Influence of Halide Anions Cl-, Br- and I- on the Zeta Potential of Oil

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Influence of Halide Anions Cl-, Br- and I- on the Zeta Potential of Oil-Wet Carbonate Surfaces Ahmed sadeed, Hasan Al-Hashim, Bastian Sauerer, and Wael Abdallah Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01139 • Publication Date (Web): 05 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Influence of Halide Anions Cl-, Br- and I- on the Zeta Potential of Oil-Wet Carbonate Surfaces Ahmed Sadeed a, Hasan Al-Hashim a, *, Bastian Sauerer b, Wael Abdallah b, * a

Department of Petroleum Engineering, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia b

Schlumberger Dhahran Carbonate Research Center, 31942 Dhahran, Saudi Arabia

ABSTRACT The benefits of injecting dynamic water (fluids with designed ionic composition) in carbonate reservoirs have been confirmed in various studies. The underlying mechanisms however, resulting in enhanced oil recovery, are still not fully understood, which complicates the design of new efficient dynamic water formulations. In the current work, we study the effect of Arabian Gulf sea water and the addition of halide anions (Cl-, Br- and I-) on carbonate surface alteration. Calcite and carbonate outcrop surfaces were aged with model oils (containing asphaltene, stearic acid or a mixture of both) to render the rock more oil-wet and afterwards conditioned in different dynamic water formulations. The surface charges at different treatment stages were investigated by zeta potential measurements to identify which dynamic water is most effective in altering the aged surfaces back to a more water-wet condition, which is believed to be more beneficial for enhanced oil recovery. Aging in model oils led to an increase in the magnitude of the negative zeta potential values on calcite and carbonate surfaces, indicating the adsorption of surfaceactive components of the model oils onto the rock surface. When afterwards conditioned in Arabian Gulf sea water with added halide ions, systematic changes of the zeta potential values were observed for the rock surfaces. The iodide system showed the strongest effect among the tested halides in terms of surface charge alteration, which is in line with the Hofmeister theory. Larger, more polarizable ions with low charge density (the chaotropic ions) seem more effective in removing adsorbed surface-active species

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from the rock surface. Based on these current results, it is highly recommended to use additional iodide in sea water flooding procedures to enhance the oil recovery from carbonate formations.

Keywords:

Zeta potential; carbonate; low salinity; wettability; dynamic water; halides; iodide; EOR

INTRODUCTION One of the main mechanisms to enhance oil recovery in carbonate formations is surface wettability alteration from oil-wet to more water-wet.1-14 Low salinity water (also known as dynamic water or smart water) injection into reservoirs attracted the attention of many oil companies due to its positive financial and environmental aspects, as well as its recovery efficiency. Despite promising laboratory studies on the topic, field applications have been very limited.15,16 The main reason for the limited application is the lack of understanding regarding the underlying mechanisms responsible for the incremental oil recovery in core flooding tests. Limited mechanistic knowledge of the rock/fluid and fluid/fluid interactions renders intelligent design of efficient dynamic water formulations challenging. Several review papers have been published on the topic. Al-Shalabi and Sepehrnoori17 discussed the different aspects of low-salinity waterflooding and reported that wettability alteration to a more waterwet condition is the leading mechanism for incremental oil recovery. However, they suggested further investigation is needed to fully understand the mechanisms leading to the alteration of rock wettability to a more water-wet condition. This is in agreement with Derkani et al.18 who concluded that the principal mechanism underpinning this recovery method is not fully understood, which poses a challenge toward designing the optimal salinity and ionic composition of any injection solution. Sohal et al.19 also pointed out that wettability alteration is a main mechanism leading to improved oil recovery, but that further research in this domain is required to fully understand the mechanism. Other possible recovery mechanisms proposed for this process are the double layer expansion20 and complex chemical


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mechanisms discussed broadly in literature.21-26 However, Yutkin et al.27 reported that for carbonate rocks the diffuse double layer expansion (DLE) is not possible unless the ionic strength of the brine is below 0.1 molar. Yang et al28 suggested that the anionic groups of organic acids (carboxylate) from crude oil adsorb onto negatively charged rock surfaces mainly through calcium bridges. Reducing the salinity increases the electrostatic repulsive forces between mineral surfaces and carboxylate groups and breaks the calcium bridges to change the wettability of the rock surface to less oil-wet. Austad and co-workers carried out several studies29-34 on chalk formations to understand the effect of potential determining ions on wettability alteration and concluded that SO42- ions in the sea water adsorb onto the positive sites of the chalk surface, which reduces the overall positive charge and electrostatic repulsion on the rock surface. This is followed by the adsorption of Ca2+ ions toward the rock surface. The adsorbed Ca2+ ions interact with the adsorbed carboxylic groups to form a calcium carboxylate complex and release some carboxylate from the rock surface. At temperatures above 90oC, Mg2+ ions replace Ca2+ ions bound to carboxylic groups and thus release more carboxylic groups from the rock surface. Although it was indicated that sulfate ions act as a wettability modifier, the increase of SO42- ions in the injected brine by itself did not increase oil recovery in the absence of Ca2+ and Mg2+ ions. These findings were in agreement with the work of Abdallah and Gmira,21 who used a combination of contact angle, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) to investigate the impact of the potential determining ions on wettability alteration of calcite surfaces. They also concluded that the effect of potential determining ions on altering calcite wettability is not a single ion effect, but that interactions between sulfate, calcium, and magnesium ions play a role in changing the rock surface morphology at certain ionic ratios. A four times increased concentration of SO42- in the 50% diluted Arabian Gulf sea water was found to be the most effective brine for surface wettability alteration toward less oil-wet. Calcite dissolution is believed by some researchers to be the leading mechanism for the incremental oil recovery in low salinity flooding.35,36 Calcite dissolution, caused by injection of low salinity brines, improves the connectivity between micro-pores and


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macro-pores and alters the rock wettability leading to improvement of oil recovery by improving the areal sweep efficiency.36 Others however reported that calcite dissolution is not a dominant mechanism in low salinity flooding.4,37-41 Using contact angle measurements, Jabbar et al.1 investigated wettability alteration of aged calcite crystals and carbonate outcrop rock chips after treating them with dynamic water formulation. The results showed that the adsorption of long chain fatty acid (stearic acid) rendered the calcite surface more oil-wet as compared to the adsorption of short chain acid (heptanoic acid). Furthermore, they observed that increasing SO42- ions in the presence of Mg2+ and Ca2+ showed the strongest impact on wettability alteration toward less oil-wet. Electro-kinetic studies based on zeta potential measurements have been proposed and used to understand the mechanisms leading to the alteration of surface wettability on a microscopic level, considering that many previous studies were only considering macroscopic investigations. Jackson et al.42 determined zeta potentials of oil-water-carbonate systems and revealed a correlation between incremental oil recovery and zeta potential.42 Kasha et al.25 studied the effect of Ca2+, Mg2+ and SO42− ions on the zeta potential of calcite and dolomite particles aged with stearic acid and showed that the affinity of the individual Ca2+, Mg2+ and SO42− ions is affected by the presence of the respective other potential determining ions. The presence of Mg2+ ions significantly affects the ability of SO42− ions to modify the original surface charges of aged calcite and aged dolomite while the presence of Ca2+ ions has less significant effect on the negative surface charges developed by SO42− ions. In a continuation of that study, Al-Hashim et al.43 observed that increasing the concentration of Mg2+ and SO42− ions in diluted Arabian Gulf sea water improved the efficiency of the brine for wettability alteration to more water-wet conditions compared to original diluted sea water. Taqvi et al.44 studied the zeta potential of aqueous limestone suspensions in the presence of asphaltene at three different concentrations. They observed that zeta potential decreased exponentially by the increase of the asphaltene content.


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Few investigators studied the role of single salt solutions on the rock surface properties. Saraji et al.45 studied dynamic adsorption of asphaltene on calcite and quartz minerals in the presence of single salt solutions of NaCl, CaCl2, MgCl2 and Na2SO4 using zeta potential measurements. They found that at high CaCl2 and MgCl2 concentrations (900 mM), the adsorption of asphaltene on calcite reduced due to increase in repulsive hydration forces at the calcite surface while at 900 mM NaCl or Na2SO4, the asphaltene adsorption on calcite slightly increased due to the increase in magnitude of negative charge at the calcite/brine interface and absence of repulsive hydration forces. Bagci et al.10 investigated the effect of individual salt solutions of NaCl, CaCl2, KCl and binary combinations of these salts on limestone core using water flooding at 50oC. High oil recovery of 38 % OOIP was observed with 2 wt% KCl brine. Alotaibi et al.46 studied the role of single salt solutions of NaCl, CaCl2, MgCl2 and Na2SO4 on calcite surface using zeta potential measurements. Individual salt solutions were found to alter carbonate surface charges differently, although all brines had the same salinity. NaCl and SO42- significantly influenced the zeta potential at the calcite/water interface toward negative, while, Ca2+ and Mg2+ ions shifted the zeta potential toward positive. Rezaei Gomari et al.47 investigated the impact of Mg2+, SO42- and HCO3- ions on carbonate rock, using contact angle and zeta potential measurements at 25oC. Stearic acid and PhenolStearic acid were used to alter the wettability of calcite toward oil-wet. As the Mg2+ ion concentration increased, the zeta potential shifted toward positive values while the increased concentration of SO42- and HCO3- ions reduced the zeta potential toward more negative. The contact angle measurements showed that increasing the Mg2+ ion concentration from 0.02M to 0.06M reduced the contact angle from initial aged calcite rock with stearic acid (74o) to 28o and 15o respectively. This was attributed to the replacement of Ca2+ ions by Mg2+ and desorption of carboxylic groups from the calcite surface, resulting in a more water-wet surface. In the recent work, we focus our efforts on studying surface alterations at a microscopic level by use of zeta potential measurements. If colloidal rock particles, suspended in an electrolyte solution, carry a high


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electrical charge, a force of mutual electrostatic repulsion between adjacent particles will be present and therefore particles remain in suspension. If the charges are low, the colloidal particles will start to agglomerate. One way to control the surface charges is by modifying the suspending electrolyte solution, i.e. their ionic content. In a similar way, the right choice of a brine as a reservoir injection fluid can lead to a modification of the surface charges on the rock surfaces and thus to different attractive or repulsive behavior of this surface towards oil or water, which results in a wettability change. The main brine of interest in the current study is the Arabian Gulf sea water as this is an abundant and cost-effective medium to start the design of any dynamic water formulation. In addition, we are interested to investigate the effect of single salt addition. We focus on halides, in particular chloride (Cl-), bromide (Br-) and iodide (I-), to study monovalent anions with different polarizability. Given the fact that polarizability (size) of ions has a significant effect on the behavior of anionic solvation in polar solvents like water, it is interesting to see whether the polarizability of the anions would have a similar effect in regard to their interaction with rock surfaces. Because the polarizability of ions increases with their size, large size anions such as iodide are expected to have more chances to interact with the surface region of the carbonate particles than the smaller ones such as chloride. Calcite and carbonate particles containing carboxylic acids and/or asphaltenes adsorbed at their surface are used as our model surfaces to study the effect of several brine systems in regard to surface alteration. The main objective of this study is to identify the best ionic additive that can be used to modify the Arabian Gulf seawater for the ultimate purpose of improved oil recovery.


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MATERIALS AND METHODS Calcite and carbonate outcrop rock. Two types of rock samples, calcite crystal and carbonate outcrop representing Arab-D carbonate reservoir, were used in this study. Iceland spar calcite crystals from Chihuahua, Mexico were purchased from Ward Natural Science. The rock samples were crushed to meet the particle size requirement of the zeta potential instrument manufactured by Brookhaven Instrument Corporation, which can handle particle sizes between 10 nm to 30 µm. In order to yield small particles, both rock samples were crushed and grinded through two different instruments manufactured by Retsch. Rock samples were first crushed by a Jaw Crusher BB 51 to produce particle sizes within the acceptable range of the Ball Mill MM 400, which produces a final fineness of up to 2 µm. Both, the calcite and the carbonate powder samples were then analyzed by X-ray diffraction (XRD) and X-ray fluorescence (XRF) to assess the mineralogy of the samples. Table 1 summarizes the mineralogical content of both rocks used for the experiments. Calcite particles showed high purity (~ 98% CaCO3) whereas carbonate outcrop rock showed high calcite content (~ 95% CaCO3) with a small percentage of silica (~ 4% SiO2).

Table 1: Mineralogy and elemental composition of calcite and carbonate outcrop rock particles determined by XRD and XRF analysis. Samples Calcite

XRD analysis

XRF analysis

Calcite, CaCO3 ~ 98%

Element

Wt.%

Dolomite, CaMg(CO3)2 ~ 1%

Calcium

99.085

Albite, NaAlSi3O8 ~1%

Impurities

Quartz, SiO2 ~ Traces (< 1%)

Sulphur

0.111

Magnesium

0.128

Manganese

0.107

Silver

0.174


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Outcrop carbonate rock

Calcite, CaCO3 ~ 95%

Element

Wt.%

Quartz, SiO2 ~ 4%

Calcium

95.414

Dolomite, CaMg(CO3)2 ~ 1%

Silicon

1.895

Impurities Iron

0.926

Aluminum

0.703

Phosphorus

0.193

Sulphur

0.119

Model oils. Three model oils were used in this study: stearic acid (CH3(CH2)16COOH), asphaltenes (extracted by precipitation of Middle Eastern crude oil in n-heptane), and a combination of stearic acid and asphaltenes, all as solutions in toluene. The model oil containing stearic acid in toluene (termed TS) was prepared with a total acid number (TAN) of 2 mg KOH/g by dissolving 415 mg of stearic acid in 43.35 g of toluene. The model oil containing 0.05 wt% asphaltene in toluene (termed TA) was prepared by dissolving 21.67 mg of asphaltene in 43.35 g of toluene. The model oil containing stearic acid and asphaltene in toluene (termed TSA) was prepared with a total acid number (TAN) of 2 mg KOH/g and 0.05 wt% asphaltene by adding 415 mg of stearic acid and 21.67 mg of asphaltene in 42.9 g of toluene. Stearic acid (purity >98.5%) and toluene (purity 99.8%) were purchased from Sigma-Aldrich.


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Brines. Different brines were used in this study to investigate the impact of anions on the rock surface. These brines contain 5 kppm NaCl, 5 kppm NaI, or 5 kppm NaBr, respectively in deionized water. Synthetic Arabian Seawater had a composition of Na+ 18,043 ppm, Ca2+ 652 ppm, Mg2+ 2,159 ppm, Cl31,890 ppm, SO42- 4,450 ppm, HCO3- 173 ppm. Further, synthetic Arabian Gulf sea water with the addition of 0.5 wt% NaI, 0.5 wt% NaCl or 0.5 wt% NaBr was prepared respectively. High purity (~95% purity) salts were used in this study. NaCl and MgCl2·6H2O were purchased from Loba-Chemie, NaHCO3, NaI and NaBr were purchased from Sigma-Aldrich, CaCl2·2H2O was purchased from Scharlau, and Na2SO4 was purchased from Techno-Pharmchem. Deionized water, produced by Barnstead Ultrapure Water System, with a resistivity of 18.2 MΩ.cm at 20oC was used to prepare the brines.

Zeta potential measurements. All zeta potential (ζ) measurements were carried out using a ZetaPALS (Zeta Phase Analysis Light Scattering) instrument by Brookhaven Instruments Corporation. Zeta potential is measured by the electrophoretic mobility of a particle suspended in a solution. The instrument applies the technique of electrophoretic light scattering (ELS) in which a beam of laser light is passed through the solution.48 The charged particles of the solution move toward the positive and negative electrode. ELS is used to measure the velocity of moving particles that scatter laser light. Analyzing the direction of movement indicates electric charge on the particle and analyzing the velocity of particle indicates the mobility of the charged particle. The zeta potential is then calculated by the mobility and applied electric field as shown in Equation 1 and 2.

𝑣𝑒 = 𝑢𝑒 ∗ 𝐸 ………………... (1) where 𝑣𝑒 is the measured electrophoretic velocity and E is the applied electric field while 𝑢𝑒 is the required electrophoretic mobility to calculate zeta potential.7 By calculating the electrophoretic mobility of a particle, the zeta potential may then be determined using the Henry Equation.


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𝑢𝑒 =

2 𝜀 ζ 𝑓(𝐾𝑎) 3η

………………... (2)

where 𝑢𝑒 is the electrophoretic mobility, ε is the dielectric constant, ζ is the zeta potential, f(Ka) is Henry’s function, and η is the viscosity. Henry’s function generally has value of either 1.5 (Smoluchowski’s approximation) or 1.0 (Huckel’s approximation). Smoluchwski’s approximation considers particle size radius larger than the double layer thickness and Huckel’s approximation considers the particle size radius smaller than the double layer thickness.

In this study, zeta potential is determined by using

Smoluchowski’s approximation that is acceptable for aqueous media with moderate particle size.48 The double layer thickness is usually in the nanometer range,49-51 while our average particle sizes are 1.3 μm and 1.7 μm for calcite and carbonate outcrop, respectively. Particle sizes were determined by DLS (Dynamic Light Scattering) technique using the ZetaPALS instrument.


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Sample Preparation for zeta potential measurements. For zeta potential measurements, a solid to liquid ratio should be chosen such that the result is independent of the concentration chosen.50 Solid to liquid ratios used in previous studies for calcite particles ranged between 0.04-1wt%.

3,25,51-52

In this

study, a constant ratio of 0.5 wt% was used for all the zeta potential measurements. For that purpose, 0.15 g of unmodified calcite or carbonate powders were added to 29.85 g of the respective brine. Then, the solutions were conditioned in a multi-wrist shaker (by Burrell Scientific) at 40 rpm for 24 hours at room temperature. After conditioning, the pH of the solution was measured. To ensure the pH values of the brines are stable, the brines were allowed to incubate for additional 24 hours without shaking, the pH was measured again and showed different values as shown in Table 2. The pH values for the brines were measured again after additional 1 hour and their values were stable. Thus, a minimum of additional 24 hours was found to be reasonable to get stable pH values after the first 24 hours of conditioning using the multi-wrist shaker. A pH of 7.5 was maintained by adding 0.1 molar HCl or NaOH solution. All zeta potential measurements were conducted at 25oC.

Table 2: pH values of calcite at different times after treatment with different salt solutions. pH values Calcite

After

suspended in

conditioning for 24 h

After additional 24 h

After additional 25 h

without shaking

without shaking

DI Water

8.65

9.41

9.42

5kppm NaI

8.81

7.91

7.9

5kppm NaCl

9.07

8.29

8.31

5kppm NaBr

9.14

8.44

8.44

SW

7.81

7.83

7.85


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SW0.5wt%NaCl

7.68

7.6

7.62

SW0.5wt%NaI

7.54

7.59

6.57

SW0.5wt%NaBr

7.53

7.48

7.49

Preparation of modified rock particles. 5 g of each calcite and carbonate outcrop sample powder were added into 50 ml of each model oil, which corresponds to 8.3% stearic acid by weight for model oils containing stearic acid and 0.43% asphaltene by weight for model oils containing asphaltenes relative to the rock material. The selected amount provides 100% surface coverage of stearic acid on the carbonates.53,54 Suspensions were then allowed to condition under room temperature for 24 hours in a multi-wrist shaker. It is reported that 24 hours are sufficient for adsorption of polar components from the model oils onto the rock surfaces.4,55-59 Then, vacuum filtration was performed using a 0.7-micron filter paper to yield the modified solid rock particles. In order to confirm whether the model oil components were adsorbed on the rock particles the, a floatation test as described by Kasha et al.57 was conducted. 0.25 g of the respective modified and unmodified calcite and carbonate outcrop rock particles were added into 5 ml of distilled water, suspensions were incubated for 48 hours. It is observed from Figures 1 and 2 that unmodified calcite and carbonates outcrop particles sink to the bottom. Modified particles with model oil (TS) and model oil (TSA) completely floated at the surface due to the adsorbed surface-active components which make the particles less water-wet. However, particles with model oil (TA) partially floated at the surface and this is ascribed to a partial adsorption of asphaltene.


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Figure 1: (a) Unmodified calcite (b) calcite modified with model oil (TS) (c) calcite modified with model oil (TA) (d) calcite modified with model oil (TSA) suspensions in deionized water at 25°C. T: Toluene, S: Stearic acid, A: Asphaltene.

(a)

(b)

(c)

(d)

Figure 2: (a) Unmodified carbonate outcrop (b) carbonate outcrop modified with model oil (TS) (c) carbonate outcrop modified with model oil (TA) (d) carbonate outcrop modified with model oil (TSA) suspensions in deionized water at 25°C. T: Toluene, S: Stearic acid, A: Asphaltene.


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RESULTS AND DISCUSSION Zeta potential of unmodified calcite and carbonate rock surface in deionized water. The zeta potential of unmodified calcite and carbonate outcrop particles suspended in deionized water was initially measured at a pH of 7.5. The calcite particles showed a weak negative zeta potential of -4.70±0.60 mV while the carbonate particles showed stronger negative zeta potential of -8.05±0.48 mV. The zeta potential magnitude and charge are consistent with published results within close range of pH value: -5.33 mV at a pH of 7.5 for calcite and -10.4 mV at pH 7.3 for carbonate surface.57 The zeta potential negativity is attributed to calcite dissolution.35,58 When calcite rock particles come in contact with deionized water, CaCO3 dissociates into Ca2+ and CO32-.59 The carbonate ions may deprotonate water and produce OH- ions which increases the pH of the system and an equilibrium is established between the solid rock and the dissociated ions. The equilibrium could be disturbed upon the addition of CO2 that comes from the atmosphere. The addition of CO2 in deionized water produces carbonic acid that lowers the pH value and increases the calcite dissolution rate. Since all experiments were performed carefully in a closed system to minimize the interaction of atmospheric CO2 with the samples, the pH value stabilizes after the dissolution reached equilibrium. Upon dissolution of calcite in water, the following reactions take place:60 𝐶𝑎𝐶𝑂3 (𝑠) → 𝐶𝑎2 + (𝑎𝑞) + 𝐶𝑂23 ― (𝑎𝑞) 𝐶𝑂23 ― (𝑎𝑞) + 𝐻2𝑂 (𝑙) → 𝐻𝐶𝑂3― (𝑎𝑞) + 𝑂𝐻 ― (𝑎𝑞) In the absence of CO2, Ca2+ ions preferentially leave the calcite surface making the surface negatively charged. Similar observations were reported by Douglas and Walker who used CO2 free water with calcite and indicated the negative charge on the calcite surface occurs due to the preferential leaching of Ca2+ ions.61 Figure 3 shows a pictorial presentation of the assumed dissolution mechanism for calcite rock particles treated with deionized water. When calcite rock particles are interacting with deionized water,


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Ca2+ desorbs from the calcite crystal and the overall accumulation of negative charge increases in the diffuse layer around the charged particle, resulting in a negative zeta potential value.

Figure 3: Pictorial presentation of the mechanism when calcite is treated with DI water.61

For the samples we studied, the zeta potential value is more negative for the carbonate outcrop than for calcite, which is attributed to the increase of negative zeta potential with the decrease in CaCO3 concentration, corresponding to the mineralogical content of both rocks (calcite~98% CaCO3 and carbonate outcrop~95% CaCO3). The calcite sample in our study contains more CaCO3 than the carbonate outcrop and therefore, there are excess positive Ca2+ ions in the diffuse layer around the charge particle in the calcite rock particles which showed relatively less negative zeta potential values compared to the carbonate rock particles which showed higher negative values as less Ca2+ concentration in the diffuse layer. This observation is in agreement with the conclusion of Chen et al. on different carbonate rock


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samples where zeta potential increases with the increase of calcite content.51 Similar observations were also reported by Mahani et al., who found that pure calcite particles show higher zeta potential values than limestone particles.62 It is also possible that the additional negative charge in the carbonate outcrop rock could be due to the presence of higher content of quartz (SiO2) (~4%) which creates more negative charge around the particles as reported in literature.63,64

Surface characterization of unmodified and modified calcite rocks (TS). Calcite rock particles were analyzed by SEM-EDS to identify the constituent elements of the rock and to determine the elemental concentration at the indicated positions as shown in Figure 4. Only three elements were detected from the EDS spectra of calcite. They were calcium, oxygen and carbon confirming that the calcite rock used in this study was pure calcium carbonate (CaCO3) which is in line with the results obtained by XRD measurements. Table 3 summarizes the results of the EDS spectra and shows that the average value is in agreement with literature.65

Figure 4: Scanning Electron Microscopy (SEM) micrograph and positions where Energy Dispersive Spectroscopy (EDS) spectra were taken on the pure unmodified calcite.


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Table 3: Elemental composition for three different spectra as highlighted in SEM image of calcite with their average values and comparison to literature values. Element

Spectrum 1

Spectrum 2

Spectrum 3

Average

Karimi et al.65

C

22.54

10.12

20.12

17.59

17.10

O

24.96

34.14

37.96

32.35

35.50

Ca

52.50

55.74

41.92

50.05

47.50

To investigate and validate the surface adsorption of stearic acid, the calcite rock particles modified with model oil (TS) were also analyzed by SEM-EDS. Figure 5 shows the SEM images of the modified calcite surface indicating the different locations where the EDS spectra were measured. Table 4 summarizes the elemental composition at the different locations in addition to the average values and comparison to reported literature values.65 Comparing the results of unmodified calcite particles (Carbon~17.59%, Oxygen~32.35% and Calcium~50.05%) and calcite particles modified with model oil (TS) (Carbon~19.104%, Oxygen~48.95% and calcium~31.948%), it is observed that the modified calcite particles show comparatively less calcium concentration and high carbon and oxygen concentrations which is attributed to the adsorption of stearic acid on the calcite surface. Stearic acid contains a carboxylate (~COO-) group that has the capability to adsorb on the rock particle.66 The adsorption of the carboxylate on the calcite surface increases the concentrations of carbon and oxygen while comparatively, the surface concentration of calcium is reduced. Karimi et al.65 also reported that the adsorption of stearate increases the concentrations of oxygen and carbon, while the detected surface concentration of calcium is lowered. Ricci et al.67 reported that organic molecules such as stearic acid, adsorbing on calcite surfaces, can form monolayers or multilayers, depending on the concentration of the acid. They observed


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a rearrangement of the adsorbed stearic acid layer, driven by minimization of the hydrophobic contact with the brines, and a restructuring of calcite around the adsorbed organic patches, which become trapped in the growing crystal. Subsequent dilution of the brine re-dissolves the freshly grown material, exposing the stearic acid patches previously incorporated into the crystal during the growth.

Figure 5: Scanning Electron Microscopy (SEM) micrograph and positions where Energy Dispersive Spectroscopy (EDS) spectra were taken on calcite modified with model oil (TS). T: Toluene, S: stearic acid.


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Table 4: Elemental composition for several areas as highlighted in SEM image of modified calcite (TS), average values and comparison to literature values. T: Toluene, S: Stearic acid. Karimi et

Element

Spectrum 4

Spectrum 5

Spectrum 6

Spectrum 7

Spectrum 8

Average

C

18.30

18.20

22.13

17.43

19.46

19.10

19.40

O

50.85

46.62

52.09

48.28

46.91

48.95

48.20

Ca

30.85

35.19

25.78

34.29

33.63

31.95

32.30


al.65

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Surface characterization of unmodified and modified carbonate outcrop rock (TS). Carbonate outcrop particles were analyzed by SEM-EDS to validate the constituent elements of the rock and to determine the elemental concentration at several surface positions as shown in Figure 6. EDS spectra showed calcium, carbon and oxygen which indicate calcium carbonate (CaCO3). A small percentage of silicon can also be seen which confirms the presence of SiO2 as indicated by the XRD results. Table 5 shows the elemental composition at all analyzed positions with their average values.

Figure 6: Scanning Electron Microscopy (SEM) micrograph and positions where Energy Dispersive Spectroscopy (EDS) spectra were taken on carbonate outcrop.

Table 5: Elemental composition for different spectra as highlighted in Scanning Electron Microscopy (SEM) image of unmodified carbonate outcrop.


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Spectrum

Spectrum

Spectrum

Spectrum

Spectrum

9

10

11

12

13

C

21.34

23.24

22.46

27.15

21.85

23.21

O

20.27

14.74

14.47

13.14

8.87

14.30

Si

2.51

10.01

9.00

7.67

22.49

11.00

Ca

55.88

51.89

54.07

52.04

46.80

52.14

Elements

Average

Figure 7 shows SEM images and EDS spectra positions for carbonate outcrop modified with model oil (TS) and Table 6 summarizes the elemental composition determined by the EDS spectra. An increase in concentrations of oxygen and carbon can be observed, while calcium concentration is reduced as compared to unmodified carbonate rock outcrop. Moreover, the concentration of silicon could not be seen in most of the spectra as it was below the detection limit in the observed sites.


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Figure 7: Scanning Electron Microscopy (SEM) micrograph and positions where Energy Dispersive Spectroscopy (EDS) spectra were taken on carbonate outcrop modified with model oil (TS). T: Toluene, S: Stearic acid.

Table 6: Elemental composition for different spectra as highlighted in Scanning Electron Microscopy (SEM) image (Figure 7) of modified carbonate outcrop (TS). T: Toluene, S: Stearic acid. Spectrum Element 14

15

16

17

18

19

20

21

22

23

C

26.15

25.56

28

32.89

22.76

21.09

24.85

22.15

19.13

24

O

47.1

47.33

42.55

45.09

46.47

52.26

53.61

51.29

50.27

55.04

Si

0.67

0.66

Ca

26.08

26.45

29.45

22.02

30.77

26.65

21.53

26.56

30.6

20.96


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Effect of different model oils on the zeta potential of calcite rock surface in deionized water. To study the effect of polar species adsorption on the surface of the calcite particles, zeta potential was measured in deionized water for rock particles preconditioned with different model oils (asphaltene, stearic acid and a mixture of both) as presented in Figure 8.

Figure 8: Zeta potential (ζ) of unmodified and all modified calcite rock particles (in model oils TS, TA and TSA) suspended in deionized water at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

All modified calcite particles in this study show more negative zeta potential values compared to the unmodified particles (Figure 8). The increase in the negative charges is due to the adsorption of the polar components of the model oil (stearic acid and/or asphaltene). González and Middea studied the impact of three different asphaltenes on calcite surface charges and found that the adsorption of asphaltenes increases the surface charge negativity.68 Moreover, their values of zeta potential were in the range of -14


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to -16 mV which is within range of our results. Despite the different molecular structure and used concentration of asphaltenes (TA) and stearic acid (TS), their effect was comparable. Tabrizy et al. reported that the asphaltene renders the calcite rock more oil-wet than stearic acid.69 González and Middea also reported wettability change due to adsorption of asphaltene on the rock surface.68 Taqvi et al. also observed an increase of zeta potential negativity upon addition of asphaltenic solution in aqueous limestone rock suspension.44 The results of the current study also show that the combined effect of stearic acid and asphaltene (TSA) yielded slightly more negative charge on the calcite surface than the individual model oils (TS and TA) as shown in Figure 8. Chukwudeme et al.70 and Tabrizy et al.69 also observed that the combined effect of stearic acid and asphaltenes makes the calcite surface more oil-wet than only a single polar component, as shown by contact angle measurements and wettability index measurements, respectively. Although we observed a slightly higher magnitude of negative zeta potential for modified rock (TSA) as compared to modified rock (TA) or (TS), the difference is still within the range of the measurement error. It is also clear from the results that the individual effects of stearic acid and asphaltene are not additive for the model oil (TSA), so that we can assume that there is competition between both polar components to adsorb on the limited sites of the calcite surface. Similar observations have been made by Sauerer et al. for the competition of asphaltenes and stearic acid at the liquid/liquid interface.71 It is reported that stearic acid adsorbs chemically on the calcite surface when the active ratio is low and forms a calcium carboxylate complex.34,67,72 For the case of modified rock (TS) treated with deionized water, we propose two possible scenarios: 1.

Ca2+ ions leave the calcite surface along with adsorbed stearic acid as a result of calcite dissolution in deionized water and produce a deficiency of positive charge on the surface that increases the negative zeta potential value.


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2.

The adsorption of polar species of stearic acid (~COO-) on the opposite charge of Ca2+ increases the overall negative charge in the diffused layer at the slipping plane.

Assuming the first scenario, the measured zeta potential value should be similar to the values of unmodified calcite in deionized water. However, the zeta potential of modified calcite (TS) is -16.58±0.7 mV compared to -4.7±0.6 mV for the unmodified calcite. The large difference in zeta potential supports the second scenario. The increase of the magnitude of the negative zeta potential for modified calcite rock particles is therefore the result of the adsorption of polar oil components from the model oils and these components were not released when modified rock surfaces were conditioned in deionized water. It is therefore fair to say that deionized water alone has no strong effect to alter the surface wettability to more water-wet.1,4 We have shown previously73 that halide solutions on the other hand are capable to turn rock surfaces more water-wet, which is likely to be due to removal of adsorbed surface-active species.


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Effect of different model oils on the zeta potential of carbonate outcrop rock surface in deionized water. Figure 9 shows the zeta potential measured for all modified carbonate particles pretreated with model oils and then conditioned in deionized water. Particles with TS and TSA yielded more negative charge than unmodified carbonate and modified carbonate rocks with TA. As in the calcite case, the magnitude of zeta potential due to adsorption of stearic acid on the carbonate surface was highly negative indicating high adsorption tendency on the surface. However, the case with asphaltene model oil is different. The fact that the zeta potential is in that case similar to what was measured with unmodified surface indicates the lack of asphaltene adsorption onto the particle surfaces. This confirms the results of the floatation test, which showed very limited floating for the rock particles conditioned with asphaltene (TA).


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Figure 9: Zeta potential (ζ) of unmodified and modified carbonate rock particles (in model oils TS, TA and TSA) suspended in deionized water at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

Comparing the results of calcite and carbonate outcrop, modified calcite rock (TA) in deionized water showed an increase in the magnitude of negative zeta potential as compared to unmodified calcite rock, while the effect is not significant in the case of modified carbonate outcrop (TA) as compared to unmodified carbonate outcrop. Furthermore, it is also shown that stearic acid is highly surface-active towards the calcite and carbonate rock surface in terms of adsorption tendency while asphaltene had less tendency to adsorb on the carbonate surface. This is interesting to observe, and it shows that pure calcite might have higher affinity to change its wetting properties to more oil-wet compared to less pure carbonates. The original zeta potential in deionized water for unmodified carbonate outcrop is more negative than the one for unmodified calcite rock in our study. Due to the less negative charge on the calcite as compared to carbonate outcrop rock surface, asphaltene might tend to adsorb more on calcite rather than on carbonate outcrop. In another study, González and Middea observed that asphaltenes produced moderate changes in the electrical properties of the mineral-solution interface.68 However, these changes were not evident for minerals already exhibiting large negative zeta potential. Pourmohammadbagher and Shaw74 investigated the effects of asphaltene coating on the enthalpy of kaolinite and illite clays in deionized water and toluene. They found that clays can adsorb asphaltenes from asphaltene/toluene mixtures and that clay surfaces accordingly become saturated with asphaltenes. However, also in their case, the surfaces remain only partially coated. The outcome of the asphaltene sorption is that the clays become less hygroscopic while they increase the capacity to attract organic liquids.


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For the results of modified carbonate (TSA) in deionized water, one would expect similar negative magnitude of the zeta potential as for modified rock (TS), if there is no adsorption of asphaltene. However, the zeta potential value for the modified rock (TSA) was slightly less negative than the zeta potential for the modified rock (TS) and more negative than for the modified rock (TA). This potentially indicates that the presence of asphaltene will partially hinder the adsorption of stearic acid. Such competition between stearic acid and asphaltene has also been observed by Sauerer et al.71 Probably due to such competition, the overall negative charge on modified rock (TSA) was reduced as compared to modified rock (TS) in deionized water.

Effect of monovalent ions on the surface charge of unmodified calcite and carbonate outcrop. To evaluate the effect of different anions on the surface of unmodified rocks, zeta potential was measured for both unmodified rocks, calcite and carbonate outcrop, after conditioning in different halide brines as shown in Figure 10.


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Figure 10: Zeta potential (ζ) of unmodified calcite and carbonate outcrop rock particles treated with different brines (Deionized water, 5Kppm NaCl, 5Kppm NaBr and 5Kppm NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. The zeta potential shows more negative values as a result of conditioning the calcite and carbonate surfaces with the respective brines. Shehata and Nasr-El-Din reported a zeta potential of -9.8 mV at pH 9.5 for calcite particles conditioned in 5kppm NaCl,75 while Saraji et al. reported a zeta potential of -13.63 mV at pH 8.50 and -5.02 mV at pH 9.17 for calcite particles conditioned in 526 ppm and 52,500 ppm NaCl, respectively.45 These values are within the range of our results for the calcite surface using 5kppm NaCl (6.85±0.23 mV at pH 7.5). Alotaibi et al. reported zeta potential of -21.6 mV at pH 7 for limestone particles conditioned in 5,436 ppm NaCl.52 The zeta potential measured in this study for carbonate outcrop conditioned in 5kppm NaCl is -13.37±0.48 at pH 7.5 value. It is possible that the magnitude of zeta potential could vary with the different rock composition, experimental conditions and sample preparation


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procedures. However, it is consistent with the reported studies that the zeta potential on both rock surfaces conditioned in 5kppm NaCl, is negative at pH value close to 7.5. It can be seen from Figure 10 that the 5kppm NaBr produced the most negative zeta potential followed by 5kppm NaI, and 5kppm NaCl respectively. Ruiz-Agudo et al. observed an increase in calcite dissolution in the sequence of (I- > Cl-) in salt solutions with a constant ionic strength of 0.1 M.58 They measured calcite dissolution rate and etch pitch spreading rate by AFM on calcite chips, treated with different brines. Their results indicated that in the presence of a small concentration of NaCl (0.001 M which is equivalent to 35.5 ppm), calcite dissolution is slightly higher than in deionized water, and at higher concentration (0.1M NaCl), the dissolution rate is further enhanced. In order to confirm calcite dissolution of calcite rock incubated for 24 hours in the respective brines, the calcium ion concentrations of the brines were determined using ICP-OES. The brines were filtered using a 0.5-micron filter paper before the measurements. Table 7 shows the calcium ion concentrations present in the filtered brines. Table 7: Effect of monovalent ions on Calcite dissolution (unmodified calcite) at 25oC. Filtrate of Calcite + Deionized water Calcite + 5kppm NaCl (IS=0.086M) Calcite + 5kppm NaBr (IS=0.049M) Calcite + 5kppm NaI (IS=0.033M)

Ca2+ ions (ppm)

Zeta Potential, ζ (mV)

65.78

-4.70±0.60

3981.25

-6.85±0.23

2367.08

-12.53±0.66

1721.20

-11.81±0.27


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It is observed from Table 7 that the calcium ion concentration in the filtered brines increased in all brines, that were in contact with the calcite rock, in the sequence deionized water < NaI < NaBr < NaCl. The high concentration of Ca2+ ions resulting from dissolution of calcite in NaCl solution as compared to the case of the deionized water system is in agreement with Ruiz-Agudo et al.58 However, the observed trend of increasing dissolution for the sequence NaI < NaCl is not in agreement with their results. A possible explanation is that they used a constant concentration of 0.1 M for all salts, while our brines ionic strength is varied in the sequence NaCl (IS=0.086 M) > NaBr (IS=0.049 M) > NaI (IS=0.033 M). Comparing the zeta potential measurements with calcite dissolution as reported in Table 7, one would assume the same trend (NaCl > NaBr > NaI > DI) instead of NaBr > NaI > NaCl > DI, if calcite dissolution was the only mechanism leading to the change in zeta potential. However, the possible adsorption of I- or Br- or Cl- ions on the calcite rock surface could produce more negative charge on the calcite surface. The zeta potential of calcite rock particles in NaI and NaBr show higher negative values than in NaCl. Thus, we can conclude that adsorption of ions as well as calcite dissolution both play a major role in surface charge alteration on the rock. To confirm the adsorption of ions on the calcite surface, we measured the difference between initial concentrations of Cl-, Br- and I- ions present in the original brine solutions and the final concentration of these monovalent anions after conditioning the calcite rock in the monovalent solutions (Table 8). The results in Table 8 show that adsorption of ions onto the calcite surface takes place during conditioning with respective brines. Thus, it is clear that the increase in negative zeta potential, as shown in Table 7, represents the combined effect of calcite dissolution and adsorption of ions onto the rock surface. The adsorption of iodide ions on the calcite surface has also been reported in the literature.76


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Table 8: Difference between initial concentration of ions before adding calcite and final concentration of ions after equilibrating with calcite rock in different salt solutions (NaCl, NaBr, NaI) of 5kppm salinity.

Ion

Initial ion concentration

Final ion

Difference between initial

concentration

and final concentrations

Cl-

3037 ppm

2949 ppm

88 ppm

Br-

3885 ppm

2680 ppm

1205 ppm

I-

4234 ppm

1701 ppm

2533 ppm

Effect of different brines on the zeta potential of modified calcite rock surface. Figure 11 shows the zeta potential values for calcite rock particles modified with model oils and then conditioned in either deionized water or respectively in one of the three different single salt brines with either 5kppm NaCl, or 5kppm NaI or 5kppm NaBr. When the modified rocks are conditioned in different brines, the surface charge would be altered by rock dissolution, ion adsorption and/or the release of polar oil components. The adsorption of anions (Cl-, Br-, or I-) on the particle surface or rock dissolution will increase the negative charge while the release of polar oil components from the rock surface will reduce the negative magnitude of the zeta potential values. From the presented results in Figure 11, all modified calcite rock particles showed less negative charge when conditioned in the used brines when compared to modified calcite rocks conditioned in deionized water. However, the zeta potential values of the modified rock (TSA) conditioned in 5kppm NaCl and the modified rock (TSA) in deionized water are close, indicating the addition of NaCl did not release much model oil components from this modified calcite rock (TSA).


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Figure 11: Zeta potential (ζ) of modified calcite rock particles (in model oils TS, TA and TSA) treated with different brines (Deionized water, 5Kppm NaCl, 5Kppm NaBr and 5Kppm NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

The general decrease in negative charge for all modified rock surfaces upon condition in the respective salt solutions indicates the release of surface-active oil components from the rock surface. It is worth mentioning that the conditioning in 5kppm NaI reduced the magnitude of the negative charge strongly for all cases, thus pointing out iodide as a good agent for removing the model oil components from the rock surfaces. It was determined earlier that all modified rock particles were floating (partially or fully) on top of the deionized water phase, indicating that particles modified with model oil were rendered hydrophobic. In order to verify the desorption of oil components from the rock surface, we conditioned the modified rock particles with 5kppm NaI to observe if we would still see any floating particles. Figure


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12 shows that after conditioning in 5kppm NaI, no floating particles were observed. This confirms that conditioning of modified calcite rock particles in 5kppm NaI released oil components from the rock surface and altered the rock wettability toward less oil-wet. Al Hamad et al. found that interaction of 5kppm NaI with carbonate rock could alter the wettability of carbonate rock and reduced the contact angle of an oil droplet from 40o in deionized water to 12o in 5kppm NaI.73 They also reported an additional oil recovery of 4.5% and 15.8% during spontaneous imbibition test for two different carbonate samples, when the samples were placed in 5kppm NaI solution.

Figure 12: Floatation test of calcite particles modified with (a) model oil (TS), (b) model oil (TA) and (c) model oil (TSA) and conditioned in 5kppm NaI at 25oC. T: Toluene, S: Stearic acid, A: Asphaltene.


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Effect of different brines on the zeta potential of modified carbonate outcrop surface. Figure 13 shows the zeta potential values of carbonate outcrop rock particles modified with model oils and then conditioned in deionized water or the same brines used to condition the calcite. For the case of carbonate outcrop modified with model oil (TSA), the zeta potential value of the modified rock becomes slightly more negative when conditioned in 5kppm NaCl, NaI or NaBr as compared to conditioning in deionized water. It is most likely that the anions (Cl-, Br- or I- ions) adsorbed on the modified carbonate rock surface and added further negative charges on the modified rock surface. This effect is observed even stronger for the system modified with model oil (TA), which is possibly due to the fact that not much asphaltenes adsorbed on the carbonate, as discussed earlier. Therefore, these results show simply a strong increase in negative charge due to adsorption of the anions. For the case of carbonate outcrop rock modified with model oil (TS), all brines slightly reduced the magnitude of the negative zeta potential values as compared to modified carbonate outcrop particles conditioned in deionized water. Thus, for modified carbonate outcrop (TS), all brines are efficient (with 5kppm NaBr being the most effective brine) with regards to releasing oil components from the modified carbonate rock surface as compared to particles conditioned in deionized water. To support our observations from the zeta potential measurements, we performed a simple floatation test in which we conditioned the carbonate outcrop particles in 5kppm NaI and in deionized water. Figure 14 shows that for the modified rock (TS), we observed fewer floating particles when modified carbonate rock was conditioned in 5kppm NaI than when conditioned in deionized water. For carbonate rock modified with model oil (TA or TSA), conditioning in deionized water is showing fewer floating particles than in 5kppm NaI. This observation agrees with our analysis of zeta potential measurements.


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Figure 13: Zeta potentional (ζ) of modified carbonate outcrop rock particles treated with different brines (Deionized water, 5Kppm NaCl, 5Kppm NaBr and 5Kppm NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

On the basis of zeta potential measurements of modified carbonate outcrop treated with different brines, it is concluded that all brines (5kppm NaBr, 5kppm NaCl and 5kppm NaI) are effective in modifying the carbonate outcrop rock wettability toward less oil-wet for the modified carbonate rock (TS) case. It was determined earlier that the carbonate outcrop modified in (TA) and (TSA) and conditioned in deionized water did not show much adsorbed asphaltene. Thus, conditioning of modified carbonate rock (TA and


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TSA) in 5kppm NaCl and in 5kppm NaBr only increased the magnitude of the negative zeta potential values due to adsorption of anions on the rock surface.

Figure 14: Floatation test of modified carbonate outcrop conditioned with (a) model oil (TS), (b) model oil (TA) and model oil (TSA) conditioned in DI water (left) and in 5 kppm NaI (right) at 25oC. T: Toluene, S: Stearic acid, A: Asphaltene.


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Effect of Arabian Gulf sea water brines on zeta potential for calcite and carbonate outcrop surface. Although the effect of the tested individual single salt brines was well demonstrated in regard to the resulting surface alteration, as discussed above, the main interest for their practical use is in connection with the local Arabian Gulf sea water, which is a mixture of complex ions. Figure 15 shows the measured zeta potential for unmodified calcite and carbonate rock surfaces after conditioning in sea water and the effect of NaCl, NaBr and NaI individual addition to this sea water in regard to the measured zeta potential. Figures 16 and 17 show the zeta potential results for modified calcite and carbonate after conditioning in such sea water-based brines.

Figure 15: Zeta potentional (ζ) of unmodified calcite and carbonate rock particles treated with different brines (SW: sea water, SW0.5wt%NaCl: sea water with added 0.5 wt% NaCl, SW0.5wt%NaBr: sea water with added 0.5 wt% NaBr and SW0.5wt%NaI: sea water with added 0.5 wt% NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values.


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Zeta potential for the unmodified calcite and carbonate surfaces is discussed first. The conditioning in sea water for calcite indicated a zeta potential of +2.00±0.3 mV compared to -4.70±0.6 mV when conditioned in deionized water while for carbonate it was -6.83±1.09 mV compared to -8.05±0.48 mV in deionized water. In terms of surface alteration, the sea water and deionized water did not have a significant difference, except that for calcite the surface charge was positive in sea water and implies either the potential adsorption of divalent cations (Mg2+ or Ca2+) or/and surface dissolution and the exposure of surface Ca2+ from the calcite crystal. The effect of adding I-, Br- and Cl- individual ions to sea water altered the zeta potential to negative values for calcite and carbonate (except when calcite surface conditioned in sea water with additional Cl- ions, where the zeta potential was +4.47±1.29 mV). However, the magnitude of the negative zeta potential values for all these cases was less, compared to conditioning with either deionized water or the individual single salt brines. This indicates the potential adsorption of positive ions from the seawater which could also compete with the monovalent halide ions for adsorption sites at the mineral surface.


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Figure 16: Zeta potentional (ζ) of modified calcite rock particles (in model oils TS, TA and TSA) treated with different brines (SW: sea water, SW0.5wt%NaCl: sea water with added 0.5 wt% NaCl, SW0.5wt%NaBr: sea water with added 0.5 wt% NaBr and SW0.5wt%NaI: sea water with added 0.5 wt% NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

Since actual carbonate reservoir rocks are generally moderate to oil-wet, it is best to investigate the zeta potential at these conditions. Both model surfaces, calcite (Figure 16) and carbonate outcrop (Figure 17) were therefore treated with the model oils containing surface-active asphaltene, stearic acid or a mixture of both, and then conditioned with sea water or sea water with addition of I-, Br- and Cl-. For the modified calcite and carbonate surfaces, addition of I- ions to the sea water showed the best efficiency in altering


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the surface property. For all observed cases (TS, TA and TSA) we could see on both model surfaces that the zeta potential was increased to less negative values in the range of the unmodified calcite and carbonate surfaces respectively. This shows impressively that the respective surface-active compounds seem to have been removed almost entirely from the mineral surfaces upon treatment with the iodide enhanced sea water. Conditioning with the Cl- ions enhanced seawater was the least effective as it leads to even more negative zeta potential than the originally modified surfaces when measured in deionized water. The effect of Br- ions was comparable to the effect of iodide but did not lead such a strong reduction of the negative zeta potential as observed for the iodide system. From a theoretical standpoint, it is interesting to note that the magnitude of the zeta potential change upon treatment with the respective halide solutions is in line with the Hofmeister series (Cl- < Br- < I-).77 Larger, more polarizable ions with low charge density (the chaotropic ions) seem therefore more effective in removing adsorbed surface-active species from the rock surface than smaller ions with higher charge density (kosmotropes). From an industrial point of view, it is very encouraging to see that the addition of iodide ions to Arabian Gulf sea water shows a much better effect in regard to surface alteration compared to the single salt brines used in this study. The use of sea water as a base fluid is economical and the addition of the identified single salt is an excellent way to improve the efficiency in removing surface-active components that are known to turn the surface more oil-wet. An injection fluid (dynamic water) based on the results of this study should allow to alter carbonate reservoir surfaces to more water-wet and thus ultimately enhance oil recovery. This is very well in line with previous results obtained in our group, investigating the impact of halide ions as an additive for seawater-based injection fluids by means of spontaneous imbibition and core flooding experiments.73


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Figure 17: Zeta potentional (ζ) of modified carbonate outcrop rock particles (in model oils TS, TA and TSA) treated with different brines (SW: sea water, SW0.5wt%NaCl: sea water with added 0.5 wt% NaCl, SW0.5wt%NaBr: sea water with added 0.5 wt% NaBr and SW0.5wt%NaI: sea water with added 0.5 wt% NaI) at pH 7.5 and 25oC. Error bars show the standard deviation of the plotted mean zeta potential values. T: Toluene, S: Stearic acid, A: Asphaltene.

CONCLUSIONS Based on the results of this study the following conclusions can be drawn: 1.

Zeta potential of calcite and carbonate outcrop is negative in deionized water due to preferential

leaching of Ca2+ ions from the calcium carbonate lattice.


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2.

The magnitude of negative zeta potential is higher in carbonate outcrop as compared to calcite.

3.

The magnitude of the negative zeta potential in calcite rock increases when the rock is aged with

the model oils (TS, TA and TSA) and conditioned in deionized water, as compared to unmodified calcite rock suspensions conditioned in deionized water. This is attributed to the adsorption of surface-active components from the model oils onto the rock surface and the results of floatation tests also verify this adsorption. Thus, the increase in negative zeta potential indicates the oil-wet nature of the rock particles. 4.

The magnitude of the negative zeta potential for carbonate outcrop increases when the rock is

aged with the model oils (TS and TSA) and then conditioned in deionized water, as compared to unmodified carbonate outcrop rock conditioned in deionized water. However, particles modified with model oil (TA) show similar zeta potential as observed with unmodified carbonate outcrop. Floatation test confirms some adsorption of model oil (TA) on the carbonate outcrop rock surface before conditioning in deionized water. The results of modified carbonate outcrop (TSA) conditioned in deionized water also indicate the competition of asphaltene and stearic acid, as the magnitude of the negative zeta potential is slightly reduced for (TSA) as compared to (TS). 5.

The SEM-EDS spectra showed increasing concentration of carbon and oxygen elements when

calcite rock is modified with model oil (TS) as compared to unmodified calcite rock, which is attributed to the adsorption of stearic acid on the calcite surface. 6.

Conditioning of modified calcite rock particles in single salt low salinity brines (5kppm of NaCl,

NaBr and NaI, respectively) showed consistently less negative charge as compared to conditioning in deionized water, with the effect of 5kppm NaI being the most dominant one. The reduction of the magnitude of the negative zeta potential is attributed to the release of adsorbed model oil components that were causing the increase in the negative charges.


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7.

All single salt low salinity brines (5kppm of NaCl, NaI or NaBr, respectively) led to less negative

charge on the carbonate outcrop rock surface modified with model oils (TS) as compared to the corresponding zeta potential value in deionized water. For other modified carbonate outcrop particles (TA and TSA), the single salt low salinity brines were increasing the magnitude of the negative zeta potential, which indicates adsorption of the anions instead of release of the surface-active material. 8.

Finally, the results of the zeta potential measurements for both rock surfaces, calcite and

carbonate outcrop, modified with any of the model oils (TS, TA and TSA) and afterwards conditioned in seawater or seawater with added chloride, bromide or iodide, showed that iodide was in all cases able to reduce the magnitude of the negative zeta potential to values close to unmodified surfaces. This shows impressively that iodide is the agent of choice for a new, economical and environmentally friendly, seawater based dynamic water formulation, that should be able to render previously oil-wet carbonate formations to water-wet, which is preferred for enhanced oil recovery.

NOMENCLATURE ζ

Zeta potential

AFM

Atomic Force Microscopy

EDS

Energy Dispersive Spectroscopy

DI

Deionized

SEM

Scanning Electron Microscopy

TAN

Total Acid Number (mg KOH/g)

XPS

X-Ray Photoelectron Spectroscopy

ICP-OES

Inductively Coupled Plasma-Optical Emission Spectroscopy


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NMR

Nuclear Magnetic Resonance

XRD

X-Ray Diffraction

XRF

X-Ray Fluorescence (XRF)

ACKNOWLEDGMENT The authors wish to thank King Fahd University of Petroleum and Minerals and Schlumberger Dhahran Carbonate Research Centre for the support provided during the study.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] (W.A.) * Email: [email protected] (H.A.)

ORCID Wael Abdallah: 0000-0001-7788-6805

Notes The authors declare no competing financial interest.

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Energy & Fuels


Graphical Abstract

10 5

Sea Water

Sea Water + 0.5wt% NaBr

Sea Water + 0.5wt% NaI

Sea Water + 0.5wt% NaCl

0

Zeta Potential, mV

-5 -10 -15 -20 -25 -30 -35 -40 10 5

Calcite

Sea Water

Calcite+Asph

Calcite+SA

Sea Water + 0.5wt% NaI

Calcite+Asph+SA

Sea Water + 0.5wt% NaBr

Sea Water + 0.5wt% NaCl

0

Zeta Ptential, 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

-5 -10 -15 -20 -25 -30 -35 -40

Carbonate

Carbonate+Asph

Carbonate+SA

Carbonate+Asph+SA


Influence of Halide Anions Cl-, Br- and I- on the Zeta Potential of Oil

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