Introduction
The growth of global energy demand and the decline in oil production from available resources have led to a greater focus on enhanced oil recovery (EOR) methods. In this regard, one of the most widely used methods for increasing oil recovery is gas injection. Specifically, carbon dioxide (CO2) injection is promising because of its low costs, high potential for increasing oil recovery, and favorable environmental effects [1-2]. Since gas is highly mobile within the porous media, the adverse mobility ratio could reduce the oil displacement efficiency[3-4]. Hence, foam injection has been proposed as an effective mobility control scheme to increase the performance of gas injection-based EOR methods [5]. Foams refer to the dispersion of gas in a continuous liquid phase that is separated by thin liquid films called lamella [6-7]. In order to improve the foam performance, surfactants can be used to increase its stability by affecting the surface forces between gas and liquid, which control the capillary pressure [8-9]. However, the adsorption of surfactants on rock surfaces and their decomposition in high-temperature reservoir conditions make the foam unstable in porous media [10-11].
Commonly, nanoparticles are added to foam to improve stability, as nanoparticles can be adsorbed on the surface of the bubbles [12⇓⇓⇓⇓-17]. In fact, nanoparticles at the gas-liquid interface can minimize the direct contact between the gas and liquid on both sides of lamellae. Moreover, they can reduce the bubble size to increase the foam stability [18⇓-20]. Many studies have investigated the effects of hydrophilicity/hydrophobicity of nanoparticles on the stability of foams. It has been concluded that compared to hydrophobic nanoparticles, hydrophilic nanoparticles are superior foam stabilizers [21-22]. Some researchers proved that hydrophobic SiO2 nanoparticles are highly suitable for producing CO2 foams because the hydrophobic SiO2 nanoparticles can be adsorbed at the gas-water interface, producing a more efficient barrier around CO2 foam bubbles and preventing the destruction of the foam [23⇓-25]. Bayat et al. surveyed the effect of some nanoparticles, such as TiO2, Al2O3, CuO and SiO2, on the stability of CO2-foams, and proved that SiO2-CO2 is the most stable foam [22]. Moreover, the synergistic effect of surfactants and nanoparticles, which is the surface modification of the solid nanoparticles through physiochemical interactions with the surfactants, could increase foam stability and produce stronger foams than the surfactants alone [26⇓⇓-29]. Kumar et al. investigated the interaction between SiO2 nanoparticles having negative charges and different surfactants in aqueous solutions [30]. According to their results, in a nanoparticle-surfactant system with an opposite charge, surfactant adsorption on nanoparticle surfaces can lead to the accumulation of nanoparticles. In contrast, similarly charged nanoparticle and surfactant have no physical interaction [30]. Interestingly, nanoparticle-stabilized foams are more resistant to harsh reservoir conditions such as high salinity, high temperatures, and the presence of heavy crude oil [18⇓-20,22,31⇓ -33].
Under reservoir conditions, surfactant and nanoparticle-stabilized foam properties are affected by several parameters, including rock permeability, injection rate, pressure, temperature, salinity, and crude oil composition[34]. A major concern about using foam in petroleum reservoirs is the stability of foam in the presence of oil because crude oil can destabilize the foam in the porous media depending on its specific composition and properties [35⇓⇓⇓-39]. In the oil recovery process, when oil is contained in a foam system, a secondary film is formed between the gas and oil phases, which is called a pseudo-emulsion film [40]. If the pseudo-emulsion film is torn, the oil might form a lens at the gas-water interface. After a while, the oil drop becomes thicker and forms a bridge between the two sides of the lamella, which eventually destroys the foam [41⇓⇓-44]. Overall, foam stability in the presence of crude oil is strongly dependent on the amount of crude oil and the length of the hydrocarbon chain [45]. Among these, the heavy and polar molecules may be more effective, as they can be absorbed into the gas-liquid interfaces and interact with foam stabilizers. Additionally, the viscosity and the density of oil can affect foam stability. In fact, the lower the viscosity and density of oil, the higher the rate of bubbles coalescence and coarsening [46⇓⇓-49].
Nevertheless, some studies have shown that stable foams can be generated effectively by selecting appropriate foaming agents in the presence of oil. Vikingstad et al.[45] observed that foam stability in the presence of oil is related to the surfactant's ability to solubilize oil molecules. According to Farzaneh et al. [49], in the presence of crude oil, the anionic surfactant generates better foam than the nonionic one. The study by Nikolov et al. [37] indicated that the stability of foam increases with increasing the hydrophobic chain length of surfactants in the presence of oil. The effect of SiO2 nanoparticles and AOS (alpha olefin sulfonate) on foam stability in the absence and presence of oil was investigated by Singh and Mohanty [50]. Based on their result, nanoparticles are trapped at the boundaries of the foam plateau and lamella, improving the stability of the foam. In addition, the studies of Nguyen et al. [51] showed that surface-active nanoparticles could significantly improve the stability and oil recovery performance of foam.
As a result, the presence of crude oil negatively affects foam stability, and this adverse effect can be related to components of crude oil. Specifically, asphaltenes with their polarity, tendency to accumulate at the gas-liquid interface, and ability to interact with nanoparticles and surfactants may affect the stability of foams [52⇓⇓⇓-56]. The previous studies have shown that crude oil negatively affects foam stability, some of which focused on how oil affects foam lamella, and others tried to show how it could reduce this effect. However, to the best of our knowledge, no comprehensive study has been conducted to measure the effects of asphaltene on foam stability in the presence and absence of nanoparticles. In this study, we investigated the effect of asphaltene, the heaviest and most polar component of crude oil, on the stability of CO2 foam. Sodium dodecyl sulfate (SDS) surfactant and SiO2 nanoparticles are highly common and effective materials used for foam formation and its stability. Besides, these materials are less expensive compared to the other additives with the same effects [57-58]. Thus, they were chosen as foaming agents in the current study. Foam stability tests were first performed using SDS surfactant at different asphaltene mass fractions. Then, foam stability tests were repeated in the presence of SiO2 nanoparticles and SDS surfactant to investigate the effect of the nanoparticles. Additionally, microscopic images, UV adsorption spectrophotometric concentration measurements, and Zeta potential measurements were conducted to understand how asphaltene affects SDS surfactant and SiO2 nanoparticles, and determine the mechanism of CO2 foam stability reduction.
1. Materials and methods
1.1. Materials
The SDS surfactant, with 98% (by volume) purity, was supplied by Tetrachem Company (Iran). At 25 °C, the critical micelle concentration (CMC) of this surfactant in deionized water was 0.236% [59]. The SiO2 nanoparticles having 99% purity were purchased from US Research Nanomaterials (U.S.), which is a low-cost nanoparticle with good performance in porous media [60]. Normal heptane and toluene, with a purity greater than and equal to 99.9% (by volume), were purchased from Merck (German). Moreover, CO2 gas greater than and equal to 99.9% purity (by volume) supplied by Parsan Gas Company (Iran) was utilized. Additionally, deionized water was used in all of the experiments. In this study, a crude oil sample from a reservoir located in southwest Iran has been employed to extract asphaltene. This sample has the density of 0.969 g/cm3 and the asphaltene content of 17% in mass fraction.
1.2. Methods
1.2.1. SiO2 nanoparticles characterization
To characterize the SiO2 nanoparticles, X-ray diffraction pattern (XRD, MPD Philips X-Pert model) was utilized to distinguish the crystalline structure of SiO2 nanoparticles. Additionally, experiments with Fourier transform infrared spectroscopy (FTIR, WQF-510A model) were performed to identify the functional groups of the SiO2 nanoparticles. A scanning electron microscope (SEM) test was performed to determine the morphology of SiO2 nanoparticles. Additionally, specific surface area measurements using the Brauer-Emmett-Teller method (BET, Nano Sord model) and particle size distribution using the dynamic light scattering method (DLA, Horiba-SZ 100 model) were performed for SiO2 nanoparticles. For the BET analysis, 10 mL of a solution of SiO2 nanoparticles in deionized water at a mass fraction of 0.060% was prepared, and then underwent four periods of 30 min, 190 W ultrasonic bath.
1.2.2. Preparation of surfactant solutions and nanofluids
Monjezi et al. reported that the foam production increases with the increasing concentration of SDS surfactant. However, such phenomena can only be observed if the surfactant concentration is less than CMC value (0.236%). For concentrations above the CMC, increasing the surfactant concentration does not affect the formation and stability of the foam [61]. In this study, to cover the measurement errors of CMC and to ensure that the concentration of SDS in the base solution has reached CMC, the SDS mass fraction of 0.354% in the aqueous suspension was used to form CO2 foam. The optimal mass fraction (0.060%) of SiO2 nanoparticles in the SDS surfactant aqueous solution can improve the stability of the CO2 foam [61]. In this regard, the nanofluids have been prepared by dispersing SiO2 nanoparticles in the water solution. The solution was then put through four periods of 30-minute, 190-watt ultrasonic bath to get the suspension of nanoparticles. The nanofluid was then added to 0.354% SDS surfactant solution to obtain SDS- SiO2 system with the SiO2 nanoparticle mass fraction of 0.06% [61].
1.2.3. Asphaltene separation and characterization
To investigate the effect of oil asphaltene content on foam stability, separating asphaltenes from crude oil is a necessary step in preparing synthetic oil solutions. Here, we have followed the IP-143 standard for asphaltene separation. At each batch process of asphaltene extraction, 10 g to 15 g of crude oil was mixed with n-heptane at a 1:40 (vol/vol) ratio, and then the resulting solution was placed in a dark place. After 24 h, the solution was passed through a filter paper to separate the precipitates. Then, to remove organic impurities from the sediments, the solid deposits on the filter were washed using 100 mL of n-heptane. The Soxhlet system was utilized to separate the asphaltenes from the filter paper in the next step. In the Soxhlet, toluene was employed to dissolve the asphaltenes, and the process was halted when all the asphaltenes were dissolved in the toluene. Finally, the toluene-asphaltene solution obtained during the Soxhlet process was transferred to a large container. After 72 h of passive drying, only the pure asphaltene remained in the container. Characterization of the obtained asphaltene was implemented using CHNS analysis (Elementar, Vario El III). In addition, X-ray diffraction pattern (XRD, MPD Philips X-PERT model) was utilized to specify the structure of asphaltene, and a scanning electron microscopy (SEM) analysis was performed to determine the morphology of asphaltene.
1.2.4. Preparation of synthetic oil
As the majority of petroleum reservoir fluids which can be displaced by foam flooding have an asphaltene content in the range of 0.5% to 15%, solutions of 0.5%, 1.0%, 3.0%, 5.0%, 10.0%, 12.5% and 15.0% of asphaltene in toluene were prepared as synthetic oil samples. These samples were employed to investigate the effect of oil asphaltene content on CO2 foam stability. Asphaltene contents of 0.5%, 1.0% and 3.0% mimic relatively lighter crude oils, and the contents of 5.0%, 10.0%, 12.5% and 15.0% represent intermediate and heavier oils. In order to investigate the effect of oil concentration on foam stability, the experiments were performed at two different ratios of synthetic oil to foam-formation suspension (1:9 and 2:8 in mass ratio, roughly equal to 5% and 10% in volume ratio, respectively). The ratios of synthetic oil to foam formation suspension were selected according to the previous studies [45,48,62] and can be considered as the ratio of oil to water at two geospatial locations of a petroleum reservoir during EOR process.
1.2.5. CO2 injection flow rate
It is necessary to conduct several foam stability tests at various gas flow rates and constant asphaltene mass fraction due to the requirement to identify the optimal CO2 injection rate. For this series of tests, an aqueous solution containing 0.354% SDS surfactant was prepared. Then, a synthetic oil sample with 0.5% asphaltenes was mixed with the surfactant solution at a 1:9 ratio. Then, the CO2 flow rate which yielded the most stable foam was selected. This optimal CO2 injection rate was employed in all foam stability tests. Although the optimal rate of gas flow can change for other nanoparticles and surfactant concentrations, this rate was considered to remain the same in all our experiments.
1.2.6. Foam stability test
The purpose of the tests was to investigate and compare the effect of oil asphaltene content on the stability of the prepared foam in the presence and absence of SiO2 nanoparticles as a stabilizer. First, the stability test was implemented using SDS surfactant at different asphaltene mass fractions. Then, the stability test was repeated on SDS-SiO2 system, in order to investigate the effect of nanoparticles on foam stability.
The test requires a 250 mL graduated cylinder, a gas flow controller (GFC-1104 model), a magnetic stirrer, a cloth sparger, CO2 gas, synthetic oil and aqueous suspension for foam formation (Fig. 1 ). In this experiment, synthetic oil and foam-formation suspension (SDS or SDS-SiO2 system in water) were first poured into a 250 mL graduated cylinder at two different mass ratios of 1:9 and 2:8. Due to the presence of two organic and aqueous phases, mixing is required during the foam generation process to make the liquid phase uniform. For this purpose, a magnetic stirrer was placed under the graduated cylinder with a speed set to 400 r/min. Then, CO2 gas was injected into the foam-generating suspension and oil through a sparger. A pipette was employed to hold the sparger into the foam solution within the cylinder. The head of the graduated cylinder was closed with a cap to prevent the pipette from rotating due to the rotation of the magnet. To evaluate and compare the stability of the foams under different conditions, we measured their half-life periods. Half-life period refers to the time required for foam to decay to half of its initial height after the foam formation process stops [63-64]. In these experiments, when the height of the formed foam reached 200 mL, the gas flow was stopped, and then the foam half-life was recorded.
Fig. 1 The foam stability testing system. |
1.2.7. Spectrophotometric UV-VIS test
The purpose of the spectrophotometric tests is to investigate the asphaltene adsorption on silica in the presence and absence of SDS. It also can help to explain the mechanism of foam stability reduction in the presence of asphaltene. However, the UV-Vis apparatus can only operate when the solution is clear, i.e., the asphaltene concentration is lower than approximately 100 mg/L. Since in the field applications, at the selected concentrations for the foam stability experiments, the solutions are unclear, spectrophotometric tests here were performed at lower asphaltene concentrations and could only give us qualitative and comparative results.
In this experiment, we measured the asphaltene concentration in toluene before and after exposure to nanoparticles and surfactant. Two different methods were used to introduce nanoparticles and surfactant into the toluene-asphaltene solution (synthetic oil): one involved direct addition to the synthetic oil (AN1 and AS1 samples in Table 1 ), and the other involved preparing an aqueous suspension of SDS and SiO2 before adding it to the synthetic oil (AN2, AS2, and ANS samples in Table 1 ). After that, the samples were mixed using a reciprocating shaker at 400 r/min for 1 h, and then, the organic and aqueous phases were separated utilizing centrifugation at 4 000 r/min for 20 min [65]. Finally, the asphaltene concentration in the synthetic oil samples was measured and compared with its value before adding SiO2 and SDS. In such a procedure, if the asphaltene concentration decreases, it indicates that asphaltene-silica or asphaltene-SDS interaction has occurred. The prepared samples are listed in Table 2 , and the volume of all samples was 10 mL.
Table 1 Prepared samples for UV-VIS analysis |
| Sample | No. of liquid phases | Materials |
|---|---|---|
| A | 1 | Toluene + Asphaltene (20 mg/L) |
| AN1 | 1 | (Toluene + Asphaltene (20 mg/L)) + SiO2 (0.2 mg) |
| AN2 | 2 | (Toluene + Asphaltene (20 mg/L)) + SiO2 nanofluid (20 mg/L) |
| AS1 | 1 | (Toluene + Asphaltene (20 mg/L)) + SDS (0.2 mg) |
| AS2 | 2 | (Toluene + Asphaltene (20 mg/L)) + Aqueous SDS solution (20 mg/L) |
| ANS | 2 | (Toluene + Asphaltene (20 mg/L)) + (SiO2 nanofluid (20 mg/L) and aqueous SDS solution (20 mg/L)) |
Table 2 Samples for Zeta potential measurements |
| Sample | No. of liquid phases | Materials |
|---|---|---|
| N | 1 | Aqueous SiO2 nanofluid (0.060%) |
| NA | 2 | SiO2 nanofluid (0.060%) + (Toluene + Asphaltene (0.5%)) |
| S | 1 | Aqueous SDS solution (0.354%) |
| SA | 2 | Aqueous SDS solution (0.354%) + (Toluene + Asphaltene (0.5%)) |
| NS | 1 | SiO2 nanofluid (0.060%) and SDS (0.354%) |
| NSA | 2 | SiO2 nanofluid (0.060%) and SDS (0.354%) + (Toluene + Asphaltene (0.5%)) |
Shimadzu UV-240 spectrophotometer was used to measure the asphaltene concentration, where the interaction of asphaltenes with nanoparticles and surfactants was investigated at the wavelength of 285 nm. Then, the line equation from the calibration diagram (obtained at the same wavelength) was employed to establish the attraction relationship in terms of concentrations at the range of 5 mg/L to 30 mg/L. The amount and percentage of interacting asphaltene with nanoparticles and surfactant could be calculated from Eqs. (1) and (2), respectively.
$q=\frac{\left( {{c}_{\text{o}}}-{{c}_{\text{e}}} \right)V}{m}$
$R=\frac{{{c}_{\text{o}}}-{{c}_{\text{e}}}}{{{c}_{\text{o}}}}\times 100%$
1.2.8. Zeta potential test
The Zeta potential test examines the surface charge of particles in a solution and serves as a crucial factor in assessing dispersion stability. In this study, we employed Zeta potential analysis (using the Horiba-Sz 100 DLS-ZETA apparatus) to further investigate the mechanism behind foam degradation in the presence of asphaltene and stabilizers (SDS and SiO2). For this experiment, the Zeta potential of SiO2 and SDS in the aqueous phase was measured before and after contact with the synthetic oil samples. As most colloidal dispersions in aqueous media carry electric charge, high Zeta potential indicates that the particles are more stable because there is a strong repulsive force between them. Conversely, low Zeta potential indicates the tendency of particles to accumulate [67-68]. When asphaltene comes in contact with a suspension of surfactant and nanoparticles, the interaction of asphaltene with the nanoparticles or surfactant is expected, which reduces the surface charge of nanoparticles and surfactants. As shown in Table 2 , several samples were prepared for the Zeta potential experiments. Finally, the Zeta potential of nanoparticle suspensions, surfactant solutions, and surfactant-nanoparticle suspension before and after contact with asphaltene were evaluated and compared.
2. Results and discussion
2.1. Characterization of SiO2 nanoparticles
The XRD, FTIR and SEM tests were performed for SiO2 nanoparticles. The results are presented in Figs. 2 and 3 . The XRD analysis of SiO2 nanoparticles shows their amorphous structure (Fig. 2a ). Additionally, the SEM image in Fig. 2b indicates the near-spherical morphology of SiO2 nanoparticles. To evaluate the results of the FTIR test, the obtained graph peaks were compared with those reported in the paper of Tinio et al. [69], and the factor groups in each peak were identified (Fig. 3 ). The specific surface area (BET analysis) is 142.8 m2/g, and the average particle size measured by dynamic light scattering (DLS) is 161 nm (with maximum standard deviation of ± 5 nm) for SiO2 nanoparticles.
2.2. Asphaltene separation results
Separating asphaltene from crude oil for synthetic oil preparation is the first step in this study, as explained in the methods section. The XRD analysis of asphaltene shows their amorphous structure (Fig. 4a ) [70-71]. Moreover, the SEM image shown in Fig. 4b depicts the morphology of asphaltene. The CHNS analysis was used to determine the ingredients in the extracted asphaltene, and the percentage of carbon, hydrogen, sulfur, nitrogen and oxygen is 84.030%, 7.447%, 5.208%, 0.561%, and 2.754%, respectively. The composition of asphaltene is consistent with that of previous studies [72-73].
2.3. CO2 foam stability experiments
In order to investigate the effect of asphaltene on foam stability in the presence of foam stabilizers (SDS surfactant and SiO2), various influential variables, including gas injection flow rate, oil asphaltene content, presence of SiO2 nanoparticles, and oil to foam-formation suspension ratio were studied. Initially, we conducted experiments to determine the optimal gas injection flow rate. Subsequently, foam stability tests were carried out with varying mass fractions of asphaltene in the synthetic oil solutions, and the results were averaged from three repetitions.
2.3.1. CO2 injection flow rate
Previous studies have indicated that the longer the bubble life in foam solutions, the longer it takes for the gas and liquid to reach equilibrium [74]. The residence time of gas bubbles in the foam mixture increases by lowering the gas injection rate, which makes the number of surfactant monomers in the gas-liquid interface increase. Therefore, reducing the gas injection rate to an optimal value improves the stability of the foam [63]. As mentioned earlier, changing the concentrations of nanoparticles and surfactants can change the optimal gas flow rate; nonetheless, in this study, this rate was considered constant at 40 mL/min when the mass fraction of SDS surfactant is 0.354% and that of asphaltene is 0.5% (Fig. 5 ). This flow rate, which resulted in the most stable foam, has been employed in all other foam stability experiments.
Fig. 5 Half-life of foam at different gas injection rates (tested using 0.354% SDS and 0.5% asphaltene; the standard deviation of half-life is ±20 s, the same below). |
2.3.2. Stability of surfactant-stabilized CO2 foam in the presence of synthetic oil
In the first step, SDS surfactant foam stability was studied in the absence of oil, where its half-life was measured to be 1 825 s. Then, the stability of CO2 foam stabilized by SDS was measured against different asphaltene mass fractions in oil. This test was performed in two mass ratios of synthetic oil to SDS solution, 1:9 and 2:8, in order to investigate the effect of synthetic oil saturation on the stability of the foam.
2.3.2.1. Stability of surfactant-stabilized foam with mass ratio of synthetic oil to SDS solution at 1:9
The results of foam stability experiments under the mass ratio of synthetic oil to SDS solution at 1:9 with different asphaltene contents are shown in Fig. 6a . The stability of surfactant foam was tested in the presence of pure toluene. The half-life of SDS surfactant-stabilized is measured to be 751 s, as depicted in the 0 mass fraction of asphaltene. The half-life of the surfactant-stabilized foam at an asphaltene mass fraction of 0.5% is 509 s. As the asphaltene mass fraction increases from 0.5% to 15.0%, the half-life of the surfactant stabilized foam decreases to 239 s. Therefore, the half-life of surfactant-stabilized foam has reduced from 1 825 s in the absence of oil and 751 s in the presence of pure toluene to 239 s in the presence of synthetic oil with 15.0% of asphaltene content.
Fig. 6 Half-life of SDS-stabilized foam at mass ratio of synthetic oil to SDS solution at 1:9 (a) and 2:8 (b). |
2.3.2.2. Stability of surfactant-stabilized foam with mass ratio of synthetic oil to SDS solution at 2:8
As stated before, to investigate the effect of oil saturation on the foam stability, we increased the mass ratio of synthesized oil to SDS solution to 2:8. In the same manner, the CO2 foam stability experiments were performed in different mass fractions of asphaltene, and the results are presented in Fig. 6b . The SDS surfactant-stabilized foam half-life is 526 s in pure toluene. At an asphaltene mass fraction of 0.5% in toluene, the half-life of surfactant foam is reduced to 342 s. With increasing the asphaltene mass fraction from 0.5% to 15.0%, the half-life of foam decreases to 171 s. A comparison of the obtained results indicates increasing the mass ratio of synthetic oil to SDS solution from 1:9 to 2:8 would reduce the half-life and stability of all CO2 foams by 28.45%-32.80%.
2.3.3. Stability of SDS-SiO2-stabilized CO2 foam
Foam stability experiments were conducted with surfactant-nanoparticle system, both in the absence and presence of asphaltene, to examine the impact of SiO2 nanoparticles on stability across different asphaltene mass fractions in the oil. The half-life of SDS-SiO2-stabilized CO2 foam in the absence of oil is measured to be 2 670 s, which is increased by 46.30% in foam stability compared to the half-life of SDS surfactant-stabilized foams (1 825 s). The foam in the presence of SiO2 nanoparticles is more stable as the nanoparticles increase the viscosity of the thin layers of foam, thereby reducing the amount of liquid discharged through the lamella [75-76]. On the other hand, when nanoparticles with a specific surface charge are placed on the contact surface between liquid and gas, they create a repulsive force between the two lamella surfaces and prevent the surfaces from colliding with each other. Consequently, nanoparticles can prevent the foam lamella from disappearing [77-78]. In the current research, the stability of surfactant-nanoparticle foam against different mass fractions of asphaltene has been investigated. Foam stability experiments were conducted with two mass ratios of synthetic oil to SDS-SiO2 system at 1:9 and 2:8.
2.3.3.1. Stability of SDS-SiO2-stabilized foam with mass ratio of synthetic oil to SDS-SiO2 at 1:9
Under the mass ratio of synthetic oil to SDS-SiO2 system at 1:9, the surfactant-nanoparticle-stabilized foam stability experiments were performed at various mass fractions of asphaltene in the synthetic oil, and the results are presented in Fig. 7a . As it is observed, the half-life of SDS-SiO2-stabilized foam is 912 s in the presence of pure toluene. At the same conditions, the half-life of SDS-SiO2-stabilized foam at a mass fraction of 0.5% of asphaltene is 703 s. In addition, increasing the asphaltene mass fraction from 0.5% to 15.0% leads to a reduction of the foam half-life to 298 s. Comparing the stability of nanoparticle-SDS stabilized foam with that of SDS-only stabilized foam reveals that SiO2 nanoparticles improve CO2 foam stability.
2.3.3.2. Stability of SDS-SiO2-stabilized foam with mass ratios of synthetic oil to SDS-SiO2 at 2:8
The mass ratio of synthetic oil to SDS-SiO2 was adjusted at 2:8. Hence, the oil saturation in the system increased while the concentration of other compounds, including SiO2 and SDS, remained constant. The half-life of surfactant-nanoparticle-stabilized foam in the presence of pure toluene is obtained to be 660 s. At 0.5% of oil asphaltene content, the half-life of the foam is 460 s, and when the asphaltene mass fraction increases to 15.0%, as shown in Fig. 7b , the half-life of the foam decreases to 205 s. These results indicated that increasing the saturation of synthetic oil would lead to intensifying foam destruction. For example, at 3% of asphaltene, when the mass ratio of synthesized oil to SDS-SiO2 is 1:9, the half-life of foam is 572 s, while with 2:8, the foam half-life is 390 s.
In Fig. 8 , a comparison was made for all the various tested conditions. The results illustrate that with a lower mass fraction of asphaltene, the half-life of the foam is higher. In other words, by increasing the asphaltene content from 0.5% to 15.0%, the half-life of the foam decreases. Hence, it can be concluded that asphaltene is a destructive agent for CO2 foam, and with increasing its mass fraction, the stability of the foam decreases. On the other hand, the presence of SiO2 nanoparticles enhances the CO2 foam stability even in the presence of oil. Nanoparticles can reside at the contact interface of liquid and gas and prevent lamella from colliding [47]. Furthermore, as shown in the diagrams, increasing the saturation of the synthesized oil from 1:9 to 2:8 reduces the stability of the foam. Synthetic oil has a detrimental effect on the foam structure, and with increasing saturation, this damaging effect increased significantly [48].
Fig. 8 Overview of half-life tests at different mass fractions of asphaltene (the half-life standard deviation is ± 20 s). |
The images captured in Fig. 9 using a digital microscope at 1 000x magnification reveal the existence of scattered dark asphaltene particles within the foam lamella, signifying the presence of asphaltene within the lamella.
Fig. 9 Microscopic images of foam lamellae in different asphaltene mass fractions. |
2.4. Interaction of SDS-SiO2 with asphaltene in the lamella
The interaction of asphaltene with SiO2-SDS was investigated by asphaltene content measurements using UV/Vis absorption spectrophotometry. To perform this, the asphaltene concentration was obtained before and after contact with SDS and SiO2. To measure the concentration of asphaltene in toluene using the UV/Vis analysis, the line equation obtained from the calibration diagram shown in Fig. 10 was employed. This graph has been plotted for concentrations of 5, 10, 15, 20, and 30 mg/L asphaltene in toluene solvent.
Fig. 10 Toluene-asphaltene solution calibration diagram. |
The results of UV/Vis are presented in Table 3 . In all experiments, the initial asphaltene concentration in toluene was 20.00 mg/L. For the AN1 sample, solid SiO2 nanoparticles were added to the asphaltene-toluene solution. After the test, asphaltene concentration in the toluene solvent decreased from 20.00 mg/L to 14.99 mg/L, which indicates that the asphaltene conveys its adsorption on the nanoparticles' surfaces. In this case, the amount of asphaltene adsorbed on the nanoparticles is equal to 25.05% of the initial asphaltene concentration. For sample AN2, a suspension of SiO2 nanoparticles at a concentration of 20 mg/L was prepared and then added to the asphaltene-toluene solution. After testing, it was found that the concentration of asphaltene in the toluene solvent decreased from 20.00 mg/L to 17.62 mg/L. In other words, the amount of asphaltene adsorbed on the nanoparticles is 11.85% of the initial asphaltene concentration. In both AN1 and AN2 samples, asphaltenes are adsorbed on SiO2 nanoparticles, even if they have existed in different phases; however, the amount of adsorption is different. Adsorption of asphaltenes on nanoparticles is 52.59% higher when nanoparticles are added to the toluene-asphaltene solution in solid form than when they are added in suspension. Such a result has also been observed in previous studies [65,79]. In AS1 and AS2 samples, the interaction between asphaltene and SDS surfactant was evaluated. Asphaltene-SDS interactions might be possible; however, the asphaltene remained stable in toluene since no meaningful reduction in asphaltene concentration was observed. Thus, this method could not verify or reflect the asphaltene-SDS interaction while proving the asphaltene adsorption on SiO2 nanoparticles. Hence, additional methods and tests are required to determine if asphaltene and surfactant adsorbed each other.
Table 3 UV-VIS spectrophotometric test results |
| Sample | Asphaltene concentration in toluene after the test*/(mg•L−1) | Adsorption of asphaltene by SiO2/(mg•g−1) | Percentage of asphaltene reduction in toluene during the test/% |
|---|---|---|---|
| A | 20.00 | - | - |
| AN1 | 14.99 | 250.000 | 25.05 |
| AN2 | 17.62 | 118.535 | 11.90 |
| AS1 | 20.00 | - | - |
| AS2 | 20.00 | - | - |
| ANS | 17.56 | 121.630 | 12.20 |
Note: *The maximum amount of standard deviation is ± 0.09 mg/L; “-” means no adsorption phenomenon is observed |
Finally, the reduction of asphaltene concentration in the ASN sample was investigated. After the SDS-SiO2 suspension was prepared at a concentration of 20.00 mg/L and added to the toluene-asphaltene solution, the asphaltene concentration in the toluene solvent decreased from 20 mg/L to 17.56 mg/L. Noticeably, the results were similar to that of the AN2 sample. Thus, the asphaltene adsorption of SiO2 nanoparticles was virtually the same in the presence and absence of SDS.
2.5. The foam stability reduction mechanism in the presence of asphaltene
As explained before, asphaltenes can penetrate foam lamella, and the penetration intensifies by increasing the asphaltene content of oil. Moreover, asphaltene can be adsorbed on the surface of SiO2 nanoparticles in the lamella, even if they are in two different phases. There are van der Waals and Coulombic interactions between surfactant and asphaltene due to the presence of oxygen in their chemical structure [56]. Based on the literatures, the presence of surfactant and silica inhibits lamella destruction since they can impose repulsive forces in the liquid-gas interface between lamellae. These repulsive forces could be attributed to the surface charge of particles and steric effects [80⇓-82]. However, the interactions between CO2 foam stabilizers and asphaltene may reduce the surface charges of foam stabilizers as well as the repulsive forces on two sides of the lamella interface, decreasing the foam stability (Fig. 11 ).
Fig. 11 Schematics of foam lamellae before and after the addition of asphaltene. |
The surface charge reduction of foam stabilizers in the presence of asphaltene was studied using Zeta potential measurements. The Zeta potential of the SiO2 particles in the N sample is −60.3 mV, and for the NA sample is −47.2 mV. It should be noted that as the Zeta potential approaches zero, the surface charge decreases. Consequently, the results indicate that the surface charge of SiO2 nanoparticles was reduced by 21.72% after contact with asphaltene. Therefore, asphaltene may have been adsorbed on the nanoparticles, reducing their surface charge and, hence, their ability to stabilize foam. The Zeta potential of solution S is −80.2 mV, and for solution SA, it is −63.4 mV, which demonstrates that the charge of SDS surfactants was reduced by 20.94% due to contact with asphaltene. The same results were obtained in cases where nanoparticles were also present. The Zeta potential of the NS suspension (SiO2-SDS suspension) is −66.7 mV, whereas the Zeta potential of the NSA solution is −48.6 mV. Thus, when the samples come in contact with asphaltene, the surface charge is reduced by 27.13% due to the interaction of asphaltene with surfactants-nanoparticles.
The results confirm that asphaltenes interact with nanoparticles and surfactants and decrease their surface charge. As already mentioned, reducing the surface charge of foam stabilizers reduces the repulsive force on both sides of the lamella and brings the lamella surfaces closer together. Eventually, the lamella surfaces collide more easily, resulting in the rupture of lamellae. Due to the loss of the lamella, the stability of the foam is reduced.
3. Conclusions
The CO2 foam stability tests were performed on SDS surfactant and SDS-SiO2 as foam-formation suspensions. It is found that in the absence of synthetic oil, SDS-SiO2 can increase foam stability by 46.30% as compared with SDS surfactant only. When the mass ratio of synthetic oil to foam-formation suspension is 1:9, increasing the mass fraction of asphaltene in the synthetic oil from 0 to 15% reduced the half-life of SDS surfactant-stabilized foam from 751 s to 239 s, and that of SDS-SiO2-stabilized foam from 912 s to 298 s. When the mass ratio of synthetic oil to foam-formation suspension is 2:8, increasing the mass fraction of asphaltene in the synthetic oil from 0 to 15% reduced the half-life of SDS surfactant-stabilized foam from 526 s to 171 s, and that of SDS-SiO2-stabilized foam from 660 s to 205 s.
Concentration measurement using UV/Vis adsorption tests indicated that asphaltene could adsorb on the surface of SiO2 nanoparticles in the lamella, even if they were in two different phases (solid or suspension). In addition, the Zeta potential measurement showed that the surface charges of particles were reduced due to the interaction of asphaltene with SDS/SiO2 in the aqueous phase. Such a phenomenon led to the reduction of repulsive forces between two interfaces of lamella in the presence of asphaltene, which in turn, caused the foam stability to decrease.
Nomenclature
ce—asphaltene concentration after the adsorption on the nanoparticles, mg/L;
co—initial asphaltene concentration, mg/L;
m—amount of nanoparticles or surfactant absorbing asphaltene, g;
q—amount of asphaltene interacting with nanoparticles or surfactant, mg/g;
R—percentage of the asphaltene interacting with nanoparticles or surfactant, %;
V—asphaltene solution volume, L.