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In this study, we initially performed interfacial tension (IFT) tests to investigate the potential of using the Persian Gulf sea- water (PGSW) as smart water with different concentrations of NaCl, KCl, MgCl, CaCl , and Na SO . Next, for each salt, 2 2 2 4 at the concentration where IFT was minimum, we conducted contact angle, zeta potential, and micromodel flooding tests. The results showed that IFT is minimized if NaCl or KCl is removed from PGSW; thus, for solutions lacking NaCl and KCl, the IFT values were obtained at 26.29 and 26.56 mN/m, respectively. Conversely, in the case of divalent ions, minimum IFT occurred when the concentration of Mg Cl, CaCl , and N a SO in PGSW increased. Specifically, a threefold rise in the con - 2 2 2 4 centration of Na SO further reduced IFT as compared to optimal concentrations of MgCl or CaCl . It should be mentioned 2 4 2 2 that eliminating NaCl from PGSW resulted in the lowest IFT value compared to adding or removing other ions. Whereas the removal of NaCl caused the contact angle to decrease from 91.0° to 67.8° relative to PGSW and changed surface wettability to weakly water-wet, eliminating KCl did not considerably change the contact angle, such that it only led to a nine-degree reduction in this angle relative to PGSW and left wettability in the same neutral-wet condition. At optimal concentrations of MgCl, CaCl , and Na SO , only an increase in Na SO concentration in PGSW could change wettability from neutral-wet 2 2 2 4 2 4 to weakly water-wet. For solutions with optimal concentrations, the removal of NaCl or KCl caused the rock surface to have slightly higher negative charges, and increasing the concentration of divalent ions led to a small reduction in the negative charge of the surface. The results of micromodel flooding indicated that NaCl-free PGSW could raise oil recovery by 10.12% relative to PGSW. Furthermore, when the Na SO concentration in PGSW was tripled, the oil recovery increased by 7.34% 2 4 compared to PGSW. Accordingly, depending on the conditions, it is possible to use PGSW so as to enhance the efficiency of oil recovery by removing NaCl or by increasing the concentration of Na SO three times. 2 4 Keywords Smart water · Minimum IFT · Wettability · Zeta potential · Enhanced oil recovery 1 Introduction are used to extract remaining oil. Depending on reservoir conditions and oil characteristics, EOR methods, up to 40%, Evidence has demonstrated that about 65%–70% of the oil in can boost oil production (Sheng 2011, 2013). reservoirs is trapped when conventional oil recovery meth- In the last two decades, among EOR methods, smart water ods are used. In other words, these methods cannot over- flooding has attracted special attention because not only is come the capillary force in the porous medium and bring it a cost-effective method, but also it has fewer environmen- about significant oil production, especially in carbonate res- tal problems (Alipour Tabrizy et al. 2011; Abubacker et al. ervoirs. As a result, enhanced oil recovery (EOR) methods 2017; Austad 2013; Darvish Sarvestani et al. 2019). Smart water flooding can be defined as a novel EOR technique in which the ionic composition of brine is changed by adding Edited by Yan-Hua Sun or removing ions to yield higher oil recovery. Smart water * Amir Hossein Saeedi Dehaghani is obtained by adjusting and optimizing the concentration of email@example.com ions in the base fluid or by adding a salt containing the ions 1 that can change the balance of the oil/brine/rock system and Department of Petroleum Engineering, Faculty of Chemical improve oil recovery (Austad 2013; Abubacker et al. 2017; Engineering, Tarbiat Modares University, Tehran, Iran Vol.:(0123456789) 1 3 896 Petroleum Science (2021) 18:895–908 Darvish Sarvestani et al. 2019; Manshad et al. 2016). Smart a strongly water-wet state. Moreover, Kakati and Sangwai water can change the wettability of the rock surface from (2018) suggested that the type of oil affects the results of hydrophobic to hydrophilic and reduce interfacial tension smart water flooding; thus, compared to divalent cations, (IFT), hence allowing trapped oil to move easily (Al-sofi and monovalent cations are more effective in reducing IFT and Yousef 2013; Awolayo et al. 2016; Honarvar et al. 2020a; changing wettability when n-alkanes hydrocarbons are uti- Manshad et al. 2017; Mohanty and Chandrasekhar 2013; lized. Kedar and Bhagwat (2018) noted that MgCl was Nowrouzi et al. 2019; Saeedi Dehaghani and Badizad 2019; more capable of reducing IFT compared to CaCl and NaCl. Saeedi Dehaghani et al. 2020). Also, Lashkarbolooki et al. (2014) investigated the impact of Smart water flooding has been the subject of numerous various ions, utilizing oil which contained a high percentage studies. For instance, Austad et al. (2012) reported that ulti- of asphaltene, on IFT and wettability alteration and reported mate oil recovery in sandstone reservoirs could reach 75%, that MgCl led to the lowest IFT; moreover, they reported, if 2− − provided that seawater ions are properly adjusted. Hognesen divalent cations were bonded with SO rather than Cl , it et al. (2005) performed imbibition tests on carbonate rock could prevent them from reducing IFT. They also found that 2− and found that SO plays an important role in altering wet- although increasing ion concentration leads to an increase in tability of the rock to water-wet; besides, oil recovery rises contact angle, it leaves wettability within the range of water- even further if the concentration of this ion is tripled in the wet conditions. Furthermore, Fattahi Mehraban et al. (2019) injected brine. Exploring the effect of different ions as sin- reported reductions in the contact angle when the tempera- 2− gle components on IFT and contact angle, Gandomkar and ture increases to 90 °C, and SO is the most determining 2+ 2− Rahimpour (2017) stated that while Mg and SO were ion to change the wettability towards a more water-wet state. 2+ + able to make the limestone surface water-wet, Ca , Na , In addition, Lashkarbolooki and Ayatollahi (2018) and K could not change wettability. They also reported that reported that the IFT of the crude oil-brine system depends minimum IFT was obtained in the presence of 2,500 ppm of on the weight percentage of resin and asphaltene in the oil MgCl and Na SO salts. Al-Attar et al. (2013) also showed and the oil aromaticity; thus, heightened resin aromatic- 2 2 4 that in carbonate rock, by reducing the concentration of ions ity increases the impact of resin on IFT. Naeli et al. (2016) in the seawater to 5000 ppm, the oil recovery rose by 21.5% studied IFT changes caused by increasing the concentration 2− and the presence of SO in brine was crucial in changing of CaCl and N a SO in diluted seawater. The results illus- 4 2 2 4 2+ 2+ the pH of the brine. It was also reported that increasing Ca trated that increasing Ca concentration first reduces and 2− concentration in seawater did not result in a clear trend in then increases IFT, but increasing SO concentration ini- IFT values and reduced oil recovery. tially increases then reduces IFT. Nowrouzi et al. (2018) also Performing smart water spontaneous imbibition, Fathi examined the combined effect of different salts on IFT and et al. (2010) showed that both divalent ions and monovalent contact angle, and their results indicated that using M gCl cations play a key role in changing the wettability of chalk. and K SO together causes the lowest IFT. In addition, the 2 4 They found that removing Na from the seawater, rather than contact angle reaches its lowest point because of the com- increasing its concentration, could be the best way to maxi- bined use of MgSO and CaCl . In another study, Lashkar- 4 2 mize oil recovery. In another study, Fathi et al. (2011) con- bolooki et al. (2017) found that NaCl and KCl exhibit differ - ducted core flooding in carbonates and observed that sweep ent behavior in changing wettability, such that contact angle ec ffi iency was optimized when NaCl was removed from sea - decreases by increasing KCl concentration or reducing NaCl water, and Na SO concentration was increased quadrupled. concentration. Moreover, Zaheri et al. (2020) reported that 2 4 At a concentration of 1000 ppm, Amiri and Gandomkar the concentration of CaCl in formation water (FW) can play (2019) examined the effects of various ions on IFT, wettabil- an important role in wettability alteration. ity alteration, and recovery factor. The results revealed that It is worth mentioning that most of the previous studies although MgSO , compared to MgCl , increased IFT values have examined the effect of ions on IFT and wettability 4 2 and altered wettability from strongly water-wet to weakly alteration when they have been used alone or binary in water-wet, oil recovery was improved by the occurrence of distilled water. Thus, the impact of each ion in a mixture the snap-off phenomenon. They also found that NaCl and of salts is still unclear. Moreover, there are no studies KCl did not alter wettability, and limestone remained in the thoroughly investigating the effect of altering the con- same oil-wet state. Adding NaCl, Na SO , and CaCl to opti- centration of specific ions in the Persian Gulf seawater 2 4 2 mized seawater and examining changes in IFT and contact (PGSW) to determine the composition and ionic strength angle, Rahimi et al. (2020) reported that increasing the con- that could optimally improve oil recovery. Given that the centration of each ion led to an increasing trend followed Persian Gulf basin has multiple oil reservoirs, each of by a decrease in IFT values. In addition, increasing the con- which can benefit from the injection of seawater, we aim 2+ + centration of Ca and Na in optimized seawater raised at determining which ion in PGSW minimizes IFT and 2− contact angle, but increasing SO concentration caused contact angle and maximizes the ultimate oil recovery. To 1 3 Petroleum Science (2021) 18:895–908 897 this end, we first study IFT changes owing to altering the 2.2 Brine and rock preparation concentration of NaCl, KCl, MgCl, CaCl , and Na SO 2 2 2 4 in PGSW. Then, for each of the ions at points where IFT Once the salt required for each solution was weighed using a is minimized, we perform contact angle and zeta potential digital scale, it was poured into a beaker containing distilled tests in order to assess wettability alteration. Finally, a water; the solution was then mixed in a stirrer for 30 min. In micromodel test is conducted to identify the amount of addition, the carbonate rock used in this study was collected oil recovery at optimal concentrations. from one of the rock formations in the southwestern Iran. First, the rock was cut into thin pieces so that it can be used in the contact angle test. The rock samples were washed with toluene and methanol to remove oil and salt, respectively; 2 Materials and methods next, distilled water was used to remove toluene and metha- nol from the washed rock. Then, the rock specimens were Each experiment was repeated three times for each placed in an oven for 1 day to dry at 65 °C. Next, the dried concentration, and the mean values for each test were rock specimens were immersed in the FW solution. After- recorded. All experiments were carried out under ambient wards, the rock specimens were immersed in oil at 90 °C temperature and pressure conditions. for 15 days in order to become oil-wet. After making the rock samples oil-wet, for each salt, the solution was prepared at optimal concentration obtained from IFT tests. Next, the 2.1 Materials rock specimens were immersed in the prepared solutions for two weeks. Table 1 shows the salts used in this study. The salts, in different concentrations, were used to generate FW, 2.3 IFT, contact angle and zeta potential PGSW, and smart water. Table 2 shows the compositions measurements of PGSW and FW. Additionally, the crude oil used in this study was taken from one of the Iranian oil reser- The pendant drop method was used to measure IFT. In this voirs. Properties and components of this oil are reported method, the shape of an oil droplet, the fluid density, gravity in Table 3. Besides, a mixture of hexamethyldisilane and force, and the size of the needle were employed to evaluate toluene was used to make the micromodel oil-wet. Metha- IFT. It should be noted that this method has received wide nol was used to alter the wettability of the micromodel currency among researchers since it can precisely measure IFT and to wash the salts from the rock specimens. It should values (Saeedi Dehaghani et al. 2020; Honarvar et al. 2020a; be noted that all the materials were purchased from Merck Lashkarbolooki and Ayatollahi 2018; Manshad et al. 2016; Company (Germany). Nowrouzi et al. 2019; Lashkarbolooki et al. 2014). In this IFT Table 1 Properties of different salts 3 3 Salt Symbol Molecular weight, g/mol Solubility in water, g/100 cm Density, g/cm Potassium chloride KCl 74.55 34.02 1.98 Sodium chloride NaCl 58.44 35.89 2.16 Calcium chloride CaCl 110.99 74.50 2.15 Magnesium chloride MgCl ·6H O 230.31 20.30 2.32 2 2 Sodium sulfate Na SO 142.04 19.50 2.66 2 4 Sodium bicarbonate NaHCO 84.01 9.60 2.20 Strontium chloride SrCl 158.53 53.80 3.05 Table 2 Compositions of formation water and Persian Gulf seawater Water Ion content, % Ionic strength, + 2+ − + 2+ 2+ ‒ 2‒ K Sr HCO Na Ca Mg Cl SO 3 4 mol/L PGSW 399 3 166 12,000 440 1632 22,358 3110 0.785 FW 1986 547 579 42,215 5032 759 78,421 635 2.117 1 3 898 Petroleum Science (2021) 18:895–908 Table 3 Properties and components of oil sample Component, mol% C C C i-C n-C i-C n-C C C C C 1 2 3 4 4 5 5 6 7 8 9+ 0.01 0.02 0.01 0.78 1.14 2.57 5.95 5.82 6.22 7.21 70.27 Viscosity @ 28 °C, cP Acid number, mg KOH/g oil Asphaltene, % Resin, % 4.97 0.56 2.9 7.5 measurement, the oil droplet was suspended in the brine by (3) The micromodel was dried in an oven at 200 °C for an means of a needle; then, Drop Shape Analysis Software (Lab- hour to maintain the silicone coating. VIEW software) was used to calculate IFT from a photo which was taken by a high-resolution microscopic camera. Moreover, the sessile drop technique was used to investigate the contact 3 Results and discussion angle. In this method, a drop of oil in the presence of brine was placed on the surface of the carbonate rock by a needle; 3.1 Eec ff t of PGSW with different salinities on IFT once equilibrium was reached, a photograph was taken from the drop, and the contact angle was calculated using Digimizer Five dier ff ent salts, including NaCl, KCl, MgCl, CaCl , and 2 2 Image Analysis Software. Na SO , were used to evaluate the oil/brine IFT. First, the 2 4 The zeta potential was measured using a Zetasizer (ZEN effect of each ion, prepared in distilled water separately, on 3600, UK). To this end, the carbonated rock was ground to IFT was examined. prepare micron-size powder. Then, in the related concentra- Figure 2 presents the effect of different concentrations tions of smart water, the powder and brine were mixed and of monovalent cations on IFT. As can be seen, the IFT then sonicated by an ultrasonic device for half an hour. Finally, decreases and reaches its minimum as a result of increasing the zeta potential was measured by placing a special electrode the concentration of each ion to 1000 ppm. Specifically, the in the mixture. IFT decreases by 2.52 and 2.47 mN/m in the cases of Na and K , respectively. Then, as the concentration of these 2.4 Flooding tests two ions increases to 10,000 ppm, the IFT rises and reaches its maximum. Finally, the IFT declines if the concentration In this study, a five-spot glass micromodel was fabricated. The exceeds 10,000 ppm. For example, according to Fig. 2, Na micromodel pattern was copied from a thin section of car- shows an IFT value of 30.65 mN/m at a concentration of bonated rock using CorelDraw Software. The flooding setup 10,000 ppm, and increasing the concentration to 40,000 ppm included an injection pump, a light, a glass micromodel, a results in an IFT value of 27.27 mN/m. computer, a camera, and a waste container. The pattern, prop- Figure 3 illustrates the effect of divalent ions on IFT. erties of the glass micromodel, and schematic of flooding setup According to Fig. 3, increasing the concentration of each are shown in Fig. 1. In order to perform the micromodel flood- divalent ion from 0 to 1000 ppm reduces the IFT. The IFT 2+ 2+ ing, the oil sample was injected into the micromodel until it decreases by 4.69, 1.67, and 2.63 mN/m for Ca, Mg , 2− was saturated 100% with oil. Then, smart water was injected and SO , respectively. The minimum IFT occurs at a con- into the micromodel at a rate of 0.05 mL/h. It should be noted centration of 1000 ppm. At concentrations above 1000 ppm, that this flow rate was chosen to avoid turbulence behavior in the IFT rises, reaching a maximum at a certain concentra- 2+ 2+ the micromodel (Ghalamizade Elyaderani et al. 2019). Next, tion. Thus, the maximum IFT values for Mg, Ca , and 2− the picture of micromodel was taken by a camera at constant SO occur at 10,000, 5000, and 10,000 ppm, respectively. time intervals to measure the oil recovery factor. Before each It should be noted that a further increase in divalent ion flooding test, the micromodel was made oil-wet by the follow - concentration, above the concentration in which the maxi- ing procedure (Mofrad and Saeedi Dehaghani 2020): mum IFT occurs, causes the IFT to decrease. Comparing the 2+ capacity of all ions in minimizing IFT suggests that C a (1) The micromodel was saturated with a mixture con- causes the highest IFT reduction, which is in line with the taining 5% of hexamethyldisilane and 95% of toluene results of Honarvar et al. (2020b). for 20 min in order to make the glass surface silicone According to Figs. 2 and 3, it can be seen that the rising coated. and falling trends in IFT values are the same when each of (2) The micromodel was washed with methanol so as to the ions is present individually in distilled water. The ion eliminate siliconizing solution. concentration range used in this study could be divided into 1 3 Petroleum Science (2021) 18:895–908 899 Micromodel properties Properties Value Camera Pore volume, cm 0.22 Porosity, % 38 Permeability, D 0.89 Glass micromodel Computer Syringe pump Plate Waste container Light source Fig. 1 Schematic of the micromodel setup and properties of the glass micromodel 34 34 KCl MgCl2 NaCl Na2SO4 CaCl2 0500010000 15000 20000 25000 30000 35000 40000 45000 01 5000 0000 15000 20000 25000 30000 35000 40000 45000 Ion concentration, ppm Ion concentration, ppm Fig. 3 Effect of divalent ions on IFT between distilled water and oil Fig. 2 Effect of monovalent ions on IFT between distilled water and oil positive, thereby reducing IFT. Moreover, ions help improve 3 regions; thus, IFT decreased, increased, and decreased in the solubility of the polar component of oil based on the Region 1, 2, and 3, respectively (Fig. 4). Based on Gibb’s salting-in ee ff ct, resulting in the IFT reduction (RezaeiDoust adsorption correlation, there is a relationship between IFT et al. 2009). In addition, when ions migrate to the interface, and surface excess concentration, such that if the surface they tend to form complex ions with polar agents and boost excess is positive, the IFT reduction is visible and vice versa the solubility of asphaltenes. This, in turn, can lead to posi- (Honarvar et al. 2020a; Lashkarbolooki et al. 2014; Rahimi tive surface excess concentration and IFT reduction (Austad et al. 2020). 2013; Lashkarbolooki et al. 2014). In short, in low salt con- In Region 1, where a low concentration of ions is present centrations, the two mechanisms of the salting-in effect and in the solution (brine with a concentration below 1000 ppm), the surface excess concentration contribute to IFT reduction. the ions tend to migrate from the solution and stay at the In Region 2, where the IFT begins to increase as the con- water/oil interface. When the ions are placed at the water/ centration of ions in the brine rises, the presence of more oil interface, two things happen. First, the surface excess ions in the solution heightens only the bulk of brine con- concentration of ions increases, leading to a decrease in centration because the ions are unable to move to one side IFT. Second, natural surface-active agents in the oil such as of the saturated interface and stay there. Consequently, the polar asphaltenes move toward the interface; consequently, surface excess concentration becomes negative, and the the surface excess concentration of asphaltenes becomes IFT increases. Additionally, the presence of more than a 1 3 IFT, mN/m IFT, mN/m IFT reduction IFT reduction 900 Petroleum Science (2021) 18:895–908 certain number of ions in the aqueous phase makes it dif- as a result of the contact between the brine and the oil phase. ficult for polar agents to dissolve in water. This phenomenon Thus, ions tend to return to the bulk of the solution because is called the salting-out effect (Fattahi Mehraban et al. 2019; of energy created at the interface (Kumar 2012; Lashkar- Lashkarbolooki et al. 2014; Rahimi et al. 2020). As a result, bolooki et al. 2014). Consequently, the IFT increases owing some natural surface-active agents return to the bulk of oil to the decreased number of ions at the interface and the from the interface, and the IFT rises owing to the negative negative surface excess concentration. Therefore, in Region quality of asphaltene surface excess concentration. Rostami 2, the salting-out effect, negative quality of surface excess et al. (2019) and Honarvar et al. (2020a) showed that when concentration, molecular movement variation, and hydrogen salinity rises in the solution, the free surface energy of the bond-breaking are the main causes of the IFT increase. interface as a result of the reduction in molecular move- Finally, in Region 3, the addition of more salts leads to a ment decreases, and, consequently, it causes an increase in reduction in IFT. This decrease in IFT is probably because the IFT values. Therefore, the IFT increase, in Region 2, although some polar agents move from the interface to the can also be attributed to the reduction in molecular move- bulk of oil because of the salting-out effect, which reduces ment. Additionally, as ions are placed in the brine, water their accumulation at the interface, there are still a number of molecules form hydrogen bonds with a cage-like structure these agents at the water/oil interface. Afterward, due to the around the ions. The formed hydrogen bonds can be broken packing effect, remaining polar agents are neatly re-situated Rising ion concentration Region 1Region 2Region 3 Polar agent Ion Brine Oil Hydrogen bond Hydrogen bond-breaking Water molecule Fig. 4 Schematics of oil/water interface and IFT variations in different sections 1 3 IFT increase Complex ion IFT variation Move back from the interface Petroleum Science (2021) 18:895–908 901 at the interface, and the IFT reduces (Lashkarbolooki et al. interface and further reduce IFT. In other words, by remov- 2014). Hence, the reason for the reduction in IFT in Region ing these salts, divalent ions could migrate to the double 3 is explained by the packing effect. layer and react with polar agents, thereby reducing IFT. In After examining the effect of the presence and absence terms of ionic strength, as shown in Table 4, it can be seen of each of the ions on IFT, we modified the concentration of that removing N a from PGSW decreases ionic strength ions in PGSW in order to determine IFT changes. Table 4 from 0.785 to 0.334 mol/L. This decrease in ionic strength shows different concentrations of ions in PGSW, related can contribute to improving the solubility of polar agents ionic strength, and their density applied to assess IFT. For and IFT reduction. However, eliminating K from PGSW, each ion, we used four different concentrations, which were leading to minimum IFT, does not significantly change the 0, 2, 3, and 4 times the initial concentration (i.e., the con- ionic strength, decreasing only by 0.025 mol/L as compared centrations existing in PGSW). Figure 5 depicts the effect to PGSW. Moreover, although SW0NaCl and SW0KCl solu- of PGSW with different concentrations of monovalent ions tions have different ionic strengths, a minimum IFT occurs + + on IFT. By increasing the concentration of Na or K in the in each mentioned solution. Therefore, it can be inferred seawater, the IFT first rises and then falls slightly, and a min - that even though the reduction of ionic strength is crucial to imum IFT occurs when NaCl or KCl is removed from the have minimum IFT, other factors such as the oil composi- seawater. In fact, the minimum value of IFT was obtained tion, type of ion, and its activity can play an important role in SW0NaCl (26.29 mN/m) and SW0KCl (26.56 mN/m). In in reducing IFT. In the case of the effect of monovalent ions addition, increasing the concentration of N a up to 2 times on IFT, previous studies reported that when NaCl or KCl is (SW2NaCl) and K up to 3 times (SW3KCl) in PGSW utilized individually or binary with other divalent ions in increases the IFT by 5.01 and 2.35 mN/m, respectively. It distilled water, IFT values reduce (Honarvar et al. 2020a; should be noted that although quadrupling the concentration Kakati and Sangwai 2018; Lashkarbolooki and Ayatollahi of each monovalent ion reduces IFT, the resulting decrease 2018; Lashkarbolooki et al. 2014; Nowrouzi et al. 2018). is still much higher than the minimum IFT obtained by However, the results of our study showed that the minimum + + removing each of the salts in PGSW. The reason could be IFT occurred when N a or K was eliminated from the 2+ that the lack of NaCl or KCl in the seawater enables Ca , smart water, and increasing concentration of monovalent 2+ 2− Mg , and SO to be placed more easily at the water/oil ions could cause the IFT values to increase. Table 4 Persian Gulf seawater with varied salt concentrations Solution Density, g/cm Ion concentration in seawater, ppm Ion strength, + 2+ 2+ + 2− − 2+ − Na Ca Mg K SO HCO Sr Cl 4 3 mol/L SW0NaCl 0.9933 1452 440 1632 399 3110 166 3 6064 0.334 SW2NaCl 1.0275 22,511 440 1632 399 3110 166 3 38,653 1.252 SW3NaCl 1.044 33,059 440 1632 399 3110 166 3 54,957 1.711 SW4NaCl 1.0594 43,607 440 1632 399 3110 166 3 71,241 2.17 SW0CaCl 1.0104 12,000 0 1632 399 3110 166 3 21,577 0.76 SW2CaCl 1.0119 12,000 880 1632 399 3110 166 3 23,139 0.826 SW3CaCl 1.0123 12,000 1320 1632 399 3110 166 3 23,920 0.859 SW4CaCl 1.0135 12,000 2560 1632 399 3110 166 3 24,701 0.892 SW0MgCl 1.008 12,000 440 0 399 3110 166 3 17,601 0.592 SW2MgCl 1.0113 12,000 440 3265 399 3110 166 3 27,115 0.994 SW3MgCl 1.0184 12,000 440 4896 399 3110 166 3 31,872 1.195 SW4MgCl 1.0204 12,000 440 6528 399 3110 166 3 36,629 1.396 SW0KCl 1.0106 12,000 440 1632 0 3110 166 3 22,003 0.783 SW2KCl 1.0117 12,000 440 1632 798 3110 166 3 22,713 0.803 SW3KCl 1.0123 12,000 440 1632 1197 3110 166 3 23,068 0.813 SW4KCl 1.0135 12,000 440 1632 1596 3110 166 3 23,423 0.823 SW0Na SO 1.0088 10,622 440 1632 399 0 166 3 22,358 0.703 2 4 SW2Na SO 1.016 13,379 440 1632 399 6220 166 3 22,358 0.883 2 4 SW3Na SO 1.0197 14,758 440 1632 399 9330 166 3 22,358 0.973 2 4 SW4Na SO 1.0239 16,137 440 1632 399 12,440 166 3 22,358 1.063 2 4 1 3 902 Petroleum Science (2021) 18:895–908 before should be considered alongside ionic strength. For example, in this research, even though the S W3Na SO solu- 2 4 tion had greater ionic strength compared to S W3CaCl , it produced lower IFT values. This issue can be rooted in the 2− 2+ fact that SO has greater ion activity than Ca , and there- fore it has more ability to reduce IFT to minimum values. The results of IFT tests suggest that even though the + + absence of monovalent ions such as Na and K in seawater NaCl leads to a decrease in IFT values, the presence of divalent KCl ions is necessary for reducing IFT. Because divalent ions, SW0SW2 SW3SW4 which are active, can form complex ions with polar agents that come to the water/oil interface from the bulk of the oil, Fig. 5 Effect of monovalent ions spiking of seawater on IFT and solubility of polar agents increases. Therefore, it can be inferred that divalent ions have a great ability to reduce IFT Figure 6 illustrates the ee ff ct of PGSW with die ff rent con - and, consequently, their presence in smart water is vital. In centrations of divalent ions on IFT. As can be seen, as the the case of the effect of divalent ions on IFT, according to concentrations of CaCl, MgCl , and Na SO are increased, previous research (Honarvar et al. 2020a; Kakati and Sang- 2 2 2 4 2+ 2+ the IFT initially declines and then increases. Mg, Ca , wai 2018; Lashkarbolooki and Ayatollahi 2018; Lashkar- 2− and SO are potential determining ions (PDI); therefore, bolooki et al. 2014; Nowrouzi et al. 2018), increasing con- their presence in seawater can play a significant role in centration of the divalent ions can diminish IFT values when 2+ 2+ 2− reducing IFT. To put it die ff rently, when the concentration of Mg, Ca , and SO are utilized individually or in pair PDI increases in PGSW, they can react with carboxyls at the in distilled water. Moreover, our results also illustrated that oil/water interface, and this, in turn, can lead to an increase they are capable of reducing IFT in the presence of other in the solubility of carboxyls in both oil and water phases. divalent and monovalent ions. Thus, the IFT decreases to a minimum value. However, when According to IFT results, the optimal concentrations for the IFT reaches its minimum value, a further increase in the NaCl, KCl, MgCl, CaCl , and Na SO occur in SW0NaCl, 2 2 2 4 concentration of PDI can bring about an increase in IFT. SW0KCl, SW2MgCl, SW3CaCl , and SW3Na SO solu- 2 2 2 4 This increase in IFT values can be attributed to two reasons. tions, respectively. The IFT between oil and diluted PGSW The first reason would be that the mechanism of the salting- and FW was calculated for comparison purposes. The IFT out effect is activated if the concentration of PDI increases values for FW, PGSW, doubly diluted PGSW (SW2d), and more than a certain value. In other words, as high concentra- tenfold diluted PGSW (SW10d) are 31.14, 30.40, 27.11, 2+ 2+ 2− tions of Ca, Mg , and SO are present in the proximity and 33.36 mN/m, respectively (Table 5). Based on Table 5 to the interface, the solubility of polar agents decreases, and and Figs. 5 and 6, removing NaCl from PGSW or tripling water molecules are unable to balance the polarization of the concentration of Na SO reduces IFT more than does 2 4 divalent ions and carboxyls. Therefore, polar agents return diluted PGSW. The IFT values for SW2d, SW3Na SO , 2 4 from the interface to the bulk of the oil phase, leading to and SW0NaCl solutions are 27.11, 26.96, and 26.29 mN/m, the negative surface excess concentration and higher IFT. respectively, indicating that smart water flooding has a better Another possible reason might be that molecular move- performance in this regard. ment can greatly decrease at a high concentration of PDI. Thus, the free surface energy of the interface can diminish, and, as a result, the IFT increases. The minimum IFT for 2+ 2+ 2− Ca, Mg , and SO , corresponding to 29.95, 27.45, and MgCl2 Na2SO4 26.96 mN/m, respectively, occurs at 3, 2, and 3 times the CaCl2 initial concentration. Furthermore, according to the results, 2− SO can cause a further reduction in IFT as compared 2+ 2+ to Ca and Mg . Also, according to Table 4, the ionic strength for SW3Na SO, SW3CaCl , and SW2MgCl was 2 4 2 2 respectively 0.973, 0.859, and 0.994 mol/L. Accordingly, 2− although tripling the concentration of SO leads to the 2+ lowest IFT compared to the other divalent ions ( Mg and 2+ Ca), SW3Na SO has greater ionic strength. As a result, SW0SW2 SW3SW4 2 4 it can be concluded that seawater with lower ionic strength may not result in minimum IFT, and other factors mentioned Fig. 6 Effect of divalent ions spiking of seawater on IFT 1 3 IFT, mN/m IFT, mN/m Petroleum Science (2021) 18:895–908 903 3.2 Eec ff t of PGSW with different salinities Table 5 Effect of seawater, diluted seawater and formation water on on contact angle and zeta potential IFT Solution IFT, mN/m Suspended oil droplet In order to evaluate wettability alteration, we measured shape the contact angle for each salt at optimal concentrations PGSW 30.40 ± 0.24 obtained from IFT tests. After the rock specimens became oil-wet, the average contact angle was 123°, which confirms that the rock specimens are oil-wet. Table 6 presents the shape of the droplets in equilibrium, contact angle values, ionic strength, and zeta potential at optimal concentrations. SW2d (diluted 2 27.11 ± 0.19 The contact angle values for PGSW, SW3CaCl , SW0NaCl, times) SW0KCl, SW2MgCl , and SW3Na SO solutions are 2 2 4 91°, 85.8°, 67.8°, 81.7°, 77.9°, and 70.2°, respectively. A weakly water-wet condition occurs when the contact angle is between 30° and 75°, and a neutral-wet condition emerges SW10d (diluted 33.36 ± 0.11 when the contact angle is in the range of 75° to 105° (Meng 10 times) et al. 2018). According to Table 6, removing NaCl from the PGSW causes wettability to approach the weakly water-wet condi- FW 31.14 ± 0.31 tion. There are main reasons for this change in wettability, as a result of eliminating NaCl. Firstly, when the concentrations of the monovalent ions decrease in PGSW, the dissolution of carbonate rock occurs (Al-Nofli et al. 2018). In this case, calcium carbonate dissolves, and the rock surface becomes negatively charged based on the following reaction (Karimi et al. 2016): of the polar agents in water, and a further reduction can be 2+ − − CaCO (s) + H O ↔ Ca + HCO + OH (1) 3 2 seen in the contact angle values. Thus, when removing NaCl from the PGSW solution, the mechanism of the salting-in Therefore, more carboxyls can be detached from the sur- effect is activated, and the contact angle decreases further. face of rock because of the repulsive force existing between + It is noteworthy that as removing Na from PGSW, the ionic negative charges of rock and carboxyls. Consequently, wet- strength reduces from 0.785 to 0.334 mol/L. Therefore, the tability can be changed to weakly water-wet, and this mecha- adhesion of oil on the rock surface can be decreased, and nism is shown in Fig. 7. Secondly, it should be mentioned this, in turn, can boost water-wetness conditions. However, that the ions in the brine are in contact with the rock sur- eliminating KCl does not considerably change the contact face through an electrical double layer which is formed by angle, it only leads to a nine-degree reduction in this angle diffusive and stern layers, and they can be either adsorbed relative to PGSW and leaves wettability in the same neutral- by the attractive force on the rock or driven away from the wet condition. The reason could be that the concentration of surface by the repulsive force (Lashkarbolooki et al. 2017; + K is low in PGSW, and removing it does not significantly Shirazi et al. 2020). Therefore, when the brine has a high affect wettability. concentration of NaCl, high levels of Na in the diffusive Also, even though doubling MgCl concentration reduces layer are present, and less chance is given to divalent ions to the contact angle by 13.1° relative to PGSW, the neutral-wet be positioned in the electrical double layer so as to further condition remains in place. As discussed in the literature reduce the contact angle due to their activity. Thus, once (Fathi et al. 2010; Karimi et al. 2016), as a result of the NaCl is eliminated from the seawater, the carbonate rock 2+ presence of anions and the dissolution process, Mg can 2+ 2+ 2− surface is more readily available to Ca , Mg , and SO , get closer to the rock surface since less positive charges are which are active ions, and wettability, as a result of the 2+ available on the rock surface. Therefore, Mg can react expansion of the double layer, could change from oil-wet to with carboxyls and reduce contact angle. Besides, it can weakly water-wet (Fig. 8). Finally, decreasing concentration 2+ replace Ca via ion exchange, and, as a result, this can of ions in seawater can give rise to the salting-in effect, and detach oil droplets from the carbonate surface (Zhang and therefore more carboxyls can be desorbed from the surface Austad 2006). Nevertheless, by comparison with SW0NaCl (Karimi et al. 2016). In other words, reducing the concentra- in terms of wettability alteration, S W2MgCl solution was tion of ions in the brine leads to an increase in the solubility unable to significantly reduce the contact angle, due to the 1 3 904 Petroleum Science (2021) 18:895–908 Table 6 Effect of different smart water solutions on the contact angle and zeta potential Solution Contact angle, degree Ion strength, mol/L Oil droplet shape Zeta potential, mV PGSW 91.0 ± 1.03 0.785 − 2.7 ± 0.17 SW3CaCl 85.8 ± 1.23 0.859 − 2.4 ± 0.13 SW0NaCl 67.8 ± 1.65 0.334 − 4.9 ± 0.21 SW0KCl 81.7 ± 1.11 0.783 − 3.7 ± 0.19 SW2MgCl 77.9 ± 0.98 0.994 − 2.3 ± 0.15 SW3Na SO 70.2 ± 1.44 0.973 − 1.8 ± 0.18 2 4 Na CaCO 2+ Ca Removing Na HCO3 Brine Rock Carboxyl Fig. 7 Schematic of the mechanism of dissolution in the absence of Na high concentration of Na in the double layer. To put it dif- rock to weakly water-wet conditions. In smart water flood- ferently, the high concentration of Na hindered the posi- ing, the carbonate rock surface can have positive charges 2+ tive effects of Mg from changing the wettability towards (RezaeiDoust et al. 2009). Also, when the concentration 2+ 2− water-wet condition. Also, tripling the concentration of Ca of SO in PGSW increases, because of the adsorption of 2− shows that wettability cannot be changed to water-wet condi- SO on it, the rock surface shifts from a surface with posi- 2+ tion. Because, more than a certain concentration of Ca , the tive charges to a surface with negative charges. Therefore, salting-out effect is activated, and the contact angle does not in the presence of negative charges, divalent cations will be change substantially (Rahimi et al. 2020). In other words, the able to approach the surface of the rock and change wetta- 2+ solubility of polar agents can be decreased due to the high bility by replacing complex ions, formed between Ca and 2+ 2+ concentration of Ca in the PGSW. In fact, as the concentra- carboxyls, with Mg (Fathi et al. 2010; Rashid et al. 2015). 2+ tion of Ca increases in PGSW, a water structure, which is Moreover, when anions and cations are present in the brine, created as a result of hydrogen bonds formed between hydro- ion-pairs can be formed. In other words, based on Eqs. (2) 2+ 2+ phobic pieces of polar agents and water molecules, can be and (3), the formation of ion-pairs is between Mg, Ca , 2− broken, and the solubility of polar agents is decreased. Thus, and SO (Moosavi et al. 2019). wettability cannot be altered to a water-wet state owing to 2+ 2− 2+........... 2− Mg + SO = Mg SO (2) the decreased solubility. 4 4 Like the effect of NaCl removal on wettability, increasing Na SO concentration can alter the wettability of carbonate 2 4 1 3 Petroleum Science (2021) 18:895–908 905 Na 2+ Mg Removing Na 2+ Ca 2- SO4 Brine Carboxyl Fig. 8 Schematic of the expansion of double layer in the absence of Na 2+ 2− 2+......... 2− However, Al-Hashim et al. (2018) reported that doubling the Ca + SO = Ca SO (3) 4 4 2− concentration of SO in seawater decreases the negative 2− Therefore, as SO is adsorbed on the rock surface, surface charge of carbonate rock. Therefore, their result for 2− 2+ 2+ SO is in line with our results. This decline in the magni- owing to the formation of ion-pairs, more Mg and Ca are available in close proximity to the surface, and wettability tude of the negative zeta potential can be attributed to two reasons. Firstly, because of electrostatic screening, above can be further altered. It should be pointed out that the ionic strength values for S W3Na SO, SW3CaCl , and S W MgCl a specific concentration of Na SO , ions are unable to be 2 4 2 4 2 2 2 adsorbed onto the rock surface, and increasing the Na SO increase by 0.188, 0.074, and 0.209 mol/L, respectively. 2 4 According to previous studies (Derkani et al. 2019), lower- concentration causes the rock surface to have less negative charges (Awolayo and Sharma 2016). Secondly, the con- ing the ionic strength values can enhance the water-wetness condition of the rock surface. Our results illustrate that centrations of divalent ions and their presence in seawater can have impacts on the affinity of ions towards the surface, although the ionic strength increases, the contact angle decreases. Thus, it can be inferred that the presence of PDI and the zeta potential values can be changed from nega- tive to positive even by an increase in the concentration of plays a prominent role in contact angel reduction, and wet- 2− tability alteration can occur if the ionic strength rises. SO (Kasha et al. 2015). As Table 6 shows, in order to evaluate the surface charge of rock for optimal concentrations, we calculated zeta poten-3.3 Micromodel flooding tial at −2.7 mV when the carbonate rock was exposed to the PGSW solution. Following the removal of NaCl or KCl, the Micromodel flooding was performed at optimal concen- trations obtained for each salt to evaluate oil recovery zeta potential was −4.7 and −3.7 mV, respectively. There- fore, the removal of monovalent ions from the PGSW solu- by smart water flooding. Figure 9 shows the ultimate oil recovery through the injection of different smart solutions tion increases the magnitude of the negative zeta potential, which is consistent with the results reported by Abbasi et al. at optimal concentrations. As a result of PGSW flooding, 2+ oil recovery was 23.22%, which is the lowest oil recovery (2020). However, as the concentration of Mg in the PGSW solution is doubled, the negative charge on the rock surface compared to other solutions. The ultimate oil recovery val- ues for SW0NaCl, SW0KCl, SW2MgCl , and SW3Na SO is reduced just slightly. In this case, the zeta potential has 2 2 4 changed from −2.7 to −2.3 mV. A similar trend was observed solutions were 33.34, 27.12, 28.44, and 30.56%, respec- 2+ tively. As can be seen, the SW0NaCl solution exhibits the for SW3CaCl . As the concentration of Ca was tripled in PGSW, the zeta potential was altered from −2.7 to −2.4 mV. highest oil recovery because it has not only the lowest IFT 2+ 2+ but also the largest alteration of contact angle. In fact, the In fact, increasing concentration of Mg or Ca owing to the adsorption of these ions onto the rock reduces the nega- oil recovery was 10.12% higher than PGSW flooding. This is in line with the results reported by Fathi et al. (2010, tive charge of the surface by a small amount. Also, the zeta potential changes from −2.7 to −1.8 mV 2011), Awolayo and Sharma (2016), and Puntervold et al. (2015). The oil recovery for PGSW without K was about by tripling the concentration of Na SO in PGSW. Some 2 4 previous studies (Abbasi et al. 2020; Strand et al. 2006; 6.5% less than PGSW without N a . The difference in oil recovery of these two solutions, although IFT values are Alroudhan et al. 2016; Kasha et al. 2015; Smallwood 1977; Mahani et al. 2017) show that there is a rise in the mag- almost the same for both, is explained by the fact that 2− the SW0NaCl solution can cause a higher reduction in nitude of the negative zeta potential as the SO concen- tration increases in seawater, which contradict our results. the contact angle; thus, it produces more oil in a more 1 3 Stern Diffusive Rock layer layer Expanded-double layer 906 Petroleum Science (2021) 18:895–908 water-wet condition. In addition, the SW3Na SO solu- 4 Conclusions 2 4 tion increased oil recovery by 2.12% more than did the SW2MgCl solution because it further reduced IFT and Based on the tests performed, which included IFT, contact changed the wettability to a weakly water-water state. angle, zeta potential, and micromodel tests, the following Figure 10 illustrates micromodel images after the injec- results can be inferred: tion of 1 pore volume (PV) of smart water for PGSW and SW0NaCl solutions. As shown, the SW0NaCl solution, (1) When each ion was utilized separately in distilled 2+ compared to PGSW, was able to improve the sweep effi - water, Ca showed a greater ability to reduce IFT to ciency, resulting in less trapped oil in the micromodel. a minimum value. In the case of using different con- Thus, the lowest IFT and contact angle were obtained for centrations of monovalent and divalent ions in PGSW, the SW0NaCl solution, which allowed overcoming the eliminating Na from PGSW resulted in the lowest IFT capillary forces in the micromodel pores leading to more value. oil production. Therefore, removing NaCl or tripling the (2) Although removing each of the monovalent ions (K concentration of N a SO can be the best possible option or Na ) from PGSW causes IFT reduction, increas- 2 4 2+ 2+ 2− if one seeks to carry out smart water flooding by changing ing Ca, Mg , and SO concentrations in PGSW the concentration of salts in PGSW in order to improve decreased IFT values. Thus, it can be stated that the oil recovery. absence of monovalent ions is of importance to decline IFT, and the presence of divalent ions plays a crucial role in reducing IFT. It should be noted that seawater with lower ionic strength may not result in minimum IFT, and other factors such as the presence of PDI can have a more positive effect on minimizing the IFT. 33.34 (3) The lowest contact angle was related to the solution 30.56 28.44 30 from which NaCl was removed (67.8°). Additionally, 27.12 among all the solutions evaluated, only SW0NaCl 25 23.22 and SW3Na SO could change surface wettability to 2 4 weakly water-wet conditions, while the other solutions led to a neutral-wet condition. 10 (4) The removal of NaCl or KCl from PGSW caused the rock surface to have slightly higher negative charges. However, increasing the concentration of divalent ions 2+ 2+ 2− (Ca, Mg , and SO ) led to a reduction in the mag- PGSW SW0NaCl SW0KCl SW2MgCl SW3Na SO 2 2 4 4 nitude of the negative zeta potential. Fig. 9 Effect of different smart water solutions on the ultimate oil recovery Fig. 10 Oil displacement after the injection of 1 PV of the injected fluid: a PGSW flooding and b SW0NaCl flooding 1 3 Ultimate oil recovery, % Petroleum Science (2021) 18:895–908 907 limestone reservoirs. J Mol Liq. 2019;277:132–41. https ://doi. (5) SW0NaCl and SW3Na SO solutions, compared to 2 4 org/10.1016/j.molli q.2018.12.069. PGSW, raised ultimate oil recovery by 10.12% and Austad T. Water-based EOR in carbonates and sandstones: new chemi- 7.34%, respectively. Therefore, if smart water flooding cal understanding of the EOR potential using “Smart Water.” Hou- is to be performed in reservoirs by changing the con- ston: Gulf Professional Publishing; 2013. p. 301–35. Austad T, Shariatpanahi SF, Strand S, Black CJJ, Webb KJ. 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Petroleum Science – Springer Journals
Published: Jan 17, 2021
Keywords: Smart water; Minimum IFT; Wettability; Zeta potential; Enhanced oil recovery
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