Effects of AOT, AES on Swelling Factor and Interfacial Tension for Carbonated Water Solution, Synthetic Resinous, Asphaltenic Oil and crude oil; Subcritical and Supercritical Conditions

Milad Samaeili 1

Seyednooroldin Hosseini 2✉ EmailSeyednooroldin.Hosseini@iau.ac.ir

Mohammad Abdideh 2

Elias Ghaleh Golab 2

Department of Chemical Engineering Om.C, Islamic Azad University Omidiyeh Iran

EOR Research Center, Department of Petroleum Engineering Om.C, Islamic Azad University Omidiyeh Iran

Milad Samaeili¹, Seyednooroldin Hosseini^2,z,2, Mohammad Abdideh², Elias Ghaleh Golab²

¹Department of Chemical Engineering, Om.C., Islamic Azad University, Omidiyeh, Iran.

²EOR Research Center, Department of Petroleum Engineering, Om.C., Islamic Azad University, Omidiyeh, Iran.

^z,2Corresponding Authors

Email: Seyednooroldin.Hosseini@iau.ac.ir

Abstract

In recent years, the application of gas injection, particularly carbon dioxide (CO₂) dissolved in water, known as carbonated water (CW), has gained increasing attention. This technique has been employed in a miscible form to activate several mechanisms, such as wettability alteration, interfacial tension (IFT) reduction, and the swelling of crude oil. Although these mechanisms can be triggered, it is possible to enhance those using surfactants in conjunction with CW. The combination can optimize IFT reduction and wettability alteration due to the presence of surfactants, while the swelling effect still originates from the CO₂ in the solution. In this context, the current study is designed to examine the effect of CO₂ dissolution in water under pressures ranging from 500 psi to 4500 psi, covering subcritical to supercritical conditions, and at temperatures between 25°C-65°C. Additionally, the synergistic effects of surfactants, namely dioctyl sulfosuccinate sodium salt (AOT) and sodium dodecyl polyoxyethylene ether sulfate (AES), were examined at concentrations ranging from 0 to 700 ppm, along with the dissolved CO2 on IFT and swelling factors. The measurements revealed that as the pressure increased, the swelling factor reached a maximum value of 19.3% when it was contacted with crude oil, while the maximum swelling factor for the solutions contacted with synthetic mixed resinous and asphaltenic oil (SMRAO) was reached at a value of 22.3%. The second oil type was selected as SMRAO since crude oil comprises thousands of components, making it hard to extract any generalized conclusions based on the obtained results. In this way, using only one or two specific fractions, especially resin and asphaltene which acts as natural surfactants, providing the chance to examine the generalized interactions between chemicals and oil fractions. The measurements revealed that the presence of surfactant in the carbonated water (CW) reduced the swelling factor up to 50% for AOT and 38% for AES as the pressure and temperature and surfactant concentration increases. The reason of this observed trend was correlated to the bulky structure of AOT compared with the linear chain-like structure of AES. Besides, the measurements revealed the positive impact of pressure and temperature on a higher swelling factor regardless of the used surfactants, which can be due to the higher dissolution of CO₂ under higher pressures and better movement and migration of CO₂ molecules, which means a penetration of higher amount of CO₂ into the oil drop leading to higher swelling factors. In the next stage, the IFT of different solutions under different temperatures (25 ^oC-65 ^oC) and pressures (0-4500 psi) was measured. The obtained IFT values showed that using SMRAO instead of crude oil has a reducing impact on the IFT values with minimum value of 19.2 mN/m, while the IFT value for similar thermodynamic condition and crude oil was 23.1 mN/m. Besides, further IFT measurements revealed that although increasing pressure has a reducing impact on the IFT, increasing temperature increases the IFT values regardless of the presence of surfactant or even the type of surfactant. The measurements also revealed that the effect of AES on the IFT reduction was better than AOT, leading to a minimum IFT value of 1.1 mN/m for AES concentration of 700 ppm dissolved in CW with pressure and temperature of 4500 psi and 65 ^oC, respectively due to longer alkyl chain length and easier packing in the interface compared with AOT which has a bulky structure prevents the high number of AOT molecules to be packed in the interface. The measured IFT values revealed the linear IFT variation behavior for the systems were in contact with SMRAO compared with crude oil due to this fact that the SMRAO has less complexities than crude oil comprises of thousands of components makes the IFT variations more straightforward for SMRAO.

Keywords:

Swelling factor

dioctyl sulfosuccinate sodium salt (AOT) and sodium dodecyl polyoxyethylene ether sulfate (AES)

synthetic oil

IFT reduction

carbonated water

1. Introduction

Oil reservoir production is typically conducted in three distinct and crucial stages. Initially, producers leverage the reservoir’s inherent energy to facilitate extraction. As the process continues, the second stage introduces water or gas injection, which significantly enhances production mechanisms and optimizes sweep efficiency. Finally, specialized techniques are deployed to fine-tune both macroscopic and microscopic recovery processes, maximizing output^1,2. However, when relying solely on natural reservoir energy becomes economically impractical or when secondary water and gas injection yield only marginal oil recovery alongside substantial gas and water production, it becomes essential to consider Enhanced Oil Recovery (EOR) methods. Importantly, these advanced techniques can be implemented from the very beginning of production, depending on the reservoir's unique conditions.

Non-thermal EOR methods stand out, as they do not lead to significant increases in reservoir temperature and rely on the strategic use of water activated with chemicals and carbon dioxide (CO₂)^3,4. The integration of diverse methods not only enhances recovery efficiency but also offers a tailored approach to meet the challenges of oil extraction. A range of scenarios has demonstrated the effectiveness of combining chemical water and CO₂ injection, yielding both time-tested and innovative methodologies. Among the most impactful strategies are Simultaneous Water and Gas (SWAG) injection, carbonated water injection, and Water-Alternating-Gas (WAG) injection—all of which are critical for maximizing oil recovery and ensuring sustainable production practices^1–5.

These methods are not only technically proficient but also regarded as economically and environmentally sustainable. One of their standout features is their capacity to store substantial amounts of carbon dioxide (CO₂) within oil reservoirs, a critical factor in today’s climate-conscious landscape. As the production of hydrogen fuel from hydrocarbons has surged, it has led to a significant increase in CO₂ emissions. Therefore, returning these emissions back to reservoirs is a prominent strategy for carbon sequestration. This approach is vital in mitigating the release of CO₂ into the atmosphere, thus playing a key role in combatting global warming and preserving the environment for future generations^6,7.

The method of carbonated water injection represents a significant advancement in enhanced oil recovery (EOR) techniques, leveraging the properties of CO₂ to improve oil extraction efficiency. In this process, CO₂ is dissolved in water at pressures below the miscibility pressure, allowing for a different dynamic in the reservoir compared to traditional methods. The efficiency of carbonated water injection can be attributed to several key mechanisms including:

a) Reduction of oil viscosity as the CO₂ introduces into the water phase lead to easier flow, allowing the oil to move more freely within the porous media of the reservoir. This phenomenon is particularly beneficial in high-viscosity oil deposits, where traditional methods may fall short.

b) Wettabilityalteration of reservoir rocks which plays a crucial role in determining the distribution and movement of fluids within the reservoir. CW can alter the wettability from oil-wet to water-wet conditions, promoting the displacement of oil towards production wells. This shift can be critical in improving overall sweep efficiency, which often suffers due to poor fluid distribution.

c) Oil swelling comes from the dissolution of CO₂ into the oil phase can cause the oil to swell. This swelling increases the volume of oil that can be produced and can help mobilize trapped oil that is otherwise not accessible through conventional extraction methods. This phenomenon also contributes to maintaining reservoir pressure.

d) Interfacial tension (IFT) reduction which is one of the significant benefits of CW injection is the reduction of IFT between oil and water. Lower IFT encourages more effective capillary action, facilitating the movement of oil towards production wells. This is particularly relevant in tight formations where capillary forces can hinder oil flow.

Although carbonated water injection is beneficial for enhanced oil recovery (EOR), several important parameters must be carefully examined to optimize its efficiency. Among these key parameters are dissolved CO₂ concentration, temperature, pressure, and the characteristics of both rock and fluid.The concentration of dissolved CO₂ refers to the extent to which CO₂ dissolves in water and oil phases, significantly influencing recovery efficiency. Higher concentrations typically enhance various mechanisms involved; however, there may be an optimal range beyond which diminishing returns can occur. On the other hand, thermodynamic parameters, including temperature and pressure, directly impact the solubility of CO₂ in water and oil. It is essential to optimize these conditions for maximum CO₂ retention and effectiveness. Additionally, understanding the characteristics of rocks and fluids—such as the mineralogy of reservoir rocks and the nature of the reservoir fluids—is crucial. Different formations may respond differently to carbonated water injection based on their specific geological and chemical properties.

Ongoing research into the carbonated water injection process is crucial for optimizing EOR techniques. Investigations into the rate of CO₂ dissolution, the interactions between injected water and reservoir oil, and the long-term effects on reservoir behavior are necessary to refine operational parameters. Additionally, computational modeling and field experiments can provide insights into the potential for scaling up this technique in various reservoir conditions⁸. In conclusion, CW injection is a promising method in the field of EOR. Its ability to positively impact several key mechanisms for oil production makes it a vital area for continued research and application. As the demand for efficient energy resources increases, methods such as these will play a essential role in ensuring sustainable and effective oil recovery practices⁹.

Sohrabi et al. conducted micromodel experiments to investigate the effects of CW injection on enhancing both light and heavy oil production. Their findings indicated that the viscosity of the produced oil decreased compared to its initial state. The researchers highlighted that the swelling and reduction in viscosity of heavy oil were the primary mechanisms contributing to this enhancement¹⁰. Other studies have echoed these findings, emphasizing viscosity reduction as a critical factor in this approach^11,12.

Recent studies have delved into the effects of CW on the wettability and physical properties of various types of rocks, with a specific emphasis on carbonate formations. These investigations have revealed noteworthy insights into how carbonated water can influence rock characteristics, particularly in carbonate rocks such as limestone and dolomite^13,14. One of the primary findings from this research is that CW can cause significant changes in wettability, which refers to the ability of a liquid to maintain contact with a solid surface. The wettability changes observed in carbonate rocks were particularly striking, as researchers noted significant surface dissolution when carbonated water was introduced. This dissolution process is believed to enhance the interaction between the water and the rock surface, leading to altered physical properties.Alqam et al. conducted a comprehensive study focusing on the wettability of carbonate minerals, focusing mainly on calcite and dolomite. They explored how these minerals responded to various mixtures involving CW. Their experimental results were illuminating; they documented a substantial decrease in the contact angle (CA)—an indicator of wettability—from 97.6° to 75.5°. Such a decrease is significant, as it suggests a transition towards a more favorable interaction between the rock and the liquid, indicating a move from a more hydrophobic state to a more hydrophilic one.The findings also indicated that the presence of dolomite played a crucial role in determining the wettability of the systems studied. When dolomite concentrations were reduced, there was a noteworthy shift from an intermediate wettability state to a distinctly hydrophilic state. This shift has important implications for various applications, particularly in fields like petroleum engineering and geochemistry, where fluid-rock interactions are critical for processes such as enhanced oil recovery, carbon sequestration, and groundwater management¹⁵.The pronounced effects of carbonated water on wettability could also signal potential strategies for optimizing resource extraction and improving the efficiency of fluid flow in porous media. Understanding these changes allows for the development of methods to manipulate the wettability of rock formations deliberately, thereby enhancing the efficiency of fluid movement through these geological structures.

In summary, the exploration of CW’s impact on the wettability of carbonate rocks has opened new avenues for understanding rock-fluid interactions. As researchers continue to investigate these phenomena, the implications for industrial applications and environmental management will be profound, highlighting the importance of continued research in this area. The integration of such findings into practical applications could lead to more effective and sustainable practices in natural resource management.

The solubility characteristics of CW play a crucial role in influencing various properties of geological reservoirs, particularly wettability, porosity, and permeability. A comprehensive study conducted by Nowrouzi et al. delved into the relationships between these key factors, revealing that the dissolution of carbonate materials in carbonated brine significantly enhances both porosity and permeability. This enhancement is attributed to a shift in the CA of the oil-wet carbonated surface, moving towards a strongly hydrophilic state. This transformation occurs as a result of rock surface dissolution, which facilitates the removal of oily compounds from the surface during the interaction with carbonated water. This process not only alters the physical characteristics of the rocks but also creates a more favorable environment for fluid flow within the reservoir¹⁶. Additionally, it's important to note that the increased salinity of carbonated water diminishes its ability to dissolve carbonate rocks, which could have implications for reservoir management and fluid recovery strategies.

When it comes to oil behavior in such environments, oil swelling has emerged as a key mechanism of interest. Research conducted by Lashkarbolooki et al. focused on the dynamics of oil swelling, particularly under varying pressure conditions, and reported significant findings related to the Bond number. This study demonstrated that, at the elevated pressures, the changes in the Bond number over time were pronounced, surpassing those observed at lower pressures. The underlying mechanism was linked to the diffusion of CO₂ within the oil at the oil-water interface, a process that intensified with increasing pressure¹⁷.Their findings emphasized that oil swelling is not a static phenomenon but is influenced by multiple factors, including pressure, time, and temperature. After certain duration, the system tends to reach a state of equilibrium, at which point it has been observed that the impact of pressure on oil swelling becomes more significant than that of temperature. This highlights the complex interplay of physical processes at work in enhancing oil recovery in carbonated water systems. Understanding these interactions is vital for optimizing reservoir performance and increasing the efficiency of oil extraction techniques ¹⁷.

A related study indicated that the swelling behavior primarily prevails over extended periods, with light oil extraction occurring at various pressures irrespective of the duration of contact¹⁸. The swelling process is driven by the dispersion, dissolution, and diffusion of CO₂¹⁹. This expansion assists in releasing oil from the matrix and reducing gaps, while also allowing separate oil droplets to merge, thereby overcoming water's shielding effect. Different oil swelling values have been noted, influenced by various factors. In a review by Chen et al., they reported oil swelling values ranging from nearly zero to almost 50%²⁰.

Surfactant-based methods play a crucial role in significantly reducing IFT, often bringing it down to very low levels, sometimes close to zero. For example, research on the anionic surfactant PELS revealed its impressive ability to decrease IFT, reaching a minimal benchmark of 0.99 mN/m at a concentration of 1500 ppm. This performance underscores its potential application in enhanced oil recovery processes²¹.

In addition, studies involving modified cationic Gemini surfactants have been conducted through core flooding experiments, which demonstrated oil recovery rates ranging from 46% to 49% of the original oil in place²². This suggests that these surfactants can significantly improve the efficiency of oil extraction from wells.

Moreover, when formulations that combine betaine with extended surfactants were tested, they exhibited remarkably low IFT values, approaching zero, particularly under conditions of high salinity, such as those involving more than 10% NaCl²³. This finding indicates the efficiency of these formulations in challenging environments often encountered in oil fields.

However, it is important to note that while surfactant applications offer notable benefits, they do not actively trigger additional mechanisms such as the reduction of oil viscosity, dissolution of rock formations, or swelling of oil. The innovative approach of using carbonated water to reduce IFT was initially introduced by Yang et al. in 2005. Their findings revealed that CO₂ can lower IFT under conditions that simulate those found in oil reservoirs, presenting a valuable avenue for enhancing oil recovery efforts²⁴.

These advances in surfactant research and carbonated water applications highlight the ongoing exploration into more effective techniques for oil recovery, which is vital for maximizing resource extraction and improving the sustainability of oil production practices.

The solubility of CO₂ in both brine and water plays a significant role in lowering the IFT. When CO₂ molecules move towards the interface between oil and water, they inhibit the mobility of water molecules, which disrupts the hydrogen bonding network that typically stabilizes water. This disruption leads to a reduction in IFT²⁵. Research has shown a strong consensus on the effectiveness of carbonated water in facilitating the in-situ production of surfactants, which further contributes to the lowering of IFT. When CO₂ is dissolved in water, it creates an acidic environment. This acidification prompts interactions with nitrogenous bases present in oil, resulting in the formation of surfactants right at the interface of the oil and water phases. Consequently, this interaction helps to diminish the oil-water IFT to a certain degree. Moreover, temperature also influences the total entropy at the interface between the two phases. As temperature increases, the molecular mobility of components at the interface rises, which brings about an increase in interface entropy. This change in entropy has a direct effect on the free energy of the system, leading to a further decrease in IFT²⁶. Overall, the interplay between CO₂ solubility, the formation of surfactants, and temperature effects offers a complex yet effective mechanism for reducing interfacial tension, enhancing the efficiency of processes such as oil recovery or the separation of oil from water²⁵.

Given the relatively elevated temperatures typically found in reservoirs, the significance of this mechanism becomes evident. While it has not been conclusively demonstrated that the final IFT can be significantly lowered—reaching values under 1 mN/m—without the use of any chemical additives, such low values are indeed achievable. This can be accomplished through the precise engineering of the injected water, which involves manipulating its ionic composition and incorporating surfactants or solvents^27,28. Furthermore, the interactions between the injected fluid and the reservoir rock, particularly with carbonate minerals, play a crucial role that should not be overlooked. The introduction of carbonated water not only facilitates the dissolution of the rock but also enhances its properties, such as porosity and permeability. This process also leads to a transformation in the rock's wettability, shifting towards medium or strong hydrophilicity, which can significantly influence oil recovery dynamics^16,29.

The effectiveness of CW can be attributed to its ability to induce oil swelling, promote rock dissolution, and alter wettability characteristics. Research has explored various components in carbonated water enrichment, including the utilization of nanoparticles (NPs)^30,31, which offer unique surface properties; dissolved ions³², which can modify water chemistry; traditional surfactants with hydrophobic carbon-hydrogen chains that enhance interfacial interactions²⁸; and polymers³³that may provide additional stability and functionality. to sum up, Water-Alternating-Gas (WAG) injection and surfactant-nanoparticle hybrid systems, the application of a CO₂-surfactan in carbonated water emerges as an innovative and economically viable strategy for improving oil recovery. WAG injection, which involves alternating between gas and water injections to enhance oil displacement, often encounters several drawbacks, including the phenomenon of gas fingering, imbalances in phase mobility, and significant operational expenditures stemming from the requirement for specialized injection apparatus. These challenges can inhibit the overall efficiency of the EOR process and lead to suboptimal results.Conversely, surfactant-nanoparticle systems have demonstrated their capability to reduce IFT and stabilize emulsions. However, they also present their own set of challenges, including elevated material costs and instability when subjected to harsh reservoir conditions. Additionally, their penetration efficiency into the rock matrix is often limited, particularly in fractured reservoirs where effective fluid displacement is crucial.The use of carbonated water for injection has already gained recognition as a feasible EOR technique, particularly in fractured carbonate reservoirs.

The introduction of a CO₂-surfactant into this process enhances its effectiveness by concurrently decreasing IFT, promoting oil swelling, and modifying wettability characteristics all of which can be accomplished utilizing low concentrations of the surfactant.

In the light of these facts, the current study investigates the enhancement of CW(in the pressure and temperature ranges of 500–4500 psi, and 25–65 ^oC, respectively) by incorporating two surfactants of dioctyl sulfosuccinate sodium salt (AOT) and sodium dodecyl polyoxyethylene ether sulfate (AES) in the range of 0-700 ppm to address its limitations in IFT reduction while augmenting other functional mechanisms such as oil swelling. By tailoring the properties of the surfactant, dissolved CO₂, and examining the interactions with crude oil, the research aims to optimize the performance of CW in enhanced oil recovery applications. Experimental tests, including IFT measurement, and oil swelling assessmentwere conducted to evaluate the surfactant's effectiveness. While the use of traditional surfactants, polymers, and nanoparticles in combination with CW has been explored, the application of these surfactants represents a novel approach in this research field, thereby enhancing its significance and potential impact since these two surfactants are different from structure and also two types of oil including crude oil and synthetic mixed resinous and asphaltenic oil (SMRAO) were used to find the interactions of these chemicals for the first time.

2. Experimental procedure

2.1. Chemicals and crude oil

The sample crude oil in the current work was kindly supplied from National Iranian South Oil Company (NISOC) with specific gravityof 0.86 @ 15 ^oC with asphaltene and resin fractions of 8.5% and 11.1%, respectively. The required CO₂ was supplied from Dubai Industrial Gases, Sharjah, UAE with purity better than 99.8% to ensure about e minimum impurities which can greatly affect the IFT. To facilitate the experiments, the required surfactant, AES with a CAS number of 9004-82-4 and a stipulated purity greater than 70%, this surfactant provided by Apollo Scientific (UK) is known for its effective surface-active properties, making it suitable for various applications in oil recovery and emulsification processes. Furthermore, AOT was sourced from Sigma Aldrich (USA) with a remarkable purity exceeding 97%. The point is that both of the surfactants were used without any further purifications as they received.

The other point to note is that the necessary Persian Gulf water for dilution to prepare the chemical solutions was collected from Asaluyeh Port. This water maintained the following composition: sodium (13,360 ppm), potassium (505 ppm), magnesium (1,580 ppm), calcium (438 ppm), chloride (25,012 ppm), and sulfate (3,410 ppm), with a total pH of 8.1.

2.2. PVT equipment

The process of preparing carbonated brine and distilled water was conducted within a PVT (Pressure-Volume-Temperature) cell with a maximum capacity of 400 cc, designed to withstand pressures up to 600 bar and temperatures reaching 426 K. This setup was provided by Fanavari Atiyeh Pouyandegan Exir Company (APEX Tech. Co.) in Arak, Iran. Initially, the desired aqueous solution comprising distilled water and MgSO4 was introduced into the main PVT cell. Following this, carbon dioxide (CO₂) was pressurized to 60 bar and injected into the cell through a designated middle port. After the addition of CO₂, the inlet discharge valve, manufactured by Autoclave in the USA, was securely closed. Subsequently, a piston inside the PVT cell was utilized to apply pressure to the internal contents, enabling the system to reach the targeted pressure levels. Monitoring and data collection throughout the procedure were accomplished using a digital pressure transmitter from Keller, Switzerland, offering precision to within 0.05% of the full scale. Additionally, temperature control was managed through a heating element linked to a PT-100 sensor, maintaining accuracy within approximately 0.1 K. An important feature of the PVT cell is its integration with a CO₂ booster, allowing for pressure enhancement up to 600 bar. This booster is crucial for achieving pressure levels that exceed 1500 psi, especially when natural compression from the gaseous phase within the cell falls short. For the operation of the CO₂ booster, gaseous CO₂ was sourced from a capsule and directed into a refrigeration system, where it underwent a phase change to become liquid CO₂ in an initial stage. The liquefied CO₂ was subsequently pumped to the desired pressure using an air-driven oil-free reciprocating pump from Haskel, USA, before entering the PVT cell. The main PVT cell also features two sapphire sight glasses, enabling real-time visual monitoring of the internal contents. Inspection is further supported by a CCD camera outfitted with a macro lens from Computar, Japan, which is essential for confirming that a proper equilibrium is achieved between the dissolved CO₂ and the aqueous solution. If undissolved CO₂ remains in the cell, ventilation or purging is necessary to maintain system integrity. To ensure effective mixing of the contents within the PVT cell, an internal magnetic mixing mechanism has been installed. This feature is paired with a motorized rocking mechanism, enhancing the mixing process by allowing the PVT cell to rotate. This rotational movement promotes more rapid equilibration of the components inside the cell, thereby improving the overall efficiency and effectiveness of the carbonation process.

Figure 1. Schematic of the used PVT cell along to the CO₂ booster unit³⁴.

The final step in the process involves carefully transferring the prepared solution to additional equipment for comprehensive analysis. This is critical, as any disturbance during this transfer could result in the undesirable separation of CO₂ from the aqueous phase, which can compromise the integrity of the results. To facilitate this transfer, we employ advanced high pressure-high temperature (HP-HT) IFT pendant drop equipment, specifically designed for such sensitive analyses. This equipment allows for precise measurements under controlled conditions, ensuring that the properties of the solution remain stable throughout the process. A micro-metering valve, provided by Autoclave Engineers in the USA, is utilized to manage the flow of the solution adeptly. This valve enables us to finely control the transfer rate and pressure, minimizing any potential disturbances that could affect the equilibrium of the CO₂ within the solution. By maintaining strict control over these variables, we can achieve more accurate and reliable analytical results in our subsequent experiments. The careful approach ensures that the solution remains homogeneous, allowing for effective further investigation of its properties and behavior under varying conditions.

2.3. Swelling factor measurement

To investigate the swelling characteristics of crude oil, we employed a sophisticated pendant drop apparatus, enhanced with advanced drop shape analysis software developed by Fanavari Atiyeh Pouyandegan Exir Company (APEX Tech. Co.), located in Arak, Iran. This apparatus is specifically engineered to function under high pressure and temperature conditions, allowing for precise measurements in challenging environments. While a comprehensive description of the equipment is provided elsewhere, a brief overview highlights its key components^35–37.

The apparatus includes several integral sections, such as state-of-the-art image processing software, high-pressure injection pumps, and the main measuring chamber where measurements take place. Initially, we introduced carefully prepared CW into the main measuring chamber from the PVT cell. This bulk fluid was maintained at a constant pressure, meticulously controlled between 500 and 5000 psi, ensuring optimal conditions for the experiment.Once the requisite pressure and temperature were achieved within the main chamber, we proceeded to inject crude oil with precision using a high-pressure pump. This process allowed us to form a distinct drop at the tip of the injection needle. As the drop was created, we continuously monitored its volume and analyzed changes over time, ultimately converting our observations into a swelling factor or swelling percentage using the specified equation. This detailed approach provided valuable insights into the behavior of crude oil under various conditions.

Where, crude oil drop volume at time t is V_t and initial time is V₀

Fig. 2

The used IFT measurement Apparatus for swelling factor measurement, (a) schematic (1, view cell; 2, pressure generator; 3, pressure manometer; 4, bulk tank; 5, drop tank), (b) HP-HT pendant drop interfacial tension measurement equipment^35–37.

2.4. Extraction of Asphaltene and Resin

In recent years, there has been a growing emphasis among researchers on isolating specific components from crude oil to create synthetic oils for various scientific experiments and measurements. Crude oil is comprised of a complex mixture of compounds, which can introduce uncertainties and impact the reliability of experimental outcomes. As a result, this study is focused on extracting resin and asphaltene fractions, recognized as the most active components of crude oil. These fractions function as natural surfactants, making it possible to develop resinous and asphaltenic synthetic oils that can be utilized in experimental settings. This targeted approach aims to enhance the precision and clarity of research findings in the field.Crude oil is composed of four principal fractions: resins, asphaltenes, saturates, and aromatics. The asphaltene and resin fractions are especially significant due to their surfactant-like properties, which can modify surface characteristics, particularly those of rock surfaces and interfacial tension. These alterations can affect various processes in fields such as petroleum engineering and reservoir optimization by influencing oil recovery and fluid behavior in reservoirs. Understanding the roles of these fractions is crucial for enhancing the efficiency of extraction and production methods in the oil industry³⁸. The stability of crude oil/water emulsions is heavily influenced by the presence of various fractions in a solution. This is primarily due to their unique structural characteristics and the presence of heteroatoms within their molecular composition. These factors play a crucial role in modulating interfacial tension, which ultimately affects the stability and behavior of the emulsions. Understanding these interactions is key to optimizing the properties of crude oil/water emulsions in various applications^39,40.

To effectively differentiate between asphaltene and resin fractions, it is essential to utilize the hydrogen-to-carbon (H/C) ratio criterion. This ratio serves as a useful indicator of the molecular structure of these two components, which are often found together in crude oil and other petroleum products. The H/C ratio is a fundamental measure in organic chemistry that provides insight into the degree of branching in a molecule’s structure. Specifically, when the H/C ratio is close to 1, it indicates a non-branched structure, which is characteristic of certain heavier molecular fractions. On the other hand, a significant deviation from this ratio suggests a branched structure, which affects the physical and chemical properties of the molecules.Typically, resin fractions exhibit an H/C ratio that ranges between 1.2 and 1.7. This range reflects the higher degree of branching and complexity in the resin molecules, which generally possess more functional groups, such as naphthenic acids. These acids, which can impact the acidity and solubility of the resin fractions, contribute to their distinct chemical behavior in different environments.

Conversely, the H/C ratio for asphaltene fractions tends to fall between 0.9 and 1.2. Asphaltene molecules are less branched and tend to have a more complex aromatic structure, which results in a lower H/C ratio. This can influence their solubility and behavior in petroleum systems, often leading to challenges such as precipitation or stability issues in crude oils.Despite their similarities, there are notable distinctions between resin and asphaltene molecules. Resins are typically smaller in size compared to asphaltenes, which allows them to be more fluid and less viscous. The higher branching in resin molecules contributes to their ability to dissolve in lighter oils, making them more compatible in certain refining processes. This structural difference not only affects their physical behavior but also plays a significant role in the overall processing and utilization of petroleum products.Understanding these differences, particularly through the lens of the H/C ratio, provides valuable insights into the composition and characteristics of crude oil, aiding in the efficient processing, treatment, and application of the various fractions derived from it⁴¹.

The IP 143/90 method was employed to isolate the resin and asphaltene components. Subsequently, both fractions were dissolved in toluene to form synthetic oils for studies focused on resinous and asphaltenic characteristics. It is noteworthy that while the resin and asphaltene fractions play significant roles due to their intricate structures and surface activity, their impact on lowering interfacial tension (IFT) and modifying wettability across different operating conditions has not been extensively explored⁴².

These two fractions primarily contain hydroxyl groups, esters, acids, carbonyl functions, and long paraffinic chains, along with naphthenic rings and polar functions in theirstructures^43,44. However, despite their similar chemical compositions, they differ in terms of aromaticity, size, polarity, and even physical appearance ⁴⁵. By employing a standard method to separate the asphaltene fraction using n-heptane at a precise ratio of 40:1, we can set the stage for deeper analysis. This initial step, followed by thorough purification using Soxhlet recycling, ensures that we obtain a high-quality product suitable for our investigations.Moreover, what remains after removing the asphaltenes—the de-asphalted residue—is not just waste; it presents an invaluable opportunity. By utilizing column chromatography for the isolation of the resin fraction, we can unlock additional insights that have been elaborated upon in previous studies. Therefore, this methodical approach not only enhances our understanding of crude oil components but also paves the way for innovative applications in synthetic oil research. Embracing these techniques can lead to significant advancements in a field that promises both academic and practical benefits^46,47.

The process of separating different components from de-asphalted oil through a silica gel column is both intricate and essential for obtaining desired fractions. Initially, the de-asphalted oil, characterized by its complex hydrocarbon composition, is blended with n-heptane. This mixture forms a thick, molten-like residue that serves as a suitable feedstock for the adsorption process.Following the preparation of the mixture, it is introduced into a silica gel column. The choice of silica gel with a mesh size of 35–70 is significant, as it adheres to ASTM (American Society for Testing and Materials) standards for adsorption processes. This carefully calibrated mesh size allows for effective separation based on molecular size and polarity. As the mixture traverses the column, the different components interact with the silica gel, allowing for selective adsorption.

To enhance the separation and purification, the column is then rinsed with a solvent solution composed of 70% n-heptane and 30% toluene. This step is crucial for eliminating saturates and aromatics that could interfere with the subsequent extraction of valuable compounds. The combination of n-heptane and toluene works synergistically; n-heptane, a high-purity hydrocarbon solvent, effectively dissolves non-polar compounds, while toluene, being more polar, assists in solubilizing certain aromatic compounds.After rinsing, the next phase of the extraction process involves the use of a ternary solvent mixture consisting of acetone, dichloromethane, and toluene in a specific ratio of 40:30:30. This mixture is strategically chosen for its ability to selectively extract resins from the silica gel column. Each solvent plays an essential role: acetone provides moderate polarity, enhancing solubility for a range of organic compounds; dichloromethane, with its excellent extraction capabilities, helps to dissolve more challenging substances, and toluene contributes additional aromatic solubilization.

The outcome of this meticulously designed process is a series of purified fractions, each rich in specific hydrocarbon types. These fractions can then be further analyzed or utilized in various applications, from fuel production to the creation of specialty chemicals, underscoring the importance of effective separation techniques in petroleum refining and chemical engineering^48–50 (see Table 1).

Table 1

Elemental analysis of heteroatoms of resin and asphaltenes fractions
H/C ratio	wt% % of heteroatoms of resin and asphaltene fractions				Fractions	Total acid number
H/C ratio	S, O and N	S	O	N	Fractions
1.16	12.12	3.81	8.31	0.00	Asphaltene	0.58
1.51	10.67	6.81	3.87	0.00	Resin	0.69

A detailed look at Table 1 indicates that the asphaltene fraction exhibits less branching than the resin fraction. The asphaltene fraction has a hydrogen-to-carbon (H/C) ratio or aromaticity index of 1.16, while the resin fraction has an H/C ratio or aromaticity of around 1.51. This increased branching in the resin structure relative to that of the asphaltene fraction enhances its surface activity. As a result, the resin fraction is more adept at interacting with other charged particles and surfaces, which improves its ability to reduce interfacial tension (IFT) and modify wettability. Generally, both asphaltene and resin fractions contain heteroatoms, such as sulfur and oxygen, which possess negative charges. This feature makes resin molecules akin to surfactant molecules, as they also exhibit distinct "head" and "tail" regions.

3. Results and discussions

3.1. Effect of pressure and temperature on the swelling factor of crude oil and SMRAO

In the initial phase of this investigation, we focused on examining how different types of oil impact the swelling factor, particularly through the lens of CO₂ adsorption. The two oil types analyzed were crude oil and SMRAO. The objective was to understand how varying oil fractions influence both CO₂ adsorption capabilities and the swelling factor, which is crucial for applications in enhanced oil recovery and carbon sequestration.

The results, as illustrated in Fig. 3, indicated a significant difference in the swelling factors between the two oils across various temperature and pressure conditions. Specifically, it was found that substituting crude oil with SMRAO resulted in a notable enhancement in the swelling factor. For instance, under conditions of 65°C and 4500 psi, the maximum swelling factor recorded for crude oil was approximately 19.3%. In contrast, SMRAO exhibited a swelling factor of around 22.3%. This increase signifies a more effective interaction between the oil and CO₂ when SMRAO is utilized instead of crude oil.

The underlying reasons for this observed trend can be attributed to the molecular composition of the two oil types. Crude oil is composed of a complex mixture of four primary fractions: saturates, aromatics, resins, and asphaltenes. Among these, the saturate and aromatic fractions tend to be less polar and exhibit a lower affinity for CO₂. This characteristic reduces their capacity to effectively adsorb CO₂ molecules, which are polar in nature. Consequently, the presence of these less reactive fractions in crude oil diminishes its overall ability to enhance CO₂ uptake.

Fig. 3

The effect of pressure and temperature and oil type on the swelling factor

Conversely, SMRAO is predominantly comprised of resin and asphaltene fractions, which are highly polar and rich in nitrogen, sulfur, and oxygen elements. These characteristics promote stronger interactions with CO₂ molecules at the molecular interface. The presence of more charged surfaces in SMRAO creates an environment that is more conducive to binding CO₂, thus allowing for a higher CO₂ retention within the structure. This increased adsorption capability directly correlates with the elevated swelling factor observed, as the effective CO₂ interaction leads to greater swelling.

Overall, this investigation underscores the significant variability in CO₂ adsorption and swelling behavior between different oil types. The findings suggest that the molecular composition of oil plays a crucial role in its ability to interact with CO₂, which has important implications for strategies aimed at enhancing oil recovery and optimizing carbon capture technologies. By utilizing oils like SMRAO, which are better suited for CO₂ interactions, there is potential for improving efficiency in these applications.

The results of our analysis provide compelling insights into the interplay between pressure, temperature, and the swelling factor of oils, particularly highlighting the pronounced effects observed with SMRAO. As we increased the pressure, we consistently observed an increase in the swelling factor across all oil types tested. However, what stands out is that this effect was significantly more pronounced in SMRAO, underscoring its unique properties.

Furthermore, temperature emerged as a critical variable influencing the swelling factor. Our investigation revealed that elevating the temperature from 25°C to 65°C produced noteworthy changes. Initially, one might expect that increased temperature would inhibit CO₂ solubilization due to a decrease in density. This suggests less CO₂ would be available for penetration into the oil. Yet, the story is more complex. Higher temperatures also impart greater energy to CO₂ molecules, enhancing their kinetic energy and mobility within the system. This increased energy means that, despite a reduced solubility, CO₂ has a better chance of diffusing into the oil, resulting in a greater overall swelling effect.

In this study, the influence of kinetic energy appears to outweigh the drawbacks posed by reduced CO₂ availability. The remarkable finding here is that, as we reach 65°C—well above the critical temperature—the kinetic energy of CO₂ molecules significantly enhances their ability to penetrate the oil, leading to substantial swelling. We observed that this temperature played a crucial role in amplifying the effects of pressure on the swelling factor, particularly when raising the pressure to 1000 psi. The results demonstrated a sharper increase in swelling factor in this range compared to higher pressure increments.

An important aspect of our findings is the transition to supercritical CO₂. When pressure was increased from 500 psi to 1500 psi, CO₂ transitioned into its supercritical state, where it exhibits a remarkable ability to expand and penetrate, leading to a steep increase in the swelling factor. This phase transition underlies why we observed a significant increase in swelling during this pressure range. In contrast, when pressure was raised from 1500 psi to 3500 psi without a phase transition, the rate of change in the swelling factor became less pronounced.

However, an unexpected resurgence in the slope of swelling factor variation was noted when the pressure was increased from 3500 psi to 4500 psi. This was attributed to the enhanced solubilization of CO₂ at 4500 psi compared to 3500 psi, resulting in a greater concentration of CO₂ available for diffusion and penetration into the oil drop. In conclusion, these analyses underscore the intricate balance between temperature and pressure in influencing the swelling of oils, particularly within the context of CO₂ solubilization. The findings suggest that optimizing these parameters, particularly in systems like SMRAO, could lead to significantly enhanced performance in applications such as enhanced oil recovery. These insights not only deepen our understanding of the underlying mechanisms but also pave the way for practical applications that leverage the unique properties of CO₂ under varying conditions. Emphasizing this knowledge could lead to more effective extraction techniques and innovations in petroleum engineering.

In contrast to the findings of the current study, Lashkarbolooki et al. ⁵¹identified a crossover pressure within the system they investigated. They attributed these observed trends to two distinct mechanisms. The first mechanism emphasized the role of CO₂ solubility as the primary factor influencing pressures below the crossover threshold. In contrast, for pressures exceeding this crossover point, they suggested that the mobility and disruption of hydrogen bonds among water molecules facilitate a more effective partitioning of CO₂.

A critical consideration that should not be overlooked is that while the presence of salinity tends to diminish CO₂ solubility, the inclusion of ionic liquids (IL) can enhance this solubility. This enhancement occurs as IL structures can adsorb some CO₂ molecules, thereby increasing the potential for crude oil swelling. If these adsorbed CO₂ molecules eventually escape from the aqueous phase and diffuse into the crude oil interface, it could partly explain the emergence of crossover pressure. In light of this mechanistic explanation, it becomes evident that, although Lashkarbolooki et al. ⁵¹documented the presence of a crossover pressure, such a phenomenon was not observed in the present investigation. This discrepancy suggests that the dominant mechanism at play in our study is primarily related to the impact of temperature on weakening the hydrogen bonding interactions between CO₂ and water molecules. This weakening facilitates a more rapid release and liberation of CO₂ from the confines of water molecules, promoting its migration towards the crude oil interface, which in turn results in a greater swelling of the crude oil. Thus, the current findings underscore the significance of temperature in enhancing the transfer of CO₂ into crude oil, as opposed to the competing effects of salinity and ionic liquids that were highlighted in previous works.

3.2. Effects of pressure, temperature and surfactants on the swelling factor of SMRAO

In the second phase of this investigation, the influence of surfactants, in conjunction with temperature and pressure, on the swelling factor of the SMRAO was meticulously examined. This phase builds upon insights gained during the initial investigation, wherein the impact of different oil types on the swelling factor was analyzed. The prior findings provided a foundational understanding of how temperature and pressure influence the swelling, allowing this phase to focus purely on the SMRAO. Aiming for broader applicability of the results, experiments were conducted using specific fractions of crude oil—with an emphasis on resin and asphaltenes—thus ensuring that the findings are relevant to a wider range of applications within the field.

During these experiments, the temperature was varied from 25 to 65°C, while pressure levels were adjusted between 500 and 4500 psi. The concentration of surfactants—specifically Aerosol OT (AOT) and Alkyl Ether Sulfate (AES)—was systematically altered within the range of 0 to 700 parts per million (ppm). The experimental results, illustrated in Figs. 4 to 6, demonstrated a consistent trend: as the pressure increased, the swelling factor exhibited a corresponding rise. This phenomenon can be attributed to the enhanced solubilization of carbon dioxide (CO₂) at higher pressures, leading to an increased availability of CO₂ for penetration into the oil droplet, which is crucial for swelling behavior.

Fig. 4

Effects of temperature and pressure on the swelling factor using AOT, a: 25°C, b: 45°C and c: 65°C

Further analysis indicated distinct patterns in the response of the swelling factor to pressure changes, particularly at the two isotherms of 25°C and 45°C. Notably, when the pressure was elevated from 500 psi to 1500 psi—surpassing the critical pressure—the slope of the swelling factor's variation exhibited a pronounced increase. This sharp gradient can be explained by the substantial rise in solubilized CO₂, which plays a pivotal role in enhancing the interaction with the oil matrix. However, as the pressure continued to rise from 1500 psi to 3500 psi, the change in the swelling factor became less pronounced, demonstrating diminishing returns in swelling response from this particular range of pressure.

Interestingly, when pressures were increased from 3500 psi to 4500 psi, a renewed and sharp increasing trend in the swelling factor was observed. This resurgence can again be linked to the increased solubilization of CO₂, suggesting an intricate relationship between pressure, temperature, and surfactant concentration. The insights gained from these experiments underscore the importance of finely tuning the operational parameters to optimize the swelling factor of SMRAO in applications such as enhanced oil recovery, where understanding the interplay of these variables can lead to more effective techniques for oil extraction and reservoir management.

These findings contribute valuable knowledge to the field, offering pathways for future research to explore the underlying mechanisms that govern the interactions between surfactants, CO₂ solubilization, and crude oil properties under varying thermodynamic conditions. This enhancement in understanding not only facilitates improved recovery strategies but also supports the development of tailored formulations for specific reservoir conditions.

Fig. 5

Effects of temperature and pressure on the swelling factor using AES, a: 25°C, b: 45°C and c: 65°C

One notable observation derived from the results is that the swelling factor of carbonated water without any surfactants is greater than that of carbonated water treated with various types of surfactants. This trend holds regardless of the specific surfactant used. The underlying reason for this phenomenon can be linked to the behavior of CO₂ molecules in the presence of surfactants.

Specifically, when surfactants are introduced, the adsorption of CO₂ onto the surfactant molecules hinders the mobility of CO₂. This adsorption effectively slows down the transfer of CO₂ molecules from the aqueous phase to the oil-water interface. In contrast, without surfactants, CO₂ can migrate more freely and reach the interface more efficiently. This unrestricted movement allows for a greater volume of CO₂ to penetrate the oil droplets, resulting in a significant expansion of those droplets.Moreover, the dynamics of gas-solvent interactions play a crucial role in applications such as enhanced oil recovery, where controlling swelling factors can impact the efficiency of extraction processes. Understanding the relationship between surfactant presence and CO₂ behavior can help optimize formulations used in various industrial applications, leading to improved outcomes in processes involving gas-liquid interactions.

Surfactants play a pivotal role in the capture of CO₂ by acting similarly to sponges that absorb and retain CO₂ molecules within their structure. This interaction allows surfactants to create stronger bonds with CO₂ than the bonds formed between CO₂ and water at the interface. As a result, the presence of surfactants leads to a reduction in the swelling factor of the system.

Moreover, the effect of different surfactants on the swelling factor is not uniform. Specifically, AOT shows a significantly greater reduction in the swelling factor compared to AES. Research has demonstrated that when AOT is used at a concentration of 700 ppm under conditions of 4500 psi pressure and a temperature of 65°C, the maximum swelling factor is around 10.8%. In contrast, AES under the same conditions exhibits a swelling factor of approximately 15.2%. This indicates that the swelling factor for AES is about 50% higher than that of AOT. The difference in performance can be linked to the structural characteristics of these surfactants.AOT has a bulky and sponge-like structure that enhances its capacity to adsorb CO₂ effectively. This structural advantage permits solutions containing AOT to dissolve a larger volume of CO₂ compared to those containing AES, which has a more linear and elongated structure that limits CO₂ adsorption capabilities.

Furthermore, the effectiveness of ionic liquids (ILs) and surfactants displaying IL-like characteristics in adsorbing CO₂ is well established. These substances emerge as promising candidates for various applications involving CO₂ capture in the chemical industry, as well as for the storage and sequestration of CO₂ in EOR techniques.

In a noteworthy study conducted by Jang et al.⁵², CO₂ solubility in the ionic liquid [BMIM][Cl] was investigated over a pressure range of 2.45 to 36.94 MPa and temperatures between 353.15 to 373.15 K. The results revealed a clear relationship between pressure and CO₂ adsorption—where increasing pressure leads to enhanced CO₂ retention. Conversely, while higher temperatures can lead to a decrease in CO₂ solubility due to weakened bonds between CO₂ and the ionic liquid, increased thermal energy also elevates molecular movement, leading to the potential escape of CO₂ from the ionic liquid structure⁵².

This interplay between temperature and pressure highlights the complexity of CO₂ adsorption processes and underscores the importance of selecting the appropriate surfactant or ionic liquid to optimize carbon capture methodologies. These insights are vital for advancing strategies in carbon management, ultimately contributing to the mitigation of climate change effects through improved carbon capture technologies.

The concept of free volume in materials significantly enhances the potential of AOT as a superior candidate for CO₂ entrapment when compared to AES. This advantage arises because the unique structure of AOT allows for an efficient and effective interaction with CO₂, making the entrapment process more reliable and robust.AOT's architecture features cations and anions that engage in Coulombic interactions, creating a more rigid and stable arrangement than that found in traditional molecular solvents. This stability is key: the cations and anions form a dense network where, despite the presence of both positive and negative charges, their interactions enhance the structural integrity, resulting in lower mobility.

As CO₂ is introduced, this rigid network exhibits remarkable adaptability. The anions can rearrange themselves to create larger voids that accommodate the incoming CO₂ molecules without compromising the overall structure. This means that CO₂ can efficiently diffuse through the established network, occupying the newly formed free volumes while maintaining the ion arrangement.In light of these properties, AOT's capability to trap CO₂ becomes increasingly evident. Its unique structural features not only promote better interactions with CO₂, but also ensure that the process does not disrupt the underlying network. This makes AOT a promising candidate for applications in CO₂ sequestration and energy efficiency technologies, supporting efforts to address climate change and promote sustainable practices.

3.3. Effect of AES and AOT on the IFT reduction

In the last stage of the investigation, the effect of AES and AOT as a function of concentration under different thermodynamic conditions of 25–65°C and 500–4500 psi was examined. Before measuring the IFT under different thermodynamic conditions and surfactant concentration, the effect of oil type, including crude oil and SMRAO, was examined on the IFT variation at different temperatures and pressures (Fig. 7).

The measurements shown in Fig. 7 indicate that as pressure increases, the interfacial tension (IFT) decreases, reaching minimum values of 14.2 mN/m for SMRAO and 16.7 mN/m for crude oil at a temperature of 25°C and a pressure of 4500 psi. The results suggest that while increasing pressure lowers the IFT due to enhanced CO₂ solubilization in water, increasing temperature has the opposite effect on IFT values. In other words, the minimum IFT at a constant pressure is consistently observed at lower temperatures, regardless of the type of oil.

The findings indicate that the IFT of CO₂ in contact with liquid phases, such as crude oil or water, is significantly influenced by the pressure of carbon dioxide. As the pressure of CO₂ rises, a greater quantity of CO₂ dissolves into the adjacent liquid phase, which in turn leads to a pronounced decline in IFT. This relationship emphasizes the solubility dynamics of CO₂; at higher pressures, the solubility capacity of the liquid phase increases, allowing for more CO₂ molecules to be integrated, and thereby altering the physical properties at the interface.

Fig. 7

The effect of oil type on the IFT variation under different pressures and temperatures

Detailed analysis of the results, as illustrated in Fig. 7, shows a distinct pattern: an initial sharp decrease in IFT occurs below the critical pressure of CO₂, likely due to the rapid dissolution process at lower pressures where the gas begins to significantly interact with the liquid phase. However, as the pressure continues to rise and approaches the critical zone, the rate at which IFT decreases begins to taper off, eventually converging toward an asymptotic value. This behavior is indicative of the critical phenomena associated with CO₂, where once critical pressure conditions are reached, the properties of CO₂ transition from a gaseous state to a supercritical state. In this supercritical phase, CO₂ exhibits unique characteristics, such as enhanced diffusivity and lower viscosity, resulting in diminished changes to IFT with further pressure increases.

Conversely, the influence of temperature on IFT is multifaceted and demands a more nuanced understanding. Generally speaking, the reductions in IFT attributable to temperature changes are often overshadowed by those induced by variations in pressure. However, the effect of temperature should not be disregarded, particularly in the contexts where temperatures deviate significantly from the CO₂ critical temperature. In these scenarios, the physical behavior of CO₂—pertaining to its expansion and diffusion characteristics—undergoes notable shifts.

For instance, in systems involving CO₂ and crude oil, an increase in temperature is typically associated with an increase in IFT. This behavior can be explained by several factors: as the temperature rises, the molecular interactions within the liquid phase may strengthen, leading to a more complex interface. Higher temperatures can also enhance the thermal motion of molecules, which can affect the stability of the interface and potentially promote phase separation, thereby increasing IFT. Further exploration of the intricate interplay between pressure and temperature on IFT is essential for optimizing processes in various applications, such as enhanced oil recovery and CO₂ sequestration methods. Understanding these phenomena could lead to improved strategies for manipulating interfacial properties to achieve desired outcomes in fluid systems.From Fig. 7, it can be observed that for temperatures below the critical point (25°C), the variation of IFT with pressure is more pronounced compared to temperatures well above the critical point (45°C). This behavior is attributed to the effects of phase transition to supercritical conditions, which influences molecular movement and packing at the interface, resulting in a smoother change in the IFT trend over time.

Further investigations revealed that increasing the temperature to 65°C does not significantly alter the slope variation of IFT compared to 45°C. In summary, when the temperature is below the critical point, CO₂ behaves like a standard pressurized gas. However, as the temperature surpasses the critical point, the expansion characteristics of CO₂ change, leading to a deviation from typical gas behavior. This change directly impacts the penetration of CO₂ into oil droplets and the variation of IFT.

The final insight derived from the results illustrated in Fig. 7 is the noteworthy transformation in IFT behavior as crude oil is substituted with SMRAO. Specifically, the IFT variation for the crude oil system exhibited considerable fluctuations during its reducing trend. In contrast, the IFT variation in the presence of SMRAO demonstrated a much smoother progression, approximating a linear trend with a significantly steeper reduction slope.

This observed difference can be attributed to the varying components of the oil samples analyzed. SMRAO primarily consists of the resin and asphaltene fractions, characterized by a reduced level of impurities as a result of an extraction process. Conversely, crude oil is a complex mixture comprising four distinct fractions—namely, asphaltenes, resins, saturates, and aromatics—along with potential impurities. These additional components can exacerbate the effects of temperature and pressure on IFT variations, resulting in greater fluctuations and non-uniform patterns, despite the overall trend appearing as either increasing or decreasing. For SMRAO, however, the IFT trend exhibited a continuous reduction without any significant fluctuations.

Despite the smoother reducing pattern seen with SMRAO, it is important to note that the IFT values were consistently lower than those observed with crude oil. This phenomenon can be linked to the precipitation of asphaltenes in the presence of CO₂, particularly under elevated temperature and pressure conditions. As asphaltenes precipitate, they migrate toward the interface, where their concentration can increase significantly. This accumulation of asphaltene particles at the interface acts as a surface-active agent or surfactant, contributing to the lowered IFT values.

Fig. 8

Effects of temperature and pressure on the IFT using AOT,(a) 25°C, (b) 45°C, and c: 65°C

Moreover, it is critical to recognize that while an increase in CO₂ content during pressure enhancement directly influences asphaltene precipitation, elevated temperatures can destabilize the resin-asphaltene balance. This instability has implications for the onset and severity of asphaltene deposition during CO₂ injection processes.

In light of these findings, subsequent experiments were conducted to evaluate the impact of surfactants, specifically AES and AOT, on the IFT reduction in carbonated water/SMRAO mixtures. These experiments involved varying both temperature (ranging from 25°C to 65°C) and pressure (from 500 to 4500 psi), while independently adjusting the concentrations of AES and AOT between 0 to 700 ppm, as depicted in Figs. 8 and 9. This systematic approach aims to elucidate the role of surfactants in optimizing IFT reduction in these systems.The impact of surfactants on interfacial tension (IFT) reduction is a critical area of study, especially in the context of enhancing fluid properties for applications in various fields such as enhanced oil recovery, detergency, and pharmaceuticals. Among the surfactants studied, AOT and AES have shown distinct behaviors in their effect on IFT as illustrated in the detailed analysis of their performance.

One of the most striking observations is the pronounced effect of even a low concentration of surfactant, specifically at 100 ppm, on the IFT reduction. This finding suggests that introducing surfactants can significantly alter the interaction between fluids at the molecular level, initiating a series of improvements in fluid mobility and performance. This is particularly relevant in applications that require the minimization of interfacial tension to enhance the efficiency of fluid flow.

Notably, while both AOT and AES demonstrated effective IFT reduction, the results highlight a more pronounced effect with AES. The trend observed with AOT shows a gradual alteration in IFT as its concentration increases from 0 to 700 ppm, indicating a smooth transition without a clearly defined critical micelle concentration (CMC). This gradual reduction suggests that AOT operates effectively across a range of concentrations, but lacks a specific point of saturation where its efficiency experiences a sudden change.

In stark contrast, AES displays a more dramatic performance with a sharp reduction in IFT as the concentration reaches approximately 300 ppm. This indicates that the CMC for AES under conditions of high temperature and pressure is around this concentration, leading to a significant increase in the surfactant's activity and effectiveness. Once this CMC is surpassed, the behavior of AES suggests that it can establish a more stable and favorable interface, contributing to enhanced fluid performance.

A direct comparison of the IFT reduction capacities of these two surfactants reveals that at a concentration of 700 ppm, AES is able to reduce the IFT to a minimum value of 0.44 mN/m at a pressure of 4500 psi and a temperature of 25°C. In comparison, AOT achieves a minimum IFT of 0.67 mN/m under identical conditions. This substantial difference highlights the superiority of AES for applications requiring significant IFT reduction.

Additionally, considering the implications of using these surfactants in activating carbonated water, the results support the notion that AES is a more suitable candidate. The marked efficiency in IFT reduction with AES not only presents it as a more effective surfactant option but also emphasizes the potential benefits in optimizing processes that involve fluid interactions, such as those encountered in various industrial applications.

Fig. 9

Effects of temperature and pressure on the IFT using AES,(a) 25°C, (b) 45°C and (c) 65°C

In conclusion, this analysis underscores the importance of selecting the right surfactant based on desired outcomes, particularly IFT reduction. The distinct behaviors of AOT and AES serve as a reminder of how critical surfactant choice is in achieving optimal performance in fluid systems. Future research should continue to explore the mechanisms behind these differences and the potential for further optimizing surfactant formulations to enhance fluid behavior in a variety of applications.

4. Conclusions

he research investigates the significant impact of thermodynamic parameters—including temperature, pressure, surfactant concentration, and oil type—on the effectiveness of enhanced oil recovery (EOR) using a hybrid carbonated water approach. Key aspects of the study consider temperature and pressure settings ranging from 25°C to 65°C and 500 psi to 4500 psi, respectively, along with their effects on interfacial tension (IFT) and swelling factors. Various surfactants, such as dioctyl sulfosuccinate sodium salt (AOT) and sodium dodecyl polyoxyethylene ether sulfate (AES), were used in concentrations ranging from 0 to 700 ppm. The research also highlights how different oil types affect CO2 solubilization behaviors, which, along with temperature and pressure conditions, have a significant impact on the stability and precipitation of resins and asphaltenes. This, in turn, directly influences the IFT and swelling factors that are critical for EOR processes. Two types of crude oil were studied, along with synthetic mixed resinous and asphaltenic oil (SMRAO), developed to mimic the resin and asphaltene content typically found in crude oil using extracted resin and asphaltene fractions dissolved in toluene. This comprehensive analysis aims to provide insights into optimizing EOR methodologies through a better understanding of these interrelated factors.

Based on measurements of IFT and swelling factors, the following results were obtained:

The data revealed that increasing pressure leads to higher CO2 dissolution and an enhanced swelling factor, which reached a maximum value of 22.3% for SMRAO and 19.3% for crude oil. These findings suggest that resin and asphaltene fractions significantly impact the swelling factor, indicating that the presence of other fractions, including saturates and aromatics, reduces the potential binding between CO2 and the resin and asphaltene fractions.

Additionally, temperature was found to positively influence the swelling factor for both types of oil examined, although the effect was more significant for SMRAO than for crude oil. The study showed that regardless of oil type, as the temperature increased to 65°C, the variation in the swelling factor became significantly more pronounced, displaying a steeper slope compared to the 25°C and 45°C isotherms.

The findings also indicated that the addition of surfactants to the aqueous solutions had a reducing effect on the swelling factor. This occurs because surfactants tend to retain CO2 within their structure, which limits the penetration of CO2 into the interface and oleic phase. Notably, the effect of AOT on reducing the swelling factor was greater than that of AES due to its bulkier structure, which acts as a cage for CO2 molecules, resulting in stronger binding between AOT and CO2, thereby lowering the swelling factor.

Furthermore, it was observed that as pressure increased from 500 psi to 1500 psi—exceeding the critical pressure—the slope of the swelling factor's variation exhibited a pronounced increase. This sharp gradient can be attributed to the substantial rise in solubilized CO2, which significantly enhances interactions with the oil matrix. However, as pressure continued to rise from 1500 psi to 3500 psi, the change in swelling factor became less pronounced, indicating diminishing returns in swelling responses within that pressure range.

IFT measurements demonstrated that the minimum IFT values for crude oil and SMRAO were approximately 16.7 mN/m and 14.2 mN/m, respectively, at a temperature of 25°C and pressure of 4500 psi. The data also revealed that the IFT variation for systems in contact with SMRAO was more linear compared to those with crude oil. This is because SMRAO is more consistent in composition (containing two fractions of resin and asphaltene) than crude oil, which comprises four distinct fractions: saturates, aromatics, resin, and asphaltenes.

The better reduction in IFT observed in the presence of SMRAO is linked to the precipitation of asphaltenes in the presence of carbon dioxide (CO2), particularly under high temperature and pressure conditions. As asphaltenes precipitate, they migrate toward the interface, where their concentration increases substantially. This accumulation of asphaltene particles at the interface acts as a surface-active agent or surfactant, contributing to the reduction in IFT values.

The addition of surfactants at a maximum concentration of 700 ppm demonstrated that AES could reduce the IFT to a minimum value of 0.44 mN/m at a pressure of 4500 psi and a temperature of 25°C. In contrast, AOT achieved a minimum IFT of 0.67 mN/m under the same conditions. This substantial difference underscores the effectiveness of AES for applications requiring significant IFT reduction.

Funding and Competing Interests

Funding:

No funding was received to assist with the preparation of this manuscript.

Competing Interests: The authors declare that they have no competing interests.

• Data Availability

The datasets used and/or analyzed during the current study available from the corresponding author (Dr. Seyednooroldin Hosseini) on reasonable request sent by email to masihanoor@gmail.com.

Author Contribution

Milad Samaeili: Conceptualization, Methodology, Experimental Design, Investigation, Data Curation, Formal Analysis, Visualization, Writing – Original Draft.Seyednooroldin Hosseini: Methodology, Experimental Setup Development, Resources, Data Validation, Writing – Review & Editing.Mohammad Abdideh: Supervision, Project Administration, Technical Guidance, Interpretation of Results, Writing – Review & Editing.Elias Ghaleh Golab: Conceptualization, Funding Acquisition, Supervision, Scientific Interpretation, Writing – Review & Editing, Final Approval of Manuscript.

Data Availability

The datasets used and/or analyzed during the current study available from the corresponding author (Dr. Seyednooroldin Hosseini) on reasonable request sent by email to masihanoor@gmail.com.

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