Impact of Modified Seawater on Zeta Potential and Morphology of

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Impact of Modified Seawater on Zeta Potential and Morphology of Calcite and Dolomite Aged with Stearic Acid Hasan Al-Hashim, Ahmed Amara Kasha, Wael Abdallah, and Bastian Sauerer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03753 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Impact of Modified Seawater on Zeta Potential and Morphology of Calcite and Dolomite Aged with Stearic Acid Hasan Al-Hashim a,*, Ahmed Kasha a, Wael Abdallah b, Bastian Sauerer b a

Department of Petroleum Engineering, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Saudi Arabia b Schlumberger Dhahran Carbonate Research Center, 31942 Dhahran, Saudi Arabia

ABSTRACT Zeta potential measurements and microscopic surface characterization and imaging were conducted on calcite and dolomite crystals aged in stearic acid model oil and exposed to different synthetic brines representing different potential scenarios of injected seawater from the Arabian Gulf. Calcite particles were negatively charged in deionized water and maintained negative surface charges in all tested brines, except in diluted Arabian Gulf seawater that contained higher concentration of Ca2+ and Mg2+ ions. Dolomite particles were positively charged in deionized water as well as in all tested brines, except in diluted Arabian Gulf seawater that contained four times higher concentration of SO42- ions. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) experiments on cleaved calcite and dolomite chips showed different morphological changes when both samples are aged in model oil and then treated with brines. Calcite surface dissolution was observed in addition to stearic acid deposition. Surface elemental analysis using Energy-Dispersive Spectroscopy (EDS) showed Mg2+ and SO42- ions adsorb preferably on locations where stearic acid is deposited. The finding that stearic acid was adsorbing stronger on dolomite than on calcite could indicate why the tested brines were less efficient to change the zeta potential of the dolomite systems. The current study concludes that manipulating the concentration of potential determining ions present in the Arabian Gulf seawater, especially Mg2+ and SO42- ions, will alter the surface charges of aged calcite and dolomite samples as well as their surface morphology. Keywords:

Zeta Potential; AFM; Carbonate Rock; Dynamic Water; Wettability; Low Salinity

1 INTRODUCTION Improving oil recovery by injecting a smart water (a specific brine with controlled ionic content and optimized salinity, also known as dynamic water or low salinity water) has gained a considerable momentum in the oil and gas industry after many successful laboratory results and few field pilots1-6. AlShalabi and Sepehrnoori7 presented a comprehensive review of more than 150 technical papers on different aspects of low salinity waterflooding. One of the main conclusions they reached is that wettability alteration to a more water-wet condition is believed to be the main drive mechanism for the incremental oil recovery. They called for more research to identify the mechanisms leading to the alteration of wettability to a more water-wet condition. Mohammed and Babadagli8 carried out another comprehensive review of more than 100 papers covering the various methods on wettability alteration by low salinity, thermal and chemical methods and the methods used to measure wettability alterations. They also tested some potential wettability alteration chemicals on bitumen containing Grosmont carbonate, and pointed out that wettability alteration

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of a given rock is a unique process, requiring understanding of the mechanisms leading to the alteration of reservoir rock to a more water-wet state. For these reasons, the number of publication on the use of low salinity water, or brines having tuned ionic content of some potential determining ions for improving oil recovery is growing steadily. Many published studies9-34 consider wettability a critical factor for the success of any oil recovery as a result of brine injection. In fact, pre-knowledge of the carbonate rock wettability state is a pre-design factor for selecting the right salinity and ionic content of the injected water to ensure successful oil recovery. Jackson and Vinogradov34 measured the streaming potential of carbonate core samples, saturated with high salinity formation brine and natural crude oil. They showed that changes in wettability led to measureable changes in the zeta potential and that the wetting state of the carbonate during low salinity water flooding could be quantified by streaming potential measurements. Alroudhan et al.35 showed that zeta potential measurements using the streaming potential method on limestone intact core samples and measurements using the electrophoretic mobility method on crushed samples from the same rock produce the same results. They also showed that there is a correlation between the incremental oil recovery and the cumulative change in the magnitude of the zeta potential. In another study, Jackson et al.36 showed that low salinity water (LSW) could result in incremental oil recovery or not depending on the type of oil. They also reported a positively charged oil-water interface at high pH and ionic strength, and attributed the failure of the conventional dilution LSW to recover additional oil to this positively charged oil-water interface. Mahani et al.13-14 investigated the underlying mechanism for improved oil recovery from carbonates during low salinity water flooding using contact angle measurements, geochemical modeling and zeta potential measurements. They showed that changes in the carbonates surface charges are responsible for altering the wettability towards more waterwet, which enhances the recovery of oil. In the Middle East, carbonate reservoirs host more than 70% of the crude oil reserve and therefore, it is crucial to better understand the surface properties of this rock and how it interacts with crude oil components and injected brines to allow prediction and design of recovery schemes. Zeta potential measurements give a close insight on the modification of the surface electrical properties of these rock surfaces as a result of the chemical interactions with the contacting fluids, which could relate to wettability alteration of the pores in carbonate formations.10,15-16 The major minerals of carbonate rocks, limestone (calcite) and dolomite, were widely studied in presence of electrolyte solutions. Chen et al.37 showed that zeta potential rises toward the positive with the increase of the concentration of inorganic salt on limestone powders (main size distribution between 3–5 µm), but the increase rate gets very low in the high concentration brines and the overall zeta potential value stays negative. They also showed that increasing the salinity reduces the effect of the pH, oil polar fractions and naphthenic acid on the zeta potential and is nearly not affected by the above in formation water. Furthermore, they indicated that the zeta potential is closely related to the mineral composition of the rock, and increasing the calcite content results in an increase of the zeta potential. Alotaibi et al.17,38 reported that changing the surface charges of the carbonate rocks from positive to negative, by the injecting brine, enhances the water-wetness of the carbonate rock surfaces and therefore improves the recovery of oil. Many studies conducted on chalk samples showed that the interactions between Ca2+, Mg2+ and SO42- ions present in the injected brine and the carbonate surface are responsible for the desorption of the adsorbed organic polar compounds (carboxylic acids) from the rock surface.18-21 Karoussi et al. studied wettability of calcite chips aged in stearic acid when exposed to various brines using atomic force microscopy (AFM) and contact angle measurements. They concluded that the presence of Mg2+ and SO42- ions increases the water wetness of the treated calcite and reduces the adhesion forces of the adsorbed stearic acid.22-23 Abdallah and Gmira


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studied calcite surface aged in crude oil and treated with different brines, using AFM and x-ray photoelectron spectroscopy (XPS) and concluded that the effect of potential determining ions on altering the surface wettability is not a single ion effect because there are affinities between sulfate, calcium, and magnesium that will change surface morphology at certain ratios. Their data showed that Mg2+ and SO42ions increase the water-wetness, but a certain ratio between sulfate ion and magnesium ion is needed in the brine mixture to maximize the recovery.24 In a previous study,25 we reported the changes in zeta potential of calcite and dolomite particles aged in model oil (stearic acid) when exposed to different brines with different concentration of Ca2+, Mg2+ and SO42- ions. The results showed the affinity of Ca2+, Mg2+ and SO42- ions is affected by the presence of other 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. Karimi et al.26 looked at the effect of sulfate ions with and without cationic surfactant as one of the most promising wettability influencing ions on the wetting properties of oil-wet calcite. They indicated the interaction between sulfate ions and the calcium ions attached to carboxylate groups on the surface is believed to be the main active mechanism of wettability alteration at high concentration of sulfate ions. The co-presence of sulfate ions and cationic surfactant resulted in a further reduction in the amount of adsorbed carboxylate on the surface which was attributed to the release of adsorbed carboxylate groups on the surface through ion pair formation with the cationic surfactant. Although natural outcrop carbonate rocks have been reported to be water-wet,28 carbonate oil reservoirs are generally characterized to be neutral to oil-wet.29,30-40 The change in carbonate rock wetness has been attributed to the adsorption of carboxylic acids that are present in crude oil.27, 41-43 Stearic acid in particular has been used widely as a model carboxylic acid in studying reservoir rock wettability alteration due to its ability to alter the carbonate rock wettability to hydrophobic conditions.27, 42-51 Ricci et al.42 showed that stearic acid molecules tend to act as “pinning points” on pure calcite surfaces, initiating a potential monolayer or bilayer growth and slow down the crystal restructuring kinetics. Once immersed into the supersaturated brines, crystal growth around the stearic acid patches was induced. The process can be reversed upon subsequent dilution of the brine, recovering the organic patches embedded inside the rock. Fatty acid such as the long chain stearic acid adsorbs on calcite surface by chemisorption and physisorption, resulting in monolayer and/or multilayer depending on the concentration of the acid used and the active ratio. Thomas et al.47 reported that fatty acids form a surface monolayer, and carboxylate polymers also appear to coat the surface completely, chemisorbed as ionic species. Fekete et al.52, reported that monolayer coverage can be achieved with 0.5% stearic acid content, and higher acid concentration of stearic acid may result in building additional free acid layers on top of the monolayer. Rezaei Gomari et al.28,30 reported that an adsorption plateau of fatty acids on calcite surfaces can be achieved at an acid concentration of 0.01%. Mihajlović et al.43,44 studied stearic acid adsorption on calcite surface using dry and wet adsorption methods. In the “dry” method, they proposed surface dissociation of stearic acid occurs where H+ ions go to a surface carbonate ion and the stearate ion is chemisorbed on the primary surface center of the Ca2+ ion which is only available for chemisorption. In the “wet” method, the resulting stearate ions from micelle dissociation can be chemisorbed on primary centers of Ca2+ ions or participate in ion exchange with OH- ions from secondary surface centers.


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To gain further insights to the surface alteration of stearic acid conditioned carbonate rocks based on the injected brines and specifically Ca2+, Mg2+ and SO4 2- ions, we measured zeta potential of conditioned calcite and dolomite samples when all three ions are present in the same brine. Brines were synthesized based on the ionic composition of the Arabian Gulf seawater, considering that this is the most injected fluid in Middle Eastern reservoirs. In addition to zeta potential measurements, optical microscopy and atomic force microscopy were used to examine the effect of the brines on the morphology alteration of stearic acid conditioned calcite and dolomite chips, while scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to provide elemental analysis for the surface.

2 SAMPLES AND EXPERIMENTAL METHODS 2.1 Calcite and dolomite crystals Iceland spar calcite crystals from Creel, Chihuahua, Mexico and dolomite crystals from Butte, Montana (both purchased from Ward’s Natural Science) were used in the current study. Mineralogy of both crystals was analyzed by x-ray diffraction (XRD) analysis on fine powders and by SEM coupled with EDS on cleaved chips. The calcite sample showed very high purity (~ 100% CaCO3) while small percentages of quartz and ankerite (< 4 wt. %) were detected in the dolomite sample by the XRD. Table 1 summarizes the EDS and XRD results for both rocks. 2.2

Model oil

5.0 g of stearic acid (CH3(CH2)16COOH) was dissolved in 493.0 g of toluene to develop a stock of model oil with a calculated Total Acid Number (TAN) of 2.0 mg KOH/g which was used to represent the oil phase in all experiments in the current study. The mixture was stirred for 24 hours to dissolve the stearic acid in the toluene. To verify the calculated total acid number, a color-indicator titration test was conducted on the prepared model oil according to ASTM Standard D974-1.53 A value of 1.952 mg/KOH was measured which is in agreement with the calculated TAN considering the experimental error within the test. The model oil was made from high purity materials purchased from Sigma-Aldrich (> 98.5 % and ~99.8% purity for stearic acid and toluene respectively). In all the experimental investigations carried out in this study, stearic acid solutions in toluene with a concentration of 1% were used to modify the calcite and dolomite surfaces from hydrophilic to hydrophobic conditions. The 1% concentration is selected based on a previously reported study showing that 1% stearic acid can achieve over 80% active ratio.43 A flotation test confirmed calcite and dolomite are oil-wet as they both floated on the water surface. 2.3

Brines

Synthetic brines were prepared based on the ionic composition of the Arabian Gulf seawater using pure chemical reagents (NaCl, CaCl2·2H2O, MgCl2·6H2O, Na2SO4, NaHCO3) purchased from PanReac and deionized water (DI) produced by Barnstead Ultrapure Water System which showed a resistivity of 18.6 MΩ·cm at 23 °C. Table 2 summarizes the ionic composition of the formulated Arabian Gulf seawater (termed SW), diluted Arabian Gulf seawater (termed SW*) and the formulated brines for the effect of increasing the potential determining ions concentrations on diluted synthetic Arabian Gulf seawater.


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2.4

Zeta Potential Measurements

As described in our previous study,25 calcite and dolomite crystals were crushed into fine powders and then soaked in model oil in closed jars. The mixtures were slowly stirred for 24 hours, after which the contents were filtered through 0.45 µm filter papers using a vacuum pump. The filtered powders were allowed to dry at room temperature for 24 hours, and then kept in a closed jar for conditioning with the formulated synthetic brines listed in Table 2. For the zeta potential measurements, a solid-to-liquid ratio of 0.5 wt% was used by adding 0.125 g of the aged powder to 25.0 ml of the aqueous brines in a 40 ml test tube. The test tube was put on a multi-wrist shaker for conditioning at room temperature for 24 hours. The conditioned suspensions were allowed to settle at room temperature for a minimum of one hour until a clear suspension is visually seen through the tube. The suspended particles were carefully drained by a syringe needle and then filtered through a 5 µm syringe filter to produce the final sample for the zeta potential measurement. All zeta potential measurements were conducted at 25 °C using a zeta potential analyzer manufactured by Brookhaven Instruments. Measurements were carried out as described in our previous study.25 2.5

Microscopic surface imaging & characterization of cleaved calcite and dolomite chips

Cleaved chips were prepared from each crystal by gently scratching along a cleavage direction using a sharp scalpel. For each rock, cleaved chips were first soaked in model oil for aging in a closed jar for 24 hours, after which, a single chip was taken and immersed in the desired brine for 24 hours. The chip was then allowed to dry at room temperature for 24 hours before being inspected by different instruments that included optical microscope (Leica Microsystems), atomic force microscope (AFM, Bruker MultiMode), scanning electron microscope (SEM, JEOL) and energy dispersive spectroscope (EDS, Oxford Instruments); all for comprehensive morphological and elemental characterization. The aged chips were not rinsed with deionized/distilled water before drying, in order to keep the surface imaging experiments consistent with the zeta potential experiments when correlating the results of both experiments. Recrystallization of NaCl was a concern. However, extra tests were made and reproducibility of the morphological changes was observed with or without the rinsing step. 3 RESULTS AND DISCUSSION 3.1 Zeta Potential Zeta potential measurement were conducted at 25 °C and at initial unadjusted pH values of the conditioned suspensions (pH 9.5 for deionized water and pH 8.0 for SW, SW* and modified SW* brines). The average hydrodynamic diameter was 1.23 µm for the calcite particles and 0.34 µm for the dolomite particles. These measurements were conducted to investigate the surface charges of aged calcite and aged dolomite in the presence of different brines that are potential for surface wettability alteration to enhance the oil recovery in Middle East carbonate formation. The aged carbonate particles suspended in different brines form a Stern layer of adsorbed ions while the hydrated ions stay in solution. At the same time, a diffuse electric double layer is formed around the particles. Figure 1 shows the zeta potential values for calcite and dolomite particles aged in stearic acid and conditioned in the brines listed in Table 2. Dolomite surfaces generally show positive zeta potential in all


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studied brines with the exception when SO42- ions are added (SW*2SO4 and SW*4SO4). Comparing the data for seawater, diluted seawater and deionized water, no apparent trend is obvious. Zeta potential of dolomite particles in seawater is slightly higher than when conditioned in the diluted seawater and deionized water. This could be due to a very slow dissolution process of the dolomite when contacted by low salinity brines.33 Increasing the concentration of Ca2+ ions in the diluted seawater decreased the magnitude of the dolomite positive surface charges but when further Ca2+ ions were added (SW*4Ca), the magnitude of the positive zeta potential of the dolomite particles increased and became close its original value measured in the original seawater. This is most likely caused by an induced adsorption of Ca2+ ions from the contacting brine on the dolomite surface. The addition of Mg2+ ions to the diluted seawater caused the zeta potential positive magnitude to increase, resulting in the highest value measured for all tested brines (+19.44 mV ± 1.09 mV with SW*2Mg brine, and +16.04 mV ± 3.14 mV with SW*4Mg brine). We would expect the addition of Ca2+ and Mg2+ ions in the diluted seawater will increase the positive surface charges to the dolomite particles as a result of the increased adsorption of these positive ions from the brines on the particles’ surfaces. Nonetheless, SW*2Ca decreased in terms of positivity and Mg2+ ions seem to show more adsorption activity compared to Ca2+ ions at the studied conditions. We have expected the addition of SO42- ions to the diluted seawater to alter the original surface charges of the dolomite particles from positive to negative. Only at high concentration of SO42- ions (SW*4SO4) this was observed (-6.97 mV ± 1.45 mV), slight additions of SO42- ions (SW*2SO4) slightly dropped zeta potential compared to seawater. To alter the surface wettability of dolomite, high sulfate concentration is required in the injected brines. This might cause desorption of carboxylate salts and excess adsorption of sulfate ions. Our result does not match with what was published by Alotaibi et al.17 who reported negative zeta potential values for pure dolomite particles when exposed to deionized water and aquifer water. In their study, the aquifer water, with the major salts been removed, was also used to assess the zeta potential of dolomite particles. In all cases, the dolomite particles possessed negative surface charges it their study. In the current study, as well as in our previous study,25 most of the measured zeta potential values of the dolomite particles were positive. The differences in zeta potential could be due to the nature of the dolomite material used and its surface preparation. Calcite particles showed different results compared to dolomite. Generally, all zeta potential recorded showed negative values except if the calcite was exposed to SW*4Ca, SW*2Mg and SW*4Mg solutions. The highest negative zeta potential is measured in deionized water (-18.70 mV ± 0.34 mV) which is most likely caused by the preferential dissolution of calcite lattice ions (Ca2+).4 Calcite dissolution in reservoir formations is very common and investigated by many researchers.54,55 Klasa et al.54 showed after flowing water over a calcite surface, that dissolution took place by the formation of characteristic rhombohedral shallow etch pits. Once formed the etch pits progressively retreat with increasing contact time with water. As a result, etch pits eventually merged with adjacent pits, effectively removing the entire unit cell layer. Occasionally, deeper etch pits formed. Ruiz-Agudo et al.55 studied the role of halide salts on the mechanism and kinetics of calcite dissolution far out from equilibrium and showed that all the electrolytes tested enhance the calcite dissolution rate with different magnitude, based on the used electrolyte and its concentration. Highly hydrated ions increased the etch pit nucleation density on calcite surfaces compared to pure water, which might be related to a reduction in the energy barrier for etch pit nucleation due to disruption of the surface hydration layer.


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Calcite particles also showed negative surface charges when conditioned in the seawater (-10.78 mV ± 0.89 mV) and diluted seawater (-9.80 mV ± 2.26 mV). This is close to the zeta potential (-8.0 mV ± 0.5) reported by Alroudhan et al.35 for Portland Limestone in twice diluted SW. Diluting seawater will expand the electrical double layers around the calcite particles, which will result in increasing the magnitude of the negative surface charges. Nonetheless, in the case of increasing the concentration of Ca2+ ions in the diluted seawater, it decreased the magnitude of the calcite negative surface charges. Further increase in the Ca2+ ion concentration resulted in a complete alteration of the calcite surface charges from negative to positive. Additions of Mg2+ ions to diluted seawater, directly altered the negative surface charges of the calcite particles to positive, and the further increase in the Mg2+ ion concentration increased the magnitude of the positive surface charges (from 6.15 mV ± 0.65 mV in SW*2Mg to 9.64 mV ± 2.12 mV in SW*4Mg). This trend is in agreement with the observed trend reported by Chen et al.37. Increasing the concentration of SO42ions increased the magnitude of the negative zeta potential of calcite particles. At very high concentration of SO42- ions (SW*4SO4), the zeta potential of the calcite particles was within range of measured zeta potential in the seawater (-11.20 mV ± 2.22 mV in SW*4SO4 compared to -10.78 mV ± 0.89 mV in SW). In previous studies using similar synthetic brines and calcite sources, Jabbar et al.27 assessed the wettability alteration of a calcite crystals aged in stearic acid model oil and conditioned in synthetic brines similar to the ones used in the current study. Contact angle measurements were performed by using a drop of model oil in a bulk of brine on aged calcite chips. The lowest contact angles, indicating a water-wet condition, were measured on the calcite chips treated with seawater (SW) and diluted seawater with twice the concentration of SO42- ions (SW*2SO4). The study also showed that increasing the concentration of Mg2+ and SO42- ions in diluted seawater would improve the efficiency of this brine for wettability alteration to more water-wet conditions compared to pure diluted seawater. Figure 2 shows the reported contact angle values compared to current obtained zeta potential. It also shows the measured contact angle for similar brines using calcite surface aged in crude oil.24 The Figure does not show any particular trend between zeta potential and the presented contact angles. Rezaei Gomari and Hamouda30 showed that the adsorption of the carboxylate anion is influenced by the ionic composition of the water, ionic strength and pH. The presence of Mg2+ and SO42- ions increases the water-wetness of the calcite and the degree of wetting is dependent on the pH. At pH 7 in a stearic acid/n-decane/water/calcite system, both ions reduce the contact angle. Higher reduction is obtained in presence of SO42- compared to Mg2+. These results also agree with the measurements reported by Abdallah and Gmira24 studying the effect of dynamic water on calcite aged in crude oil. They concluded that the presence of SO42- and Mg2+ ions and their relative ratio are very crucial for enhancing the water wetness of the calcite surface. 3.2

Microscopic imaging and characterization

The surfaces of the calcite and dolomite chips were characterized using SEM and EDS after aging in stearic acid as well as after exposing them in SW and SW*. Figure 3 shows the SEM image and Table 3 shows the results of the EDS spectra for the calcite chip in different locations after it was aged in stearic acid and dried. The white deposits on the surface are confirmed to be the adsorbed acid as indicated by the high concentration of carbon (Table 3, Spectrum 2) compared to the calcite surface that has no apparent deposits of acid (Table 3, Spectrum 1) which matches the original EDS analysis of dry calcite before aging as shown in Table 1. Similarly, stearic acid adsorption was also detected on the dolomite chip that was aged in the


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model oil as apparent from white spots of the SEM image (Figure 4) and confirmed by EDS spectra (Table 4, Spectrum 2). The detected Si, Al and Fe ions in Table 4 result from the quartz and ankerite minerals which are found to be less than 3% wt. as indicated in Table 1. When the aged calcite chip was immersed in seawater, very clear morphological changes were developed as shown in the SEM image (Figure 5) compared to the aged calcite surface (Figure 3). The initially adsorbed stearic acid crystals shown in Figure 3 were dispersed into small separated patches over the calcite surface as visible in Figure 5 and confirmed by the EDS results in Table 5. To better understand the calcite surface changes as shown in Figure 5, modified SW brines were prepared to study the individual effect of ions present in the seawater. Three brines were prepared using deionized water and CaCl2·2H2O salt (for Ca2+ ions), MgCl2·6H2O salt (for Mg2+ ions) and Na2SO4 salt (for SO42-ions). These brines were formulated to be equivalent to the individual concentration level of each equivalent divalent ions present in the Arabian Gulf seawater as presented in Table 6. Three cleaved calcite chips were first aged in stearic acid model oil and then immersed in each of the brines. Three other cleaved calcite chips were directly immersed in the same brines without prior aging in stearic acid. All six chips were dried and analyzed under an optical microscope. Figures 6a, 6b, and 6c show the optical microscopic images for three calcite chips immersed in CaCl2, MgCl2, and NaSO4 brines without aging in model oil, while Figures 6d, 6e, and 6f show the optical microscopic images for three calcite chips aged in stearic acid and then immersed in CaCl2, MgCl2, and NaSO4 brines. Clearly, Figures 6a, 6b and 6c show that all calcite chips exhibited the development of rhombohedral shapes with different characteristics representing calcite dissolutions in the presence of Ca2+, Mg2+ and SO42- ions. This is consistent with the results demonstrated by Klasa et al.54 and Ruiz-Agudo et al.55-57 Figure 6a shows few irregular etch pits exhibiting rounding of +/+ etch pit corners (red arrows). Figure 7b shows shallower rhombohedral etch pits that are adjacent to each other with rounding of the etch pits corners (red arrows) due to the presence of Mg2+ ions. Ruiz-Agudo et al.56 also observed that increasing Mg2+ results in an increase of etch pits density and depth. They also reported that rounding of +/+ corners was more significant in MgCl2 solutions as compared to MgSO4 solutions having the same concentration. In a later study, Ruiz-Agudo et al.61 studied the role of halide salts on the mechanism and kinetics of calcite dissolution and showed that all the electrolytes tested enhance the calcite dissolution rate with different magnitude based on the used electrolyte and its concentration. Ruiz-Agudo et al.56 also studied the mechanisms by which background electrolytes modify the kinetics of non-equivalent step propagation during calcite growth using AFM at constant driving force and solution stoichiometry. Their results suggest that the acute step spreading rate is controlled by kink-site nucleation and, ultimately, by the dehydration of surface sites (increasing the relative amount of carbonate ions in solution), while the velocity of obtuse step advancement is mainly determined by hydration of calcium ions in solution. However, dehydration of calcium ions in solution seems to be rate-limiting for the spreading of obtuse steps. Changes in cation hydration in solution will occur in the presence of different background anions. Arvidson et al.58 showed similar behavior for calcite dissolution in water in the presence of Mg2+ ions. They attributed the curvedshape of etch pits to the presence of Mg2+ ions. They also discussed the inhabitation of Mg2+ ion on calcite growth. The morphological influence of Mg2+ ions on calcite crystal, and the rounding edges of regular rhombohedral shape were also observed by Gratz et al.59 as a result of calcite surface dissolution. Figure 6c shows very clear development of rhombohedral etch pits with no round corners. Ruiz-Agudo et al.55 reported similar behavior for unaged calcite when immersed in 1M NaSO4 solution. They also reported that increasing the concentration of NaSO4 has no effect on the etch pit density. The developed rhombohedral shapes on the calcite surface treated by CaCl2·2H2O brine are smaller and shallower compared to the


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rhombohedral shapes developed on the calcite surfaces treated by MgCl2·6H2O and Na2SO4 brines. The observed differences in the rhombohedral shapes associated with CaCl2·2H2O brine could be due to the low concentration of Ca2+ ions in the Arabian Gulf seawater compared to Mg2+ and SO42- ions. In our previous study,25 we observed a development of homogenous patterns of rhombohedral etch pits as a result of calcite surface dissolution when a cleaved calcite chip was treated by deionized water. In the current paper, the dissolution patterns shown in Figures 6a and 6c have irregular etch pits. Some studies reported curved shapes of calcite surface dissolutions in deionized water and Mg2+ rich solutions with a time dependent step retreat.58 Difference in the dissolution kinetics and the orientation of the lattice ions are among the controlling factors that affect the dissolution of the calcite surface and therefore surface activity.60 In presence of stearic acid, the rhombohedral shapes on the calcite surface were only observed for the case where the calcite chip was treated with MgCl2·6H2O brine as indicated by the red arrows in Figure 6e. However, the most interesting observation was the development of a new layer on the calcite surface when the calcite chips were treated with MgCl2·6H2O and Na2SO4 brines as indicated by the white arrows in Figures 6e, and 6f. The developed layer appears as an aggregation of rounded microscopic black deposits. These black deposits are adsorbed stearic acid molecules which were seen as white deposits on the SEM image in Figure 5a. In the MgCl2·6H2O brine (Figure 6e), the black deposits were much more apparent compared to the observed deposits in the CaCl2·2H2O brine (Figure 6d). For the calcite surface treated with Na2SO4 brine, the population of the black deposits was the highest and tends to develop dense and larger deposits at different locations on the surface. Based on optical microscope images shown in Figure 6, it is believed that the interplay between the divalent ions present in the contacting seawater (Mg2+ and SO42ions), and the rock mineral together with the adsorbed stearic acid, is responsible for the micromorphological changes observed on the aged calcite chip when exposed to the Arabian Gulf seawater (Figure 5). The surface morphology of the aged dolomite when exposed to the seawater was different from the calcite surface. As it can be seen in Figure 7, the stearic acid deposits do show a less obvious distribution pattern of adsorbed species on the dolomite surface compared to the adsorbed stearic acid on the calcite (Figure 5). EDS results shown in Table 7 do not indicate significant adsorption of Ca2+ and Mg2+ ions from the SW onto the dolomite surface when compared to the EDS characterization of the original surfaces before being exposed to model oil or SW as shown in Table 1. This is consistent with the observed measurement of zeta potential for dolomite when exposed to different brines which showed less changes in surface charges. Especially Spectrum 2 (Table 7) shows however the presence of adsorbed stearic acid, resulting in a high percentage of carbon. AFM was acquired on the same calcite and dolomite chips to investigate the topological changes on both samples after being immersed in the seawater. Figure 8 shows the AFM height images on a 15 x 15 µm scanning area on the calcite (left) and the dolomite (right). On both samples, surface adsorption composed of stearic acid molecules are observed as the brighter shades while the original surface is visible by the darker shade referenced by the height scale bar located at the right of each image. As shown in Figure 8, the adsorbed molecules on the calcite are agglomerated in separate large patches in agreement with what was observed in the microscopic image (Figure 5). The height of the deposits outgrows in some cases the range of 1.2 µm, leading to the observed image artefacts when scanning over these high patches. On the dolomite, the adsorbed acid molecules are more scattered and cover most of the surface with only few high patches.


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Comparing the surface morphology observed on the dolomite surfaces (Figure 7 and Figure 8 (right)) with those observed on the calcite surfaces (Figure 5 and Figure 8 (left)), it indicates that for these seawatertreated samples the interaction between the surface and the stearic acid is much stronger in the dolomite case compared to the calcite case. On calcite, stearic acid tends to adsorb where other stearic acid molecules are already present on the surface, building patchy aggregates. On dolomite, the strong affinity of the stearic acid for the surface leads to a broad coverage of the surface. The strong positive zeta potential values obtained for the stearic acid treated dolomite particles in DI water did not change much when seawater was utilized (Figure 1). This could be an indication that seawater does not lead to a significant release of such strongly adsorbed organic molecules on dolomite surfaces. However, it is not very accurate to make a direct correlation between the observed morphological changes on the microscopic surface images and the corresponding changes in the zeta potential unless the same morphological changes happen at the nano-scale on the suspended particles. Most of the observed surface etching and growth on the microscopic surface images are much larger than the average size of the suspended calcite particles from which the zeta potential values were measured. In addition to the seawater case, SEM and EDS were also used to evaluate the morphological changes on aged calcite and dolomite chips treated with diluted seawater (SW*) and diluted seawater with increased concentration of divalent ions. Figures 9a, 9b, 9c, and 9d show four SEM images for aged calcite chips immersed in SW*, SW*4Ca, SW*2Mg and SW*2SO4 brines, respectively. All calcite chips exhibited a noticeable induced roughness on the original surface with varied degrees of surface growth and depositions, which is caused by the interactions between the divalent ions present in the brines, the calcite surfaces and the adsorbed stearic acid molecules. Ruiz-Agudo et al.56 used AFM to study the growth of calcite at a constant supersaturation and constant solution stoichiometry in the pH range 7.5–12 and observed the calcite growth rate decreased with increasing pH. He indicated these observations can be explained considering the effect of OH- ions on solute hydration. Ricci et al.42 used AFM to investigate dissolution and growth of calcite with low and high stearic acid coverage and exposed to supersaturated and diluted brines. For the case of low stearic acid coverage, they observed initial rearrangement of the stearic acid layer with the formation of stable bilayer or multilayer patches on the calcite surface. On the other hand, calcite growth was found to concentrate the adsorbed molecules of stearic acid in confined areas forming thick patches of stearic acid. When the treated calcite samples were immersed into the supersaturated brines, crystal growth around the dense stearic acid patches was induced. They also reported that the process of crystal growth can be reversed upon subsequent dilution of the brine, recovering the organic patches embedded inside the rock. In the diluted seawater (Figure 9a), most of the stearic acid deposits are connected to each other and denser compared to the aged calcite treated in seawater (Figure 5). On the surface shown in Figure 9a, very small percentages of Mg2+ ions were detected by the EDS on the locations where the stearic acid was deposited, while no significant adsorption of SO42- ions or significant increase of the Ca2+ ions was found on all locations analyzed by the EDS (Table 8). Increasing the concentration of Ca2+ ions (SW*4Ca) resulted in less intensive and more segregated stearic acid deposits on the surface (Figure 9b) compared to both seawater and diluted seawater. In Figure 9b, the EDS spectrum on a relatively clear site (Spectrum 2) showed mainly original surface. On the other hand, Spectrum 1 and 4 showed significant increase of carbon associated with significant decrease in the calcium ions confirming the coverage of the surface with stearic acid. Exposing the aged calcite chip to the diluted seawater with increased concentration of magnesium ions (SW*2Mg) resulted in a more even distribution of the stearic acid patches, resulting also in a lower carbon concentration in the individual EDS spectra measured (Figure 9c, Table 8). For the case of diluted seawater


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with increased concentration of sulfate ions (SW*2SO4), large surface deposits are visible on the calcite surface (Figure 9d). EDS spectra showed significant amount of Mg2+ and SO42- ions on the surface growth where the stearic acid deposits are located (Table 8). Apart from the obvious massive deposits, clearly visible in Figure 9, less apparent, less aggregated surface deposition was noticed on the clear appearing locations on the calcite chips. To investigate these positions further, the same chips were scanned by AFM, locating the AFM tip on positions similar to those indicated by the red squares on Figures 9b, 9c and 9d. Figure 10 shows the AFM height images for the three selected calcite chips. As indicated by the height scale bar, connected surface growth was found on the top of the original calcite surface which was exposed to the SW*4Ca brine (Figure 10a). On the calcite chip exposed to SW*2Mg brine, the original calcite surface appeared as elongated parallel channels beneath the surface growth (Figure 10b). The SW*2SO4 brine resulted in less coverage of the surface growth (Figure 10c). This indicates that SW*2SO4 brine was able to decrease the amount of the adsorbed stearic acids. Recent AFM studies used chemical force mapping to explain the effect of salinity changes on the adhesion of organic compounds on carbonate and sandstone surfaces. For sandstone rocks, it was found that adhesion force is significantly influenced by the pH, regardless of the salinity.61 In another study for carbonate, however, adhesion decreased in some areas as salinity decreased, while the opposite was noticed in other areas on the same surface. The varying behavior was mainly attributed to the type and quantity of adsorbed organic compounds.62 Both studies emphasized that the surface charges behavior has a significant influence on the adhesion of the organic compounds on carbonate as well as on sandstone surfaces. As before for the calcite chips, cleaved dolomite chips were aged in stearic acid and then exposed to diluted seawater (SW*) and diluted seawater with increased concentrations of divalent ions (SW*4Ca, SW*2Mg and SW*2SO4). The chips were then analyzed by SEM and EDS. As shown in Figure 11, immersing the aged dolomite chip in the diluted seawater resulted in appearance of the same pattern of deposits which was noticed when the dolomite chip was immersed in the seawater (Figure 7). However, with the diluted seawater (SW*), the stearic acid deposits on the dolomite surface were more evenly distributed (Figure 11a) compared to the seawater case (Figure 7). Increasing the concentration of Ca2+ ions in the SW* brine (SW*4Ca) resulted in a slight decrease of the amount of surface deposits (Figure 11b). With increased concentration of Mg2+ ions (SW*2Mg), very intensive and connected surface deposits were developed on the dolomite surface (Figure 11c). On the other hand, when the SO42- ions concentration was increased (SW*2SO4), the size of the developed surface deposits significantly decreased (Figure 11d) compared to other three brines. Table 9 shows EDS results for the marked locations in Figure 11. From all the EDS spectra shown on Figure 9 and Figure 11, it was noticed that most of the adsorbed ions from the studied brines are detected on locations where the stearic acid molecules are deposited. 4

CONCLUSIONS

In the present work, the zeta potential and the morphology of calcite and dolomite crystals aged in model oil were studied when both rocks are exposed to Arabian Gulf seawater and diluted Arabian Gulf seawater brines with modified ionic content. The major findings are summarized below: •

Normal Arabian Gulf seawater and diluted Arabian Gulf seawater maintain the original negative surface charges of the aged calcite and do not alter the original positive surface charges of the aged dolomite.


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•

•

• •

•

5

Increasing the concentrations of Ca2+ and Mg2+ ions in the diluted Arabian Gulf seawater altered the original calcite surface charges from negative to positive and increased the magnitude of the positive dolomite surface charges, while at high concentration of SO42- ions, the diluted seawater completely altered the original positive surface charge of the dolomite and increased the magnitude of the negative surface charges of the calcite. The SEM and AFM images showed clear morphological changes of the original surface and of the adsorbed stearic acid patches when the aged calcite and dolomite samples are exposed to the Arabian Gulf seawater and the modified versions of this seawater. The main observations were the induced roughness on the original surface of each sample in addition to a distribution of the adsorbed stearic acid into smaller, separated and scattered deposits. Mg2+ and SO42- ions had a profound effect on the observed morphological changes on both samples compared to Ca2+ ions. The EDS analysis showed that most of the adsorbed divalent ions from the contacting brines are located on or around the adsorbed stearic acid molecules rather than anywhere else on the calcite and the dolomite chips. Interpretation of the zeta potential measurements and the observed morphological changes indicate that organic material adsorbs much stronger on dolomite surfaces. This might decrease the efficiency of different contacting brines in releasing these adsorbed materials.

ACKNOWLEDGMENTS

We would like to thank the Petroleum Engineering Department at King Fahd University of Petroleum & Minerals for allowing us to use their facilities, Dr. Ahmed Dogan from the Earth Science Department at King Fahd University of Petroleum and Minerals (KFUPM) for the preparation of cleaved chips. We also extend our appreciation to the Center for Engineering Research at the Research Institute of KFUPM for allowing us to use their surface characterization instruments. 6

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http://dx.doi.org/10.1016/j.apsusc.2016.03.205. 49. Thomas, M. M.; Clouse, J. A.; Longo, J. M. Adsorption of organic compounds on carbonate minerals:

3.

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http://dx.doi.org/10.1016/0009-2541(93)90072-Q. 50. Legens, C.; Palermo, T.; Toulhoat, H.; Fafet, A.; Koutsoukos, P. Carbonate rock wettability changes induced

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http://dx.doi.org/10.1016/S0920-4105(98)00031-X. 51. Yang, Y-C.; Jeong, S-B.; Yang, S-Y.; Chae, Y-B.; Kim, H-S. The Changes in Surface Properties of the Calcite Powder with Stearic Acid Treatment. Materials Transactions 2009, 50, 695-701. http://dx.doi.org/10.2320/matertrans.MER2008388. 52. Fekete, E.; Pukánszky, B.; Tóth, A.; Bertóti, I. Surface modification and characterization of particulate

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http://dx.doi.org/10.1016/0021-9797(90)90300-D. 53. ASTM Standard D974-12, Standard Test Method for Acid and Base Number by Color-Indicator Titration, ASTM International, West Conshohocken, AP, 2012. http://dx.doi.org/10.1520/D0974-12. 54. Klasa, J.; Ruiz-Agudo, E.; Wang, L. J.; Putnis,C. V.; Valsami-Jones, E.; Menneken, N.; Putnis, A. An atomic force microscopy study of the dissolution of calcite in the presence of phosphate ions. Geochim. Cosmochim. Acta 2013, 117, 115-128. http://dx.doi.org/10.1016/j.gca.2013.03.025. 55. Ruiz-Agudo, E.; Putnis, C. V.; Jiménez-López, C.; Rodriguez-Navarro, C. An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions. Geochim.


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Cosmochim. Acta 2009, 73, 3201-3217. http://dx.doi.org/10.1016/j.gca.2009.03.016. 56. Ruiz-Agudo, E.; Kowacz, M.; Putnis, C.V.; Putnis, A. The role of background electrolytes on the kinetics and mechanism of calcite dissolution. Geochim. Cosmochim. Acta 2010, 74, 1256-1267. http://dx.doi.org/10.1016/j.gca.2009.11.004. 57. Ruiz-Agudo, E.; Putnis, C. V; Wang, L.; Putnis, A. Specific effects of background electrolytes on the kinetics of step propagation during calcite growth. Geochim. Cosmochim. Acta 2011, 75, 38033814. http://dx.doi.org/10.1016/j.gca.2011.04.012. 58. Arvidson, R. S.; Collier, M.; Davis, K. J.; Vinson, M. D.; Amonette, J. E.; Luttge, A. Magnesium Inhibition of Calcite Dissolution Kinetics. Geochim. Cosmochim. Acta 2006, 70, 583-594. http://dx.doi.org/10.1016/j.gca.2005.10.005. 59. Gratz, A. J.; Hillner, P. E.; Hansma, P. K. Step dynamics and spiral growth on calcite. Geochim. Cosmochim. Acta 1993, 57, 491 - 495. http://dx.doi.org/10.1016/0016-7037(93)90449-7. 60. Stipp, S. L. S.; Eggleston, C. M.; Nielsen, B. S. Calcite surface structure observed at microtopographic and molecular scales with atomic force microscopy (AFM), Geochim. Cosmochim. Acta 1994, 58, 3023-3033. http://dx.doi.org/10.1016/0016-7037(94)90176-7. 61. Shi, L.; Olsson, M. H. M.; Hassenkam, T.; Stipp, S. L. S. A pH-Resolved View of the Low Salinity Effect

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Table 1: Summary of XRD and EDS results of the used calcite and dolomite crystals. Rock

XRD Results

EDS Results


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Calcite

Calcite, CaCO3 ~ 100

Dolomite, CaMg(CO3)2 ~ 96.2% wt. Dolomite

Other impurities ~ 3.8 % wt. (Quartz, Ankerite)

Element O Ca C

% wt. 43 42 15

Element O C Ca Mg

% wt. 34 32 22 12

Table 2: Ionic composition of all used synthetic brines. Ion Na+ Ca2+ Mg2+ SO42ClHCO3TDS, g/l Ionic Strength (M)

SW, g/l 18.04 0.65 2.16 4.45 31.81 0.17 57.29 1.15

SW*, g/l (50% diluted) 9.02 0.33 1.08 2.23 15.90 0.09 28.64 0.57

SW*2Ca g/l 9.02 0.65 1.08 2.23 14.90 0.09 27.96 0.57

SW*4Ca g/l 8.40 1.30 1.08 2.23 13.50 0.09 26.60 0.57

SW*2Mg g/l 7.55 0.33 2.16 2.23 12.00 0.09 24.35 0.57

SW*4Mg g/l 4.40 0.33 4.32 2.23 4.30 0.09 15.66 0.57

SW*2SO4 g/l 8.55 0.33 1.08 4.45 13.50 0.09 27.99 0.57

Table 3: EDS readings on calcite surface conditioned in model oil as located in Figure 3.

Spectrum # 1 2 3 4 5

Detected elements (wt%) C O Ca 15.0 43.1 41.9 74.3 16.2 9.5 66.1 20.3 13.6 41.4 17.7 40.9 34.9 32.9 32.2

Table 4: EDS readings on dolomite surface conditioned in model oil as located in Figure 4.

Spectrum # 1 2 3 4 5

C 31.3 91.5 64.0 58.5 34.9

O 34.1 6.4 16.0 18.9 42.7

Detected elements (wt%) Ca Mg 22.1 11.9 1.5 0.6 12.0 6.0 15.1 6.6 13.1 8.9

Fe 0.5 0.0 0.7 0.7 0.3

Si 0.0 0.0 1.1 0.3 0.2

Al 0.0 0.0 0.2 0.0 0.0

Table 5: EDS readings on calcite surface conditioned in model oil and seawater as located in Figure 5.


SW*4SO4 g/l 6.85 0.33 1.08 8.90 9.50 0.09 26.74 0.57

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Detected elements (wt%) C O Ca 19.9 43.0 37.1 45.8 35.4 18.7 22.9 41.3 35.8

Spectrum # 1 2 3

Table 6: Ionic composition of the formulated brines for the individual effect of divalent ions of the Arabia Gulf seawater. Brine # 1 2 3

Salt CaCl2·2H2O MgCl2·2H2O Na2SO4

TDS, g/l 2.39 18.06 6.58

Concentration of the divalent ions (as in the Arabian gulf seawater), g/l [Ca2+] = 0.65 g/l [Mg2+] = 2.16 g/l [SO42-] = 4.45 g/l

Table 7: EDS readings on dolomite surface conditioned in model oil and seawater as located in Figure 7.

Spectrum # 1 2

C 28.0 58.5

O 37.1 24.3

Detected elements (wt%) Ca Mg Fe Si 20.3 13.3 0.8 0.0 9.3 6.4 0.0 0.0

Al 0.0 0.0

Mn 0.4 0.0

Table 8: Detected elements by the EDS on stearic acid depositions on aged calcite chips.

Spectrum No. Original Calcite Surface 1 2 3 a SW* 4 5 6 1 2 SW*4C b a 3 4 1 2 SW*2M c 3 g 4 5 1 2 3 SW*2S d 4 O4 5 6 7

Fig. 9

Brine

C 15 38.5 42.6 43.8 38.6 35.7 28.0 70.3 24.2 28.1 64.6 16.1 65.6 61.8 22.4 31.1 15.0 20.8 19.1 23.0 39.1 28.0 52.1

O 43 24.0 29.4 28.2 31.3 32.5 38.6 19.6 36.3 32.7 21.8 27.0 21.6 14.1 29.3 27.5 40.3 37.2 38.8 36.6 30.4 50.9 31.6

Detected elements by the EDS (wt. %) Ca Mg S Na Cl Mo 42 25.8 0.2 6.5 5.1 25.3 0.5 0.5 1.0 0.6 25.1 0.2 1.5 1.3 25.2 0.7 0.2 1.2 2.7 28.0 0.2 1.8 1.3 33.3 0.1 9.0 0.2 0.3 0.5 39.5 39.2 13.0 0.4 0.3 56.9 7.2 2.0 0.3 3.3 20.4 1.1 0.3 2.3 47.5 0.4 0.4 35.8 1.8 44.7 42.0 42.1 38.4 0.2 4.1 0.9 0.5 20.9 4.1 4.1 0.2 1.1 2.0 9.0 0.8 1.7 6.5 7.4 0.1 0.6 -


Si -

F -

1.1 -

0.7

Cl 0.3 1.5

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Table 9: Detected elements by the EDS on stearic acid depositions on aged dolomite chips.

Fig. Spectrum Brine 11 No. Original Dolomite Surface 1 a SW* 2 1 b SW*4Ca 2 c SW*2Mg 1 1 d SW*2SO4 2

C 32 64.8 30.7 32.8 59.5 43.6 43.2 32.8

Detected elements by the EDS (wt. %) O Ca Mg S Na Cl 34 22 12 21.2 8.2 4.4 1.3 35.9 20.0 12.4 0.4 34.0 21,6 10.8 0.4 18.5 14.2 6.2 0.8 34.8 7.5 7.9 0.7 5.4 35.1 12.6 8.4 0.7 41.3 15.1 10.3 0.3


Si 0.3 -

Fe 0.7 0.5 0.4 0.3

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-11.20

SW*4SO4

-6.97 -6.10

SW*2SO4

12.99 9.64

Calcite Dolomite

6.15

SW*2Mg

-6.62

15.23

SW*2Ca

-9.80

8.89

SW*

-10.78

12.31

SW

-18.70

-20

19.44

6.48

SW*4Ca

-23

16.04

SW*4Mg

15.07

DI -18

-15

-13

-10

-8

-5

-3

13.41 0

3

5

8

10

13

15

18

20

23

25

Zeta Potential (mV)

Figure 1. Zeta potential of calcite and dolomite particles aged in model oil (stearic acid) and conditioned in deionized water (DI) and different brines including seawater (SW), diluted seawater (SW*) and increased concentrations of ions (Ca2+, Mg2+ and SO42). All measurement were conducted at 25 °C and at initial unadjusted pH values of the conditioned suspensions (pH 9.5 for DI water and pH 8.0 for SW, SW* and modified SW* brines). The average hydrodynamic diameter was 1.23 µm for the calcite particles and 0.34 µm for the dolomite particles.


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Abdallah and Gmira

Zeta Potential

60

15

50

10

40

5

30

0

20

-5

10

-10

0

Zeta Potential (mV)

Jabber et al

Contact Angle (degree)

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

-15

SW

SW*

SW*4Mg

SW*2SO4

SW*4SO4

Brine Figure 2. Measured contact angle of calcite aged in stearic acid model oil by Jabbar et al.27 and aged in crude oil by Abdallah and Gmira24 compared to measured zeta potential for various brine systems on calcite chips.


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Figure 3. SEM image and positions of EDS spectra for different locations on a calcite chip aged in model oil.

Figure 4. SEM image and positions of EDS spectra for different locations on a dolomite chip aged in model oil.


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Figure 5. SEM image and positions of EDS spectra on different locations on a calcite chip aged in model oil and treated in seawater (SW).

Figure 6. Optical microscopic surface images for pure calcite chips immersed in a) CaCl2·2H2O, b) MgCl2·6H2O and c) Na2SO4 without aging in model oil compared to calcite chips aged in model oil and then immersed in the same brines d) CaCl2·2H2O, e) MgCl2·6H2O and f) Na2SO4.


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Figure 7. SEM image and positions of EDS spectra on different locations on a dolomite chip aged in model oil and immersed in seawater (SW).


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Figure 8. AFM height images for calcite (left) and dolomite (right) chips aged in model oil and immersed in the Arabian Gulf seawater (SW).


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Figure 9. SEM images on cleaved calcite chips aged in model oil and immersed in diluted seawater (a) and diluted seawater brines with increased concentrations of Ca2+, Mg2+ and SO42- ions (b, c and d). Red squares indicate positions on which the AFM height images were taken (Figure 10). EDS results of selected spectra are shown in Table 8.


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Figure 10. AFM height images on plain surfaces on aged calcite chips exposed to SW*4Ca (a), SW*2Mg (b) and SW*2SO (c) brines respectively as indicated by the red squares in Figure 9.


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Figure 11. SEM images on cleaved dolomite chips aged in model oil and immersed in diluted seawater brine (a) and diluted seawater brines with increased concentrations of Ca2+, Mg2+ and SO42- ions (b, c and d). EDS results of selected spectra are shown in Table 9.


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