Introduction
Nanotechnology has been the turning point in developing novel technologies that rely on advanced materials. Accordingly, huge successful investments have been made in employing nanotechnology to solve critical engineering issues [1⇓-3]. The main classes of nanomaterials exploited in engineering applications are metal oxides and carbon-based nanostructures which provide specific properties at different scales and morphologies. In addition, the simple synthesis methods, abundance, ease of scaling-up, and low cost make the metal-oxide nanomaterials the suitable agent in the enhanced oil recovery (EOR) investigations.
The chemical EOR (C-EOR) is divided into exploiting polymers, alkali, or surfactants, and the oil displacement mechanism includes microscopic displacement and volumetric sweep. Considering the critical conditions during the C-EOR methods (i.e., high temperature and salinity), there have been limitations in employing C-EOR. Moreover, since chemical flooding is expensive, as well as other factors such as formation damages, nanomaterials have attracted additional interest for enhanced oil production.
In recent years, with the advancement in nanotechnology and realization of its potential capabilities, researchers have conducted studies on the use of nanoparticles to enhance oil production [4-5]. Pickering emulsion has been proposed as a new method of C-EOR where the stability of the emulsion is improved by employing solid particles [6⇓⇓-9]. Silica and titanium are the most studied metallic nanoparticles for which it is confirmed that the synthesis conditions, functionalizing, and other factors can greatly impact their performance. Qin et al. [10] proposed a novel method to in-situ synthesize silica nanoparticles in microemulsions with less tendency to agglomeration and a better Pickering formation activity and therefore the better performance in EOR. Yoon et al.[11] stabilized the Pickering emulsion by using a colloidal layer that was composed of silica nanoparticles, dodecyl trimethyl ammonium bromide (DTAB) and poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSS-co-MA). They observed that the colloidal dispersion raised the oil recovery factor by 4 percentage points in comparison to that of water flooding. In another work, Jia et al. [12] stabilized the Pickering emulsions by dendritic silica nanoparticles and hybrids of dendritic mesoporous silica and titanium. In other attempts, Junus-SiO2 nanoparticles [13], AlO(OH) nanoparticles on sodium dodecyl benzene sulfonate [14], mixed AlO(OH)/SiO2 aqueous dispersions [15], and silica nanoparticles/nonionic surfactant have been examined in stabilizing the Pickering emulsion and the EOR investigation. TiO2 has also been a fascinating choice for EOR [16-17]; however, TiO2 has not been assessed in stabilizing the Pickering emulsions.
In addition to the metallic nanoparticles, carbon nanomaterials have been extensively studied in stabilizing the Pickering emulsions and EOR studies. These efforts include the use of multi-walled carbon nanotubes (MWCNT), single-walled CNTs (SWCNTs), active carbon [18], functionalized CNT/silica [19], and nanoporous graphene/silica [18] in stabilizing the Pickering emulsion. Afzalitabar et al. [20] reported that the nanoporous graphene/silica has better performance than CNT/silica in preparing the Pickering emulsions.
According to the literatures, to benefit from the advantages of both metal-oxides and carbon nanomaterials, it is worthy to focus on carbon/metal-oxide nanocomposites. In this work, CNT/TiO2 nanocomposite is synthesized and used in stabilizing the Pickering emulsion. Its performance on oil displacement in porous media through contact angle, interfacial tension (IFT), and emulsion stability is evaluated. Finally, the application of the synthetized CNT/TiO2 nanocomposite in EOR application has been evaluated through microfluidic experiments.
1. Experiment
1.1. Materials and methods
The chemical reagents comprising benzyl alcohol, titanium isopropoxide (TTIP), ethanol, and heptane (with a high purity) were purchased from Merck and used. Deionized water (with electrical conductivity of 1.8 µS/cm) was used throughout the experiments. Multiwall CNT (MWCNT) with a diameter of 20-25 nm was prepared by chemical vapor deposition (CVD) of methane using CoMoCAT method. The crude oil, as received from a current oil field in Iran, was used. In addition, prior to experiments, separation of the asphaltene from crude oil was carried out. The crude oil has a density of 0.84 g/cm3, and a viscosity of 4.2 mPa•s.
1.2. Pretreatment of multiwall CNT
Since CNTs are hydrophobic, it is required to functionalize them with hydrophilic organic groups to provide attractive interaction with the titanium sol. These groups are normally attached via covalent bonding after ultrasonication in strong acids. The thickness of TiO2 coating on CNTs is highly nonuniform and a great amount of oxide is present as particles. Therefore, the improvement of the interface between TiO2 and the CNT surface, and the control of size and morphology of the TiO2 particles during crystallization are the challenges. Adding benzyl alcohol (BA) as surfactant enables TiO2 to uniformly coat pristine CNTs without the need for covalent functionalization. BA acts as a weak surfactant and enhances the dispersion of CNTs in the reaction solution, which helps to achieve a uniform distribution of TiO2. After sonication, the CNTs were filtered out and washed with deionized water (DI water) to reach neutral pH, then dried in the oven [21].
1.3. Synthesis of CNT/TiO2 nanocomposites
To synthesize CNT/TiO2 nanocomposites, 0.25 g CNT was dispersed in 164 mL ethanol using an ultrasonic bath for 10 min at the power of 80 W, followed by 5 min resting, and repeating this cycle two more times (Solution 1). In the next solution, 10.8 g benzyl alcohol was mixed with DI water, while stirring (Solution 2). Then, Solution 1 was poured into a volumetric flask, placed in ice. When, the temperature of Solution 1 reached 0 ˚C, Solution 2 was added to it, which were then mixed for 2 h (Solution 3). Afterward, the titanium oxide precursor (TTIP) was added to ethanol dropwise and stirred for 30 min (Solution 4). Then, Solution 4 was added to the Solution 3 dropwise, which was then mixed for 2 h. In the next step, the solution was kept at room temperature to be aged, followed by filtering and drying in the oven.
1.4. Characterization techniques
1.4.1. Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FTIR) analysis is performed using a FTIR spectroscopy in the range between 400 cm−1 and 4000 cm−1 (Tensor 27, Bruker Inc., Germany). FTIR is an effective analytical instrument for detecting functional groups and characterizing covalent bonding information. Chemical bonds can be detected through their characteristic infrared absorption frequencies or wavelengths.
1.4.2. Scanning electron microscope
A scanning electron microscope (SEM) is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample’s surface topography, composition and other properties such as electrical conductivity.
1.4.3. X-ray diffraction analysis
X-ray diffraction analysis (XRD) is a technique used in materials science to determine the crystallographic structure of a material. XRD works by irradiating a material with incident X-rays and then measuring the intensities and scattering angles of the X-rays that leave the material. A primary use of XRD analysis is the identification of materials based on their diffraction pattern. As well as phase identification, XRD also yields information on how the actual structure deviates from the ideal one, owing to internal stresses and defects.
1.4.4. Brunauer-Emmet-Teller analysis
Specific surface area of nanohybrid was measured based on the principle of N2 adsorption-desorption with Brunauer-Emmet-Teller (BET) surface analyzer (SA 3100, Coulter, USA).
1.4.5. Zeta potential measurement
In order to determine the surface charge of nanoparticles in colloidal solution, Zeta potential is an analytical measurement that is generally used. Malvern Zetasizer was used to measure the Zeta potential of nanofluid. The instrument is capable of measuring Zeta potential with particle size ranging from 0.3 nm to 8 µm and temperatures up to 90 °C.
1.4.6. Thermal gravity analysis (TGA)
Thermal stability of the synthetized nanocomposite was investigated using Shimadzu TG50 analyzer by measuring the weight loss and heat flow (differential scanning calorimetry) at temperature up to 800 °C with heating rate 10 °C/min.
1.5. Contact angle measurement
To investigate the wetting behavior of the rocks after being exposed to the nanofluids, the contact angle measurement tests were conducted. To this end, high-resolution images were taken from a drop over a slice of rock. The oil-wet carbonate slabs were used for contact angle measurement, which was polished before the tests and then fixed in the cell which was filled by the solutions of 0.1% CNT/TiO2 nanofluid (dispersed in DI water and brine). In the next step, a small oil drop was directed toward the carbonate surface and the photographs were taken to assess the contact angle.
1.6. IFT measurement
The drop profile analysis tensiometer (PAT, SINTERFACE Co. Germany) [22] was employed to record the dynamic interfacial between oleic and aqueous phases, i.e., n-heptane and the nanofluid containing varied amounts of CNT/TiO2 nanofluid (within 4% NaCl). Monitored by image acquisition with time, all the IFT measurement experiments were performed at ambient condition.
1.7. Emulsion preparation
To study the effect of varied parameters on the stability of the emulsion, the emulsion with varied amounts of CNT/TiO2 nanocomposites were prepared. To disperse the CNT/TiO2 nanocomposites in the aqueous phase of the emulsions, the ultrasonication with the power of 70 W for 10 min was employed. In the next step, n-heptane was added to the prepared nanofluid as the oil phase. Then, magnetic stirring was used for 10 min at 1400 r/min, followed by placing the samples in an ultrasonic bath for 30 min. To monitor the stability of the prepared emulsions, emulsion volume and particle size were studied by Digitizer software of the images taken by optical microscope after 24 h, 72 h, and 7 d.
1.8. Microfluidic oil displacement experiments
To investigate the effect of nanofluids on oil displacement inside the porous media and the effective EOR mechanisms, the glass micromodel experiments were considered. To this end, a two-dimensional model of the reservoir rock was used, which was a simplified simulation of a porous medium with a pores network. The experimental setup comprises of a syringe pump system capable of injecting fluids into the micromodel at very low rate (0.000 1-3.000 0 mL/min). A backlight source was placed under the micromodel, while a high-resolution video camera was installed above the micromodel and connected to a computer to schedule imaging times to record the variations of fluid saturations within the micromodel. To collect effluent, a storage tank was connected to the outlet port of the micromodel. Fig. 1 shows the schematic elements of the apparatus. The dimension of the micromodel is 3.5 cm×3.5 cm, and the pore diameter is 16-270 µm, the etched depth is 166 µm, with a total pore volume of 0.08 mL and absolute permeability of 1.1 µm2.
Fig. 1. Schematic representation of fluid injection. |
To analyze the obtained data, ImageJ software was utilized, and according to Eq. (1), the recovery factor was calculated.
Prior to each test, toluene was utilized to clean the horizontal glass micromodel, followed by DI water and evacuating with a vacuum pump. In addition, wettability was altered to the oil-wet using salinized process [22-23]. Afterward, the micromodel was saturated by the crude oil. In the following, DI water was injected into the micro- model with the injection rate of 0.000 6 mL/min, and then 0.01%, 0.05% and 0.50% CNT/TiO2 nanofluids were injected at the same rate. The rate is chosen based on the capillary number and reasonably simulates fluid flow velocity in the underground reservoirs. Fluid flow behavior in porous media during nanofluid flooding and waterflooding (deionized water) is recorded using a video camera and the resulting images are processed to assess the effective mechanisms of nanoparticles on oil displacement in porous media.
2. Results and discussion
2.1. Characterization of CNT/TiO2 nanocomposite
The XRD pattern of the CNT/TiO2 nanocomposite is represented in Fig. 2 , in where distinct peaks confirm the formation of crystalline phases. The obtained pattern is similar to the JCPDS-card-No-21-1272 corresponding to TiO2 [24]. Since the synthetized nanocomposite contains a low amount of the CNT, it cannot be detected by the XRD analysis. In the other word, the XRD analysis is only able to detect the surface of the CNT/TiO2 nanocomposite, as it is observed in the XRD analysis that the TiO2 nanoparticles cover the CNTs’ surface.
Fig. 2. The XRD pattern of the synthetized CNT/TiO2 nanocomposite. |
N2 adsorption/desorption isotherm of the CNT/TiO2 nanocomposite is presented in Fig. 3 a. The specific surface area and total pore volume of the sample is obtained as 32.34 m2/g and 0.078 cm3/g, respectively. The N2 adsorption/desorption isotherm of the CNT/TiO2 nanocomposite corresponds to Type IV isotherm, demonstrating that the synthetized nanocomposite is mainly composed of mesopores. Moreover, the pore size distribution shows peaks in the range of 2-5 nm, which is the indication of the presence of mesopores in the synthetized nanocomposite (Fig. 3 b).
Fig. 3. The N2 adsorption/desorption isotherm (a) and pore size distribution of the CNT/TiO2 sample (b). |
The morphology and structure of the CNT/TiO2 nanocomposite is investigated through the FESEM images that are given in Fig. 4 . In this Figure, the presence of the CNTs is detected, that are covered by TiO2 particles. In addition, the SEM images confirm that a TiO2-based nanocomposite in which CNTs act as the reinforcement phase. Therefore, a nanocomposite with possibly higher mechanical and thermal stability is formed which may be effective as an EOR agent.
Fig. 4. FESEM images of the CNT/TiO2 nanocomposite. |
Considering the harsh conditions (high temperature and pressure) in the reservoirs, the TGA analysis is used to evaluate the thermal stability of the CNT/TiO2 nanocomposite. The TGA curve is plotted in Fig. 5 indicating the great thermal stability of the synthetized nanocomposite. Up to 800 °C the nanocomposite retains about 98% of its initial mass. This indicates that the synthetized nanocomposite comprising the TiO2 matrix and CNT reinforcement possesses suitable thermal stability which is a good choice for the reservoir condition in the EOR investigation.
Fig. 5. The TGA curve of the CNT/TiO2 nanocomposite. |
The Zeta potential values of the 0.1% CNT/TiO2 nanocomposite dispersed in the DI and saline water are −38.5 mV and −32.5 mV, respectively. Both values confirm the high stability of the nanocomposite dispersed in the DI and saline water, indicating that the CNT/TiO2 is an effective EOR agent. In addition, the average particle size of CNT/TiO2 nanocomposites dispersed in the DI and saline water is 50.00 nm and 48.30 nm, respectively.
2.2. EOR performance
To evaluate the performance of the synthetized CNT/TiO2 nanocomposite in the EOR application, different factors are considered. In this paper, the contact angle, interfacial tension and emulsion stability have clarified the performance of the nanomaterials in the EOR. The microfluidics experiments are carried out to reveal the performance of CNT/TiO2 nanocomposite in oil production.
2.2.1. Contact angle
The measured contact angle between the oil droplet and an oil-wet rock slice is 15˚, implying that the rock slices are strongly oil-wet. The contact angle between oil-wet rock slices, which was initially aged in different fluids (0.1% CNT-TiO2 dispersed in the DI and saline water) and oil drop is measured, which resulted in a considerable increase of the contact angle. As represented in Fig. 6 b, the contact angle is increased to 135.5˚ while using CNT/TiO2 nanocomposite dispersed in DI water. Moreover, nanofluid with the saline water as a base fluid is also able to increase the contact angle to 134.0˚ (Fig. 6 c). Therefore, the collected data show the significant effect of nanofluid on wettability alteration from the oil-wet surfaces to water-wet surfaces, which is considered as an effective mechanism in the EOR investigations.
Fig. 6. The contact angle between oil drop and rock slice treated with different CNT/TiO2 nanofluids. |
2.2.2. Interfacial tension measurement
To assess the nanomaterials performance in the oil displacement, IFT reduction between displacing and displaced fluid (i.e., the nanofluid and the crude oil, respectively) is one of the fundamental EOR mechanisms. To evaluate the performance of the CNT/TiO2 nanocomposite, the IFT between n-heptane and different mass fractions of CNT/TiO2 nanofluids have been evaluated over time. The IFT assessment results are represented in Fig. 7 . The IFT between n-heptane and the base fluid (DI water) is measured at about 51 mN/m, which shows no distinct variations in IFT over time. However, varied mass fractions of the CNT/TiO2 nanocomposites are able to reduce the IFT between nanofluid and n-heptane. As is observed, the IFT values increased with the reduction of the nanofluid mass fractions i.e., the mass fraction of 0.01% CNT/TiO2 leads to the IFT reduction to 41-43 mN/m while increasing the CNT/TiO2 mass fraction to 0.1% resulting in the IFT reduction of 36-37 mN/m. Therefore, the lowest IFT value is related to the sample with the highest nanofluid mass fraction. The absorption of nanoparticles at the interface between nanofluid and the crude oil can change the IFT value of the nanofluid-oil systems. As the nanofluid mass fraction increases, the repulsive electrostatic force between nanoparticles causes the nanoparticles to diffuse from the aqueous brine solution to the nanofluid-oil interface. Consequently, the interface is more saturated with nanoparticles and IFT between the injected fluid and the crude oil decreases. Therefore, it can be inferred that the nanofluid has the potential to produce more oil from the reservoir in comparison to the base fluid [25].
Fig. 7. The IFT evaluation between n-heptane and various mass fractions of CNT/TiO2 nanocomposites. |
2.2.3. Emulsion stability
As well known, in chemical EOR, the formation of stable Pickering emulsions is of high technical importance. To investigate the effect of volume ratio of water and heptane on the emulsions, three different emulsions by nanofluid-heptane volume ratios of 5:5, 7:3, and 3:7 are prepared. To differentiate between the aqueous and oleic phase, two colored substances, i.e., Sudan red and methylene blue are used. The aqueous and oleic phases are shown by the color of blue and red respectively. In addition, the stability of the emulsion and emulsions type (oil-in-water or water-in-oil) are studied at different nanofluid-oil volume ratios. Oil-in-water emulsion forms if 7 mL nanofluid and 3 mL n-heptane are used (Fig. 8 a). In contrast, using 3 mL water and 7 mL n-heptane, the water-in-oil emulsion is formed. Therefore, the emulsion type is a function of the aqueous-oleic phase volume ratio in which the dominant phase forms the continuous phase however the minor phase is considered as the dispersed phase. Moreover, it is concluded that the synthetized CNT/TiO2 nanocomposite has an amphiphilic structure which is able to form both emulsion types. Due to the dominant hydrophilicity, it is observed that at the nanofluid/heptane volume ratio of 5:5, the oil-in-water emulsion is formed (Fig. 8 c) with higher stability, indicating that the CNT/TiO2 nanocomposites migrate to the interface between two phases. The average particle diameter of the emulsions that are formed by CNT/TiO2 nanofluid-heptane volume ratios of 5:5, 3:7, and 7:3, are 2.9, 3.3, 4.8 µm, respectively.
Fig. 8. The microscopic images of the emulsions formed by nanofluid and n-heptane with different volume ratios. |
Emulsions are also prepared using varied contents of CNT/TiO2 nanocomposite including 0.05%, 0.10%, and 0.50%. To investigate the effect of nanofluid mass fraction on the stability of the emulsion, the emulsion is monitored to evaluate the average emulsion particle diameter and total volume of the emulsions. The microscopic images of the emulsions at varied time i.e., 24 h, 72 h and 7 d, are displayed in Fig. 9 . The average emulsion particle diameter and total volume of the emulsions are listed in Table 1 . The average particle diameter of the emulsion is considered to be related with the stability of the emulsion, the smaller the emulsion particle diameter, the higher the stability of the prepared emulsion. It is found that that the prepared emulsions with varied nanofluid mass fraction are stable, which confirms that the CNT/TiO2 nanocomposite is successfully able to stabilize the emulsions and keep the stability for more than 7 d. The emulsion with 0.05% CNT/TiO2 nanocomposite has the lowest average particle diameter is able to stay stable even after 7 d. As can be seen in Table 1 , the average particle diameter of emulsion is increased with the increase of the nanofluid mass fraction, implying that the nanofluid mass fraction plays a negative role on the stability of the emulsions. The total volume of the emulsions is monitored to evaluate the long-term stability. In this respect, the sample with the highest total volume provides the higher stability which is useful for the EOR application. This confirms that the emulsion with 0.05% CNT/TiO2 nanocomposite provides the highest stability of emulsions. Therefore, the emulsion contains 0.05% CNT/TiO2 is investigated in further experiments.
Fig. 9. The microscopic images of the emulsions formed by different mass fractions of CNT/ TiO2 at different times. |
Table 1. The average particle diameter and total volume of the emulsions containing varied amounts of CNT/TiO2 nanocomposites |
| CNT-TiO2 mass fraction/% | Average particle diameter of emulsions after different times/µm | Total volume of the emulsions after different times/% | ||||
|---|---|---|---|---|---|---|
| 24 h | 72 h | 7 d | 24 h | 72 h | 7 d | |
| 0.05 | 2.7 | 3.2 | 4.3 | 95 | 93 | 80 |
| 0.10 | 5.2 | 11.7 | 15.4 | 86 | 66 | 66 |
| 0.50 | 6.7 | 14.3 | 20.6 | 73 | 62 | 58 |
Considering the severe conditions in practical applications, it is crucial for nanomaterials to form a stable emulsion even at high temperature and high salinity. Therefore, the formation of the emulsion with the optimum amount of CNT/TiO2, 0.05%, was investigated at 90°C. As it is shown in Fig. 10 , the emulsion has remained stable at high temperature after 24 h. This confirms the great potential of the CNT/TiO2 nanocomposite in stabilizing the emulsions even at high temperatures which is well suited for practical applications. Then, the effect of salinity on emulsion formation is investigated. The formation of the emulsion containing 0.05% CNT/TiO2 at three different NaCl mass fractions, i.e., 1%, 2%, and 4% is investigated. The microscopic images of the formed emulsions after 24 h, 72h, and 7 d are shown in Fig. 11 . As can be seen in Table 2 , by increasing the NaCl mass fraction, the emulsion volume dropped considerably. However, for NaCl mass fraction of up to 2%, a stable emulsion can be formed even after 7 d. These experiments confirm the promising potential of the CNT/TiO2 nanocomposite in the EOR application by forming Pickering emulsion.
Fig. 10. The microscopic image of the emulsions contains 0.05% CNT/TiO2 at 90 ˚C after 24 h. |
Fig. 11. The microscopic images of the emulsions formed at different salinities with CNT/TiO2 nannofluid after different times. |
Table 2. The average particle diameter and total volume of the emulsions prepared with 0.05% CNT/TiO2 nanocomposite and different mass fractions of NaCl after different times |
| NaCl mass fraction/% | The average particle diameter of emulsions after different times/µm | The total volume of the emulsions after different times/% | ||||
|---|---|---|---|---|---|---|
| 24 h | 72 h | 7 d | 24 h | 72 h | 7 d | |
| 1 | 4.6 | 10.1 | 12.2 | 95 | 91 | 55 |
| 2 | 5.9 | 11.7 | 15.6 | 95 | 84 | 50 |
| 4 | 6.3 | 12.9 | 35.4 | 95 | 62 | 24 |
2.2.4. Microfluidic oil displacement experiments
The effect of nanofluid flooding on oil displacement in the porous media is investigated by performing several runs at varied nanocomposite mass fractions. During the nanofluid flooding, the oil recovery factor is calculated using image processing. Fig. 12 shows the performance of three DI-based nanofluids with varied nanocomposite mass fractions (i.e., 0.01%, 0.05% and 0.50%) in comparison to DI water flooding during injection into the glass micromodel. Increasing the nanocomposite mass fraction improves the displacement performance and delays the breakthrough time. The breakthrough is defined as the time at which the first drop of injected fluid is observed at the outlet port of the micromodel. Therefore, nanofluid with a mass fraction of 0.50% provides a higher recovery factor in comparison to deionized water injection.
Fig. 12. The oil recovery degree after injecting DI water and nanofluids with various amounts of CNT/TiO2 nanocomposite at different times. |
From microfluidic investigation, it is inferred that while using a low mass fraction of CNT/TiO2 nanocomposite (0.01% and 0.05%), the wettability alteration is considered as the main mechanism in enhancing oil recovery. In contrast, by increasing the nanocomposite mass fraction, the IFT reduction is also an effective mechanism as well as wettability alteration which lead to the increase of oil production. Since the nanoparticles migrate to the oil-water interface and reduce the IFT, accordingly making the oil to be displaced from more paths and increasing oil recovery factor up to 50% for the nanofluid with 0.50% CNT/TiO2 (Figs. 12 and 13 ).
Fig. 13. Residual oil distribution of the micromodel after injecting DI water, and nanofluids with 0.01% and 0.50% CNT/TiO2 nanocomposites. |
In addition, it is worth mentioning that since the micromodel is different from rock structure, it is suggested to evaluate the potential of the synthetized nanocomposite in oil production in the core plug samples which is expected to provide a higher oil recovery factor.
3. Conclusions
In this study, a novel nanocomposite (CNT/TiO2) is synthesized successfully. Characterization methods confirmed that the CNT/TiO2 nanocomposite is composed of TiO2 matrix and CNT reinforcement phases with high thermal stability. It is found that anatase titania is the dominant crystalline phase and TiO2 nanoparticles covered the CNTs.
Results show that the CNT/TiO2 is migrated to the interface between water and oil to alter the rock wettability from oil-wet to water-wet conditions and effectively reduce the IFT. The optimum amount of CNT/TiO2 nanocomposite (0.05%) results in the average particle diameter and total volume of emulsions as 2.7 µm and 95%, respectively, denoting the synthetized nanocomposite plays an effective role in stabilizing the emulsions. Emulsifying role of the CNT/TiO2 nanocomposite was studied at high salinity and high temperature. It was observed that the emulsion formed with CNT/TiO2 nanocomposite at NaCl mass fraction of up to 2% kept stable emulsion even after 7 d at 90 °C. These experiments confirm the promising potential of the CNT/TiO2 nanocomposite in the EOR application by forming Pickering emulsion.
The performance of nanocomposite with different mass fractions was evaluated by conducting oil recovery experiments in a glass micromodel. Compared with the deionized water flooding with the oil recovery of about 38%, 0.50% nanofluid provides a higher oil recovery of 50%.
Acknowledgments
The authors are immensely grateful to Dr. Fatemeh Razavirad (Research Institute of Petroleum Industry (RIPI), Iran) for carrying out an independent review of the manuscript. The authors also acknowledge Dr. Mohammad Hossein Akhlaghi (Research Institute of Petroleum Industry (RIPI), Iran) for technical support to synthetize nanoparticle.
Nomenclature
p/p0—relative pressure, dimensionless;
R—oil recovery, %;
rp—pore diameter, nm;
Soi—initial oil saturation, %;
Sor—residual oil saturation, %;
Va—N2 absorption volume, cm3/g;
Vp—pore volume, cm3/g;
θ—diffraction angle, (°).