Response of Non-Polar Oil Component on Low Salinity E ﬀ ect in Carbonate Reservoirs: Adhesion Force Measurement Using Atomic Force Microscopy

: While the e ﬀ ect of polar-oil component on oil-brine-carbonate system wettability has been extensively investigated, there has been little quantitative analysis of the e ﬀ ect of non-polar components on system wettability, in particular as a function of pH. In this context, we measured the contact angle of non-polar oil on calcite surface in the presence of 10,000 ppm NaCl at pH values of 6.5, 9.5 and 11. We also measured the adhesion of non-polar oil group (–CH 3 ) and calcite using atomic force microscopy (AFM) under the same conditions of contact angle measurements. Furthermore, to gain a deeper understanding, we performed zeta potential measurements of the non-polar oil-brine and brine-calcite interfaces, and calculated the total disjoining pressure. Our results show that the contact angle decreases from 125 ◦ to 78 ◦ with an increase in pH from 6.5 to 11. AFM measurements show that the adhesion force decreases with increasing pH. Zeta potential results indicate that an increase in pH would change the zeta potential of the non-polar oil-brine and calcite-brine interfaces towards more negative values, resulting in an increase of electrical double layer forces. The total disjoining pressure and results of AFM adhesion tests predict the same trend, showing that adhesion forces decrease with increasing pH. Our results show that the pH increase during low-salinity waterﬂooding in carbonate reservoirs would lift o ﬀ non-polar components, thereby lowering residual oil saturation. This physiochemical process can even occur in reservoirs with low concentration of polar components in crude oils. of pH increase on non-polar oil-calcite adhesion during low-salinity water injection, and provides insights into the quantiﬁcation of carbonate reservoir wettability.


Introduction
Carbonate reservoirs host roughly >60% of hydrocarbons (oil and gas) in the world [1], but only up to 40% of the oil can be recovered from the reservoirs [2]. The petroleum industry is therefore constantly working to develop economically feasible techniques to enhance oil recovery in order to meet the growing global demand for energy [3]. One technique that has attracted interest within the industry is low-salinity water flooding [4]. Extensive research has been done to show that lowering injected brine salinity can improve oil recovery from carbonate reservoir at secondary and tertiary mode from 5 to 30% of the original oil in place (OOIP) [5][6][7][8][9][10][11][12]. Wettability alteration towards a more water-wet state is believed to be one of the main physiochemical processes behind the low salinity effect, which shifts oil relative permeability towards a lower residual oil saturation [9,[13][14][15], thus enhancing oil recovery. To understand the factors controlling the wettability alteration process in carbonate reservoirs, several explanations have been developed and proposed to predict and quantify the wettability alteration, such as electrostatic bridging [7,8,[16][17][18], electrical double layer [13,[19][20][21], and surface complexation modelling [22,23].
To be more specific, the existing literature shows that the adsorption of the sulphate onto the pore surface would likely shift the surface charge from positive to negative, thus leading to a repulsion force between the oil (base and acidic functional groups) and carbonate pore surfaces [7]. Additionally, previous studies show that electrical double layer force also plays an important role in wettability alteration and release of the oil component from the carbonate surface [13,18,20,21,23]. For example, Mahani et al. [18] measured the zeta potential of the oil-brine and brine-carbonate (limestone) interfaces. They found that decreasing the salinity leads to an increase in the negative charge of the polar oil-brine and brine-carbonate (limestone) interfaces, suggesting a weaker electrostatic adhesion between the polar oil-brine and brine-carbonate (limestone) interfaces, and hence recession of the three-phase contact line. Furthermore, to gain a deeper understanding at the molecular level, atomic force microscopy (AFM) [24,25] and quartz crystal microbalance (QCM) [26] have been used to examine the effect of brine chemistry on polar oil-rock adhesion. For example, Liu et al. [24] used AFM to study the effect of salinity on the adhesion force between functional oil groups and hydrophobic surfaces. They concluded that the electrical double layer was at least one of the controlling factors of adhesion forces, and thus the wettability of oil-brine-rock. In addition, Joonaki et al. [26] used QCM to investigate the impact of brine composition on the deposition of the asphaltene oil species on the rock surface at elevated temperature and pressure conditions. They found that brine composition and salinity play an important role in the deposition of the asphaltene oil species on the rock surface, thereby affecting the wettability of the oil-brine-rock system.
To better quantify and predict the wettability alteration process, surface complexation modelling has been implemented to model chemical surface species from oil and calcite surfaces, thus characterizing the adhesion of oil and calcite [22,23,[27][28][29][30]. For example, Brady et al. [22] developed a surface complexation model to characterize the wettability of oil-brine-calcite using a bond product sum concept, which is equal to [ 3 Ca + ]. They suggest that the potential adhesion between the polar oil and carbonate surface increases with increasing bond product (more oil-wet system), and the potential adhesion between the polar oil and carbonate surface decreases with decreasing bond product (less oil-wet system). However, the above geochemical models were proposed to characterize oil-brine-carbonate wettability in light of the interaction of polar oil groups (-NH + and -COO − ) on carbonate surfaces. Few research have been done to draw on any structured research into the response of non-polar oil on wettability alteration when low-salinity waterflooding in carbonate reservoirs, and fewer works have been performed to investigate the effect of pH on the adhesion of non-polar oil and calcite, although local pH likely increases with low-salinity water flooding in carbonate reservoirs [31][32][33][34]. For example, Generosi et al. [35] used AFM to investigate the effect of the potential determining ions on the calcite wettability, although the pH effect was not examined. They found that the presence of Mg 2+ and SO 4 2− ions decreased the adhesion forces between CH 3 -terminated tip and calcite surface, thus increasing hydrophilicity.
In conclusion, while the influence of polar oil components on wettability alteration has been extensively investigated, in particular in geochemical modelling and molecular-level force measurements, researchers have not treated the response of non-polar oil on wettability alteration in much detail. Also, we know that the binding/bridging between polar oil component-calcite surface would likely decrease with increasing pH, but how non-polar oil responds to the increase in pH remains unclear. We therefore hypothesized that non-polar oil would be lifted off from calcite surfaces due to the increase in pH during low-salinity water flooding. To test this hypothesis, we measured the contact angle of non-polar oil on the calcite surface in the presence of an aqueous ionic solution (10,000 ppm NaCl) at pH values of 6.5, 9.5, and 11. We also measured the adhesion of the non-polar oil Energies 2020, 13, 77 3 of 14 group (-CH 3 ) and calcite using AFM under the same conditions of the contact angle measurement. Furthermore, to gain a deeper understanding of the isothermal thermodynamic, we performed zeta potential measurements of -CH 3 -brine and brine-calcite interfaces, and calculated the total disjoining pressure using a sphere to flat thermodynamic model under constant potential condition.

Brine
To validate our hypothesis and study the influence of pH on the adhesion force between non-polar oil and carbonate surface, we used NaCl salt (Analytical reagent, 99.9%) and ultrapure water (Resistivity 18.2 MΩ) to prepare NaCl brine with 10,000 ppm salinity. It is noteworthy that divalent cations (Ca 2+ & Mg 2+ ) were not used in this study because of their competition with H + /OH − to adsorb on calcite surfaces, which likely compensates for the pH effect on non-polar oil [36]. Rather, the monovalent ion can better help us to focus on the pH effect. Also, to avoid any variation of the pH due to calcite dissolution during the measurements, a piece of calcite crystal was submerged in the solution for two days to reach calcite dissolution equilibrium. It is worth noting that calcite dissolution can reach equilibrium within 4 h [37]. Finally, we used 0.250 mol/L of hydrochloric acid and sodium hydroxide solutions to adjust the pH of the brine to 9.5 and 11.

Oil
Given that this work aims to understand the response of non-polar component on the adhesion at calcite surface with the presence of various brines, we used model oil compound containing -CH 3 groups to represent hydrocarbon functional groups [27,38,39]. In this work, we used Octadecane (C 18 H 38 ) for contact angle measurements, and 1-Octadecanethiol (C 18 H 38 S) for AFM measurements. The strong chemisorption of the thiol group on the gold helps to represent the oil on the gold AFM tip [40,41].

Calcite
To examine our hypothesis, we used calcite crystal (Iceland Spar from Ward's Science) as substrates for the contact angle measurements and the AFM experiments. To ensure the integrity of experimental measurements, and to avert the surface roughness effect on the contact angle and AFM measurements [42,43], the calcite crystals were cleaved to obtain a new clean smooth calcite surface [29], then rinsed with deionized water saturated with calcite to remove any small calcite pieces. It is worth noting that we saturated the deionized water with calcite to prevent any effect on the new calcite surface during cleaning. Also, it is noteworthy that the surface roughness of the new calcite surface used in this work is in a range of 4 to 7 nm, which has been reported in our previous work [44].
Also, we prepared calcite powders with diameters in the range of 10 to 100 µm for the zeta potential measurements at the interface of brine-calcite as a function of pH (6.5, 9.5 and 11). The calcite powders were prepared using the following steps. Deionized water saturated with calcite was used to clean the calcite crystal. Then, the calcite crystal was kept in an oven at 60 • to remove any adsorbed water at surfaces. Afterwards, mortar and pestle were used to crush the calcite crystal to fine powders [21].

Contact Angle Measurements
In this study, we used the sessile drop method (Figure 1) to measure the contact angle of non-polar oil in the presence of aqueous ionic solution (10,000 ppm NaCl) at pH values of 6.5, 9.5 and 11 on calcite crystal surfaces. Given that pressure likely plays a negligible role in the oil-brine-rock system interactions [45], we measured the contact angle at ambient pressure. Also, we performed the contact angle measurements at 27 • C to avoid any solidification of the non-polar oil (Octadecane). However, Energies 2020, 13, 77 4 of 14 a clean and new calcite substrate was used in the contact angle experiments. The substrate was obtained by cleaving a clean calcite sample, which was then rinsed with ultrapure water pre-saturated with CaCO 3 to remove all the remaining small pieces of calcite from the new surface. Afterward, the new calcite surface was placed into the wettability measurement cell which was pre-filled with NaCl solution. After that, a droplet of the non-polar oil was then introduced onto the calcite substrate using a syringe. The contact angle was monitored and recorded until the contact angle variation with time was negligible. It is worth noting that to solely focus on the pH effect on non-polar oil adhesion on calcite surfaces, and to compare contact angle results with adhesion force measured by atomic force microscopy, we did not use a porous carbonate rock to measure contact angle [46]. filled with NaCl solution. After that, a droplet of the non-polar oil was then introduced onto the calcite substrate using a syringe. The contact angle was monitored and recorded until the contact angle variation with time was negligible. It is worth noting that to solely focus on the pH effect on non-polar oil adhesion on calcite surfaces, and to compare contact angle results with adhesion force measured by atomic force microscopy, we did not use a porous carbonate rock to measure contact angle [46].

AFM Measurements
Atomic force microscopy (AFM) is an effective tool for directly measuring the adhesion force of the oil-brine-rock system [24,25,[47][48][49][50][51][52]. In this work, we used AFM (WITec alpha 300 SAR) to measure the adhesion force between the model oil compound and calcite in the presence of NaCl brine (10,000 ppm) at pH values of 9.5 and 11 at ambient conditions. It is worth noting that we did not measure the adhesion force at pH 6.5 due to the fact that calcite dissolution at the surface may significantly affect the oil-brine-calcite adhesion at nano-scale. To achieve realistic adhesion force results, we performed about one thousand force-distance measurements for each of the experiments with one thousand data points for each force-distance curve. The mean average value is reported in the results section. However, WITec ProjectFOUR software was used to collect the AFM experimental data. Also, we modified the MatLab program developed by Yu et al. [53], and used it to extract adhesion force data for multiple measurements at the same time.
To functionalize the model oil compound onto the AFM tip surface, in all the AFM experiments, we used the AFM tips (NPG-10 from Bruker Corporation) coated with gold and with a spring constant of 0.06 N/m. A chemical (1-Octadecanethiol, C18H38S) was used to functionalize the tip to represent the non-polar oil group (-CH3) [52]. To successfully functionalize the tip, we firstly used a plasma cleaner to clean the AFM tip for 30 min. Then, we immersed the AFM tip directly into ethanol at ambient temperature to minimize the gold oxide (Au2O3) that forms during the cleaning to metallic gold (Au) [54]. Thereafter, we submerged the AFM tip in 1mM of 1-Octadecanethiol (C18H38S) ethanol solution for one day. On the completion of the functionalization, the functionalized tip was washed by pure ethanol, and then dried by a stream of nitrogen gas prior to AFM measurements [55].

Zeta Potential Measurements
Electrical double layer force may play an important role in the wettability alteration of oil-brinemineral systems [21,56,57], and this can be computed using the zeta potential of the oil-brine and brine-mineral interfaces. Thus, we used a Malvern Zetasizer ZS Nano series to measure the zeta potential of -CH3-brine and brine-calcite interfaces at different pH values (6.5, 9.5 and 11). However, the zeta potential experiment methodologies are well demonstrated by Yang et al. [58]. It is worth

AFM Measurements
Atomic force microscopy (AFM) is an effective tool for directly measuring the adhesion force of the oil-brine-rock system [24,25,[47][48][49][50][51][52]. In this work, we used AFM (WITec alpha 300 SAR) to measure the adhesion force between the model oil compound and calcite in the presence of NaCl brine (10,000 ppm) at pH values of 9.5 and 11 at ambient conditions. It is worth noting that we did not measure the adhesion force at pH 6.5 due to the fact that calcite dissolution at the surface may significantly affect the oil-brine-calcite adhesion at nano-scale. To achieve realistic adhesion force results, we performed about one thousand force-distance measurements for each of the experiments with one thousand data points for each force-distance curve. The mean average value is reported in the results section. However, WITec ProjectFOUR software was used to collect the AFM experimental data. Also, we modified the MatLab program developed by Yu et al. [53], and used it to extract adhesion force data for multiple measurements at the same time.
To functionalize the model oil compound onto the AFM tip surface, in all the AFM experiments, we used the AFM tips (NPG-10 from Bruker Corporation) coated with gold and with a spring constant of 0.06 N/m. A chemical (1-Octadecanethiol, C 18 H 38 S) was used to functionalize the tip to represent the non-polar oil group (-CH 3 ) [52]. To successfully functionalize the tip, we firstly used a plasma cleaner to clean the AFM tip for 30 min. Then, we immersed the AFM tip directly into ethanol at ambient temperature to minimize the gold oxide (Au 2 O 3 ) that forms during the cleaning to metallic gold (Au) [54]. Thereafter, we submerged the AFM tip in 1mM of 1-Octadecanethiol (C 18 H 38 S) ethanol solution for one day. On the completion of the functionalization, the functionalized tip was washed by pure ethanol, and then dried by a stream of nitrogen gas prior to AFM measurements [55].

Zeta Potential Measurements
Electrical double layer force may play an important role in the wettability alteration of oil-brine-mineral systems [21,56,57], and this can be computed using the zeta potential of the oil-brine and brine-mineral interfaces. Thus, we used a Malvern Zetasizer ZS Nano series to measure the zeta potential of -CH 3 -brine and brine-calcite interfaces at different pH values (6.5, 9.5 and 11). However, the zeta potential experiment methodologies are well demonstrated by Yang et al. [58]. It is worth noting that all the measurements were conducted at 27 • C, and the value of each zeta potential was taken from the mean average of four measurements.

Total Disjoining Pressure
To expand our understanding of the isothermal thermodynamic, we calculated the intermolecular forces (IMF) between the non-polar oil-brine and brine-calcite interfaces using Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory [59,60].
To interpret the AFM adhesion measurements better, we used the sphere to flat geometry model to compute the total disjoining pressure at the constant potential condition [52]. It is worth noting that our previous study showed that the sphere to flat geometry model could better interpret the adhesion between the tip and flat substrate in AFM experiments compared to flat to flat geometry model [52].

Van der Waals Forces
Van der Waals force is a long-range interaction, and it can be an attractive or repulsive force, depending on the sign of the calculated Hamaker constant [61]. In this research, the Van der Waals intermolecular force was an attractive force, and we computed the Van der Waals intermolecular force using the sphere to flat surface geometry model using the following model [61]: where R is the sphere radius (m), h the film thickness between the tip and flat substrate (m), and A is the Hamaker constant (range between 10 −21 to10 −19 ) calculated using Equation (3), below [36]: where k B is the Boltzman constant (1.381 × 10 −23 J·K −1 ); T is the temperature in kelvin; ε o is the permittivity of oil (2·C·V −1 ·m −1 ) [36]; ε w is the permittivity of water (78.3 C·V −1 ·m −1 ), and ε s is the permittivity of rock (7 C·V −1 ·m −1 ) [36].

Electric Double Layer Forces
Electric double layer forces are a function of the surface charges and brine ionic strength, which strongly affects the total disjoining pressure [59]. It is noteworthy that we calculated electrostatic potential using the Debye-Huckel equation although some of our zeta potential values are a bit greater than 50 mV [62].
where ψ 1 and ψ 2 are the zeta potential of the brine-calcite and -CH 3 -brine interfaces, respectively; ε is the permittivity of vacuum (8.85 × 10 −12 C·V −1 .m −1 ); R is the radius of the sphere (m), and k is the Debye length of solution (m −1 ). The Debye length is computed using equation 5 below [59]: where e is the electron charge (1.60 × 10 −19 C); z is the valence of a symmetrical electrolyte solution; and n b is the ion density in the bulk solution.

Structural Forces
The structural forces are a short-range interaction (h < 5 nm) compared to the long-range interaction forces (Van der Waals and Electric double layer forces). In this research, the structural forces were attractive interaction (hydrophobic forces) [60,63], and we computed these using Equation (6), below [63]: where A s is the constant range between 10 6 to 5 × 10 8 Pa [64]; and λ 0 is the structural forces decay length range between 0.2 to 1 nm [64]. In our study, we assumed that A s = 10 8 Pa [64] and the λ 0 = 0.3 nm [65,66].

Effect of pH on the Contact Angle of the Non-Polar Oil-Brine-Calcite System
Our contact angle measurement (measured from the brine phase) shows that increasing the pH of the bulk brine leads to a decrease in the contact angle of the non-polar oil-brine-calcite system, indicating a more water-wet system. For example, Figure 2 shows that the contact angle of the non-polar oil-brine-calcite system decreases from 125 • to 97 • when pH increases from 6.5 to 9.5, and decreases further to 78 • when the pH increases to 11. This is largely due to the fact that pH influences the magnitude and polarity of electrostatic forces at the non-polar oil-brine and brine-calcite interfaces [28,29]. Our results are in agreement with Rezaei et al. [67], who reported that increasing the pH of n-decane (non-polar oil)-brine-calcite system from 5 to 10 reduces the contact angle from 148 • to 51 • , suggesting that the wettability of n-decane-brine-calcite system is dependent on pH. Alameri et al. [13] also reported a similar pH effect on the contact angle of the oil-brine-carbonate system. It is also worth noting that pH likely significantly affects the interaction of the polar-oil-brine-carbonate system, and thus contact angle and wettability. For example, Xie et al. [29] found that increasing the pH from 3 to 8 increased the contact angle of the oil-brine (Na 2 SO 4 )-carbonate system from 53 • to 175 • . Moreover, the trend of the contact angle variation with pH can be predicted by the concept of bond product sum [28,29]. is the ion density in the bulk solution.

Structural Forces
The structural forces are a short-range interaction (h < 5 nm) compared to the long-range interaction forces (Van der Waals and Electric double layer forces). In this research, the structural forces were attractive interaction (hydrophobic forces) [60,63], and we computed these using Equation (6), below [63]: where is the constant range between 10 6 to 5 × 10 8 Pa [64]; and is the structural forces decay length range between 0.2 to 1 nm [64]. In our study, we assumed that = 10 8 Pa [64] and the = 0.3 nm [65,66].

Effect of pH on the Contact Angle of the Non-Polar Oil-Brine-Calcite System
Our contact angle measurement (measured from the brine phase) shows that increasing the pH of the bulk brine leads to a decrease in the contact angle of the non-polar oil-brine-calcite system, indicating a more water-wet system. For example, Figure 2 shows that the contact angle of the nonpolar oil-brine-calcite system decreases from 125° to 97° when pH increases from 6.5 to 9.5, and decreases further to 78° when the pH increases to 11. This is largely due to the fact that pH influences the magnitude and polarity of electrostatic forces at the non-polar oil-brine and brine-calcite interfaces [28,29]. Our results are in agreement with Rezaei et al. [67], who reported that increasing the pH of n-decane (non-polar oil)-brine-calcite system from 5 to 10 reduces the contact angle from 148° to 51°, suggesting that the wettability of n-decane-brine-calcite system is dependent on pH. Alameri et al. [13] also reported a similar pH effect on the contact angle of the oil-brine-carbonate system. It is also worth noting that pH likely significantly affects the interaction of the polar-oil-brinecarbonate system, and thus contact angle and wettability. For example, Xie et al. [29] found that increasing the pH from 3 to 8 increased the contact angle of the oil-brine (Na2SO4)-carbonate system from 53° to 175°. Moreover, the trend of the contact angle variation with pH can be predicted by the concept of bond product sum [28,29].

Effect of pH on Non-Polar Oil-Calcite Interaction
The AFM results show that the adhesion force between non-polar-oil and calcite decrease with increasing pH (Figures 3 and 4), supporting our contact angle measurements and confirming that the increase of pH caused by low-salinity injection likely prompts non-polar oil lifting off from pore

Effect of pH on Non-Polar Oil-Calcite Interaction
The AFM results show that the adhesion force between non-polar-oil and calcite decrease with increasing pH (Figures 3 and 4), supporting our contact angle measurements and confirming that the increase of pH caused by low-salinity injection likely prompts non-polar oil lifting off from pore surfaces. Figure 3 shows the average force-distance curves between non-polar oil group and calcite in the presence of sodium chloride brine (10,000 ppm salinity) at pH 9.5 and 11. The force-distance curves show that the adhesion forces of the non-polar oil group (-CH 3 ) decreases from about 9.2 to 2.8 nN (a 3-fold decrease) with an increase in pH from 9.5 to 11. While few studies have been done to examine the impact of pH on non-polar calcite adhesion with the aid of AFM, our new results are in agreement with our previous work [52], where we found that increasing the pH from 7 to 11 would decrease the adhesion force five times (from 890 to 180 pN) between non-polar oil (-CH 3 ) and muscovite in the presence of sodium chloride brine with 10,000 ppm salinity. Also, Juhl et al. [62] found that increasing the pH from 2 to 8 decreased the adhesion force between non-polar oil (-CH 3 ) and muscovite from 1200 to 100 pN. Additionally, they noticed a similar pH impact on the adhesion force between non-polar oil (-CH 3 ) and quartz. For example, in their measurements, when increasing the pH from 2 to 8, the adhesion force between the non-polar oil (-CH 3 ) and quartz decreased from 300 to ≈10 pN. Therefore, we tentatively conclude that increasing the pH increases the negative charges of the non-polar oil (-CH 3 )-brine interface and brine-calcite interface, thus giving rise to the electrical double layer force, which is discussed in the section below. surfaces. Figure 3 shows the average force-distance curves between non-polar oil group and calcite in the presence of sodium chloride brine (10,000 ppm salinity) at pH 9.5 and 11. The force-distance curves show that the adhesion forces of the non-polar oil group (-CH3) decreases from about 9.2 to 2.8 nN (a 3-fold decrease) with an increase in pH from 9.5 to 11. While few studies have been done to examine the impact of pH on non-polar calcite adhesion with the aid of AFM, our new results are in agreement with our previous work [52], where we found that increasing the pH from 7 to 11 would decrease the adhesion force five times (from 890 to 180 pN) between non-polar oil (-CH3) and muscovite in the presence of sodium chloride brine with 10,000 ppm salinity. Also, Juhl et al. [62] found that increasing the pH from 2 to 8 decreased the adhesion force between non-polar oil (-CH3) and muscovite from 1200 to 100 pN. Additionally, they noticed a similar pH impact on the adhesion force between non-polar oil (-CH3) and quartz. For example, in their measurements, when increasing the pH from 2 to 8, the adhesion force between the non-polar oil (-CH3) and quartz decreased from 300 to ≈10 pN. Therefore, we tentatively conclude that increasing the pH increases the negative charges of the non-polar oil (-CH3)-brine interface and brine-calcite interface, thus giving rise to the electrical double layer force, which is discussed in the section below.

Effect of pH on Zeta Potential of Brine-Calcite and Non-Polar Oil-Brine Interfaces
The zeta potential of the brine-calcite interface is shifted in the negative direction with increasing

Effect of pH on Zeta Potential of Brine-Calcite and Non-Polar Oil-Brine Interfaces
The zeta potential of the brine-calcite interface is shifted in the negative direction with increasing pH (Figure 5), which is in line with previous works [68][69][70]. To be more specific, the zeta potential of

Effect of pH on Zeta Potential of Brine-Calcite and Non-Polar Oil-Brine Interfaces
The zeta potential of the brine-calcite interface is shifted in the negative direction with increasing pH (Figure 5), which is in line with previous works [68][69][70]. To be more specific, the zeta potential of the brine-calcite decreases from −8.0 mV to −12.0 mV with increasing pH from 6.5 to 9.5, and decreases further to −18.3 mV when increasing the pH to 11. This is consistent with the work of Thompson et al. [69], who found that increasing the pH from 9 to 11 decreased the zeta potential of the brine-calcite interface from −19 to −31 mV. Similarly, Siffert et al. [68] found that the zeta potential of NaCl brine (1mM)-calcite interface decreased from −2 to −22 mV when increasing pH from 9 to 11. Additionally, Vdovic et al. [70] reported that zeta potential of calcite in 1 mM NaCl brine decreased from −15 to −18 mV when increasing the pH from 9 to 10.5. However, the increase of zeta potential negativity with pH is mainly due to the deprotonation of the calcite surface sites and the increase in the negative calcite surface species >CaO − and >CO 3 − according to the following geochemical reactions [19,71]: the brine-calcite decreases from −8.0 mV to −12.0 mV with increasing pH from 6.5 to 9.5, and decreases further to −18.3 mV when increasing the pH to 11. This is consistent with the work of Thompson et al. [69], who found that increasing the pH from 9 to 11 decreased the zeta potential of the brine-calcite interface from −19 to −31 mV. Similarly, Siffert et al. [68] found that the zeta potential of NaCl brine (1mM)-calcite interface decreased from −2 to −22 mV when increasing pH from 9 to 11. Additionally, Vdovic et al. [70] reported that zeta potential of calcite in 1 mM NaCl brine decreased from −15 to −18 mV when increasing the pH from 9 to 10.5. However, the increase of zeta potential negativity with pH is mainly due to the deprotonation of the calcite surface sites and the increase in the negative calcite surface species >CaO − and >CO3 − according to the following geochemical reactions [19,71]: >CO3H 0 +OH − = >CO3 − + H2O Similar to the brine-calcite interface, increasing pH yields strongly negative zeta potential of non-polar oil (-CH3)-brine interface. For example, increasing the pH from 6.5 to 9.5 ( Figure 6) decreases the zeta potential of the non-polar oil (-CH3)-brine interface from −35 to −45 mV, and it decreases further to −64 mV when increasing the pH to 11. This is consistent with the work of Jozef et al. [72], who observed that increasing the pH from 6 to 11 leads to a decrease in the zeta potential  Similar to the brine-calcite interface, increasing pH yields strongly negative zeta potential of non-polar oil (-CH 3 )-brine interface. For example, increasing the pH from 6.5 to 9.5 ( Figure 6) decreases the zeta potential of the non-polar oil (-CH 3 )-brine interface from −35 to −45 mV, and it decreases further to −64 mV when increasing the pH to 11. This is consistent with the work of Jozef et al. [72], who observed that increasing the pH from 6 to 11 leads to a decrease in the zeta potential of the non-polar oil (C 6 H 14 )-brine from −30 to −35 mV. Also, Marinova et al. [73] observed that raising the pH from 7 to 9 leads to a decrease in the zeta potential of the non-polar oil interface from −70 to −100 mV. Additionally, Beattie et al. [74] reported that zeta potential of the non-polar oil (hexadecane)-brine decreased from −95 to −125 mV when increasing the pH from 7 to 9.5. Taken together, the existing zeta potential of non-polar oil-brine implies that increasing pH leads to the adsorption of excess OH − on the -CH 3 -brine interface, thus further shifting the zeta potential to be more negative [72][73][74].

Effect of pH on Disjoining Pressure of the Sphere to Flat Model
To provide deeper thermodynamic insight into the experiments (e.g., contact angle, AFM adhesion force and zeta potential measurements), total disjoining pressure calculation was performed under constant potential conditions. It is worth noting that positive and negative disjoining pressure represents repulsive and attractive forces, respectively. Figure 7 shows that all pHs (6.5, 9.5 and 11) give negative disjoining pressure (Figure 7) in line with the AFM measurement. Additionally, the total disjoining pressure increases with increasing pH, suggesting greater repulsion in the non-polar oil-brine-calcite systems, which is in agreement with adhesion force results. For example, increasing the pH from 9.5 to 11 increases the repulsion of non-polar oil-brine-calcite systems in line with the AFM measurement.

Effect of pH on Disjoining Pressure of the Sphere to Flat Model
To provide deeper thermodynamic insight into the experiments (e.g., contact angle, AFM adhesion force and zeta potential measurements), total disjoining pressure calculation was performed under constant potential conditions. It is worth noting that positive and negative disjoining pressure represents repulsive and attractive forces, respectively. Figure 7 shows that all pHs (6.5, 9.5 and 11) give negative disjoining pressure (Figure 7) in line with the AFM measurement. Additionally, the total disjoining pressure increases with increasing pH, suggesting greater repulsion in the non-polar oil-brine-calcite systems, which is in agreement with adhesion force results. For example, increasing the pH from 9.5 to 11 increases the repulsion of non-polar oil-brine-calcite systems in line with the AFM measurement.
In general, our results show that increases of pH during low-salinity flooding in carbonate reservoirs lead to detachment of the non-polar oil components (the major oil components) from carbonate surface. This physicochemical process may not be modelled using the existing geochemical reactions because only the polar functional groups are captured in the geochemical models [22,28,75]. However, our results show that the pH increase triggered by low-salinity waterflooding may lead to the detachment of non-polar component. This implies that low-salinity waterflooding may also be applied to carbonate reservoirs with low concentration of polar component such as low acid number and base number. disjoining pressure represents repulsive and attractive forces, respectively. Figure 7 shows that all pHs (6.5, 9.5 and 11) give negative disjoining pressure (Figure 7) in line with the AFM measurement. Additionally, the total disjoining pressure increases with increasing pH, suggesting greater repulsion in the non-polar oil-brine-calcite systems, which is in agreement with adhesion force results. For example, increasing the pH from 9.5 to 11 increases the repulsion of non-polar oil-brine-calcite systems in line with the AFM measurement. In general, our results show that increases of pH during low-salinity flooding in carbonate reservoirs lead to detachment of the non-polar oil components (the major oil components) from carbonate surface. This physicochemical process may not be modelled using the existing geochemical reactions because only the polar functional groups are captured in the geochemical models [22,28,75]. However, our results show that the pH increase triggered by low-salinity waterflooding may lead to the detachment of non-polar component. This implies that low-salinity waterflooding may also be

Summary and Conclusions
Wettability alteration is believed to be a major physicochemical factor in the application of low-salinity water injection in carbonate formations. The effect of polar oil components on wettability alteration has been extensively investigated as a function of pH, but researchers have not evaluated the response of non-polar oil on wettability alteration in much detail, although crude oils are rich in non-polar components. Therefore, we measured the contact angle of non-polar oil on the calcite surface in the presence of an aqueous ionic solution (10,000 ppm NaCl) at pH values of 6.5, 9.5 and 11. In addition, we used atomic force microscopy (AFM) to measure the adhesion of the non-polar oil group (-CH 3 ) and calcite at different pH. Furthermore, to gain a deeper understanding of the isothermal thermodynamic, we performed zeta potential measurements of non-polar oil (-CH 3 )-brine and brine-calcite interfaces, and calculated the total disjoining pressure using a sphere to flat thermodynamic model under constant potential conditions.
Contact angle measurements showed that the contact angle decreases from 125 • to 97 • with increasing pH from 6.5 to 9.5, and decreases further, to 78 • , when increasing the pH to 11, implying a more water-wet system. AFM adhesion force measurements between the non-polar oil group (-CH 3 ) and calcite showed that the adhesion force decreases with an increase in pH from 9.5 to 11. The zeta potential data measured indicate that an increase in pH would cause a change in the zeta potential of the non-polar oil (-CH 3 )-brine and calcite-brine interfaces towards more negative values which would then result in an increase in the electrical double layer force. The total disjoining pressure and results of AFM adhesion tests follow similar trends, showing that adhesion force decreases with increasing pH due to electrical double layer expansion thus leads to a water-wet system. Taken together, our results confirm that detachment of non-polar oil from carbonate rock continues to take place during low-salinity water injection, although desorption of non-polar oil from carbonate could not be modeled using existing surface complexation models. Also, this work sheds further light on the significance of pH increase on non-polar oil-calcite adhesion during low-salinity water injection, and provides insights into the quantification of carbonate reservoir wettability.