Effect of binary mixtures on the IFT of synthetic oilsIn the first stage of this investigation, the dynamic IFT of synthetic model oils (3% wt of resin or asphaltenic fraction) in contact with different binary aqueous solutions with NaCl as the common salt of binary solutions was measured. After that, the impact of using binary solutions with NaCl as common salt was compared with the results reported by Hamidian et al.41 examined the sole effect of NaCl on the crude oil and synthetic oils IFT. In Fig. 1, the IFT of binary solutions of NaCl/KCl (with ionic strength of 0.7) in contact with ASO and RSO were compared with the IFT values reported by Hamidian et al.41 measured with only NaCl dissolved in the aqueous solution. Similar to the results reported by Hamidian et al.41 for NaCl, the results obtained for the mixture of NaCl and KCl revealed higher IFT reduction for the RSO than ASO. In detail, following the results depicted in Fig. 1 revealed that IFT values reduces as a function of time for both ASO and RSO using KCl/NaCl solution while the IFT reduction was more obvious in the case of RSO. Besides, a closer look into the results depicted in Fig. 1 revealed that the dynamic behavior of the RSO system is higher than the ASO system although they rather reach an equilibrium value at the same time. The point is that the observed trend can be related to the different affinity of the ASO and RSO and the different interactions between their polar groups and ions at the interface. It would be obvious that the variation of equilibrium IFT values for the ASO sample in the presence of NaCl was lower than 2 mN/m coming from the lower interaction and affinity of asphaltene and ions while a larger reduction in IFT was observed for resinous model oil which is directly related to the higher surface activity of resin molecules.Figure 1Evaluation of dynamic interfacial tension of asphaltenic oil and resinous oil in the presence of monovalent cation and chloride anion.The other point that can be extracted from Fig. 1 is that the existence of KCl is concomitant with the NaCl leading to an increase in the IFT compared with the solution that was individually prepared by NaCl. Considering these findings, it seems that the presence of potassium cation has impaired the performance of the sodium cation in the presence of the chloride anion for more IFT reduction due to a reduction in the activity of natural surfactants (asphaltene and resin) with an aqueous solution. A closer look into the results of Fig. 1 revealed that the presence of potassium cations directly affected the initial value of the ASO while the initial value for RSO remained constant. The reason behind this observed trend can be correlated to the resistive effect of RSO to dampen the fluctuations that may occur due to the presence of KCl salt. In other words, due to the surface activity of the resin molecules than the asphaltene molecules, RSO can digest the impact of KCl salt in the preliminary stage although its overall impact could not be eliminated. In other words, although the initial value for RSO remained constant for both NaCl and KCl/NaCl solutions, the equilibrium IFT values were significantly impacted. In detail, the individual presence of NaCl can reduce the IFT value from about 7 mN/m to 10 mN/ which is about 7 mN/m reduction in IFT while the concomitant presence of NaCl and KCl led to a reduction in IFT value from about 17 mN/m to 14 mN/m which is about 42% higher than the case was examined by Hamidian et al.41. However, the IFT variation for both NaCl and NaCl/KCl solutions using ASO was rather the same (about 2 mN/m) although the initial values were different. The other point that must be mentioned is that the presence of KCl salt leading to a significant impact on the IFT variation for RSO while the IFT variation trend was rather the same for ASO contacted with NaCl solution or NaCl/KCl. In detail, a glance into the results depicted in Fig. 1 revealed that using KCl/NaCl instead of NaCl in the solution led to a faster equilibrium due to the higher reduction in repulsive forces that existed between the two salts and resin molecules moved toward the interface.In the second stage, the effects of CaCl2 and MgCl2 concomitant with the NaCl were investigated on the IFT reduction. In contrast to the results obtained for NaCl/KCl in the presence of asphaltene and resin fractions, the presence of CaCl2 concomitant with NaCl has a considerable impact on the IFT reduction and dynamic IFT variation behavior of the studied systems especially for the RSO. In detail, in the case of using CaCl2 for both ASO and RSO, not only the initial IFT value was changed for both RSO and ASO, but also the equilibrium IFT was reduced especially for the ASO. A glance into the results depicted in the Fig. 2 revealed that the presence of NaCl/CaCl2 can move the equilibrium IFT of ASO to about 13 mN/m compared with the equilibrium IFT of individual NaCl solution which was about 17 mN/m while the IFT variation from about 10 to 9 mN/m was observed for RSO. The other point is that although the impact of CaCl2 on the IFT reduction of equilibrium IFT was more evident in the case of ASO, the presence of CaCl2 led to a sharp reduction and change in the IFT pattern of aqueous solution/RSO. The point is that a closer look into the results revealed rather similar IFT variation trend for the solution including CaCl2 and NaCl and the solution contains only NaCl.Figure 2Evaluation of dynamic interfacial tension of asphaltenic oil and resinous oil contact with NaCl in the presence of divalent cation.A closer look into Fig. 2 revealed that although the presence of CaCl2 led to IFT reduction for both synthetic oils, the presence of MgCl2 led to a more complicated pattern for IFT variation. In detail, in contrast to the patterns observed for the effect of KCl and CaCl2 presence concomitant with NaCl, the presence of MgCl2 concomitant with NaCl led to IFT reduction for the synthetic oil prepared by asphaltene while this combination led to an increase in the IFT values for RSO. The reason behind this observed trend can be correlated to two different phenomena. The first one is that in the absence of affinity of active materials to the ions of aqueous solution (i.e. surface excess concentration of surface active materials unaffected by salinity), IFT increases as a function of salinity. But, there is a cage-like hydrogen-bonded structure of water molecules around the salt ions including Na+, Mg2+, and Cl− which can manipulate this general trend since the salts can be depleted near the interface in the shadow of a greater energy environment of the ions which disrupts due to the hydrogen bonding42. On the other side, it is possible to correlate the observed trend to the crystal radii of ions which directly affects the hydration numbers and hydrated radii of the system. In detail, it is well accepted that the ions with smaller crystal radii (here Mg2+ < Ca2+) introduce higher hydration numbers and larger hydrated radii that affect the arrangement of ions into the interface. According to these facts, it seems that in the case of Ca2+, both resin and asphaltene molecules can conquer the interactions that existed between water molecules and Ca2+ cations and force them to move toward the interface leading to IFT reduction while the situation was more complicated in the case of Mg2+. So, it seems that in the case of RSO, the affinity of the Mg2+ ions to be remained dissolved in the water is higher than ASO which directly leads to higher IFT values for the Mg2+/NaCl/RSO than Mg2+/NaCl/ASO.The noteworthy point is that similar results were reported by Lashkarbolooki et al.9 regarding the IFT value of NaCl/MgCl2 solution in contact with synthetic oil prepared by dissolution of 8 wt% of extracted asphaltene and resin in toluene (higher effect on the IFT reduction for asphaltenic oil compared with resinous oil). The higher IFT reduction of NaCl/MgCl2 can be correlated to the higher affinity of Mg2+ toward oxygen as a heteroatom in asphaltene compared with resin. Investigation on the oxygen content of resin and asphaltene revealed a similar amount for the current study and the investigation performed by Lashkarbolooki et al.21. However, the IFT reduction for asphaltene was more differentiable which means the existence of another effective parameter such as aromaticity of the asphaltene and resin or functional groups. The other point is that the ratio of NaCl to MgCl2 was about 1.6 in the investigation performed by Lashkarbolooki et al.9 while this ratio is 3.6 for the current investigation. The point is that similar to the observed IFT trend for NaCl/MgCl2/asphaltenic oil, the IFT value of solution including NaCl/CaCl2 and asphaltenic synthetic oil was decreased, while a reverse trend was observed for the effect of CaCl2 salt compared with the MgCl2 salt for the resinous oil.In general, the IFT reduction for NaCl/CaCl2 aqueous solution (especially for asphaltenic oil) was larger than the other studied system due to the more affinity of Ca2+ towards fluid/fluid interface in the presence of Na+ and its proper interactions with asphaltene and resin components to produce complex ions compared to that occurred for Mg2+. Generally, the most important reason for the different IFT behavior of oil samples in different brines can be associated with the different packing and orientations of asphaltene and resin molecules in different electrolytes. If complex ions form between the polar organic components of asphaltene and resin fractions and cations of brine, IFT experiences a reduction due to the enhancement of their affinity to the interface and their larger solubility in the aqueous phase. Complex ions are commonly known as a charged molecular aggregate consisting of metallic atoms or ions attached to one or more electron-donating molecules. Proper complex ions would be constructed if divalent calcium cations existed in the presence of monovalent sodium cations and a polar organic component, i.e. asphaltene and resin which donating their heteroatoms enhances the surface excess concentration of active agents leading to IFT reduction9,43.On the other hand, it is reported that the formation of metal/ligand complexation involving acetate with divalent metal ions can be occurred by the creation of bounding between metal ions and the carboxylate oxygen44,45,46. Besides, it was reported that acetic acid in the presence of Ca2+ and Mg2+ exclusively forms the mononuclear complexes44. Moreover, the resinous oil leads to higher initial and equilibrium IFT reduction than the synthetic oil prepared by asphaltene. The remarkable point about comparing crude oil and resinous oil is that the IFT variations for both are about the same, so it can be concluded that the IFT of crude oil in the presence of NaCl/CaCl2 brine is a function of the resin, not the asphaltene. The other point that can be extracted is that the other existing fractions such as saturates and aromatics provide resistance to the salts and retard the effects of the ions on the IFT reduction.In the last stage of IFT measurement, the impact of NaCl/MgSO4 and NaCl/Na2SO4 pairs was investigated on the IFT variation of ASO and RSO. Investigating these two pairs is important and applicable since one of them has monovalent cations while the other one has divalent cations leading to different behaviors during IFT variation. According to the results given in Fig. 3, the results revealed that the addition of MgSO4 and Na2SO4 has a positive impact on the IFT reduction of ASO compared with the solution comprised of only NaCl although the impact of Na2SO4 is negligible. The point is that similar to the results observed for KCl, MgCl2, and CaCl2, the presence of ASO in the system leading to different and distinguished initial points for IFT while for the RSO system the initial points are rather the same regardless of the used salts. The reason behind this observed trend can be correlated to the stronger impact of asphaltene molecules on the ions solvation which rapidly grabs them from the bulk phase into the interface leading to rapid change in the IFT value while the RSO was not capable to introduce rapid interaction with the ions for the faster IFT change. Moreover, this observed trend can be correlated to the fact that the solvation energy of the ions in the water compared with the interactions that existed between asphaltene molecules and ions in the interface is weaker and consequently moving the ions toward the interface. However, the resin molecules are not capable to overcome the solvation energy of ions in the water to force the ions moving toward the interface. As a consequence of this weakness, the IFT for the RSO system is higher than the solution comprised of only NaCl while the situation is completely different for ASO.Figure 3Evaluation of dynamic interfacial tension of asphaltenic oil and resinous oil contact with NaCl in the presence of sulfate anion.The other possible reason behind the observed trend can be correlated to the higher aromaticity of the asphaltene fraction (i.e. 1.02) compared to the resin fraction (i.e. 1.46). As aforementioned, more polar heteroatoms in the resin structure and its higher H/C make it possible for more diffusion in water and adsorption at the interface between model oil and water, as a result, lower IFT value was observed for resinous model oil compared to the asphaltenic oil. Despite of high affinity of resin toward the water phase, the polar group of resin fraction shows no synergistic effect by the salts consisting of chloride and sulfate anions simultaneously. This low tendency can be correlated to the enhancement of intermolecular distance resulting in the decrease of the coverage fraction of the lateral chains of resin in the presence of SO42− and Cl−. Therefore, the trend of NaCl/Na2SO4 and NaCl/MgSO4 salts is similar to the presence of MgCl2 salt in NaCl salt (see Fig. 2), so the reasons explained for this salt in Fig. 3 can be applied to the simultaneous presence of Na2SO4 or MgSO4 salts with NaCl.Equilibrium IFT (EIFT) values for all the studied brines and both model oils at equilibrium state which are depicted in Fig. 3 revealed that the EIFT values depend on the kind of salts and natural surfactants. For both studied model oils, all of the brines revealed insignificant effects in the IFT reduction and even increasing effects in some cases although the lowest EIFT value was observed for NaCl/CaCl2 brine. The remarkable point is that the presence of different salts concomitant with NaCl, leads to different behaviors of the brine solutions in contact with the synthetic oil namely resinous or asphaltenic oil. For asphaltenic oil, except KCl, the presence of other salts in NaCl has a decreasing effect on IFT probably due to bonds between the divalent cations and chloride anion. In more detail, it seems that in the case of Mg2+ and sulfate anions, strong bonds due to the higher affinity of the polar functional groups to the interface and enhancement of coverage and packing at the interface lead to better reduction in IFT, while in the case of resinous oil in contact with different salts except for CaCl2, the presence of other salts in NaCl has an increasing effect on IFT.In the next stage of this investigation, the obtained results were modeled using a mono-decay model with two different approaches of x = e and x = 10 (see Figs. 4 and 5 and Tables 5 and 6). A closer look into Table 5 where the average absolute relative deviation percent (AARD %) as well as standard deviation of equilibrium IFT measurements were tabulated, revealed the acceptable level of accuracy of the used model besides its capability to evaluate the adsorption or relaxation time as a single adjustable parameter of the model. The point is that the mono-decay model was capable to well correlated the relaxation time with more accuracy using x = e leading to τ = 600s while using x = 10 leading to underestimation of relaxation time with τ = 250s regardless of their accuracy to predict the IFT values which are the same. Besides the performed analysis using two different x values of e and 10 for the mono-decay model, the modeled IFT values versus log (\(\frac{{\gamma }_{0}-{\gamma }_{t}}{{\gamma }_{t}-{\gamma }_{e}}\)) provide the chance of observing the different regions of induction time, rapid fall time, meso-equilibrium time, and equilibrium time. In the first stage, as it is obvious, the utilized mono-decay model was satisfactorily capable of not only predicting the IFT values but also differentiating the different regions as aforementioned and depicted in Fig. 4b. Moreover, the further analysis which their results depicted in Fig. 5 revealed that although using x = e leading to more accurate prediction of different times regions, it is possible to correlate the relaxation time of using x = e and x = 10 with a simple linear regression with an acceptable correlation coefficient of 0.9976. In other words, using the obtained linear correlation depicted in Fig. 5, one can easily convert the relaxation time of the mono-decay model with different powers to each other with an acceptable level of accuracy no matter what type of synthetic oil was used. In other words, the performed analysis revealed that the mono-decay model is well capable to predict the IFT values and the corresponding time zones of induction, rapid fall, etc. with high accuracy regardless of the used oil type. Respecting the accuracy of the proposed mono-decay model, the EIFT values of different solutions were calculated and tabulated in Table 5 along with the standard deviation of the calculated EIFT values.Figure 4Capability of decay model to predict the dynamic interfacial tension, (a) IFT vs. time, and (b) IFT vs. different time steps.Figure 5Capability of decay model to predict the dynamic interfacial tension using x = 10 and x = e.Table 5 The standard deviation of EIFT and prediction error of mono-exponential decay model.Table 6 Comparison of the relaxation time of two considered models.The characteristic decay times (τ) of asphaltenic and resinous model oils for different aqueous solutions were shown in Fig. 4. A glance into this figure revealed that the adsorption times are extremely longer than with the adsorption time of the aqueous solution comprised of surfactants. In detail, the performed measurements by Bauget et al.17, Zhang et al.22 and Wang et al.18 for asphaltene/toluene mixture and water revealed an adsorption time of in a few seconds. It is also found that the adsorption time values were strongly dependent on the salt and oil types. In detail, the presence of salts in NaCl brine, for asphaltenic model oil, except for CaCl2, led to an increase in adsorption time, while adsorption time values of resinous model oil, except for MgCl2 experienced a reduction. One of the most important points of this figure is the high amount of NaCl/Na2SO4 adsorption time that can represent the slow rate of polar functional groups and ions interaction on the surface when this salt is in contact with asphaltenic oil. For both studied model oils, the obtained adsorption time was considerably lower in the presence of CaCl2 since the higher affinity of the polar functional groups toward the interface reduces the required time for the coverage and packing of active agents at the interface.Moreover, investigating the results tabulated in Table 6 revealed that the proposed model for calculating the adsorption time for different binary solutions is accurate since the average AARD% is below 2% except for the NaCl/CaCl2 solution led to the lowest IFT values for both ASO and RSO. In other words, it seems that the proposed model for adsorption time prediction is highly sensitive to the IFT value and the adsorption behavior. In other words, this model loses its accuracy as the IFT value reduces since as the equilibrium IFT value reduces, the dynamic zone of IFT for the examined solutions narrows.Wettability alterationIn the last stage of this investigation, the effects of different salts in binary conditions were examined on the wettability alteration of rock surfaces using the contact angle measurement (see Figs. 6 and 7). In this way, different thin sections were prepared using a rock sample which the majority was comprised of dolomite compounds based on the performed mapping image analysis which revealed the high content of silicon (Si), carbon, calcium, and magnesium. Since more than half of the world’s hydrocarbon resources are stored in carbonate reservoirs, a large number of investigations were performed regarding the wettability alteration of carbonate rock surface toward the strongly water-wet conditions since the oil-wet nature of the carbonate rocks greatly hinders the oil production. In light of this fact and since a substantial proportion of the Earth’s crust comprised of dolomite which is a carbonate mineral several academic researchers were concentered on the precipitation and dissolution of the dolomite mineral known as the “dolomite problem” in the presence of different ions and chemicals47,48,49. The research revealed that similar to the carbonate rocks, dolomite can be rendered oil-wet by carboxylates and that the oil-wetness can be reversed more effectively using different chemicals and ions50,51,52,53,54,55. Unfortunately compared to the large number of performed investigations regarding the wettability alteration of carbonate rock surfaces using potential determining ions (PDIs), very limited systematic studies on the effect of PDIs on dolomite rock surfaces for wettability alteration have been reported56,57.Figure 6Wettability alteration of dolomite rock sample with aging in asphaltenic model oil.Figure 7Wettability alteration of dolomite rock sample with aging in resinous model oil.In this way, the current investigation is aimed to find the possible effect of different salts in binary solutions on the wettability alteration of dolomite rock surfaces. According to the performed measurements, the lowest contact angle value (the strongly water-wet condition) was observed for the system in which the distilled water was used as the aqueous solution. But, it is impossible to use the distilled water injection in the field scale due to economic issues (highly expensive) even if the distilled water injection leads to the highest oil recovery due to wettability alteration or IFT reduction. For a better understanding of the effect of the oleic phase, the equilibrium contact angle values using synthetic oil samples were measured and compared (see Fig. 8) with the values obtained using nonacidic crude oil with the same aqueous solutions (ionic strength of 0.7 including NaCl, Na2SO4, KCl, CaCl2, MgCl2, and MgSO4)10. The results revealed the superiority of Na2SO4 and MgSO4 for wettability alteration to strongly water-wet conditions while the other salts can change the wettability only toward neutral wet states. The phenomenon that is called the double layer is possible for any surface with charges to form a layer near the surface. In this layer, the consequence of ionization, ionic adsorption, and ionic dissolution can occur via particle attachment between the particles with opposite charges of their own (counter-ions) or escape from the particles that have charges similar to their own (co-ions)58.Figure 8Comparison of equilibrium contact angle of this study obtained for asphaltenic and resinous model oil with its base crude oil (10).Comparing the results of binary mixtures obtained for resinous and asphaltenic oils, one can conclude that the presence of NaCl–MgSO4 and NaCl–Na2SO4 has a similar effect as well as the application of crude oil while the application of NaCl–KCl, NaCl–CaCl2, NaCl–MgCl2 at the presence of resinous and asphaltenic oils leading to extremely better effects for wettability alteration toward strongly water-wet compared with the used crude oil (see Fig. 7).Several mechanisms have been proposed for the effects of engineered brine solutions such as calcium ion can be substituted with the magnesium ion on the surface, and it is possible to expect to substitute magnesium ion with the calcium ion existing in the carboxylic compounds stick to the rock surface. Another point that should be considered and analyzed is the existence of \(So_{4}^{2 – }\) that can enhance the concentration of Mg2+ on the surface, which can substitute by Mg2+ on the surface and released in the solution7. In contrast, the concomitant existence of Mg2+ and Ca2+ while \(So_{4}^{2 – }\) exist may loosen the bonding between carboxylic compounds of crude oil and the carbonate rock surface that consequently leads to the unavoidable elimination of carboxylic compound from the rock surface rendering the surface towards the water-wet states.So, it can be claimed that where the positive charge of carbonate surfaces adsorption of carboxylic groups to carbonate surface is unavoidable, and due to the injection of engineered brine solution that comprises of a high level of sulfate ion that can modify the surface. Furthermore, sulfate ion is a divalent ion and uses one of its capacities to react with the positive charge of the rock surface. While the other capacity utilizes to react with the carboxylic group that consequently leads to the release of oil droplets from the rock surface, even though it is possible to produce higher oil in the shadow of bonding between the oil droplet and the existing cations in the aqueous solution59.In detail, Boumedjane et al.8 reported that wettability alteration of calcite surface occurs due to the light electrostatic attraction existing between SO42- ions and the mineral calcite surfaces which possess positive water-wet sites. At this point, the sulfate ions could approach and enhance the negative charge of the rock surface, while on the other side, it may lead to a reduction in electrostatic repulsion between magnesium ions and the calcite surface making it easier for magnesium ions to reach the surface. At this point, they can easily interact with the organic compounds, or replace the calcium ions that are bonded to the carboxylate group in order to desorb them from the calcite surface. In this way, it can be concluded that the existence of sulfate ions can boost the wettability alteration effect of Mg2+ or Ca2+ with its catalytic role.According to the results depicted in Fig. 8, it is revealed that the presence of saturates and aromatics in the crude oil act as a resistance to the dissolved salts which retards the wettability alteration. In detail, the measured contact angles for crude oil which is comprised of four different fractions of resin, asphaltene, saturates and aromatics revealed a slight effect of salts on the contact angle while using asphaltenic or resinous synthetic oils revealed a significant effect of salts for wettability alteration toward strongly water-wet conditions. In this way, it is obvious that two fractions of saturates and aromatics are two fractions that retard the effect of salts on the wettability alteration. The point is that the results reported by Saputra and his coworkers60 revealed a similar trend regarding the effect of the aromatic fraction of the crude oil on the wettability alteration toward oil-wet condition while a contradicting trend regarding the effect of resin fraction was observed compared with the results obtained in the current investigation. They reported that higher contents of aromatic, resin, and asphaltene fractions lead to higher adsorption of crude oil components on the rock surface which can be directly explained by mutual solubility/polarity theory. In detail, as the first layer is formed by adsorption of the aforementioned fractions, the additional adsorption sites would be formed as a consequence of first layer formation leading to adsorption of the remainder of the oil components, i.e., the nonpolar saturates. So, the crude oil with a lower amount of aromatic, resin, and asphaltene and higher saturate contents faced with essential initial oil adsorption limitation on the rock surface leading to the more water-wet surface created on these crude oil samples.
