Introduction

With an increasing world population, the demand for energy is becoming a critical issue. According to “International Energy Outlook” (2021), it is projected that world energy consumption will increase by almost 50% between 2020 and 2050 [1]. The need for an increasing supply of fossil fuels will also continue to play a substantial role in fulfilling a significant part of the world’s energy needs in the years ahead, mainly in the field of transportation [2]. However, globally, the oil and gas industry has been marked by turbulent conditions, characterized by the instability of oil prices, which makes it challenging to accurately predict the future economic climate [3]. Considering that the viable lifespan of an oil well, from its initial planning to its final abandonment, is on average 15 years but can sometimes extend up to 50 years, it is important for oil production and exploration activities to consider different economic scenarios when developing viable and consistent plans [4].

Conventional oil recovery mechanisms can be divided into two stages. The first stage, known as primary oil recovery, involves the extraction of oil through a combination of differential pressure and artificial lift techniques. However, this stage only contributes approximately 10–15% to the total amount of oil present in the reservoir (Original Oil in Place - OOIP) [5]. When the differential pressure becomes insufficient to extract oil, a new recovery mechanism needs to be implemented. In the second stage, water is injected into the reservoir through injection wells, increasing the differential pressure and dragging the oil towards the producing wells in a process known as waterflooding or secondary oil recovery. However, water mobility is greater than that of oil, leading to the formation of preferential paths in the reservoir rock, known as fingers, which compromise the recovery of hydrocarbons. This phenomenon is caused by the high interfacial tension within the water/oil system, defined as the amount of energy required to stabilize the interface between the two fluids [6]. High values of interfacial tension result in high capillary pressure, meaning that more energy is needed in the system for the injection fluids to flow into the pores, resulting in lower displacement efficiency in the rock.

In cases where the rock is oil-wet or has mixed wettability, water injection becomes even less effective. Wettability determines the interactions between rock and liquids in reservoirs and is recognized as one of the main factors controlling residual oil saturation [7]. It is worth noting that even when conventional recovery methods are implemented, approximately 30% of the OOIP remains in reservoirs worldwide [8]. Given the limitations of these conventional recovery methods, it becomes necessary to utilize other techniques to recover the remaining oil in the reservoir.

Enhanced Oil Recovery (EOR) can be classified into chemical injection, thermal injection, gas injection, or microbiological injection. Thermal EOR methods reduce the viscosity of fluids in the reservoir and are widely used in onshore fields but have disadvantages such as unwanted gas emissions, excessive energy consumption, and higher costs. Gas injection techniques are more economical but are dependent on a continuous supply of gas and may face mobility issues in the reservoir [9, 10]. Finally, there is chemical injection, a proposed technique for advanced oil recovery that involves the application of surfactants, polymers, and alkalis as primary features of the recovery process.

Chemical injection began in the 1960s but gained prominence in the 1980s when oil prices increased, leading to the need to understand the mechanisms behind this technique. Its effectiveness lies in chemically enhancing injection fluids to increase the yield of recovered oil. The use of polymers increases the efficiency of volumetric scanning by reducing the water-oil mobility ratio, while surfactants and alkalis reduce residual oil saturation by decreasing the water-oil interfacial tension [11].

Although synthetic compounds are the predominant choice for this injection method, the use of bioproducts offers several advantages over synthetics, including low toxicity and biodegradability. Moreover, many synthetic compounds are petroleum derivatives, and the economic variability of their source may compromise their viability as an EOR mechanism. In contrast, bioproducts are derived from biomass and microorganisms [12]. Microbial Enhanced Oil Recovery (MEOR) utilizes microorganisms that generate chemical products to promote oil production [9].

The industry’s growing commitment to using techniques that have less impact on the environment, combined with government incentives through regulatory measures, has led researchers to study more ecological mechanisms for enhanced oil recovery. Biosurfactants are compounds with both hydrophilic and hydrophobic properties, enabling them to act at the interface between two fluids such as oil and water, reducing interfacial tension [13, 14]. The potential applications of these bioproducts in the oil industry are vast, as they can be utilized not only in enhanced oil recovery but also in other areas such as improving oil transportation in pipelines, cleaning storage tanks, and mitigating oil spills [15, 16].

Biosurfactants can be interesting candidates for enhanced oil recovery, as they offer many advantages over synthetic surfactants, including biodegradability and low toxicity. Additionally, they have other characteristics such as the availability of raw materials, strong resistance to severe salinity and temperature conditions, and a high capacity to reduce water/oil interfacial tension [14, 17]. Biosurfactants can be produced as byproducts of bacteria like Pseudomonas aeruginosa and Bacillus subtilis [18,19,20], fungi such as Candida bombicola [21] and Trichoderma reesei [22], and plants like Zyziphus spina christi [23, 24] and Cordia myxa [25]. Recent laboratory-scale studies have shown that biosurfactants are effective for EOR, but large-scale production is still limited by high costs [26]. To address this, researchers are actively working to reduce these costs by using cheaper substrates, such as agro-industrial waste [27, 28], selecting efficient strains [18, 29], improving productivity [28, 30], and optimizing processing [31].

This article provides information on the applications of ex-situ biosurfactants from both microbial and biomass sources in enhanced oil recovery since 1999. Although the studies were performed on a laboratory scale using carbonate and sandstone rocks, it is possible to assess the potential of this natural product in reducing interfacial tension and increasing oil displacement. Furthermore, this article discusses the main organisms that produce biosurfactants, such as Bacillus and Pseudomonas bacteria, plants, and fungi, and analyzes the results of critical micellar concentration, surface tension, and interfacial tension.

Enhanced oil recovery

When oil reservoirs can no longer produce through pressure differential alone, water or gas injection is applied as a conventional method of secondary recovery. Water is cost-effective for this method and easy to inject into the reservoir, making waterflooding a very attractive mechanism.

The first water injection project dates back over 100 years, but it wasn’t until the 1950s that this technique saw more widespread use. Typically, oil recovery initially increases with water injection until a plateau is reached, and then it starts to decline due to increased water production. When oil production becomes less economically viable through water injection alone, applying alternative special production methods becomes beneficial. These methods are called EOR [10].

The EOR mechanisms can also be applied as a secondary method in specific cases, where it is already known that waterflooding will not be efficient. On account of this, the term “tertiary recovery” to designate EOR mechanisms fell into disuse. The term Improved Oil Recovery (IOR) can also be found in literature, but more generically, incorporating several activities such as reservoir characterization and management, among others [32].

EOR processes can be classified into the following injection categories: chemical, thermal, gas, and, more recently, microbial [33], as illustrated in Fig. 1. The injected fluids increase the reservoir energy and interact with the rock/oil/water system, creating favorable conditions for oil displacement.

Fig. 1
figure 1

 Classification of enhanced oil recovery techniques

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Chemical injection for EOR involves the injection of polymers, surfactants, alkalis, foam, and more recently, nanofluids, which facilitate oil recovery [11]. These techniques have proven to be quite effective, showing substantial results in mature and depleted reservoirs and have been supported by laboratory studies [9, 34].

Gas injection, which began in 1864 in the United States, remains a widely applied method [9]. Gaseous organic compounds, carbon dioxide, and nitrogen are the most commonly used gases in this technique [35]. The success of gas injection relies on two critical factors: displacement efficiency and sweep efficiency. Displacement efficiency refers to the percentage of oil displaced by the injected fluid, while sweep efficiency takes into account the volume of the reservoir penetrated by the injected fluid [36]. Gas injection is primarily used in light and condensed oil carbonate reservoirs [33].

Thermal injection processes utilize thermal energy to increase the reservoir’s temperature and reduce the oil’s viscosity [37]. In addition to modifying the physical properties of the oil, thermal injection induces chemical changes in oil components through cracking and dehydrogenation, among other reactions. These properties make thermal injection more suitable for heavy oil reservoirs, where it is predominantly applied [38].

MEOR can be applied in two ways: in-situ and ex-situ. MEOR in-situ offers versatility in application. Biosurfactant-producing microorganisms, along with the necessary nutrients to sustain their metabolism, are injected into the reservoir to yield the desired chemical products for oil recovery. Alternatively, if analysis confirms the presence of microorganisms in the reservoir, only the required nutrients need to be injected to sustain continued production of the necessary chemical products [39]. MEOR ex-situ is a powerful technique when implemented on a field scale, providing the advantages of a comparatively short application time and being a more sustainable alternative to synthetic surfactants. In this approach, bioproducts (biosurfactants, biopolymers, biomass, and biosolvents) are produced outside the reservoir before being injected into the well for enhanced oil recovery, sometimes combined with other fluid compositions [40, 41]. This type of biosurfactant injection will be discussed in more detail in the following topics.

Laboratory surfactant studies

Defining the laboratory techniques used to investigate the effects of surfactants and biosurfactants is fundamental for comprehending the studies carried out in the literature. The techniques employed include surface and interfacial tension analyses, rock wettability through fluid/rock contact angle measurements, and oil displacement testing and analysis.

One of the initial analyses that provides a clear indication of the surfactant’s potential is the study of surface tension. At the atomic level, it is more energetically favorable for molecules to be surrounded by others with similar molecular characteristics. They are attracted by Van der Waals forces, hydrogen bonds, or even metallic bonds (such as with mercury) [42]. At the surface/interface, the region becomes unstable due to limited interaction among similar molecules. Therefore, work is required to reduce this limited interaction and bring the molecules from the bulk to the surface. In this context, surface tension or interfacial tension can be defined as the amount of work required to bring molecules from the bulk to the surface. When surfactant molecules cover this surface or interface, there is a reduction in surface/interfacial tension. The degree of surfactant adsorption at the interface depends on both the surfactant’s structure and the nature of the phases involved. Thus, there is no universally optimal surfactant that is effective in all possible systems. However, it can be stated that a good surfactant must have low solubility in the bulk of liquids, as the molecules need to be concentrated at the interface between them [43].

There is a plateau where the concentration of molecules on the surface affects the reduction of surface tension, eventually reaching a maximum effect. This point is called the critical micelle concentration (CMC), where surfactant molecules begin to form micelles in the bulk. CMC can be determined by using surfactant solutions at varying concentrations and measuring the surface tension values of each solution. Below the CMC, surface tension values decrease with increasing surfactant concentration in the system. Above the CMC, the surface tension value remains practically constant regardless of the surfactant concentration applied to the system, as the surfactant concentration at the interface becomes saturated. The amount of micelles present influences the solubility of organic hydrocarbons and oils in aqueous solutions and the viscosity of the system but does not affect the surface tension [44]. Therefore, the lower the CMC, the lower the amount of surfactant required to have an effect on interfacial tension. Another way to indirectly measure the CMC value is through the observation of the critical micelle dilution (CMD). This technique is usually applied to biosurfactants and involves diluting a concentrated fluid into different volumes and determining their respective surface tensions. For concentrations above the CMD, the surface tension remains nearly constant, while below the CMD, the tension varies and is proportional to the concentration present [45].

In the laboratory, the most commonly used techniques to measure surface and interfacial tension are the Du Nouy ring or Wilhelmy plate, hanging drop, and spinning drop methods, which are illustrated in Fig. 2.

Fig. 2
figure 2

Techniques for measuring surface and interfacial tension: (a) Du Nouy ring method / Wilhelmy plate method, (b) Hanging drop method, and (c) Spinning drop method

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The Du Nouy ring method involves the insertion of a ring below the surface of a liquid. The ring is then moved upward across the interface between the liquid and the air (or from a liquid of higher density to one of lower density). The tensiometer measures the force required to release the ring from the surface or interface of the substances. The Wilhelmy plate method is very similar and can usually be performed on the same tensiometer. When the plate is lowered to touch the liquid, surface tension produces negative pressure on the plate and pulls it down. The measured force is then used to calculate the surface tension of the liquid [46].

The hanging drop technique involves quantifying the profile of a hanging drop of a liquid suspended in another immiscible liquid. The balance between gravitational forces and interfacial forces determines the profile of the drop. Although considered a simple technique, the hanging drop method is effective and highly applied in laboratories [47].

The spinning drop method involves introducing a drop of a liquid into another liquid of higher density in a horizontal tube that rotates around its own axis at a known speed. The drop undergoes deformation, becoming elongated. The interfacial tension is then determined through calculations based on the difference in fluid density, rotational velocity, and deformation of the droplet [48].

Contact angle tests are used to calculate rock-fluid interactions. This technique involves applying a drop of fluid to a surface and determining the contact angle formed between the two phases, as showed in Fig. 3. If the rock is wetted by the fluid, it will spread out on the surface. However, if the rock is not wettable by the fluid, the drop will tend to retain a shape as close to a sphere as possible [49]. In studies of EOR, it is interesting to observe how the injected fluids will affect the wettability of the system because the lower the oil-wettability of the rock, the greater the volume of oil detachment and thus an increase in displacement efficiency.

Fig. 3
figure 3

(a) Contact angle measurement of oil on rock slab in an air medium, and (b) oil under rock slab in a water medium

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Oil displacement tests are conducted in an environment meant to simulate that of the reservoir, with pressure and temperature differences from ambient conditions. These tests can be spontaneous imbibition or forced displacement (coreflooding) tests, illustrated in Fig. 4. In imbibition tests, a rock plug is saturated with formation water and an oleic phase, which can be either crude oil or mineral oil. It is then placed in a graduated container called an Amott cell, where the rock is submerged in the injection fluid to be studied. The fluid enters the pores of the rock through spontaneous imbibition and displaces the oil contained therein. This test can last for weeks and even months [50].

Fig. 4
figure 4

Schematic illustration of (a) spontaneous imbibition and (b) forced displacement (coreflooding) tests

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In coreflooding tests, the rock is prepared in a similar way and placed in a core holder, where the fluid to be studied is injected using a pump, producing forced displacement. Usually, the solid medium is a plug-shaped rock, but many authors use a column of compacted sand to simulate the porous medium of the reservoir (sand-pack) [51,52,53].

In addition to the aforementioned tests, other simpler tests in the literature have been used to characterize biosurfactants applied in EOR, such as pH, temperature, and salinity stability tests, which are properties of known reservoirs, and the emulsification index, which indicates the interfacial tension between two fluids. The following sections present studies that address the application of biosurfactants in EOR, as well as the main types of producers.

Biosurfactants

Studies on natural surfactants have been a source of motivation for the scientific community since the 1960s. Biosurfactants, primarily derived from microorganisms, have garnered industry attention due to their favorable qualities, including diverse structures, low toxicity, biodegradability, and resilience to pH, temperature, and salinity variations [54, 55]. The pharmaceutical, food, and cosmetic industries have displayed significant interest in developing biosurfactants for their products [14].

Biosurfactants encompass a group of molecules that function on surfaces and are produced by microorganisms such as bacteria and fungi, or derived from plants. Figure 5 illustrates the role of biosurfactants in oil recovery. They promote a change in wettability at the oil/rock interface, transitioning from oil-wet to mixed or water-wet conditions, thereby releasing oil trapped in the smaller pores of rocks. Additionally, biosurfactants reduce interfacial tension by emulsifying the oil into smaller droplets, which facilitates its displacement by the injected fluid [56].

Fig. 5
figure 5

Mechanism for enhanced oil recovery by biosurfactants

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This section will provide a detailed account of the primary laboratory studies focusing on the utilization of biosurfactants. The advancements in MEOR ex-situ through biosurfactant injection, wherein the product is introduced into the reservoir as a solution, will be discussed [56]. Consequently, the subsequent topics will be categorized based on the origin of the biosurfactant under examination, distinguishing between microbiological and plant-derived sources.

a. Biosurfactants of Microbial Origin

These biosurfactants are categorized based on their chemical composition and microbial source, resulting in five distinct groups: lipopeptides, glycolipids, phospholipids and fatty acids, polymeric biosurfactants, and particulate biosurfactants [15, 57]. In the literature, this classification is predominantly applied to biosurfactants of microbial origin, while plant-derived biosurfactants are commonly referred to as saponins [58]. Concerning oil recovery specifically, lipopeptides and glycolipids derived from bacteria are the most extensively employed biosurfactants [59]. Further details regarding these aspects will be elaborated upon in the subsequent sections.

i) Lipopeptides

They are recognized as a highly fascinating class of biosurfactants, both in scientific and commercial contexts. Lipopeptides are cyclic peptides that are linked to fatty acids with diverse lengths and compositions. They are commonly produced by various classes of microorganisms, including Bacillus sp. and Pseudomonas sp. [60, 61].

One of the well-known lipopeptides is surfactin, which is produced by the bacteria Bacillus subtilis. It possesses remarkably effective anionic surfactant properties, capable of reducing the surface tension of water from 72 mN/m to as low as 26 mN/m, even at low concentrations such as 0.005% w/w [62].

Makkar and Cameotra (1999) examined the characteristics of the biosurfactant produced by Bacillus subtilis bacteria at 45ºC for its potential application in EOR. At a CMC of 35 mg/L, the biosurfactant was able to reduce the surface tension of water from 72 mN/m to 28 mN/m, displaying excellent surface activity. The natural surfactant exhibited resistance in environments with pH values ranging from 4 to 10. When tested in a synthetic porous medium (sand-pack), it recovered approximately 62% of the OOIP, thereby demonstrating its EOR potential [53].

Another experiment evaluating the activity of surfactin produced by Bacillus subtilis was conducted by Schaller et al. (2004). The biosurfactant was produced using effluents from the potato process as a carbon source. They analyzed the biosurfactant’s stability through surface tension tests under various conditions, including NaCl concentration (0–10%), pH (3–10), and temperature (21ºC–70ºC). Even under extreme conditions of pH equal to 10, temperature equal to 70ºC, and salinity of 80 g of NaCl/L, the surface tension remained low at approximately 25.8 mM/m. These results indicate that this type of surfactant delivers exceptional performance, even in harsh conditions [63].

Das and Mukherjee (2007) also employed low-cost sources from potato peel as a substrate for biosurfactant production using two strains of Bacillus subtilis bacteria. They observed that both strains produced a biosurfactant with potential for application in EOR, achieving surface tensions lower than 40 mN/m. In sand-pack tests, the oil recovery ranged between 67% and 74% of the OOIP. Several studies have reported the use of low-cost carbon and nitrogen sources to reduce the production cost of lipopeptide biosurfactants [52, 64,65,66,67].

Therefore, one of the main goals when applying biosurfactant in enhanced oil recovery is to reduce its production cost. Souayeh et al. (2014) determined the minimum concentration of biosurfactant produced by Bacillus subtilis using critical micellar dilution to make the process more economically feasible. The results indicated that the interfacial tension in the system with heavy crude oil reached values of up to 1.8 mN/m, with an observed change in wettability in sandstones. Even after diluting the biosurfactant 20 times, it was still able to recover at least 14% additional oil (at the CMC value of 100 mg/L) [28].

Joshi et al. (2008) also studied biosurfactants produced by different strains of Bacillus sp. using economically sourced carbon from sugarcane molasses and whey. The biosurfactant exhibited resistance at a temperature of 80ºC for the tested period of 9 days, with pH ranging from 6 to 12 and up to 7% salinity. Precipitation occurred in a highly acidic medium (pH equal to 2), which aligns with studies conducted by other authors [68]. Sand-pack column experiments were performed, showing the recovery of approximately 30% of additional OOIP when the solution containing the biosurfactant was injected. Despite the simplicity of the sand-pack column tests, the authors considered the results positive and significant. They also suggested that low-purity biosurfactants or the fermented broth itself could be applied in the oil industry to reduce production costs, which would not be viable in the pharmaceutical or food industries [57, 67, 69, 70].

Salehi et al. (2008) compared the performance of anionic biosurfactant produced by Bacillus subtilis bacteria with cationic (C12TAB) and anionic (SLS) synthetic surfactants in oil-wet sandstones. The anionic surfactants exhibited better performance than C12TAB in all tests conducted on this type of rock, with surfactin showing superiority over SLS [71]. The study confirmed the wettability change mechanism proposed by Standnes and Austad (2000) for oil recovery, involving the formation of ionic pairs between oil and surfactant molecules. The authors further explained that surfactin’s superior performance compared to SLS may be attributed to the ionic charge density of the molecule’s head, which interacts more strongly with oil compounds [72]. Similar results were obtained by Al-Wahaibi et al. (2016) and Saito et al. (2016) [73, 74].

While most studies focus on Bacillus subtilis, other Bacillus species have also been investigated for their ability to produce biosurfactants applicable in EOR, such as Bacillus licheniformis and Bacillus mojavensis, among others. In 2010, Al-Sulaimani et al. (2010) studied biosurfactants produced by the bacteria Bacillus licheniformis using different carbon sources. The most effective biosurfactant strain was able to reduce the interfacial tension with n-heptane from 46.6 to 3.28 mN/m [75]. Joshi et al. (2015) characterized the broth containing biosurfactant produced by Bacillus licheniformis and investigated its potential for enhanced oil recovery through stability, surface tension, and coreflooding experiments. The biosurfactant exhibited excellent stability at a temperature of 85ºC for 90 days, salinity of 10% NaCl, and pH variations from 5 to 12 for 10 days. Additionally, it was capable of reducing the surface tension from 70 mN/m to 28 mN/m. In coreflooding tests with sandstone rock, the biosurfactant recovered more than 37% of additional OOIP [76].

Armstrong and Wildenschild (2012) utilized microtomography to study the effect of injecting a biosurfactant produced by Bacillus mojavensis on the wettability of glass samples. They captured images of columns with different oil wettability indices, allowing them to observe the curvature difference between the fluids in each system with predetermined wettability, indicating the interfacial tension of the water-oil system. Overall, the results demonstrated that MEOR was more effective than water injection in terms of oil recovery from systems with mixed wettability, achieving a recovery factor of between 44% and 80% of additional oil. The highest recovery factors through MEOR were observed in systems with stronger oil-wet characteristics [77].

Other experimental studies with the lipopeptide biosurfactant produced by the bacteria Bacillus mojavencis were conducted by Ghojavand et al. (2012). The results indicated that the biosurfactant could reduce the interfacial tension even in the presence of high salinity (240 g of NaCl/L) and exhibited oil recovery capabilities even in oil-wet carbonates. The addition of the biosurfactant reduced the surface tension of water from 72 mN/m to 26.7 mN/m [78].

In addition to the authors mentioned, further studies have been carried out with Bacillus licheniformis and Bacillus atrophaeus [41, 79,80,81,82,83,84,85]. Although the number of publications is substantially small compared to Bacillus subtilis, these authors have demonstrated the potential of other Bacillus species in biosurfactant production for EOR.

Table 1 presents the studies conducted on the lipopeptide biosurfactant of bacterial origin for ex-situ MEOR. As mentioned earlier, the majority of the microorganisms used belong to the Bacillus species, and the results demonstrate their resistance to harsh conditions of salinity, temperature, and pH variations. The surface tension (SF) values in aqueous media ranged from 24 to 36 mN/m, indicating excellent surface activity, while the interfacial tension (IFT) ranged from 0.05 to 5.7 mN/m. It is important to note that the interfacial tension varies based on the non-polar component as well.

Table 1 Summary of published studies on the use of microbiologically derived lipopeptide biosurfactants in ex-situ MEOR applications
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Another widely used biosurfactant in EOR research is glycolipidic surfactant, as described in the following topic.

ii) Glycolipids

Glycolipids can be rhamnolipids, sophorolipids, and trehalolipids. These types of biosurfactants are carbohydrates such as mono-, di-, tri-, and tetrasaccharides that include glucose, mannose, galactose, glucuronic acid, rhamnose, and galactose sulfate combined with long-chain aliphatic acids or aliphatic hydroxy acids [54]. The most commonly used glycolipids in EOR are produced by Pseudomonas sp.

Özdemir et al. (2004) studied and compared two rhamnolipid biosurfactants produced by Pseudomonas aeruginosa of commercial origin, analyzing their behavior against the oil phase composed of decane and hexadecane. They also studied the effect of pH on changing surface and interfacial tensions. The main results indicated that neither the CMC value nor the minimum surface tension in the CMC are significantly affected by the type of rhamnolipid. However, the superficial concentration of biosurfactants was significantly affected by modifying the pH of the medium [106].

Bordoloi and Konwar (2008) isolated the bacteria Pseudomonas aeruginosa from contaminated soil samples of three different fields in India. With a controlled growth medium, the microorganisms produced a biosurfactant that was stable at temperatures of up to 100ºC and with a pH ranging from 2.5 to 11.0. The biosurfactant managed to reduce the surface tension of the water to a value of 30 mN/m and was stable in these scenarios. The authors tested the fermented broth containing the biosurfactant in oil recovery through a sand-pack column. At ambient conditions, the biosurfactant recovered 50% of the OOIP. At 70ºC, there was a recovery of 53%, and at 90ºC, the recovery was 60% [107].

Pornsunthorntawee et al. (2008) studied two producing biosurfactant bacteria (Bacillus subtilis and Pseudomonas aeruginosa) using the same carbon sources and production techniques. The CMC of the lipopeptide biosurfactant produced by B. subtilis was 25 mg/L, while the CMC of the glycolipid biosurfactant from P. aeruginosa was 120 mg/L. In CMC, biosurfactants produced by B. subtilis and P. aeruginosa reduced the surface tension of water up to 26.4 mN/m and 28.3 mN/m, respectively. Coreflooding tests were carried out in compacted sand tubes at room temperature. After injecting distilled water, surface oil displacement tests were conducted applying two natural surfactants and three synthetic surfactants, all with a concentration three times greater than the CMC. The results indicated that the lipopeptide obtained a more substantial recovery than the glycolipid, being 61.65% and 57.01% of the OOIP, respectively. Both natural surfactants when tested recovered more than the three synthetic surfactants, which recovered 51-55% of the OOIP [87].

Duarte (2018) compared the potential of the rhamnolipid biosurfactant produced by the bacterium Pseudomonas aeruginosa with a commercial synthetic surfactant (Ultrasperse II) to evaluate its performance in enhanced oil recovery. The results showed that the rhamnolipids reduced the ST of water to values between 25.1 and 17.4 mN/m at a CMC of 89.5 mg/L, while Ultrasperse reduced the ST to 28.7–29.3 at a CMC of 1049 mg/L. Therefore, both compounds were effective in reducing interfacial tension, but the biosurfactant was considered more effective due to the more prominent results even with the lowest concentration used [108].

Sakthipriya et al. (2021) also compared the application of biosurfactants produced by Bacillus subtilis YB7 and Pseudomonas aeruginosa CPCL with synthetic surfactants SDS and CTAB. At a concentration of 200 ppm for all surfactants, biosurfactants recovered more oil (surfactin-15.43% and rhamnolipid-15.47% of the OOIP) than synthetic surfactants (CTAB-7% and SDS-8.82% of the OOIP). The study reported that the efficiency of biosurfactants was due to interfacial tension reduction [18].

There are a few studies with other types of bacteria that produce glycolipids in addition to Pseudomonas sp. In the research carried out by Zheng et al. (2012), the injection of trehalolipid produced by Rhodococcus ruber Z25 was able to reduce the surface tension of water from 68.57 mN/m to 29.54 mN/m. It also reduced the interfacial tension with n-hexadecane from 43.62 mN/m to 1 mN/m. In sand-pack applications, the fermented biosurfactant broth recovered between 8.88% and 25.78% of OOIP after conventional water injection. The authors reported mechanisms involving hydrocarbon degradation, improvement of oil mobility, change in wettability, and selective displacement [70].

Dhasayan et al. (2014) isolated the bacteria Holomonas sp. MB-30 from a marine sponge and verified the potential of the produced biosurfactant by applying it in EOR. The glycolipid showed excellent results, having a low CMC (250 mg/L) and a DMC of 1/10, which means that the broth showed good surface activity even when diluted 10 times. In addition, the saline medium emulsion containing kerosene showed stability for the studied period of 1 month at a temperature of 80 °C. In the sand-pack column tests, the biosurfactant additionally recovered 62% of the residual oil [109].

Dong et al. (2016) studied a rhamnolipid biosurfactant produced by Acinetobacter junii using NaNO3 and soybean oil as sources of nitrogen and carbon, comparing the in-situ injection with the ex-situ injection. Micromodel tests indicated that the surfactant was effective in both models, but there was greater oil recovery in the ex-situ than in-situ injection, recovering respectively 13.4% and 9.6% of the oil [110].

Bhattacharya et al. (2019) used the bacteria Ochrobactrum pseudintermedium and Bacillus cereus to produce glycolipids and glycoproteins that were subsequently applied in MEOR ex-situ. The biosurfactant reduced the oil/water interfacial tension from 46 mN/m to 14.5 mN/m. In the sand-pack column tests, there was an additional oil recovery of 40.93% at 40 °C and 46.85% at 70 °C when the biosurfactant was applied [111].

Table 2 shows several works that were carried out using glycolipid biosurfactant for EOR. The Pseudomonas species predominates among the microorganisms used, showing resistance to severe conditions of salinity, temperature, and pH variations. In aqueous media, the surface tension values ranged from 19 to 31 mN/m, indicating surface activity nearly as remarkable as lipopeptides. The interfacial tension (IFT) ranged from 1 to 21 mN/m.

Table 2 Summary of published studies on the use of microbiologically derived glycolipidic biosurfactants in ex-situ MEOR applications
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iii) Biosurfactant from fungi

Biosurfactants derived from the production of fungi and yeasts have not yet been as tested in EOR studies as those derived from bacteria and plants [117]. Most of the application of this type of biosurfactant is concentrated in the pharmaceutical, cosmetic, and food industries. Despite their promising characteristics, their potential in the oil and gas sector remains underexplored [118]. Qazi et al. (2013) investigated the potential for oil recovery by the biosurfactant produced by the fungi Fusarium sp. In addition to showing stability under reservoir conditions, the biosurfactant recovered an additional 46% of oil when applied to a sand-pack column [119]. This finding suggests that with further research and optimization, fungal-derived biosurfactants could play a crucial role in EOR processes.

After using several carbon sources, Elshafie et al. (2015) optimized the production of a natural surfactant by Candida bombicola yeast. The biosurfactant was classified as a mixture of sophorolipids and demonstrated stability in salinity levels ranging from 13 to 15% w/w, across a pH range of 2–12, and at temperatures as high as 100ºC, making it a robust candidate for MEOR applications. The free-cell broth containing the biosurfactant was applied in coreflooding tests using samples of sandstone at a temperature of 60ºC. These sandstone samples were not aged and therefore exhibited a water-wet condition, which typically favors water-based recovery methods. Despite these conditions, the biosurfactant solution still managed to recover 27.27% of additional oil following conventional water injection [21].

El-Sheshtawy et al. (2016) conducted a detailed investigation into the potential of biosurfactants produced by Bacillus licheniformis bacteria and Candida albicans yeast for EOR. They focused on lipopeptides, produced by the bacteria, and sophorolipids, produced by the yeast, comparing their efficacy in improving oil recovery from reservoirs. The study revealed that the lipopeptide biosurfactants demonstrated superior performance compared to the sophorolipids. Specifically, the lipopeptides significantly increased emulsion stability by 96%, whereas the sophorolipids enhanced it by 65%. This difference highlights the higher efficiency of lipopeptides in stabilizing emulsions. In addition, the impact of these biosurfactants on surface tension was also evaluated. The surface tension of water decreased to 36 mN/m after 3 days of exposure to the lipopeptide, while it fell to 45 mN/m with the sophorolipid. Furthermore, the oil recovery rates achieved with the biosurfactants were significantly different. The biosurfactant derived from bacteria resulted in a substantial oil recovery of 16.6%, whereas the biosurfactant from yeast achieved a lower recovery rate of 8.6%. These results underscore the higher efficiency of bacterial-derived biosurfactants in enhancing oil recovery, making them a more effective choice for applications in oil reservoirs [97].

Andrade, (2016), Andrade and Pastore (2016) and Zanotto (2018) studied the surface activity of two biosurfactants, surfactin and MEL-B, produced by the bacteria Bacillus subtilis and the yeast Pseudozyma tsukubaensis, respectively. Their research aimed to evaluate the efficacy of these biosurfactants in the context of MEOR, utilizing three different types of oil: light, medium, and heavy. The studies revealed that surfactin and MEL-B exhibited varying responses to experimental conditions, particularly with respect to saline concentration and temperature. Specifically, surfactin’s surface activity was significantly impacted by changes in saline concentration, demonstrating a strong sensitivity to salinity variations. In contrast, MEL-B’s surface activity was less affected by saline concentration but was notably dependent on temperature. The research found that while temperature did not alter surfactin’s effectiveness, it had a considerable impact on MEL-B’s surface activity, particularly within the temperature range of 100 to 121 °C. By displacement tests, both surfactin and MEL-B showed superior efficiency in enhancing the recovery of heavy oils compared to light and medium oils [94,95,96].

Table 3 shows several works that were carried out using biosurfactants derived from fungal production. Despite the relatively limited number of published works specifically addressing biosurfactants produced by fungi in the context of MEOR ex-situ, the existing research highlights some promising results. Similar to those produced by bacteria, fungal-derived biosurfactants have shown resistance to the harsh conditions encountered in oil reservoirs. This includes their ability to maintain functionality and effectiveness in environments characterized by high salinity, extreme pH levels, and elevated temperatures.

Table 3 List of published works using biosurfactant produced by fungi and yeasts in EOR
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b. Plants and biomass-derived biosurfactants

So far, biosurfactants produced by bacteria and yeast have been presented. However, there are other natural sources of biosurfactants that have been applied in the literature, such as production through biomass or plants. Although not considered MEOR, plant-derived biosurfactants have been widely studied for application in EOR [120]. This type of biosurfactant is called saponin, whose name comes from the Latin and means foaming agent from plants [121]. Normally, saponin can be found in different parts of the organism such as seeds, leaves, flowers, roots, and fruits [122]. There is a wide variety of biosurfactant-producing plants whose products have been applied in EOR, and typically, research revolves around the flora of the region being studied.

Daoshan et al. (2004) reported the application of broth containing biosurfactant produced by an acidic plant found in the Chinese province of Heilongjiang. The broth was employed as a secondary (sacrificing) agent to reduce adsorption loss of the anionic surfactant alkylbenzene sulfonate (AS) in a sandstone reservoir rock. Interfacial tension results with wort and AS solutions were measured with different concentrations of NaOH and crude oil, indicating excellent affinity between the compounds. Static adsorption tests indicated that the presence of broth in the solution reduced the adsorption of AS by 25–30%. The authors reported the usage of biosurfactant as being very promising and economical since it costs around 15% of the AS and obtained an excellent result [123].

Chhetri et al. (2009) evaluated the ability of the biosurfactant extracted from the peel of the Sapindus mukorossi fruit to reduce the water/oil interfacial tension. The surfactant was able to reduce the interfacial tension from 19 to 2.5 mN/m as the biosurfactant concentration ranged from 0 to 12% (w/w). In addition, the authors studied the effect of temperature and observed that there was a greater reduction in interfacial tension in the tests at 50ºC [124].

Several authors have reported the application of the Zyziphus Spina Christi plant for enhanced oil recovery on a laboratory scale. Daghlian Sofla et al. (2016) investigated the application of natural surfactant extracted from the leaves of this plant in changing the wettability of calcites, dolomites, and sandstones, comparing its potential with synthetic surfactants (CTAB, SDS, and sodium alpha-olefin sulfonate - AOS). The results indicated that the biosurfactant called Cedar was able to change the wettability of carbonate and sandstone rock as well as the synthetic surfactants. However, synthetic surfactants were more stable at high salinities. Finally, in coreflooding tests, Cedar, DTAB, and AOS were able to recover, respectively, 15, 17, and 13% of OOIP, indicating that the biosurfactant can be an ecological alternative to synthetic surfactants in oil recovery [125].

Ahmadi et al. (2012), Ahmadi and Shadizadeh (2013a) and Zendehboudi et al. (2013) studied the adsorption kinetics of biosurfactants produced by Glycyrrhiza Glabra and Zyziphus Spina Christi plants through conductivity tests. The crushed rock used was limestone from a Persian Gulf formation. The authors concluded that due to the adsorption of the biosurfactant in the rock, the concentration necessary to start the formation of micelles in the reservoir rock is much higher than the CMC. Through tests with variable temperatures, they could see that the amount of adsorbed surfactant decreased with increasing temperature. At higher temperatures, therefore, the surface activity of the rock decreased, qualifying a process of an exothermic nature [126,127,128]. Following the same line of work Ahmadi and Shadizadeh (2018) studied the influence of salinity on the saponin adsorption profile in the reservoir rock and concluded that monovalent and divalent ions further intensified this effect. On the other hand, the same authors had stated that the use of nanosilica combined with saponin can reduce the adsorption of the surfactant in the rock. The results indicated that the saponin adsorbs on the nanosilica surface through the mechanism of hydrogen bridges between the hydroxyl groups of the two compounds, resulting in less loss of surfactant by adsorption on the sandstone rock [129].

Pordel Shahri et al. (2012) also used the biosurfactant produced by Zyziphus Spina Christi. Hanging drop tests were performed to determine the interfacial tension of solutions with saponin concentrations ranging from 0 to 10% by mass, with kerosene simulating the oleic phase. The results indicated that saponin was able to reduce the interfacial tension from 48 to 9 mN/m. Although a large amount of the product is required compared to other synthetic surfactants to reduce the interfacial tension to these levels, the authors claimed that the production of this biosurfactant is cheap and its source is plentiful [120].

Another biosurfactant of biomass origin called alkyl polyglycoside (APG) has also been applied in the literature for EOR purposes. Yin and Zhang (2013) used APG derived from a sugar source in IFT, wettability, and oil displacement tests. Applied at a mass concentration of 0.5%, combined with 0.5% of NaHCO3, the interfacial tension of the oil and solution can be reduced to values of 2.3 × 10−3 mN/m. The coreflooding tests with carbonate rocks indicated that the injection of the solution containing 0.5% in mass of APG combined with 0.5% of NaHCO3 in mass managed to recover from 6.4 to 7.1% of the OOIP additionally. The authors state that the materials used to produce the APG are found in abundance and are economical, justifying their use in EOR [130].

Li et al. (2019) compared APG with seven other synthetic surfactants. They used a high salinity and high-temperature heavy oil environment. Compared to the other synthetic surfactants, APG showed the highest resistance to temperature and salinity. Oil recovery was 10.1%, about 2 times greater than synthetic surfactants [131]. do Vale et al. (2020) supported previous results where the APG biosurfactant was able to recover up to 52.1% of OOIP after conventional water injection in sand-pack tests [132].

Wei et al. (2020) studied the effect of salinity and temperature on the adsorption of APG biosurfactant in sandstones. At 20 °C, the calculated biosurfactant adsorption was 24.5 mg/g, while at 115 °C, this value dropped to 21.6 mg/g. The increase in the salinity of the solution as well as the concentration of APG caused an increase in adsorption in the rock. The opposite behavior could be observed with increasing temperature. However, the authors considered that despite the loss of biosurfactant by adsorption, its economic cost, the environmental advantage, and the fact that it alters the wettability of the rock even at concentrations as low as 0.01% by mass, the biosurfactant has great potential for applicability in EOR [133].

As has been reported, there is a wide variety of biosurfactant-producing plants whose product has been applied in EOR. Ahmadi et al. (2014) extracted a natural surfactant from mulberry leaves. IFT studies demonstrated that a solution containing 1% by mass of the produced biosurfactant was able to reduce the interfacial tension in the water/kerosene system by approximately 60%. The biosurfactant was then used in coreflooding tests using crude oil and carbonate rocks from the Persian Gulf. With only 2 PV of injected fluid, the solution containing 1% biosurfactant recovered 68.6% of the oil compared to water injection, which recovered only 49%. The authors also observed through the pressure differential of the sample that the biosurfactant increased breakthrough time, i.e., the injected solution took longer to be produced [134].

Company et al. (2019) developed a new biosurfactant using coconut and palm kernel oil as sources that indicated great potential for enhanced oil recovery. The natural surfactant was able to reach interfacial tensions in the water/n-dodecane system in the order of 3.5 × 10−4 mN/m. In coreflooding tests, the injection of water resulted in a residual oil saturation (Sor) of 0.27, and after the injection of the biosurfactant, the Sor was 0.01. Therefore, the oil recovery by this biosurfactant was 96% of the OOIP [135].

The effect of the divalent ions Ca2+, Mg2+ and SO42− (smart water) on the activity of the non-ionic biosurfactant derived from alfalfa was studied by Eslahati et al. (2020). With a concentration of 4% w/w (CMC), the isolated biosurfactant was able to reduce the IFT by 63% and the contact angle by 50%. Solutions containing calcium, magnesium, or sulfate were prepared to investigate the influence of each ion on IFT and the contact angle. Among them, calcium showed better synergistic effects with the biosurfactant solution for changing wettability and reducing IFT. Finally, spontaneous imbibition tests with solutions containing 4% biosurfactant + three times the Ca2+ concentration of seawater recovered an additional 19.2% of OOIP when applied after conventional water injection [136].

Dashtaki et al. (2020) extracted a biosurfactant from a medicinal plant called agnocasto (Vitagnus) and studied its application in EOR. In this study, the natural surfactant was used to minimize the interfacial tension and change the rock wettability towards water-wet, thus improving oil recovery in the carbonate rock. Conductivity, pH, and turbidity measurements were performed to identify the CMC of the biosurfactant. The experimental results obtained show that the CMC value of the biosurfactant used was 3000 ppm. At this CMC value, the IFT was reduced from 29.5 to 5.28 mN/m, and the contact angle of the rock surface was reduced from 131.5° to 40.5°, indicating an improvement in rock wettability. Finally, the coreflooding experiments confirmed that the biosurfactant was able to significantly increase the oil recovery factor, increasing the incremental oil recovery factor to 30% [137].

Table 4 shows the work that has been done using plant-derived biosurfactant for enhanced oil recovery. Similar to the biosurfactants discussed in previous sections, these products have demonstrated resistance to reservoir conditions. It is noteworthy that their CMC is higher compared to surfactants produced by microorganisms. Additionally, both surface and interfacial tension are relatively higher; however, they still show potential for application in oil recovery, subject to the availability of raw materials in the region.

Table 4 List of published papers using plant-derived biosurfactant in EOR
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Therefore, there is a wide variety of plants that can be used for biosurfactant production. The work presented is exclusively aimed at application in enhanced oil recovery, but there are many other sources of natural surfactants that have not yet been tested for use in the oil industry.

Comparison of biosurfactants

The previous topics have demonstrated that there is a broad range of microorganisms and plants capable of producing natural surfactants, which yield different results when applied in EOR. Most of the studied biosurfactants have shown remarkable resilience to conditions of high temperature, salinity, and pH changes, justifying their utilization in this field.

It is worth noting that there is a greater abundance of studies focusing on biosurfactants produced by bacteria compared to fungi and plants. Figure 6 illustrates the number of articles that have applied biosurfactants from different production sources between 1999 and 2024. More than 90 studies were analyzed, revealing that the majority of them employed species of Bacillus bacteria. Bacillus sp. are known for producing the lipopeptide surfactant surfactin, which exhibits a higher capacity for reducing interfacial tension. There is also a considerable number of studies on biosurfactants derived from plants or biomass.

In the present paper, surfactants produced by more than 20 different plants were reported, highlighting the wide range of producers. Some articles have also mentioned the use of Pseudomonas sp., though in smaller quantities. This genus of bacteria primarily produces glycolipid biosurfactants. Finally, compared to the aforementioned groups, there are relatively few studies reporting the utilization of fungi and other bacteria for surfactant production in EOR applications.

Fig. 6
figure 6

Number of publications on natural surfactants applied in EOR

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One of the most important characteristics of a surfactant or biosurfactant is its critical micellar concentration. Once the surfactant concentration reaches the CMC, the formation of micelles commences. Micelles are molecular clusters formed by surfactant molecules in a liquid medium [151]. A significant advantage of using biosurfactants produced by microorganisms, in addition to their biodegradability, is their lower CMC value. As previously mentioned, the CMC represents the concentration of surfactant required to achieve the maximum potential for surface or interfacial tension change. Beyond this point, micelle formation occurs in the bulk of the liquid, and the tension at the surface or interface remains constant. Thus, a lower CMC indicates a lower amount of surfactant needed to exploit the effect on interfacial tension. According to Desai and Banat (1997), biosurfactants produced by microorganisms generally have a CMC approximately 10 to 40 times lower than that of synthetic surfactants [57].

Based on the results presented in the previous sections, the CMC of surfactants produced by bacteria, such as lipopeptides and glycolipids, is approximately 100 mg/L, while those produced by plants are about 35 times higher. Despite the abundance of plants and their derivatives capable of producing biosurfactants, a significant amount of material is required to reach the CMC and, consequently, achieve the maximum reduction in surface tension.

In the literature, the authors compared the surface tension of pure water (68–72 mN/m) with the solution containing biosurfactant. The values obtained ranged between 27 and 33 mN/m for each type of producing organism. Glycolipids, primarily produced by Pseudomonas sp. bacteria, exhibited a lower surface tension of approximately 27.7 mN/m. Lipopeptides, predominantly produced by Bacillus sp. bacteria, also demonstrated high efficiency in reducing surface tension, with an average of 29.3 mN/m. Some works have reported that glycolipids exhibit less pronounced interfacial activity compared to lipopeptides. They suggest that this is because surfactin is more sensitive to the ionic strength of the medium, which reduces the number of molecules interacting at the surface or interface [95, 152, 153]. Lastly, the biosurfactants produced by plants and fungi reduced the surface tension to an average value between 31 and 33 mN/m. Overall, all these surfactant types exhibited excellent surface activity.

Several authors have also conducted interfacial tension tests between hydrocarbons and water. The purpose of this methodology is to examine how the surfactant will act in the reservoir, considering that the rock contains both oil and water. Interfacial tension is a characteristic of the system, specifically the two phases involved [154]. Consequently, interfacial tension results may vary depending on whether crude oil or solvent is used as the non-polar phase. The analyzed studies utilized various solvents such as kerosene, n-decane, diesel, n-heptane. In contrast to surface tension, lipopeptides exhibited higher interfacial activity compared to glycolipids in both non-polar phases. When a solvent was used instead of crude oil, the interfacial tension was lower for both types of biosurfactants produced by microorganisms. However, the biosurfactant derived from plants displayed a contrasting and more notable behavior, with lower interfacial tension observed when dead oil was employed. This difference in IFT may be attributed to the presence of resins and asphaltenes in crude oil, which are not present in commercial solvents [155].

The interfacial tension exhibited the same trend as the surface tension, indicating that the biosurfactant derived from bacterial production possesses higher surface activity compared to the one produced by plants. In the reservoir system, when the IFT is too high, the oil tends to form larger droplets and becomes trapped within the rock’s pores. However, if the injected fluid reduces the interfacial tension, these larger oil droplets can transform into emulsions of smaller droplets, facilitating the removal of oil from the pores.

Conclusion

Biosurfactants are promising candidates for EOR due to their biodegradability, low toxicity, availability of raw materials, and resistance to harsh reservoir conditions. They effectively reduce water/oil interfacial tension, making them advantageous over synthetic surfactants. Over the past 30 years, the use of biosurfactants in ex-situ EOR has been extensively studied. Research has focused on various producing organisms, particularly bacteria and plants, which account for 93% of studies. These biosurfactants have been tested through techniques like surface and interfacial tension tests, coreflooding, and contact angle measurements. Based on the findings, several conclusions can be drawn:

  • All types of biosurfactants demonstrated stability across a range of pH, salinity, and temperature conditions.

  • Plant-derived biosurfactants have a CMC approximately 35 times higher than those from bacteria, meaning more surfactant is needed to form micelles.

  • Microbial-origin surfactants have proven more effective in reducing interfacial tension.

  • To improve economic viability, researchers have utilized low-cost carbon and nitrogen sources, such as agro-industrial effluents, and applied crude broth directly without the need for purification.

  • Surfactant adsorption on reservoir rocks, influenced by factors like rock type and solution salinity, can be managed by incorporating additives such as alkalis and silica nanoparticles, enhancing the effectiveness of biosurfactants in EOR applications.

This review has demonstrated that there are many ways to produce a natural, biodegradable product with surfactant properties suitable for the oil industry. While the emphasis has been on biosurfactants for EOR, these bioproducts also have significant potential for other applications, such as pipeline transportation, tank cleaning, and oil spill mitigation.