Investigation of the Impact of Potential Determining Ions from Surface

Aug 9, 2018 - Recently, low salinity water flooding and ion tuned water flooding have gained a lot of attention in the petroleum industry for their pr...
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Investigation of the Impact of Potential Determining Ions from Surface Complexation Modeling Hongna Ding, and Sheik Rahman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02131 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Investigation of the Impact of Potential Determining Ions from Surface Complexation Modeling Hongna Ding, Sheik R. Rahman School of Petroleum Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Abstract: Recently, low salinity water flooding and ion tuned water flooding have gained a lot of attentions in petroleum industry for their promising applications in enhancing the oil recovery of carbonate reservoirs. The fundamental mechanism for low salinity waters (LSW) to enhance the oil recovery is most possibly due to an expansion of electrical double layer whereas it could be attributed to surface charge change/modification for ion tuned waters (ITW). However, the simulation studies in investigating the electro-chemical interactions between the ions in the injection water and the minerals are simplified to double layer model (DLM) which is physically not appropriate in the cases of ITW. Therefore, a more sophisticated model-basic Stern model (BSM) is evaluated in this paper. The feasibility of DLM and BSM in reproducing the experimental zeta potential results from our measurements and literatures are discussed. Furthermore, the influences of potential determining ions (PDI, Ca2+, Mg2+, SO42-) on the electro-chemical interactions are evaluated based on the surface complexation results. Our results suggest that the DLM can be applied to reproduce or predict the zeta potential results of LSW whereas the BSM can be employed to reproduce or predict the zeta potential results of ITW. Moreover, the modeling results show a parallel change in the adsorption of Mg2+ and SO42-, a competitive relationship between Ca2+ and Mg2+ and a compensation relationship between Ca2+ and SO42-. Consequently, it is possible to choose the most appropriate LSW or ITW for water flooding program in carbonate reservoirs by predicting the zeta potential results with corresponding model and as well estimate the magnitude of the electrical double expansion and the surface charge change in the presence of LSW and ITW, respectively.

Keywords: Zeta Potential, Calcite, Low Salinity Water, Ion Tuned Water, and Surface Complexation Model.

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1. Introduction Low salinity water flooding (LSWF) and ion tuned water flooding (ITWF) have recently gained much attention in the petroleum industry for their high potentials of enhancing the oil recovery in carbonate reservoirs. In a representative study by Yousef et al. 1, 2, it was reported that a 18%-19.5% increase in oil recovery was obtained from carbonate samples by injecting diluted-seawaters (3,0000-600 ppm) in sequential flooding experiments. Strand et al. 3 4, Gupta et al. 5 and Nasr et al. 6 have observed an improved oil recovery of 15% OOIP from North Sea chalk with ion tuned seawaters (ITW) such as seawater with four times of the SO42- concentration (SW4SO). This enhanced oil recovery by LSWF or ITWF has been attributed microscopically to the electrical and chemical interactions between the oil, injected water and carbonate minerals, or more specifically, double layer expansion (DLE) in the case of LSWF 7 and surface charge change/modification due to SO42- adsorption in the case of ITWF 8. Accordingly, zeta potential measurements and surface complexation modeling (SCM) have become compelling approaches for quantitatively evaluating the influences of ion concentration and ion species on DLE and surface charge change. Yousef et al. 2, Chen et al. 9 and Alroudhan et al. 10 have observed that the zeta potential of limestone was gradually changed to more negative values by diluted-seawaters (2100 times dilutions or 2dSW-100dSW), causing an expansion of the electrical double layer at the rock surface. Mahani et al. have investigated the zeta potential changes of different rock types in SW and 25dSW 11, 12. It was found that the electronegativity of these rocks was in the order of chalk>limestone>calcite>dolomite. Strand et al. 13 and Zhang et al. 8, 14 have measured the zeta potentials of chalk as a function of Ca2+, Mg2+ and SO42- (potential determining ions, PDI) concentrations in background NaCl solutions. They found that the zeta potential could be changed to more negative values by increasing the SO42- concentration or decreasing the Ca2+ or Mg2+ concentration. Alroudhan et al. 10 reported that the zeta potential of fresh or aged limestone varied identically toward Ca2+ and Mg2+ in NaCl solutions compared to that toward SO42-. Additionally, it was observed that the isoelectric point (IEP) decreased with the increased NaCl concentration even though Na+ and Cl- were indifferent ions to carbonate rock surface 10. Similarly, Al Mahrouqi et al. have reached the same conclusions with calcite, and furthermore, they showed that the surface charge of calcite was weakly dependent on the pH and that the zeta potential was controlled by the adsorptions of PDI at the Stern layer 15. Kasha et al. 16 have investigated the mutual influences of two PDI in background NaCl solutions and they found that Mg2+ had a more noticeable effect on SO42- than Ca2+. Two surface complexation models, the double layer model (DLM) and the basic Stern model (BSM), are mostly used to reproduce the experimental zeta potentials (see Figure 1). Hiorth et al. proposed that the experimental zeta potentials of chalk powders can be predicted by DLM using the standard thermodynamic data 17. Song et al. also showed that DLM could model the zeta potentials of calcite particles in NaCl solutions that 2 ACS Paragon Plus Environment

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contained an individual PDI 18. The BSM developed by Heberling et al. 19, 20 21 has successfully reproduced the experimental zeta potentials of calcite in NaCl solutions. Besides its application in zeta potential prediction, SCM has been used to simulate the fluid transport in porous media. For example, Qiao et al. have successfully incorporated DLM into a systematic reaction network of an oil-brine-rock system to reproduce the results of the core flooding experiments 22, 23. Unfortunately, no attempt to fit the zeta potential results was made in the studies by Qiao et al.

Fig.1. Schematic descriptions of the surface complexation models. The left figure shows the double layer model (DLM) and the right figure shows the basic Stern model (BSM). More detailed explanations of these two models can be found in Section 3.

The electrostatics and the surface complexations of carbonate surfaces interacting with LSW and ITW are crucial in understanding the wettability alteration and improved oil recovery in carbonate reservoirs. However, most studies to date only focus on the zeta potential measurements in single electrolytes (e.g., Na2SO4 or CaCl2) 9 or at most on solutions containing one or two PDI dissolved in background NaCl solutions 10 15 16. Consequently, the studies covering the zeta potential measurement in complex brines such as those LSW (e.g., 10dSW) and ITW (e.g., SW4SO) used in water flooding programs have been limited 2 9 10 13 16 24. For SCM, despite some successful applications in predicting the zeta potential results of single electrolytes (e.g., NaCl and CaCl2) 17 18 19 20 21, SCM especially DLM could not predict the trend of zeta potential vs pH for synthetic solutions (e.g., SW4SO) 12. More importantly, to the best of the author’s knowledge, no studies have examined the feasibility of different SCMs in predicting the zeta potentials in synthetic brines. Therefore, this study aims to provide some insights into these gaps. Accordingly, this paper is divided into two parts. The first part includes the zeta potential measurements of calcite particles in both LSW and ITW, and in the second part, the feasibility of DLM and BSM for reproducing the experimental zeta potentials is evaluated. Furthermore, the mutual influences of PDI are discussed. 2. Materials and Methods

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2.1. Sample Preparations The composition of synthetic SW is the same as that in the study by Yousef et al. 1. SW is prepared with high purity NaCl (>99.5%), Na2SO4 (>99%), MgCl2·6H2O (>99%), and CaCl2 (>96%, all from Sigma Aldrich) dissolved in MilliQ water (resistivity of 18.2 MΩ at 22.4℃, pH ~5.6). SW is then diluted from 2-20 times to obtain the LSW-2dSW, 4dSW, 10dSW, and 20dSW (see Table 1). SW is also modified to obtain six ITW-SW2SO and SW4SO, which include two and four times of the SO42- concentration in SW, SW0.5Mg, SW0.25Mg, SW0.5Ca and SW0.25Ca, including one-half and one-fourth times of the Ca2+ and Mg2+ concentration, respectively. Charge balance is maintained by adjusting the background NaCl concentration (see Table 1). Natural Iceland spar (~98% CaCO3 measured by Axios advanced XRF analysis, see Table 2) is used to represent the carbonate rock. Table 1. Compositions of Synthetic LSW and ITW (in ppm) Ions

SW

2dSW

4dSW

10dSW

20dSW

SW2SO

SW4SO

SW0.5Mg

SW0.25Mg

SW0.5Ca

SW0.25Ca

Na+

18300

9150

4575

1830

915

20356

24467

18300

18300

18300

18300

Ca2+ Mg2+ SO42ClTDS IS (M)

650 2110 4290 32399 57749 1.118

325 1055 2145 16199 28874 0.549

163 528 1073 8100 14437 0.273

65 211 429 3240 5775 0.109

33 106 215 1620 2887 0.055

650 2110 8580 32399 64095 1.194

650 2110 17160 32399 76786 1.358

650 1055 4290 29278 53573 0.993

650 528 4290 27717 51485 0.933

325 2110 4290 31822 56847 1.094

163 2110 4290 31534 56396 1.082

Table 2. XRF Analysis of Iceland spar (%) Sample ICELAND

Na2O 0.04

MgO 0.15

Al2O3 CaCO3-, >CaOH2+ and >CO3-). The inner Stern layer (0-1 plane) includes the binding of Ca2+, CO32- or HCO3-. The outer Stern layer (1-2 plane) defines the adsorption of large hydrated PDI such as Ca2+, Mg2+, SO42- and CO32- (HCO3-). The diffuse layer is where both excess PDI and indifferent ions such as Na+ and Cl- ions loosely bond to the calcite surface (reproduced from reference [15] 15).

3.2. Description of SCM The detailed description of SCM for this study is provided in Table 3. The DLM in this work is based on the studies by Qiao et al. 22 23. The parameters specifying their DLM were obtained from reproducing the results of core flooding experiments such as the relative permeability curves, the breakthrough curves and the oil recovery profiles (22 23 and the references therein). Similar surface specification and corresponding parameters can be found in other reports 12 17. DLM is a simplified description of the electrical double layer and it has a particular characteristic in which there are no coordination distinctions between the PDI and the H+ (OH-); rather, all anions and cations are assigned to the 0-plane whereas indifferent Na+ and Cl- ions are assigned to the diffuse layer. It is noteworthy that this treatment may overestimate the pH effect, possibly giving unreasonable results 15. The surface charge density σ0 is assumed to be equal to the diffuse charge density σd (σ0=σd, see Figure 1). An exponential decay of the diffuse potential ψd is assumed at the diffuse layer and it can be described by the Grahame equation (|ψd|CaOH

>CO3H

>CaOH1.5

Number C1 C2 C3 C4 C5 C6 CH Cin1 Cin2

Reaction After Qiao et al. 22, 23 (DLM) >CaOH + H+ = >CaOH+2 >CaOH2+ + SO4-2 = >CaSO4- + H2O >CaOH2+ + CO3-2 = >CaCO3- + H2O >CO3H = >CO3- + H+ >CO3- + Ca+2 = >CO3Ca+ >CO3- + Mg+2 = >CO3Mg+ After Heberling et al. 19, 20 (BSM) >CaOH1.5 = >CaOH-0.5 + 0.5H+ >CaOH-0.5 + H+ = >CaOH2+0.5 >CaOH2+0.5 + HCO3- = >CaOH3CO3-0.5

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logK 11.8 -2.10 6.00 -5.10 2.85 0.68 20.0 0.50 0.04

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Cin3 >CaOH2+0.5 + HCO3- = >CaOH2CO3-1.5 + H+ -7.07 Cou1 >CaOH-0.5 + Ca+2 = >CaOHCa+1.5 1.68 Cou2 >CaOH-0.5 + Mg+2 = >CaOHMg+1.5 0.68 Cou3 >CaOH2+0.5 + HCO3- = >CaOH3CO3-0.5 0.04 Cou4 >CaOH2+0.5 + HCO3- = >CaOH2CO3-1.5 + H+ -7.07 Cou5 >CaOH2+0.5 + SO4-2 = >CaOH2SO4-1.5 2.10 CH >CO3H0.5 = >CO3-0.5 + 0.5H+ 20.0 Cin >CO3-0.5 + Ca+2 = >CO3Ca+1.5 1.68 >CO3H0.5 Cou1 >CO3-0.5 + Ca+2 = >CO3Ca+1.5 1.68 Cou2 >CO3-0.5 + Mg+2 = >CO3Mg+1.5 0.68 Note: ion valence is displayed following the charge sign ±, for example >CaOH2+0.5 represents complex >CaOH2 with +0.5 charge and SO4-2 represents ion SO42-.

4. Results and Discussions 4.1. Low Salinity Waters (LSW) The normal practice of the preparation of LSW in an oilfield is to dilute the SW to 2 times, 10 times or more. During the dilution process, not only the concentrations of monovalent ions (e.g., Na+, K+ and Cl-) but also the concentrations of divalent ions (e.g., Ca2+, Mg2+ and SO42-) are reduced. Therefore, the ionic strength is decreased with dilution resulting in an expansion of the double layer at the calcite surface; the Debye length κ-1 increases from ~0.3 nm to 1.3 nm (see Table 4). Meanwhile, the measured zeta potential ζEXP decreases gradually with dilutions, from -5.5 mV in SW to -10.1 mV in 20dSW. The overall change in ζEXP is hence -4.6 mV in this study, which is comparable to the reported values in the literatures 2, 11. What’s more, the changes in ζEXP between successive dilutions are less than -2 mV (see Table 4). The same observation has been reported by Yousef et al. 2. Table 4. Zeta Potential Results in LSW from Experiments and SCM Simulation Brine IS (M) κ-1 (nm) ζEXP (mV) ζDLM (mV) ζBSM (mV) SW 1.118 0.288 -5.53 -5.42 -5.39 2dSW 0.549 0.411 -7.56 -7.80 -7.56 4dSW 0.273 0.583 -8.10 -8.56 -8.20 10dSW 0.109 0.921 -9.21 -10.01 -9.42 20dSW 0.055 1.298 -10.13 -10.61 -10.59 The zeta potential results in LSW are modeled by SCM simulation (ζDLM and ζBSM). As shown in Figure 3, both models have reproduced the ζEXP successfully. The main difference between DLM and BSM is that DLM is more sensitive to pH than BSM, especially at a pH>8 (see SI.1). Thus, the predicted pH values obtained by DLM are slightly higher than those predicted by BSM but are still in the reasonable range of 8.0±0.1. This difference is attributed primarily to the different treatments of PDI adsorptions by these two models as discussed previously. In the experiments, especially in studying the influences of specific ions, the pH is normally fixed to a constant value as 8 ACS Paragon Plus Environment

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pH may complex the adsorption of ions to the mineral surface and as well mask the effects of specific ions by triggering unwanted mineral dissolution or precipitation (see SI.1). In consequence, the zeta potential results may not be comparable for the cases involving PDI. Nevertheless, the successful reproduction of the IEP of calcite mineral and the trend of zeta potential vs pH in 1000dSW (see SI.1) provides us confidence to employ the DLM and the BSM at alkaline solutions. Besides the successful reproduction of the zeta potentials of LSW in this study, both DLM and BSM are able to predict the results in the literatures (see SI.1).

-5

EXP DLM BSM

-6 -7

ζ (mV)

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-8 -9 -10 -11 SW

2dSW

4dSW

10dSW 20dSW

Fig.3. Zeta potential results of calcite particles in SW and LSW (2dSW-20dSW) from the experiments (EXP) and DLM and BSM simulations. Error bars for experimental data represent the 95% confidence level of 5 measurements.

The surface speciation in LSW by SCM simulation is demonstrated in Figure 4. We have extended the simulation to 100dSW because this highly diluted seawater was used in other studies 1, 2. In the DLM simulation (left figure), the molarities of >CO3and >CaOH2+ increase with the decreased IS, indicating that the calcite dissolution is increased due to dilution. The molarities of >CaCO3- and >CO3Ca+ are found to be changed in opposite ways by the decrease of the IS; the molarity of >CaCO3- decreases whereas the molarity of >CO3Ca+ increases. In contrast, the molarities of >CO3Mg+ and >CaSO4- are simultaneously decreased by the decrease of the IS. Moreover, the variations in the surface complexes become insignificant above 10dSW. It can be inferred from these observations that calcite dissolution is promoted by dilution whereas PDI adsorption is suppressed due to the decrease in PDI concentration. The molarity changes of surface complexes are more dramatic for BSM than those for DLM (right figure), but the general trend of these changes for BSM are similar to those for DLM. However, minor differences from DLM were observed in BSM; for example, the molarities of >CO3-0.5 and >CaOH2+0.5&>CaOH-0.5 in BSM are far larger than the molarities of >CO3- and >CaOH2+ in DLM. Additionally, the molarity of >CaOH2CO39 ACS Paragon Plus Environment

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1.5&>CaOH3CO3-0.5 first decreases from 2dSW to 10dSW and then starts to increase from 10dSW to 100dSW due to the reduced dissolution of calcite above 10dSW. The changes of >CaOHMg+1.5&>CO3Mg+1.5 and >CaOHSO4-1.5&>CaOH3SO4-0.5 are more pronounced for BSM than those for DLM because calcite dissolution and PDI adsorption occur at separate layers in BLM. The decrease in PDI concentration influences both processes and accordingly causes more dramatic changes in the molarities of surface complexes. 4.0e-7

4.0e-7

3.0e-7

3.0e-7

Molarity (mol—L-1)

Molarity (mol—L-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0e-7

1.0e-7

0.0

-1.0e-7

2.0e-7

1.0e-7

0.0

SW

-1.0e-7

1 2 1 4 2d SW dSW 0dS 0dS 00d SW W W

>CaOH&>CO3H >CaOH2+ >CO3>CO3Ca+ >CO3Mg+ >CaSO4>CaCO3-

SW

1 2 1 4 2d SW dSW 0dS 0dS 00d SW W W

>CaOH1.5&>CO3H0.5 >CaOH2+0.5->CaOH-0.5 >CO3-0.5 >CaOHCa+1.5&>CO3Ca+1.5 >CaOHMg+1.5&>CO3Mg+1.5 >CaOH2SO4-1.5 >CaOH2CO3-1.5&>CaOH3CO3-0.5

Fig.4. Molarities of surface complexes in SW and LSW from DLM (left) and BSM (right) simulations.

4.2. Ion Tuned Waters (ITW) The zeta potential results in ITW are shown in Table 5. The preparation of ITW used in this study is somewhat arbitrary; for example, SW4SO was prepared by increasing the amount of the Na2SO4 salt added to the SW by a factor of four. Consequently, the IS of the ITW varies in a range of 0.93-1.36 M. The differences in IS cause the Debye length κ-1 to vary from 0.26 nm to 0.32 nm (see Table 5). This operation of preparing the ITW is more practical from an engineering point of view. More specifically, the ITW for the water flooding program in carbonate reservoirs has been normally prepared by mixing two kinds of membrane-treated waters together to make a specific cocktail suitable for a particular oil reservoir 36. The ITW prepared in this way thus differ in IS, monovalent ion content and divalent ion content (or both ion contents). It is neither practical nor recommended to strictly control the IS of ITW in field application. However, keeping the IS constant is more important and meaningful for scientific research to enable direct comparisons between the zeta potentials in different ITW. Nevertheless, it is assumed that the zeta potential results in different ITW are still comparable because the changes 10 ACS Paragon Plus Environment

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in IS are not significant in this study. In addition, the magnitude of κ-1 is approximately equal to the diameter of one water molecule (~0.27 nm), suggesting that the diffuse layer in ITW is completely suppressed due to the strong ionic screening effect. Accordingly, the approximation of ζEXP as ψd will be more reasonable in ITW compared to LSW as the contribution of the diffuse layer can be neglected negligible. In other words, the adsorption of the PDI plays the dominant roles in determining the zeta potentials in ITW. Table 5. Zeta Potential Results in ITW from Experiments and SCM Simulation Brine IS (M) κ-1 (nm) ζEXP (mV) ζDLM (mV) ζBSM (mV) SW 1.118 0.288 -5.53 -5.52 -5.39 SW2SO 1.194 0.279 -8.12 -9.47 -9.54 SW4SO 1.358 0.261 -13.80 -13.59 -13.26 SW0.5Mg 0.993 0.306 -8.23 -10.31 -9.77 SW0.25Mg 0.933 0.315 -14.17 -12.81 -13.19 SW0.5Ca 1.094 0.291 -9.42 -10.31 -6.54 SW0.25Ca 1.082 0.293 -17.21 -15.94 -6.13 For ζEXP in ITW, it is found that the ζEXP in SW (-5.5 mV) can be changed to more negative values by all ITW; for example, ζEXP is changed to -8.1 mV by SW2SO and to -8.23 mV by SW0.5Mg (see Table 5). This observation is consistent with the findings by Strand et al. 3 4. It is also found that ζEXP can be changed to the same magnitude by increasing the SO42concentration and decreasing the Ca2+ or Mg2+ concentration; for example, ζEXP≈-8 mV in SW2SO, SW0.5Mg and SW0.5Ca. The concentrations of Ca2+, Mg2+ and SO42- in SW are 0.045 M, 0.088 M and 0.016 M, respectively; this outcome suggests that ζEXP is more sensitive to Ca2+ than it is to Mg2+ and SO42- at room temperature, and the sensitivity order is Ca2+>Mg2+>SO42-. This conclusion is also reached by reproducing the zeta potential results of Alroudhan et al. with SCM simulation (see SI.1). On the other hand, ζEXP, ζDLM and ζBSM are compared in Figure 5. An examination of Figure 5 shows that both DLM and BSM have successfully reproduced the zeta potential results. However, the predicted pH in SW0.25Ca is 8.2 according to DLM, and the equilibrium constant of SO42adsorption is 2.8 for SW0.5Ca and SW0.25Ca according to BSM. As discussed above, it is possible for DLM to overestimate the pH effect because it has been reported that the PDI of calcite was not H+ and OH- but Ca+ and CO32- (HCO3-) 27. The underestimation of zeta potentials by BSM may be due to the non-incorporation of the inner-sphere adsorption of Mg2+ or SO42- in the development of this model. As pointed out by Heberling et al. 20, it is possible that the inner-sphere adsorption at step or kink sites was not captured by surface diffraction experiments, and more importantly, this inner-sphere adsorption was different from the outer-sphere adsorption. Moreover, it has been confirmed in AFM studies that both Mg2+ and SO42- could be incorporated into the calcite lattice 37 38 39 40 24.

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-2 -4 -6 -8

ζ (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-10 -12 -14

EXP DLM BSM

-16 -18 -20

SW

SW 2

SO

SW 4

SO

SW 0

5M g

SW 0

25 M

SW 0 g

5C a

SW 02 5C a

Fig.5. Zeta potential results of calcite in SW and ITW from the experiments (EXP) and DLM and BSM simulations. Error bars on experimental data represent the 95% confidence level of 5 measurements.

Similar to the LSW cases, the molarity changes of the surface complexes in BSM are more significant than those in DLM. SCM results also suggest that the three PDI are mutually influenced by each other (see Figure 6). In the DLM simulation, the molarity of >CO3Ca+ is decreased, while the molarity of >CO3Mg+ is significantly increased by increasing the SO42- concentration to 2 and 4 times. The molarity of >CO3Mg+ is significantly increased, while the molarity of >CaSO4- is largely decreased by the reduction in the Ca2+ concentration. Similarly, the molarity of >CO3Ca+ is slightly decreased, whereas the molarity of >CaSO4- is substantially decreased due to the decrease in the Mg2+ concentration. In BSM simulations, similar trends to those of DLM are observed by varying the SO42- and Mg2+ concentrations. However, the molarities of both >CaOHCa+1.5&>CO3Ca+1.5 and >CaOH2SO4-1.5 are increased by decreasing the Ca2+ concentration, which is contrary to the variations of the corresponding complexes (>CO3Ca+ and >CaSO4-, respectively) in DLM. It is noteworthy that the adsorption of SO42- ions at the inner Stern layer is manually enhanced in the cases of SW0.5Ca and SW0.25Ca in order to fit the zeta potential results. As a consequence, it leads to an increase in the Ca2+ adsorption, as well. Despite this difference, the relationships of PDI from SCM can be generally summarized as: (a) a competitive relationship between Ca2+ and Mg2+ due to their adsorptions on the same sites on the calcite surface and (b) a compensation relationship between Ca2+ and SO42- as determined by the electrical attraction; that is, if the adsorption of SO42- increases (decreases), the adsorption of Ca2+ also increases (decreases). This simulation result agrees with the experimental observation in the study by Al Mahrouqi et al. in which pSO4 tends to moderate the pCa 15. It is noteworthy that this outcome is precisely what has been observed in BSM, while an opposite trend is observed in DLM. Since DLM uses a simplified description of electrical structure, some intrinsic phenomena may be hindered during the simulation, 12 ACS Paragon Plus Environment

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resulting in (c) a parallel relationship between Mg2+ and SO42-. Both models predict parallel change in the adsorption of Mg2+ and SO42- to the calcite surface because of electrical attraction. This relationship as well suggests that Mg2+ and SO42- have stronger effects on each other, being identical to the observations from another study 16. 4.0e-7

4.0e-7

3.0e-7

3.0e-7

Molarity (mol—L-1)

Molarity (mol—L-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0e-7

1.0e-7

0.0

-1.0e-7

2.0e-7

1.0e-7

0.0

SW

SW

-1.0e-7

SW SW SW SW SW 0 2S 4 0 0 0 O SO 5Mg 25M 5Ca 25C a g

SW

>CaOH&>CO3H >CaOH2+ >CO3>CO3Ca+ >CO3Mg+ >CaSO4>CaCO3-

SW

SW SW SW SW SW 2S 4 0 0 0 0 O SO 5Mg 25M 5Ca 25C a g

>CaOH1.5&>CO3H0.5 >CaOH2+0.5->CaOH-0.5 >CO3-0.5 >CaOHCa+1.5&>CO3Ca+1.5 >CaOHMg+1.5&>CO3Mg+1.5 >CaOH2SO4-1.5 >CaOH2CO3-1.5&>CaOH3CO3-0.5

Fig.6. Molarities of surface complexes in SW and ITW from the DLM (left) and BSM (right) simulations.

5. Conclusions This paper presents the zeta potential measurements and surface complexation modeling in LSW and ITW. It is observed that the zeta potential of calcite particles in SW is modified to more negative values by seawater dilutions, and the overall change is about -5 mV from SW to 20dSW. However, the differences in zeta potentials between successive dilutions are less than -2 mV. Additionally, zeta potential is found to become more negative with the increased SO42- concentration or the decreased Ca2+ and Mg2+ concentrations. Moreover, the zeta potential is more sensitive to Ca2+ and Mg2+ than it is to SO42- at room temperature. The zeta potential results are reproduced by two surface complexation models. It is found that both DLM and BSM can successfully reproduce the experimental results in LSW. It is also found that the molarities of surface complexes change more dramatically with ionic strength in BSM than DLM, while both models suggest that the molarities of all surface complexes change insignificantly above 10dSW. On the other hand, DLM and BSM have successfully reproduced the zeta potential results in four ITW (SW2SO, SW4SO, SW0.5Mg and SW0.25Mg). However, the zeta potentials at SW0.5Ca and SW0.25Ca are underestimated by the two models. The origins of the discrepancies lie in 13 ACS Paragon Plus Environment

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the intrinsic inefficiency of these two models; DLM overestimates the effect of pH while an inner incorporation of SO42- adsorption (or Mg2+ adsorption) is neglected by BSM. It is recommended that the DLM is more suitable to predict the zeta potential results in LSW such as 10dSW whereas BSM is more efficient to estimate the zeta potential results in ITW such as SW4SO. The mutual interactions between PDI from SCM simulation can be generally summarized as a competitive relationship between Ca2+ and Mg2+, a compensation relationship between Ca2+ and SO42- and a parallel relationship between Mg2+ and SO42- at room temperature. Therefore, it is intuitive to conclude that more Mg2+ should be included into the sulfate-rich ITW to promote the adsorption of SO42ions whereas Ca2+ should be reduced or removed from the injection water in case of precipitation. On the other hand, Ca2+ ions are advised to be reduced or depleted from the injection water for either promoting the calcite dissolution or weakening the competition between Mg2+ which could enhance the adsorption of SO42-. In a sense, SCM simulation may be able to assist petroleum engineers to identify promising candidate water for LSWF or ITWF in carbonate reservoirs. Supporting Information Acknowledgement We thank two anonymous reviewers provided valuable comments on this paper. Special thanks are given to one anonymous reviewer whose comments helped enrich the manuscript a lot. References 1. Yousef, A. A.; Al-Saleh, S. H.; Al-Kaabi, A.; Al-Jawfi, M. S., Laboratory investigation of the impact of injection-water salinity and ionic content on oil recovery from carbonate reservoirs. SPE Reservoir Evaluation & Engineering 2011, 14, (05), 578-593. 2. Yousef, A. A.; Al-Saleh, S.; Al-Jawfi, M. S. In The impact of the injection water chemistry on oil recovery from carbonate reservoirs, SPE EOR Conference at Oil and Gas West Asia, 2012; Society of Petroleum Engineers: 2012. 3. Strand, S.; Austad, T.; Puntervold, T.; Høgnesen, E. J.; Olsen, M.; Barstad, S. M. F., “Smart water” for oil recovery from fractured limestone: a preliminary study. Energy & fuels 2008, 22, (5), 3126-3133. 4. Shariatpanahi, S. F.; Strand, S.; Austad, T., Initial wetting properties of carbonate oil reservoirs: effect of the temperature and presence of sulfate in formation water. Energy & fuels 2011, 25, (7), 3021-3028. 5. Gupta, R.; Mohanty, K., Wettability alteration mechanism for oil recovery from fractured carbonate rocks. Transport in porous media 2011, 87, (2), 635-652. 6. Zekri, A. Y.; Nasr, M.; Al-Arabai, Z. In Effect of EOR technology on wettability and oil recovery of carbonate and sandstone formation, IPTC 2012: International Petroleum Technology Conference, 2012; 2012. 7. Ligthelm, D. J.; Gronsveld, J.; Hofman, J.; Brussee, N.; Marcelis, F.; van der Linde, H. In Novel Waterflooding Strategy By Manipulation Of Injection Brine Composition, EUROPEC/EAGE Conference and Exhibition, 2009; Society of Petroleum Engineers: 2009. 8. Zhang, P.; Tweheyo, M. T.; Austad, T., Wettability alteration and improved oil recovery by spontaneous imbibition of seawater into chalk: Impact of the potential determining ions Ca

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