Impact of Sulfate Ions on Wettability Alteration of Oil-Wet Calcite in the

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Impact of Sulfate Ions on Wettability Alteration of Oil-wet Calcite in the Absence and Presence of Cationic Surfactant Mahvash Karimi, Rashid S. Al-Maamari, Shahab Ayatollahi, and Nasir Mehranbod Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02175 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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Impact of Sulfate Ions on Wettability Alteration of Oil-wet Calcite in the Absence and Presence of Cationic Surfactant Mahvash Karimi1, Rashid S. Al-Maamari2,∗, Shahab Ayatollahi3 and Nasir Mehranbod1

1

School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran

2

Petroleum and Chemical Engineering Department, Sultan Qaboos University, Muscat, Oman.

3

School of Petroleum and Chemical Engineering, Sharif University of Technology, Tehran, Iran

KEYWORDS: Wettability alteration, Oil-wet, Carbonate, Sulfate ion, Surfactant.

∗ Corresponding author at: Department of Petroleum and Chemical Engineering, College of Engineering, Sultan Qaboos University, Muscat, Oman. Tel.: +96824141361. E-mail address: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The modification of the surface wetting characteristics in fractured oil-wet carbonate reservoirs, by reversing wettability from oil-wet to water-wet leads to improved oil recovery. However, in order to obtain a successful oil recovery process, it is crucial to understand the active mechanisms of wettability alteration. This study looks at the effect of sulfate ions as one of the most promising wettability influencing ions on the wetting properties of oil-wet calcite; the effect is studied both with and without the presence of cationic surfactant and possible mechanisms of wettability alteration are explored. A number of analytical techniques were utilized to analyze the mineral surface before and after treatment. The study presents a thorough discussion of the influence of sulfate ions in displacing adsorbed carboxylate from the oil-wet surface, both with and without the presence of cationic surfactant are discussed thoroughly. 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. Ion-exchange between the hydroxide group and the adsorbed stearate ion on the calcite surface is shown to act as a supplementary mechanism that desorbs stearate ions from the surface. In the treatment of an aged calcite surface with sulfate ions, a combination of these two mechanisms resulted in a more water-wet surface. The co-presence of sulfate ions and cationic surfactant resulted in a further reduction in the amount of adsorbed carboxylate on the surface. This can be attributed to the release of adsorbed carboxylate groups on the surface through ion pair formation with the cationic surfactant. The desorption of negatively charged carboxylate groups from the surface facilitates the approach of negatively charged sulfate ions to the aged calcite surface. It can therefore be concluded that sulfate ions


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accompanied by cationic surfactant molecules can alter the wetting properties towards water-wet state more effectively than sulfate ions alone.

1. INTRODUCTION Most carbonate reservoirs are characterized as neutral to preferential oil-wet and highly fractured with the chemical adsorption of carboxylic acids from crude oil onto the carbonate surface resulting in their oil-wet nature 1. In general, conventional water flooding has not proved successful in producing incremental oil from oil-wet fractured reservoirs, making oil recovery from them a great challenge 2-3. The degree of success of enhanced oil recovery (EOR) methods depends on the extent to which the spontaneous imbibition of water will displace oil from the matrix blocks to the fractures 4-6. It has been demonstrated that the wetting preference of the reservoir rock significantly affects oil recovery during the imbibition process

1,7-9

. Several researchers have reported that reversing

surface wetting characteristics from oil-wet to water-wet can lead to improved oil recovery in fractured reservoirs

3,10-16

. Surface active compounds as well as wettability influencing ions can

alter wettability and create a more favorable wetting state 4,10. It is widely believed that cationic surfactants can act as wettability modifying agents by removing adsorbed carboxylate groups from the surface and thus enhancing the spontaneous imbibition of water into the carbonate cores

10,12,17-19

. The injection of modified brine is also an

economical method for enhancing oil recovery. The modified brine used, known as smart water, contains a proper concentration of wettability influencing ions. Calcium, Ca2+, magnesium, Mg2+, and sulfate, SO42- ions have been reported to be effective in influencing wettability in oil-


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wet carbonate surfaces

3,12-13,20-21

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. The interaction between these ions and the adsorbed organic

compounds on the surface, might lead to the favorable wettability alteration. A recent study of ours

22

, examined the impact of Mg2+ on the wetting properties of oil-wet carbonate surface, as

well as the possible mechanisms of wettability alteration and found that magnesium divalent ions could remove adsorbed carboxylate materials from the surface via an ion-exchange mechanism and form a complex with the carboxylate group. It has been shown that sulfate is also one of the most promising ions for improving the water-wetness of carbonate surface. In a series of studies, for instance, Hamouda et al., have shown that sulfate ions are able to displace adsorbed fatty acids from the calcite surface and thus reduce the oil-wetness of the rock surface 9,15,23-25. Strand et al.17 have similarly pointed out that the addition of sulfate to the imbibing fluid in oil-wet chalk cores leads to improved oil recovery during the spontaneous imbibition process. Zhang et al.13 have also suggested that sulfate ions can adsorb onto the positively charged sites on the chalk surface and hence reduce its positive charge. This decrease in the electrostatic repulsion between the chalk surface and divalent cations causes an increase in the adsorption of calcium and magnesium ions on the surface. In this way, Ca2+ and Mg2+ are able to react with the adsorbed organic compounds on the surface and remove them from the oil-wet chalk surface. Hence, sulfate ions can act as a catalyst in the wettability alteration process

13,20,26

. It has also

been reported that sulfate ions alone are able to modify the wettability of chalk cores, even without the presence of surfactant 27. The combination of surfactant and smart water as an imbibing fluid to alter wettability and create a more favorable wetting state can result in high oil recovery from oil-wet carbonate reservoirs. However, to achieve the most effective oil recovery process, it is crucial that we understand the active mechanisms of wettability alteration. Although many researchers, as noted earlier, have


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suggested mechanisms for interpreting wettability alteration data obtained from contact angle measurements and/or imbibition experiments, the mechanisms of the process were still not fully understood. The present study is an attempt to shed more light on these mechanisms. This is done by the presentation of experimental data on the effect of sulfate ions on the wetting properties of the oil-wet calcite and by a discussion of the interactions between the oil-wet calcite surface, sulfate ions and cationic surfactant molecules. Different analytical tools were utilized to provide a clear insight into the effect of the sulfate ions on the wettability of the oilwet carbonate surface, used both with and without cationic surfactant. Calcite crystals (as a proxy for carbonate rock surface), a solution of stearic acid in n-decane (as model oil), sodium sulfate solutions and a cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), were utilized. The investigation is divided into two parts. The first part discusses the influence of sulfate ions on displacing adsorbed carboxylate from the oil-wet surface when no surfactant is used, while the second part examines the impact of sulfate ions, used along with surfactant, on the wettability alteration of oil-wet calcite. Contact angle measurements were performed before and after the treatment of the calcite surfaces in order to assess alterations in their wettability. Thermo gravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray spectroscopy (EDS) and zeta potential measurements were also used, so as to characterize the mineral surface.

2. MATERIALS Calcite crystals were used as a proxy for the carbonate rock surface, with “Iceland spar” calcite being provided by Ward’s Science. The calcite crystals were cut along their cleavage planes and


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then they were smoothed with a series of coarse and fine polishing plates using several grades of abrasive diamond wheels (120, 400 and 800 grit) 28. The calcite surfaces were then washed with DIW and dried overnight at 40 °C. The wetting preference of each piece of calcite crystal was checked before and after the polishing process by placing a drop of DIW on the top of the surface. There was no obvious change in the wetting properties of the surfaces during the cleaning process. Contact angle measurements were performed on the calcite crystals, while the calcite powder, whose particle size was less than 6 µm, was used in the FTIR, TGA and zeta potential experiments. Stearic acid was also used to represent the natural fatty acids in crude oil. A solution of 0.01 M stearic acid in n-decane was used in this study as a model oil for modifying the wettability of the calcite surface

9,19,23-24,29

. Both the stearic acid and the n-decane were supplied by BDH

chemicals (purity>99%). In order to investigate the effects of sulfate ions on the wettability of oil-wet calcite, anhydrous sodium sulfate (Na2SO4, >99%, Surechem products) was dissolved in distilled de-ionized water (DIW), with 0.01 and 0.10 M solutions being prepared. These solutions were applied both with and without the presence of a cationic surfactant. The cationic surfactant studied was dodecyltrimethyl ammonium bromide (DTAB), ([CH3(CH2)11N(CH3)3]Br, 99% purity, Acros OrganicsTM). The concentration of DTAB was fixed at 0.1 wt. % for all the experiments in this study.


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3. METHODS

3.1. Adsorption of Fatty Acids on Calcite Surface (Aging). The calcite crystals were placed in deionized water for 1 h at 75 ˚C. The samples were then removed from the water and immersed in the model oil (0.01 M stearic acid in n-decane solution) for 48 h at 75 ˚C. Thereafter, the calcite surfaces were removed from the model oil and immediately rinsed with DIW and nheptane and then air dried

23,25

. After the calcite surfaces had been aged in the model oil, the

wettability of the surface was expected to alter from a strongly water-wet state to an oil-wet condition. FTIR, TGA and zeta potential experiments were then carried out using calcite powder, as explained earlier. To alter the wettability of the calcite powder, 1 g of calcite powder was dispersed in 10 ml of DIW. 10 ml of model oil was added to this mixture, which was then agitated for 48 h at 75 ˚C. A centrifugation process at 5300 rpm for 30 min was then used to separate the solid powder from the liquid phase and the resulting oil-wet powder was dried overnight in an oven at 40 ˚C.

3.2. Surface Treatment. The effect of the sulfate ions on the wettability of an oil-wet calcite surface was then investigated, with the calcite crystals and the calcite powder being treated separately for the different measurements. A set of contact angle measurements were performed after the surface was treated with two different concentrations of sodium sulfate (0.01 M and 0.10 M); each treatment was done first alone and then with the addition of DTAB. The calcite


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crystals were soaked in these four different aqueous solutions for 48 h at 75 ˚C. The four sets of calcite crystals were then washed with DIW and air dried in preparation for the contact angle measurements described below.

The oil-wet powder was then prepared for the FTIR, TGA and zeta potential measurements, which aimed to examine both the impact of the sulfate ions alone on the wettability alteration of aged calcite surfaces, and also the impact of surfactant and sulfate ions together. The oil-wet powder was treated with two different sodium sulfate solution (0.01 M and 0.10 M ) for 48 h at 75 ˚C; each treatment was done first alone and then with the addition of DTAB. The powder was then separated from the liquid suspensions by centrifuging them at 5300 rpm for 30 min. The powder was dried overnight at 40 ˚C in preparation for the measurements.

3.3. Contact Angle Measurements. For all the calcite surfaces examined, the surface was immersed in the n-decane and then a droplet of DIW was placed on the surface. The water contact angle in n-decane was measured using a Krüss drop shape analyzer (DSA-100, Krüss, Germany) with the measurement resolution of ±0.1˚. After placing the drop of DIW on the surface, and the contact angle monitored every minute until the change of contact angle was less than 0.2˚.The numerical values reported for the contact angles in this study were an average of three separate measurements performed at four different locations on the surface with the overall uncertainty of these measurements estimated to be ±7˚. In addition, the pH of the aqueous solutions was measured before and after the surface treatment; this was done using a SevenMulti™ S47- dual pH / conductivity meter (Mettler Toledo, United States) with an accuracy of ±0.002.


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3.4. Thermo Gravimetric Analysis (TGA). The treated and untreated samples were also examined by thermo gravimetric analysis in order to evaluate the extent of the wettability alteration caused by sulfate ions, both with and without the presence of surfactant. The amount of carboxylate adsorbed onto the calcite surface before and after treatment was calculated from the weight loss measured by TGA. A PerkinElmer STA 6000 with the balance resolution of ±0.1 µg, was used to record the weight loss as a function of temperature. The samples were placed in an alumina pan and heated at 10 ˚C/min 19 under a constant flow of air (40 ml/min) 9,24,29-30 from 30 up to 900 ˚C.

3.5. Fourier Transform Infrared (FTIR). Fourier transform infrared (FTIR) spectroscopy was then performed to evaluate the structural changes to the functional groups during the surface treatment and also to analyze the adsorbed compounds on the calcite surface. FTIR spectra were recorded in the range of 400–4000 cm−1 wave number, with PerkinElmer FT-IR Spectrometer Frontier. For the FTIR analysis, 1 mg of sample was mixed thoroughly with 99 mg of potassium bromide. The mixture was then compressed for 15 min using a pellet die and applying a force of 10 tons. All the spectra were obtained by averaging 80 scans at a resolution of 4 cm−1.

3.6. Zeta Potential Measurements. Zeta potential measurement is used as an approximation of the surface potential

31

and a comparison of zeta potential values before and after treatment can

give some insights into the electrical changes of the surface during the adsorption process. The zeta potential of the calcite powder before and after it was treated with different solutions was measured with a MICROTRAC Nanotrac Wave particle size analyzer. The 1 wt. % calcite suspension was prepared for this analysis by mixing 0.2 g of calcite powder with 20 ml of DIW.


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The samples were then placed in a sonication bath to provide better dispersion 32-36. The mixture was homogenized by means of an ultrasonic bath for 30 min. After sonication, the suspensions were allowed to rest for an hour before the measurements were performed relatively coarser particles to settle

32,37-38

32

, allowing the

. Several studies have reported that, due to the similar

chemical nature of various sizes of particles, the zeta potential of the coarser particles is the same as that of the smaller particles

37,39-41

. The average of the two measurements is reported as the

expected zeta potential. Since the pH of the formation brine in carbonate reservoirs is slightly basic (≈8)

12,19

, the pH of

the suspensions was adjusted to a similar level utilizing 0.01 M NaOH or 0.01 M HCl solutions 33

as necessary.

3.7. Energy Dispersive Spectroscopy (EDS). Energy dispersive X-ray spectroscopy was used to provide the chemical characterization and the elemental analysis of the treated calcite powder. The EDS analysis was obtained by using a combined EDS and scanning electron microscope (SEM) (JEOL field emission scanning electron microscope JSM-7600 F). In all cases, the accelerating voltage was 20 kV, the beam current was 8 nA and the working distance was 8mm.


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4. RESULTS AND DISCUSSION 4.1. The Displacement of Carboxylate Compounds by Sulfate Ions in the Absence of Surfactant. Carbonate surfaces are usually positively charged at a pH lower than 8-9.5

1,42

and

act like weak bases. Accordingly, the acidic compounds in crude oil can adsorb onto the carbonate surface and alter its initial surface wetting properties

43

. Several studies have

concluded that carboxylic acids are adsorbed onto the calcite surface irreversibly

44-46

. This

research has used stearic acid as the wettability modifying agent, given that it is known to be one of the most important wettability altering agents in crude oil was utilized as in this research 24,29,47-48

. Mihajlović et al.49 showed that stearate groups are chemically adsorbed on the calcite

surface and form a chemisorbed stearate layer there. The initial state of the calcite surface is strongly water-wet. However after aging the calcite surface in the model oil, the wetting properties changed from a strongly water-wet state with an average contact angle of 18 ˚ to an oil-wet state with an average contact angle of 145˚. The current study investigated the effect of sulfate ions on the wettability of an oil-wet calcite surface by performing a set of contact angle measurements. Figure 1(a) represents the contact angle behavior of the oil-wet calcite surface after its treatment with solutions of sodium sulfate at different concentrations. After the aged calcite surface was soaked in DIW, the contact angle decreased slightly from 145˚ to 125˚. Treatment with sodium sulfate solutions of 0.01 and 0.10 M, altered the oil-wet surface gradually to a water-wet preference, with contact angles of 99˚ and 76˚ respectively. The results shown in Figure 1(a) are in line with the reported contact angle values in the literature 25. As the concentration of sulfate ions increased, the contact angle value decreased and the wettability of the surface shifted towards water-wetness.


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Wettability alteration of the surface was accompanied by a change in the pH of the treating solutions, with the initial and final pH values of the solutions illustrated in Figure 1(b). Soaking the aged calcite in DIW resulted in an increase in the pH value of the solution from 5.8 to 8.7. The partial dissolution of the calcite surface in DIW, which is expressed by the reaction of the equation (1), results in the release of hydroxide (OH−) ions, and it is this which causes the increase in pH: + ⇄ + +

(1)

The initial pH of both 0.01 M and 0.10 M sodium sulfate solutions is higher than that of DIW. This is due to the following reaction which leads to an increase in the concentration of hydroxide ions and hence an increase in pH: + ⇄ 2 + +

(2)

As shown in Figure 1(b), soaking the oil-wet calcite surfaces in sodium sulfate solutions causes a significant increase in the final pH of the treating solutions, which indicates the enhancement in hydroxide concentration.


12

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(a)

(b)

“Figure 1. (a) Contact angle values for oil-wet calcite surface after treatment with DIW and sodium sulfate solutions and (b) Initial and final pH values of the treating solutions.”

To verify the reliability of the contact angle measurements, TGA was used to measure the amount of adsorbed stearate on the calcite surface before and after treatment. Figure 2 presents the TGA results of calcite, aged calcite and aged calcite treated with a 0.10 M sodium sulfate solution. As shown in Figure 2, there are three main steps in the temperature ranges of 30-700 ˚C. The first step, which takes place in the temperature range of 30-210 ˚C, is associated with the removal of physically adsorbed materials from the surface. The second step, which is between 210 and 400 ˚C, is related to the removal of the chemically adsorbed materials on the surface. The amount of stearate adsorbed was estimated from the sample weight loss in the second step.


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The third step is the weight loss due to calcite decomposition which commences at about 570 ˚C. This observation of three main steps for weight loss versus temperature is consistent with data in the literature 9,19,24,29-30,47,50.

The TGA curve of the calcite sample consists of one step, which starts from 570 ˚C onwards. The single sharp weight loss is a confirmation of the purity of the sample. The weight loss for fresh calcite demonstrates the decomposition of calcium carbonate (calcite) into calcium oxide and carbon dioxide. After the calcite surface was aged in model oil, three significant weight loss steps can be recognized in the TGA curve, described earlier. The amount of adsorbed stearate on the calcite surface can be estimated using the weight loss in the temperature range of 210-400 ˚C; this is 0.5723 %. Obviously, the aged calcite sample contains a greater amount of adsorbed stearate on the surface. After the aged calcite was treated with 0.10 M sulfate solution, the weight loss due to stearate decomposition was 0.4000 wt. % which corresponds to a reduction of 30 % of the adsorbed stearate on the surface. The TGA analysis is thus in harmony with the contact angle results; both show that the presence of sulfate ions enhances the partial removal of stearate ions from the surface.


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“Figure 2. TGA analysis of calcite, aged calcite and aged calcite treated with 0.10 M sodium sulfate solution”

Based on the contact angle measurements and TGA results, it is obvious that sulfate ions are able to partially release the adsorbed stearate from the surface and then modify the surface wetting state towards water-wetness. Based on the above results, three different mechanisms can be proposed to explain the wettability alteration by sulfate ions; these are depicted pictorially in Figure 3: 1) An ion-exchange between sulfate and/or bisulfate ions and adsorbed carboxylate ions on the surface. This results in the release of carboxylate from the surface (Figure 3(a)) 2) The interaction between the sulfate and/or bisulfate ions and the calcium that is attached to the stearate ion, which leads to the removal of calcium ions along with attached carboxylate from the surface (Figure 3(b)) 3) An ion-exchange between the hydroxide ions and the adsorbed stearate ions on the calcite surface, a process which results in the displacement of the stearate ions from the surface (Figure 3(c)).


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(a)

(b)

(c)

“Figure 3. Schematic view of the proposed wettability alteration mechanisms of oil-wet calcite caused by sulfate ions: a) An ion-exchange between sulfate and/or bisulfate ions and adsorbed carboxylate ions on the surface, b) The Interaction between sulfate and/or bisulfate ions with calcium on the surface, and c) An ion-exchange between hydroxide ion and adsorbed stearate ion on the calcite surface.”


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Three other characterization methods, FTIR, EDS and zeta potential were employed to provide complementary evidence about the surface wettability alteration mechanisms, and also to investigate the performance of sulfate ions facing oil-wet calcite surface. Infrared spectroscopy is a powerful technique for observing oscillations arising from the vibration of molecules 51-54. A change in bond length (stretching) or bond angle (bending) results in changes in the vibration of a molecule. The vibrational motions of a molecule reflect characteristics of chemical bonds within the molecule, a fact which can be used to identify the structure of a molecule and also to investigate the interaction of the molecules with the surrounding environment

52

. Accordingly, FTIR spectroscopy was carried out to identify the

functional groups in the molecules and to analyze the adsorbed compounds on the calcite surface. Figure 4 demonstrates the FTIR absorbance spectra of fresh calcite, aged calcite and aged calcite treated with 0.10 M sodium sulfate solution within the wavenumber range of 4000 to 400 cm-1. In the spectrum presented for fresh calcite (Figure 4(a)), the absorption bands of the carbonate group are clear. There are four characteristic infrared bands for pure calcite (calcium carbonate): C─O symmetric stretching centered at 1799.54 cm-1, C─O asymmetric stretching located at 1429.78 cm-1, out-of-plane bending centered at 875.76 cm-1 and in-plane bending vibration located at 712.75 cm-1. A broad, symmetrical and smooth band at 1429.78 cm-1 is one of the main characteristic infrared bands of carbonate group produced by C─O stretching vibration. However, two sharp bands in the range of 900-700 cm-1 referred to the bending vibrations of the carbonate group. Depending on the attached cation, these bands indicate measurable frequency deviations. For calcium carbonate crystal, the bending vibrations are located at 875.76 cm-1 and 712.75 cm-1. These observations are in excellent agreement with the data in the literature 51,53-58.


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The water molecules adsorbed on the surface or incorporated in the lattice structure of a crystalline compound produce characteristic sharp band in the 3800–3200 cm-1 due to O–H stretching

51,53

. The absorption band at 3437.95 cm-1 is attributed to the stretching vibration of

the hydroxide group (OH−) which is referred to the water molecules adsorbed on the surface. A comparison of the FTIR spectra of calcite before and after it was aged in model oil (Figures 4(a) and 4(b)) reveals that several absorption bands were added to the calcite spectrum due to the adsorption of carboxylate on the surface after the aging. The doubled peaks at 1572.56 cm-1 and 1539.9 cm-1 are ascribed to CO asymmetric stretching vibration 51. The adsorption of stearate on the calcite surface also yields an additional four distinct bands at 2956.17 cm-1 (ascribed to methyl symmetric C─H stretching vibration), 2917.12 cm-1 (assigned to methylene asymmetric C─H stretching), 2874 cm-1 (ascribed to methyl asymmetric C─H stretching), and 2850.04 cm-1 (referred to methylene symmetric C─H stretching vibration)

53,57

. The intense symmetrical peak

centered at 1429.78 cm-1 in the FTIR spectrum of pure calcite deviates from symmetry in the FTIR spectrum of aged calcite surface (Figures 4(b)). This asymmetry indicates the presence of a mixture as well as overlapping of the characteristic in-plane C─O─H bending vibration of carboxylic acid centered at 1430 cm-1. The FTIR spectrum of aged calcite found here is in line with the findings of Osman and Suter 59. The peak at 3436 cm-1 assigned to the O–H stretching vibration is evidence of the presence of adsorbed water on the aged calcite surface.


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(a)

(b)

(c)

“Figure 4. FTIR spectra (absorbance vs. wavenumbers) of (a) calcite surface, (b) aged calcite surface and (c) aged calcite surface after treating with 0.10 M sodium sulfate solution”


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Figure 4(c) presents the FTIR spectrum for the aged calcite after it was treated with the 0.10 M sodium sulfate solution. As shown in this figure, four main absorption bands of calcite (described earlier) are present in the spectrum. There was a decrease in the intensity of the methyl C─H stretching vibration (symmetric and asymmetric vibrations at about 2956 and 2874 cm-1, respectively) as well as in that of the methylene C─H stretching absorption bands (symmetric and asymmetric vibrations at around 2850 and 2917 cm-1, respectively) after the treatment of the surface with sulfate ions. This indicates that sulfate ions partially remove adsorbed stearate from the calcite surface. The CO asymmetric stretching vibration band at 1539.9 cm-1 disappeared from the FTIR spectrum after treatment of aged calcite with sulfate ions which is due to the partial desorption of stearate ions from the surface. However, the sulfate ions produced characteristic bands in the wavenumber ranges of 1130-1080 and 680-610 cm-1

53

and there was

no change in the general pattern of the FTIR spectrum before and after the surface treatment. Moreover, the absence of any band within the wavenumber ranges of 1130-1080 and 680-610 cm-1 in the FTIR spectrum is enough evidence to conclude that no sulfate adsorption took place on the aged calcite surface. This conclusion was confirmed by performing a SEM-EDS analysis on the aged calcite sample after it had been treated with the 0.10 M sodium sulfate solution. SEM-EDS is an alternative and sensitive analytical tool that provides multi-elemental determination

60

. Figure 5 shows one of the EDS spectra as a representative of the entire

specimen among all the spectra which were obtained at different points of the pure calcite as well as the aged calcite powder after its treatment with the sodium sulfate solution described above (Other spectra are not shown here). The sample was composed of oxygen, calcium and carbon. The weight percentage of oxygen, calcium and carbon in pure calcite (Figure 5(a)) is 47.5%, 35.5 % and 17.1 % respectively, while at this point of the treated sample (Figure 5(b)), the


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weight percentages were 48.2 % for oxygen, 32.3 % for calcium and 19.4 % for carbon. The difference in the weight percentage of the elements in pure calcite and in the sample shown in figure is due to the presence of stearate ions in the sample. The presence of stearate group causes a reduction in the composition of calcium and an increase in composition of carbon and oxygen. No sulfur was found in the sample which indicates that there was no adsorption of sulfate ions on the surface. This observation is in agreement with the FTIR results.


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(a)

(b) “Figure 5. EDS spectrum of (a) pure calcite and (b) aged calcite sample after treating with 0.10 M sodium sulfate solution.”


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Zeta (electrokinetic) potential in the colloidal dispersion is defined as the electric potential difference between a dispersion medium and the stationary layer of fluid attached to the dispersed particles. Jackson and Vindogrado

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have demonstrated that wettability alteration

results in a measurable change in the value of zeta potential. Measurement of the zeta potential of the calcite samples after different type of treatments can be helpful in explaining the interactions between the aged calcite surface and its surrounding environment. The results of the zeta potential measurements are presented in Table 1. The zeta potential value for the pure calcite suspension was positive (+29.6 mV), a finding which is in line with observations found in the literature 62-64. A comparison of the zeta potential of pure calcite and that of the aged calcite suspension indicates that the adsorption of stearate ions on the calcite surface resulted in a negative zeta potential (-33.4 mV). The zeta potential of the aged calcite treated with 0.01 M sodium sulfate solution was -23.9 mV, and when more sulfate ions were added to the suspension, the zeta potential became more negative. In line with the results reported by Kasha et al.65, our study showed that sulfate ions were able to decrease the zeta potential of aged calcite particles. Zhang and colleagues also introduced the idea that sulfate ions can act as one of the potential determining ions and affect the surface charge of a chalk surface. They have also shown that the zeta potential of a chalk suspension decreased as the sulfate concentration in the suspension increased 3,13,66.


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“Table 1. Zeta potential of the aqueous dispersions” Sample Zeta Potential (mV) Calcite +29.6 Calcite after aging in model -33.4 oil (aged calcite) Aged calcite after treating with 0.01 M solution of -23.9 sodium sulfate Aged calcite after treating with 0.10 M solution of -36.4 sodium sulfate

The data given above allowed us to come to a number of conclusions about the three proposed mechanisms , shown in Figure 3 earlier in this section, by which sulfate ions (without surfactant) can alter the wettability of aged calcite. Both the FTIR and SEM-EDS analysis presented no sign of adsorbed sulfate on the surface, an absence which leads us to conclude that the ion-exchange between sulfate and/or bisulfate ions and adsorbed carboxylate ions is not the active mechanism that removes the carboxylate group from the surface. This conclusion disproves the first mechanism proposed in Figure 3(a). Moreover, if the second mechanism proposed (Figure 3(b)) was the active mechanism of stearate desorption from the surface, the net charge of the surface would become more negative as a result of the desorption of calcium ions along with the attached stearate with the net positive charge from the surface. However, the insignificant change of the net charge of the surface results from the ion-exchange between the hydroxides ion and adsorbed stearate ions on the surface which supports the predominance of the third proposed mechanism (Figure 3(c)). Here, the addition of sodium sulfate to the calcite/DIW system did cause an increase in the pH of the solutions indicating an enhancement in the hydroxide concentration (Equation (2)). As the hydroxide concentration increased, the possibility of the ion-exchange between the hydroxide and the adsorbed stearate ions on the calcite surface


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increased. The zeta potential measurements of the aged calcite demonstrated that the surface zeta potential increases after the addition of 0.01 M sulfate ions, but then decreases as the concentration of sulfate ions in the suspension is increased to 0.10 M. This suggests that at low concentration of sulfate ions, the second and third mechanisms are the active ones for wettability alteration during the treatment. However, a lower concentration of hydroxide in the solution, compared to divalent negatively charged sulfate anions, suggests that the interaction between sulfate ions and calcium ions on the surface is the main active mechanism of wettability alteration at high concentration of sulfate ions. In addition, the ion-exchange between hydroxide and adsorbed stearate ions on the calcite surface acts as a supplementary mechanism for desorbing stearate ions from the surface. It can therefore be concluded that a combination of the second and third proposed mechanisms results in a more water-wet surface, when the aged calcite surface is treated with sulfate ions.

4.2. The Displacement of Carboxylate Compounds by Sulfate Ions in the Presence of Surfactant. As discussed earlier, sulfate ions can act as a wettability modifying agent that can alter the wetting preferences of the oil-wet calcite surface towards more water-wet state. However, the study of the behavior of the sulfate ions, in the presence of cationic surfactant facing oil-wet calcite requires further analysis, which will be discussed in this section. Figure 6 illustrates the wettability alteration of oil-wet calcite caused by sulfate ions along with the cationic surfactant, DTAB, as well as the initial and final pH of the treating solutions. A contact angle of 89˚ for the control surface treated only with DTAB indicates a partial displacement of stearate from the surface. It is widely believed that cationic surfactants can reverse the wettability of oil-wet carbonate surfaces

2,10,17-19,67

. The mechanism of wettability alteration has


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been experimentally verified as the irreversible desorption of carboxylate materials from the surface caused by ion pair formation between surfactant monomers and organic carboxylate compounds 19. The cationic surfactant adsorbs on the oil-wet surface via electrostatic interactions between the positively charged head of the surfactant and the negatively charged carboxylate and then forms ion pair which results in the release of the adsorbed carboxylate from the surface 2,10,17

. Hydrophobic as well as electrostatic interactions stabilize the ion pairs in the solution

10

.

The pH of the control solution is increased after treatment due to partial dissolution of the calcite.


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(a)

(b)

“Figure 6. (a) Contact angle values for oil-wet calcite surface after treatment with sodium sulfate solution in the presence of cationic surfactant and (b) Initial and final pH of the treating solutions.”

Figure 6 also shows that contact angle values decreased when sulfate ions were added to the surfactant solution. A contact angle of 76˚ was obtained, when the surface was treated with a 0.01 M sodium sulfate solution, but a higher concentration of sodium sulfate solution led to a further reduction in the contact angle value. The most significant change was the decrease of contact angle value to 58˚ when using a 0.10 M sodium sulfate solution in the presence of DTAB. Despite the effectiveness of the sulfate ions in altering the wettability of oil-wet calcite,


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the original wetting state of the calcite surface has not been attained. As a result of the calcite partial dissolution, the pH for both sodium sulfate solutions was increased significantly. A comparison between the contact angle measurements in the absence and presence of DTAB makes it clear that the combination of sulfate ions with cationic surfactant has led to a more water-wet condition. The TGA analysis of aged calcite treated only with DTAB and aged calcite treated with a 0.10 M sodium sulfate solution in the presence of DTAB are compared in Figure 7. This figure shows that stearate decomposition commenced above 210 ˚C; the weight loss associated with the decomposition occurred at the same temperature range (210-400 ˚C) for both samples. In the aged calcite treated with DTAB, the adsorbed stearate is estimated to be 0.3441 %, as the cationic surfactant causes partial removal of the adsorbed stearate from the calcite surface. This observation is in excellent agreement with the findings of Jarrahian et al.19. However, the copresence of sulfate ions and cationic surfactant resulted in further reduction of the amount of adsorbed stearate on the surface.

“Figure 7. TGA analysis for aged calcite treated with DTAB and aged calcite treated with DTAB along with 0.10 M sodium sulfate solution.”


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The TGA analysis showed that about 30% of adsorbed carboxylate was removed from the surface when treating the aged calcite with sulfate ions alone, while 69 % of the initially adsorbed stearate on the surface could be removed by the use of sulfate ions in combination with DTAB. The observed trend is in a good agreement with the contact angle data, where the amount of adsorbed stearate on the surface of the aged calcite decreased from 0.5723 % to 0.4000% and 0.1785% for the aged calcite treated with 0.10 M Na2SO4 and 0.10 M Na2SO4-DTAB, respectively. It was also observed that, in the range of 210-400 ˚C, the minimum weight loss was for the aged calcite treated with a 0.10 M sodium sulfate solution which also contained DTAB. It is clear that sulfate ions in combination with cationic surfactant were able to displace adsorbed carboxylate more effectively than the sulfate ions alone. The other surface characterization techniques (FTIR, EDS and zeta potential) were utilized to give further insight into the surface wettability alteration behavior of sulfate ions in the presence of cationic surfactant. FTIR spectra of an aged calcite surface after interaction with a DTAB solution first alone and then in combination with a 0.10 M sodium sulfate solution are shown in Figure 8. The treatment of the aged calcite surface with DTAB alone led to no additional peak in FTIR spectrum compared to the original spectrum of the aged calcite surface, which suggests that no adsorbed DTAB existed on the samples. In other words, the absence of any absorption band in the 1220-1020 cm-1 region which was assigned to amines with aliphatic C─N (available in the head group of the surfactant) indicates that no DTAB molecules adsorbed on the surface. After the sample was treated with DTAB solution alone, the intensity of the bands attributed to methyl and methylene C─H stretching vibration in the 3000-2800 cm-1 region was reduced compared to that of the bands for the untreated aged calcite spectrum shown in Figure 4(b). This confirms that cationic surfactant can partially displace stearate groups from the surface and alter


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the surface wettability towards more water-wet. Co-presence of DTAB and sulfate ions resulted in a further reduction of the intensity of the methyl and methylene C─H stretching vibration bands in the wavenumber range of 3000-2800 cm-1 compared to the aged calcite treated only with DTAB. Analogous to the FTIR spectrum of an aged surface treated with sodium sulfate solution in the absence of DTAB (Figure 4(c)), there is a lack of any characteristic bands in the 1130-1080 and 680-610 cm-1 regions, revealing that there has been no adsorption of sulfate ions on the calcite surface. The SEM-EDS analysis verifies this conclusion (Figure 9). The EDS spectrum presented is an example of the entire sample, with oxygen, calcium and carbon being its constituent elements. There is no sign of sulfur in this specimen, showing that the presence of adsorbed stearate on the surface has caused the elemental composition of this sample to deviate from that of a sample of pure calcite. Despite the efficient removal of adsorbed carboxylate from the surface, the FTIR spectrum of aged calcite after treatment with sodium sulfate solution in the presence of DTAB (Figure 8(b)) is slightly different from the spectrum for pure calcite (Figure 4(a)) indicating that the calcite surface could not completely retrieve its initial wetting state through the treatment. These findings are in line with both the TGA results and the contact angle measurements.


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(a)

(b)

“Figure 8. FTIR spectra (absorbance vs. wavenumbers) of aged calcite surface after treating with (a) DTAB solution and (b) 0.10 M sodium sulfate solution in the presence of DTAB.”


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“Figure 9. EDS spectra of aged calcite sample after treating with 0.10 M sodium sulfate solution in the presence of DTAB.”

Table 2 summarizes the zeta potential values for aged calcite after it was treated with cationic surfactant at two different concentrations of sulfate ions. An interaction between the cationic surfactant and the adsorbed stearate on the calcite surface (ion pair formation) resulted in the desorption of stearate ions from the surface, leading in turn to a significant increase in the zeta potential value. This increase in the zeta potential is evidence that there has definitely been stearate displacement rom the surface. The results show that the presence of 0.01 M of sulfate ions in the solution caused a decrease in the value of the surface potential and even changed the sign. When the concentration of the sulfate ions in the treating solution was increased, the zeta potential decreased further.


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“Table 2. Zeta potential of the aqueous dispersions of aged calcite after treating with cationic surfactant at different concentrations of sulfate ions” Sample Zeta Potential (mV) Calcite after aging in model oil -33.4 (aged calcite) Aged calcite after treating with +9.7 DTAB Aged calcite after treating with 0.01 M solution of sodium -8.11 sulfate in the presence of DTAB Aged calcite after treating with -25.1 0.10 M solution of sodium sulfate in the presence of DTAB

Strand et al.17 proposed a model to show the effect of sulfate on the wettability of oil-wet chalk when using cationic surfactant; the model was based on contact angle and spontaneous imbibition experiments. They noted that the chalk surface charge alters as a result of sulfate adsorption on water-wet zones and the desorption of carboxylate from the surface can therefore take place more easily. However, they also reported that sulfate ions are only effective at low temperatures, but at 70 ˚C and above their impact vanishes. In our study, the FTIR and EDS analysis both showed that no adsorption of sulfate ions occurred on the calcite surface. This means that, as also shown in section 4.1, the ion-exchange between sulfate and/or bisulfate ions and adsorbed carboxylate ions on the surface is not the active mechanism that removes carboxylate from the surface. In the light of the zeta potential results and other measurements, it can be concluded that it is rather the interaction between sulfate ions and calcium ions on the surface that plays the main role in modifying the wetting preferences of aged calcite surfaces. It can also be stated that ion-exchange between the hydroxide and adsorbed stearate ions on the calcite surface is a complementary mechanism that will desorb stearate ions from the surface


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both in the absence and in the presence of cationic surfactant. However, the combination of sulfate ions and cationic surfactant for the surface treatment process did lead to a significant change in surface wettability. The TGA analysis of aged calcite treated with 0.10 M sodium sulfate and DTAB showed a strong synergic effect between the sulfate ions and DTAB molecules, which resulted in the removal of carboxylate from the surface. As mentioned earlier, adsorbed carboxylate groups on the surface can be released via ion pair formation with a cationic surfactant. The desorption of negatively charged carboxylate groups from the surface resulted in a reduction of the negative charge of the surface. This process can facilitate the approach of negatively charged sulfate ions to the aged calcite surface and can therefore enhance the contact between the two. In this way, sulfate ions can more effectively alter the wetting properties of oilwet calcite and create a more water-wet state. Thus, when the oil-wet calcite is treated with a sodium sulfate solution in the presence of cationic surfactant, both the surfactant and the sulfate ions are effective in displacing adsorbed stearate from the surface and hence in obtaining a more water-wet surface.


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5. CONCLUSIONS This study has presented experimental data on the effect of sulfate ions on the wetting properties of oil-wet calcite and has explored several possible mechanisms of wettability alteration. Different analytical tools (contact angle measurements, TGA, FTIR, EDS and zeta potential measurements) have been used to explore the interactions between an oil-wet calcite surface, sulfate ions and DTAB molecules. This work has also examined the influence of sulfate ions on the displacement of adsorbed carboxylate from an oil-wet surface using the ions both alone and in the presence of surfactant. The results show that sulfate ions are indeed capable of improving the water-wetness of carbonate surfaces in both the presence and the absence of cationic surfactant. At low concentration of sulfate ions, the interaction between sulfate ions and the calcium ions attached to the stearate on the surface and the ion-exchange between the hydroxide group and adsorbed stearate ions on the calcite surface, are both active mechanisms of wettability alteration during the treatment. However, at high concentration of sulfate ions, the interactions between the sulfate ions and the calcium ions on the surface is the most active mechanism of wettability alteration; in this situation the ion-exchange between hydroxide and adsorbed stearate ion on the calcite surface can act as a supplementary mechanism that desorbs stearate ions from the surface. A combination of sulfate ions and cationic surfactant in the surface treatment process leads to a significant change in surface wettability. Both the cationic surfactant and the sulfate ions were found to be effective in displacing the adsorbed stearate from the surface, when oil-wet calcite was treated with sodium sulfate solution in the presence of cationic surfactant. The desorption of negatively charged carboxylate groups from the surface, due to ion pair formation with the cationic surfactant, results in a reduction of the negative charge of the aged calcite surface, which


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in turn can facilitate the approach of negatively charged sulfate ions to this surface. In the presence of cationic surfactant, the sulfate ions can more effectively alter the wetting properties of oil-wet calcite, making it more water-wet.

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