 20% compared to conventional water – flooding.

6.3 The Permian Basin in the United States

In the Permian Basin, surfactants have been used in EOR processes to target the tight oil reservoirs. A research by ExxonMobil (2021) demonstrated that the use of zwitterionic surfactants in combination with carbon dioxide flooding improved the oil recovery from tight formations. The zwitterionic surfactants were able to maintain their performance in the high – temperature and high – pressure conditions of the tight reservoirs.

7. Challenges and Future Perspectives

7.1 Challenges

7.2 Future Perspectives

8. Conclusion

Surface active agents have shown great potential in enhancing oil recovery processes. Their unique properties, such as interfacial tension reduction, wettability alteration, and emulsification, make them valuable tools in the oil industry. Different types of surfactants, including anionic, non – ionic, cationic, and zwitterionic surfactants, have their own advantages and limitations in EOR applications. Field applications and case studies have demonstrated the effectiveness of surfactants in increasing oil recovery. However, challenges related to cost, surfactant loss, and environmental concerns need to be addressed. The future of surfactant – based EOR holds promise with the development of novel surfactants, the application of nanotechnology, and the integration of surfactants with other EOR techniques.

References