© A. Khalilnezhad et al., published by IFP Energies nouvelles, 2019

Nomenclature

Acronyms

APS: Average Particle Size

CCD: Charge Coupled Device

DW: Distilled Water

EOR: Enhanced Oil Recovery

FW: Formation Water

HP–HT: High Pressure–High Temperature

IFT: Interfacial Tension

NPs: Nanoparticles

Rpm: Revolution Per Minute

SSA: Specific Surface Area

TDS: Total Disolved Solid

TH: Total Hardness

1 Introduction

NPs improve the reservoir geomechanical characteristics by modifying reservoir EOR mechanisms, which can finally help with an increase in the oil recovery [1, 2]. The Nano-formulations of polymers, surfactants, and colloidal dispersion gels could be embedded in this category. For instance, NPs can help with controlling the recovery processes, which are inconsistent with current and/or past technologies. Moreover, it has been seen that the viscosity of water, carbon dioxide, and surfactant solutions are usually lower than that of oil, therefore, adding NPs can adjust the injected fluid viscosity at the optimum point [3]. As a consequence, an improvement in fluid transport and oil recovery can be achieved [4]. However, the adsorption of NPs onto the rock surface during movement in the pore throats can be harmful or beneficial to the Enhanced Oil Recovery, depending on specific conditions [5, 6].

Some NPs are capable of adsorbing asphaltene particles present in the crude oil [7–12]. This would prevent IFT increase, asphaltene precipitation and deposition, and wettability alteration toward the oil-wet condition. Catalytic NPs including NiO, Fe₃O₄, TiO₂, CuO, Al₂O₃, etc. are mostly categorized in this group [5, 13]. The adsorption process is mainly controlled by various factors:

• Type of adsorbent and its properties

Both physical and chemical properties of the adsorbent affect the adsorption process. Chemical properties involve various types of functional groups available on the adsorbent surface. Ionization of adsorbent surface also affects the adsorbent properties. Since adsorbents could differ in shape (i.e. powder or grain), physical properties of adsorbent, including specific surface area and pore distribution play an important role. Adsorbents usually present large surface pores and have grains appearing in spherical or irregular shapes. The smaller particles of the adsorbent, the faster they get in contact with the fluid phase.

• Adsorbate

The chemical properties of adsorbate are important in the adsorption process. The size of adsorbed molecules controls the adsorption rate. The bigger molecules have a lower solubility in the fluid and tend to leave the medium and adsorb onto the adsorbent more quickly. On the other hand, bigger molecules occupy larger areas when adsorbing onto the adsorbent. Therefore, smaller molecules are sometimes beneficial. The shape of molecules also affects the solubility. The solubility of adsorbate plays a key role in controlling the adsorption equilibrium. The effect of solubility on the adsorption process is based on the binding between the adsorbent and solvent. The stronger than binding, the weaker than adsorption.

• Temperature

Temperature applies two main effects to the adsorption process: on the one hand, an increase in temperature decreases the diffusion rate of adsorbate molecules into the external layers and inside the adsorbent pores. As a result, the viscosity of the solution reduces. On the other hand, the effect of temperature on the adsorption process provides some useful information on enthalpy and entropy changes during adsorption. An increase in temperature leads to the increase in the adsorption rate before the solution reaches the equilibrium. Generally, temperature elevation amplifies the movement of the molecules on the surface and impedes the formation of a monolayer on the surface. Therefore, the adsorption would become slower and repulsion results in the less surface coating. In another word, since the adsorption phenomenon is an exothermic process, adsorption would be more intense at lower temperatures. At higher temperatures, the adsorption rate increases, whereas the total adsorption capability decreases.

• pH of solution

Hydrogen ions and hydroxyl groups present in the medium are strongly adsorbed; therefore, the adsorption of the other ions is mainly affected by the pH of the solution. Acid/base ionization is also effective in the adsorption process, which itself is dependent on the pH. The adsorption mainly increases with a decrease in pH. However, it should be noted that the effect of temperature and pH generally depends on the type of adsorbent and for each adsorbent, there is an optimum temperature and pH.

Titanium dioxide (TiO₂) NPs, also known as titanium (IV) oxide or Titania NPs, hold all the properties of titanium dioxide with the difference that their smaller size makes them more efficient, which is due to the increased surface area of NPs. The photocatalytic activity of Titania improves as the particle dimensions reduce to the nanoscale, which results from the increased specific surface area [14]. These NPs are very chemically stable, cost-effective substance. There are three different crystalline phases of Titanium dioxide, namely: Rutile, Anatase, and Brookite (Fig. 1). Rutile is the most stable phase and the other two phases are converted to Rutile by temperature elevation. Rutile and Anatase phases are spatially tetragonal, while Brookite has an orthorhombic form [15, 16].

In the EOR processes, the addition of TiO₂ NPs to the injected fluid is capable of altering rock wettability and preventing asphaltene precipitation [18]. However, it has been reported that exceeding the optimum concentration does not necessarily improve the situation. Sometimes, it even results in NPs aggregation and their subsequent movement from the bulk phase toward the oil/water interface. As a result, the interface gets stronger and the IFT between two fluids rises [19, 20].

There is a vast amount of literature on the experimental investigation of NPs for EOR methods [6, 21–24]. Haroun et al. compared the injection of smart water and nanofluids. They concluded that the addition of suitable NPs to the injected water can improve the recovery factor from 46–63% to 57–85% [25]. In their analysis of ZnO NPs, Moghadam and Azizian analyzed the effect of these NPs on anionic surfactants at the fluid/fluid interface. They reported a decreased oil/surfactant IFT with the addition of NPs [26]. In a major attempt in 2016, Kazemzadeh and coworkers surveyed the asphaltene precipitation in the oil phase by IFT measurements. They announced that Fe₃O₄ NPs are able to prevent asphaltene precipitation at the oil/water interface by adsorbing large asphaltene particles. They also observed a considerable IFT reduction between oil and water in the presence of NPs [9]. Castillo et al. proved that the size of adsorbents and their shape could influence the adsorption process. They observed great variances in interactions between adsorbed asphaltene with macro surfaces and NPs [27]. In another study, Franco-Aguirre et al. used SiO₂ NPs with an anionic surfactant to alter the wettability of sandstone in gas-condensate reservoirs. They observed that the contact angle changed from 0° to 118° for water/rock/air system [28]. Rezvani et al. synthesized Chitosan-coated Fe₃O₄ nanocomposites to investigate their possible EOR effects. Based on their sand pack flooding, they observed an additional oil recovery factor of 10.8% with the addition of the synthesized nanocomposite to the seawater in the tertiary mode, which resulted in the ultimate recovery factor of 67.5%. They attributed this improvement to the decrease in oil/water IFT, prevention of asphaltene precipitation, the emulsion formation, and rock wettability alteration [7].

The current research presents a complete study of the oil/water interfacial properties in the presence of TiO₂ NPs for the first time. The IFT measurements were carried out at different pressures and temperatures in the presence and absence of NPs and the optimum concentration of NPs was determined. The same concentrations of nanofluids were also selected for contact angle measurements to determine the optimum concentration. Thereafter, the stability of NPs in the FW was analyzed by zeta potential measurements and thus compared with the results of IFTs and contact angles. Finally, the asphaltene adsorption by TiO₂ NPs was analyzed by batch adsorption experiments.

2 Materials and methods

2.1 Materials

2.1.1 Crude oil, formation water, and carbonate rock

The crude oil and Formation Water (FW) used in this research were obtained from one of the Iranian oil reservoirs. The crude oil had a gravity of 31.5 °API. The results of SARA (Saturate, Aromatic, Resin and Asphaltene) and compositional analyses are presented in Table 1.

The compositional analysis of FW is tabulated in Table 2. The total salinity was measured as 221 450 ppm. Moreover, the surface outcrop of the reservoir formation was trimmed into thin sections for evaluation of wettability alteration.

2.1.2 TiO₂ nanofluids

The solid Anatase/Rutile TiO₂ nanopowders were purchased from the US Research Nanomaterials Inc. and used as received. The XRD and SEM of these NPs are shown in Figures 2 and 3, respectively. Further detailed properties of NPs are presented in Table 3.

The nanofluid preparation process included the addition of NPs to the FW, which was followed by the ultrasonication process for 30 min with the power of 300 W. The apparatus used was the Ultrasonic Probe DA UP-400 (Development of Ultrasonic Technology Co., Iran). To prevent NPs from aggregation during sonicating process, the ultrasonication was performed in an ice bath to prevent temperature elevation of the nanofluid. This process was similarly repeated before all the following experiments.

2.2 Methods

2.2.1 Interfacial tension measurement

The pendant drop IFT–400 apparatus manufactured by Fars EOR Technology (Fig. 4) was utilized to investigate the effect of TiO₂ NPs on IFT values under reservoir conditions (i.e. high pressures and temperatures). The apparatus supported measuring from 1–72 mN m⁻¹ with an accuracy of 0.1 mN m⁻¹. The pressure and temperature accuracy of the device was 0.5% full scale and 0.1 °C, respectively. The camera used in this device was a 2-megapixel CCD-HD camera consisting of a C-mount macro lens with a magnification of ×10. The experiments were carried out at different temperatures (40, 60, and 80 °C) and pressures (14.7, 600, 1200, and 1800 psi) for six concentrations of NPs (250, 500, 750, 1000, 1500, and 2000 ppm) in FW, as presented in Table 4.

At each experiment, the pressure reduced to atmospheric pressure to compare the efficiency of NPs at atmospheric pressure (14.7 psi) with higher pressures. The equilibrium IFT at which the interfacial tension is stable was measured at each experiment.

2.2.2 Contact angle measurement

The inverted sessile drop method was adopted for analyzing the wettability alteration (Fig. 5). The apparatus used was manufactured by Fars EOR Technology and supported the measurements from 10 to 180° with an accuracy of ±3°.

Firstly, the rock slabs trimmed from reservoir outcrop were polished and washed with toluene to remove any possible impurities. Afterward, they were aged 3 days in FW to become completely water-wet. They were then aged in the reservoir’s crude oil for 21 days at the temperature and pressure of 80 °C and 2500 psi, respectively [29]. The same six nanofluids (as used in IFT evaluation) were selected to evaluate the wettability alteration.

2.2.3 Zeta potential measurement

In order to investigate the effect of ions of FW and TiO₂ NPs on the surface electrical charges and in turn wettability alteration, different zeta potential measurements were conducted. Since the existence of clay minerals, including kaolinite, calcite, quartz, and other metallic components in reservoir rock is prevalent [31], the carbonate rock samples were first pulverized to <45 μm at 1354 rpm and 4 wt.% rock powder was added to different Titania nanofluids (500, 1000, and 2000 ppm) [32, 33]. To compare the results, the experiments were followed by measuring the zeta potentials of Distilled Water (DW) and FW + 4 wt.% rock powder. The apparatus used was Microtrac Zeta Check (USA) with a measuring range of –200 mV to +200 mV and an accuracy of ±1 mV.

2.2.4 Asphaltene adsorption analysis

There are several methods to evaluate the asphaltene adsorption by NPs like Thermogravimetric Analysis (TGA) and UV–Vis spectroscopy. In this research, the latter has been adopted to evaluate the asphaltene adsorption by TiO₂ NPs and their effect on the prevention of asphaltene precipitation. In order to analyze the amount of asphaltene adsorption by TiO₂ NPs, different batch adsorption experiments were carried out. The procedure was based on the method performed by previous authors [34, 35].

TiO₂ NPs used in this research were nonporous and held a specific surface area of 10–45 m²/g. Asphaltene was extracted from the crude oil using the IP-143 standard and used in the preparation of the synthetic oil. The oil was synthesized with a toluene/n-heptane ratio of 60/40 vol% and 10–3000 mg/L asphaltene. The experiments included the addition of TiO₂ NPs to the synthetic oil with a TiO₂/oil ratio of 10:1 gr/L. The mixture was shaken at 400 rpm and then was aged to reach an equilibrium. Centrifugation was used to separate the asphaltene of the solutions at 5000 rpm. The asphaltene concentration in the supernatant was determined using Dynamica DB-20S UV/Visible Double Beam Spectrophotometer at 700 nm. The apparatus supported a full spectral scan from 190 to 1100 nm and a wavelength accuracy of 0.3 nm. The asphaltene adsorption by NPs was analyzed by monitoring the changes in the asphaltene concentration before and after combining the synthetic oil with NPs. The following equation calculates the amount of adsorbed asphaltene (eq. (1)):

$${ q}_t=\frac{C_0-C_t}{m}V,$$(1)where q _t represents the adsorbed asphaltene (mg/g), C ₀ is the initial asphaltene concentration (mg/L), C _t is the asphaltene concentration at time t (mg/L), m is the mass of solid particles (gr), and V stands for volume of solution (L).

3 Results and discussions

3.1 IFT measurements

3.1.1 Effect of ions present in FW

Based on the previous results, oil/water IFT linearly increases with an increase in pressure and decreases with an increase in temperature [36–39]. However, the presence of ions potentially alters the linear variation of IFT with pressure and temperature [30]. In this case, the IFT value is a function of temperature, pressure, salinity, total acid number, and asphaltene content. Therefore, the ultimate IFT value depends on the amount of surface active agents at the oil/water interface [40]. There are different reports regarding the effect of ions in the water on the oil/water IFT [41–43]. Generally, low ionic strength induces the salting-in effect in which potential-determining ions added to the water interact with crude oil components and a relative solubility of the oil components in the water phase occurs, as a result. This would decrease the oil/water IFT. However, some reports show that high ionic strength depletes the oil/water interface and makes the natural surfactant of the crude oil to move to the oil bulk. This is true even in the case of high potential-determining ions in the water [44].

The results on FW/oil IFT variations with pressure and temperature in the absence of TiO₂ NPs are demonstrated in Figure 6. As illustrated, the IFT between oil and FW ascends with an increase in pressure at all temperatures, which is mainly due to the high salinity of the FW and therefore the salting-out effect. However, with an increase in temperature, the possibility of hydration of potential-determining ions available in the FW (i.e. Ca²⁺, Mg²⁺, and ${\mathrm{SO}}_4^{2-}$) becomes low, which is accompanied by an improvement in their coefficient activity. These two factors together are considered as the main reason for the decrease in IFT values at higher temperatures (i.e. 60 and 80 °C). Actually, the different trends of IFT with pressure at different temperatures emanate from thermal cracking of large oil molecules at high temperatures.

3.1.2 Effect of TiO₂ NPs

Several surveys have revealed that NPs aggregation is amplified with temperature elevation due to the increased random motion of NPs in the base fluid, which in turn reduces the NPs efficiency [45–48]. However, an increasing number of studies have confirmed the role of some NPs in the reduction of the IFT between crude oil and water [2, 49–51]. Therefore, the efficiency of NPs at elevated temperature and pressure is a vital factor for practical application of nanofluids in the oil fields.

It is apparent from Figure 7 that the smallest value of IFT between crude oil and nanofluid has been observed for 1000 ppm Titania nanofluid at 600 psi and 60 °C. This concentration is regarded as the optimum concentration at all pressures. An increasing trend beyond the optimum concentration (1000 ppm) is also clear in Figure 7, which is mainly due to the increased NPs concentration and therefore augmented attractive forces between particles with increased charge density. This would ultimately increase the IFT values.

Pressure is a significant effect on IFT variations that is observed in Figure 7. Based on the experiments in this research, IFT variations are different at high and low pressures. From 14.7 up to 600 psi, the IFT values decrease as pressure increases. However, from 600 up to 1800 psi, an increase in pressure increases the IFT. With this in mind, the binary behavior of NPs at the optimum concentrations at different pressures results from intermolecular forces. In fact, at low pressures, asphaltene particles present in the crude oil drop tend to move to the oil/water interface. At this situation, NPs create a false surface that adsorbs large asphaltene particles and prevents the IFT increase, as compared to the case when NPs are absent in the system. Therefore, the intermolecular balance at the interface deducts the IFT between oil and FW. The high stability of NPs in the base fluid at low pressures results in more adsorption of asphaltene particles onto the surface of NPs, which prevents the asphaltene precipitation and controls the IFT values. On the other hand, at high pressures, the stability and therefore the efficiency of NPs in asphaltene adsorption reduces, which affects the IFT variations. Given these points, pressure and concentration of NPs in the base fluid are the fundamental parameters in NPs stability. It is also of great importance to imply that the stability reduces with an increase in pressure and NPs concentration; however, pressure increase stabilizes the asphaltene particles. As a result, the IFT reduction at optimum cases is intensified at low pressures.

Figure 8 indicates the IFT variations with pressure for different concentrations of NPs. As it is clear, the addition of TiO₂ NPs to FW intensifies the total rate of IFT increasing with pressure. It is observed that in the absence of NPs, the IFT increasing rate is 0.0031, whereas, the addition of 250, 1000, and 2000 ppm NPs to the base fluid rises IFT increasing rate to 0.00037, 0.004, and 0.0046, respectively. A relative decrease in IFT values with pressure at 1000 ppm is also observed. To that end, a deeper understanding of the relationship between IFT and temperature/pressure seems to be obligatory for the explanation of the NPs behavior at high pressures and temperatures. In fact, a compact arrangement of NPs due to the pressure lift intensifies the charge density. This would cause the NPs to move toward the interface and create a strong, rigid interface, which ultimately increases the oil/water IFT. Figure 8 confirms the significant impact of NPs on the rate of IFT increasing as the NPs concentration goes up, which is the likely reason for the low efficiency of NPs at high pressures.

Figure 9 depicts the IFT variations with TiO₂ NPs concentration at different temperatures. The figure clearly shows a decreasing IFT trend with the addition of TiO₂ NPs from 0 to 1000 ppm at all temperatures. It is also obvious that 1000 ppm is the optimum concentration of NPs at all temperatures. Beyond the optimum concentration, increasing NPs concentration increases the IFT values at all temperatures, which is mainly due to the high aggregation of NPs with an increase in concentration.

The catalytic behavior of TiO₂ NPs increases as temperature goes up, which can help to the improved thermal cracking of the large oil molecules and decreases the crude oil viscosity [52]. This would reduce the oil/water IFT. On the other hand, the temperature elevation increases the Brownian motion of NPs in the suspension that intensifies the NPs collision together. Therefore, the stability of the nanofluid may decrease owing to the NPs aggregation. Brownian motion is also responsible for liquidification at the interface. This is responsible for improving oil/water IFT. Overall, temperature elevation has multilateral effects on the ultimate oil/water IFT and depending on the conditions, the oil/water IFT may increase, decrease or does not change considerably.

3.2 Contact angle measurements

3.2.1 Effect of ions present in FW

Fine migration, rock mineral dissolution, Multicomponent Ion Exchange (MIE), alteration of surface electrical potential, Electrical Double Layer expansion (EDL), dolomitization, and pH alteration are among the well-known EOR mechanisms have been reported for injection of saline water into the oil reservoirs [53]. Potential-determining ions present in FW play a key role in the alteration of rock wettability. Previous reports have shown that even a small amount of ${\mathrm{SO}}_4^{2-}$ ions in comparison with inactive ions of FW can alter the rock wettability [44, 53].

The dynamic contact angle measurements of FW/rock/oil and DW/rock/oil systems are shown in Figure 10. The DW/oil/rock contact angle was calculated as 174°. The results show that present ions in the FW bring an additional reduction of 13° in contact angle at the first moment. A relatively good wettability alteration after 7-day treatment by FW (from 161° to 74°) is also observed in the figure. However, a relative increase on the 8th day of treatment is observed. In fact, high saline FW is prone to the salting-out effect, in which high concentrations of ions at the interface deplete the interface from natural surfactants and makes them move to the oil bulk or solid/oil interface. This would increase the oil-wet wettability of the rock as time goes on. In addition, inactive ions present in the FW including Na⁺, Cl⁻, ${\mathrm{HCO}}_3^{-}$, etc. prevent approaching of potential-determining ions to the rock surface, and hence, reduce the efficiency of active ions [44]. All in all, based on the results, it is clear to mention that wettability alteration by FW is not enough and further irreversible wettability improvement by the addition of NPs seems to be essential.

3.2.2 Effect of TiO₂ NPs

Figure 11 depicts the contact angle variations with NPs concentration after a 7 and 8-day treatment. As it is clear, two distinctive intervals can be observed for contact angle results. At low concentrations (first interval), the reduction of contact angles occurs with a steep slope. As indicated, a reduction of 140° (from 174 to 34°) is observed on the 7th day of treatment, which implies a strong wettability alteration from oil-wet to a water-wet condition. On the contrary, at high concentrations (second interval), an increase in contact angle happens due to the increase in NPs concentrations. The increasing slope of the contact angle with the second interval is considerably smaller than the reduction rate with the first interval. Therefore, it all boils down to the fact that the NPs behavior in the FW is different at the two intervals.

Figure 12 illustrates the impact of an increase in NPs concentration on the disjoining pressure and in turn wettability alteration. As shown, increasing NPs concentration induces a compact arrangement of NPs in the wedge layer. This implies an increased number of NPs in the volume unit, which applies a high disjoining pressure to the surface against adhesion force between crude oil droplet and rock surface. As a result, the crude oil droplet gradually desorbs from the rock surface and NPs can attach to the surface and, as a result, the rock wettability changes.

Foremost, a subtle understanding of disjoining pressure seems to be necessary. It is by definition the required pressure for overcoming the oil adhesion force to the solid’s surface to detach the oil from the surface. It is actually the pressure difference between the phase bulk and thin film adjacent to the rock surface, which can be considered as an EOR mechanism for NPs. In this mechanism, NPs create a wedge film with discontinuous fluid on the surface. Brownian motion and electrostatic repulsive forces between NPs are the key factors for developing this mechanism.

As the size of NPs decreases and as the concentration of NPs increases, the repulsive forces get stronger. This arrangement of NPs in the wedge layer inserts higher pressure on the fluid/solid interface. The more the number of NPs in the unit of volume, the stronger the pressure inserted in the interface. Surprisingly, this pressure can sometimes reach up to 50 000 psi. It is worth noting that with the exertion of such forces upon the interface, an oil displacement occurs to reach a balanced condition. Taken together, like in other colloidal systems, particle size, temperature, the salinity of carrier fluid, and properties of available phases all affect the intensity of disjoining pressure.

As noted earlier, NPs accumulation near the rock surface detaches the oil droplet from the surface. Thereupon, NPs adsorb onto the rock surface and reduce the contact angle. This reduction of the contact angle is continuous until reaching the optimum concentration. At the optimum concentration, the highest wettability alteration occurs, as shown in Figure 12. Beyond this concentration, NPs stack in a laminated structure near the rock surface, which no longer affects the wettability. The adsorption of NPs onto the rock surface increases with an increase in NPs concentration. However, at high concentrations (second interval), the added NPs adsorb onto the surface of the attached NPs to the rock surface. The resulting stack of NPs without any contact with the rock surface is no longer able to further reduce the contact angle and hence alter the wettability condition (Fig. 13). That is to say, the optimum concentration is the most efficient and cost-effective concentration for wettability alteration. Therefore, concentrations at the second interval sometimes increase the contact angle and, more or less, exacerbates the condition.

3.3 Zeta potential measurements

The electrical charges developed by clay minerals at the solid/solid or solid/fluid interface can lead to a re-arrangement of the charges near the rock surface [31], in a way that neutralization or adsorption of the charges at the interface may happen [54]. Importantly, there is a dependency between contact angle and zeta potential values. In fact, high negative zeta potentials indicate more adsorption of NPs onto the rock surface and thus a higher wettability alteration toward the water-wet condition [55].

Table 5 presents zeta potential measurements for DW and FW + 4 wt.% rock powder, as well as three different nanofluids used in this research. Owing to the positive surface charge of carbonate rocks, the addition of 4 wt.% rock powder to the DW brings a positive zeta potential. This can alter the carbonate surface to the more oil-wet condition due to the adsorption of negative carbocyclic groups present in the crude oil onto the rock surface. With the addition of 4 wt.% rock powder to the FW, potential-determining ions (including but not limited to ${\mathrm{SO}}_4^{2-}$, Ca²⁺, Mg²⁺) present in the FW act as wettability modifier and modify carbonate surface charges. This would result in an inverse zeta potential value from positive (+41.6 mV) to negative (–10.3 mV). However, the results indicate that the wettability alteration by FW is not noticeably high. Adding 500, 1000, and 2000 ppm NPs to the FW + 4 wt.% rock powder supercharge the zeta potential values to –41.4, –46.9, and –31.0, respectively, which implies a strong wettability alteration, as compared to that of FW. The results also are in good accordance with contact angle measurements, in which the fluid included 1000 ppm of NPs was the optimum concentration.

In light of the evidence from previous studies, zeta potential measurements are also good indicators of NPs stability in the base fluid. According to the previous reports, zeta potentials above ±30 mV are in the good stability region [56, 57]. Stable nanofluids are more efficient in the modification of interfacial properties. As it can be seen in Table 5, all nanofluids present a good stability. However, 2000 ppm nanofluid is prone to instability with time, as its zeta potential is on the edge. According to the obtained results, the highest stability belongs to 1000 ppm nanofluid, which is in good agreement with IFT and contact angle results, in which 1000 ppm was the optimum concentration.

3.4 Asphaltene adsorption

There are different adsorption models available for fitting experimental data resulting from batch adsorption experiments. Langmuir and Freundlich models are the most well-known ones adopted in asphaltene adsorption analyses. The former describes a monolayer adsorption, while the latter is associated with the mono/multilayer adsorption. The most important basic assumptions of the Langmuir model are summarized as follows:

• Adsorption ∆H is independent of adsorbed sites and all adsorption sites are equivalent.

• Each adsorption site can only adsorb onto one type of adsorbent.

• Adsorption occurs in a monolayer manner.

Adsorption interactions that follow this model are called “Ideal Adsorption Interactions”. In this research, the experimental data were plotted to determine the correct model which best describes the adsorption process and its type of adsorption layer. The Langmuir equation describes monolayer adsorption as follows (eq. (2)):

$$Q_{\mathrm{e}}=Q_{\mathrm{m}}\frac{K_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}.$$(2)

In this case, C _e refers to the concentration of asphaltene in toluene/heptane. The terms Q _m and K _L describe the adsorption capacity and adsorption affinity, respectively. In order to obtain the terms K _L and Q _m, the plot of 1/Q _e versus 1/C _e is drawn and the Langmuir constants could be calculated as follows (eq. (3)):

$$\begin{array}{c}\frac{1}{Q_{\mathrm{e}}}=\frac{1}{K_{\mathrm{L}}{Q}_{\mathrm{m}}{C}_{\mathrm{e}}}+\frac{1}{Q_{\mathrm{m}}}\\ Q_{\mathrm{m}}={\left(\mathrm{intercept}\right)}^{-1}, K_{\mathrm{L}}=\frac{\mathrm{intercept}}{\mathrm{slope}}\end{array}$$(3)where Q _e is the adsorbed asphaltene (mg/g) and C _e is the asphaltene concentration (mg/L) both at equilibrium. Figure 14 shows the amount of adsorbed asphaltene with asphaltene concentration based on the results obtained from the current analysis.

Figure 15 indicates the plot of C _e/Q _e versus C _e for calculation of the Langmuir constants.

Figure 16 depicts the C _e versus Q _e related to the Freundlich model, which is used in the determination of Freundlich parameters. Table 6 presents the Langmuir and Freundlich parameters calculated from each plot.

According to the R-squared values calculated from plots (Tab. 6), one can conclude that the asphaltene adsorption by TiO₂ NPs is best described by Langmuir Isotherm. Therefore, the adsorption process is a monolayer. This would confirm the role of TiO₂ NPs in the prevention of asphaltene precipitation and deposition, which in turn is effective in the reduction of oil/water IFT and prevention of inducing oil-wet wettability.

4 Conclusion

According to the obtained results, four distinctive groups of results corresponding to IFT, contact angle, zeta potential, and asphaltene adsorption analyses were obtained as follows:

• The oil/water IFT increases with pressure (with or without NPs); however, the presence of TiO₂ NPs in the FW intensifies the increasing rate of IFT with pressure. This effect is due to the increased charge density with pressure increase and therefore the higher aggregation of NPs. What’s more, the IFT alterations with pressure in the presence of NPs were found to be different at low and high pressures.

• Temperature has an inverse effect on oil/water IFT values. In fact, as the balance between two phases is established, NPs begin to penetrate relatively from water into the oil phase. As a result, the catalytic behavior of Titania NPs leads to the further reduction of IFT between two phases by thermal cracking of large oil molecules. This thermal cracking varies at different temperatures and, according to the results, is intensified at higher temperatures.

• Altogether, IFT values are different at various pressures, temperatures, as well as different concentrations of NPs in the base fluid. In this research, 1000 ppm TiO₂ nanofluid was found to be the optimum concentration in which the lowest IFT value was attained at 600 psi and 60 °C.

• Based on the obtained results, the initial DW/oil/rock contact angle was 174°, whereas the initial FW/oil/rock contact angle was 161°. The addition of Titania NPs to the FW changed the carbonate rock wettability from strongly oil-wet (174°) to the water-wet condition (34°). The contact angle measurements indicated that two different intervals could be assumed for different concentrations of NPs. At low concentrations (below the optimum concentration), the arrangement of Titania NPs between oil and rock surface induces a disjoining pressure, which overcomes the available oil/solid adhesion force and reduces the contact angles. On the other hand, at high concentrations (beyond optimum concentration), NPs stacking on previous NPs attached to the rock surface does not necessarily reduce the contact angles toward a more water-wet condition.

• Zeta potential measurements revealed that 1000 ppm nanofluid presents the highest stability among all nanofluids, which is, as expected, in good accordance with IFT and contact angle results. In other words, the most stable Titania nanofluid introduced the lowest IFT and contact angle. Therefore, in the scrutiny of interfacial properties, stability and instability of NPs are of great significance, which can considerably affect the ultimate efficiency. The results also showed that the addition of TiO₂ NPs inverses the zeta potential values from positive to the negative, which confirms the alteration of surface electrical charges for wettability alteration.

• Batch adsorption experiments showed that catalytic TiO₂ NPs presented a monolayer adsorption onto the asphaltene particles since it was best fitted to the Langmuir Isotherm. This would be an asset to the prevention of asphaltene precipitation and deposition, IFT increase, and oil-wet wettability.

References

All Tables

All Figures