Potential Application of Silica Nanoparticles for Wettability Alteration of

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The Potential Application of Silica Nanoparticles for Wettability Alteration of Oil-Wet Calcite: A Mechanistic Study Abolfazl Dehghan Monfared, Mohammad Hossein Ghazanfari, Mohammad Jamialahmadi, and Abbas Helalizadeh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00477 • Publication Date (Web): 29 Apr 2016 Downloaded from http://pubs.acs.org on April 30, 2016

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The Potential Application of Silica Nanoparticles for Wettability Alteration of Oil-Wet Calcite: A Mechanistic Study Abolfazl Dehghan Monfared 1,2, Mohammad Hossein Ghazanfari 1,*, Mohammad Jamialahmadi 2, Abbas Helalizadeh 2 1

Chemical and Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran 2

Petroleum Engineering Department, Petroleum University of Technology, Ahwaz, Iran

ABSTRACT Oil recovery from carbonate reservoirs can be enhanced by altering the wettability from oilwet toward water-wet state. Recently, silica nanoparticles (SNP) suspensions are considered as an attractive wettability alteration agent in enhanced oil recovery applications. However, their performance along with underlying mechanism for wettability alteration in carbonate rocks is not well discussed. In this work, the ability of SNP suspension, in the presence/absence of salt, for altering the wettability of oil-wet calcite substrates to a waterwet condition was investigated. In the first step, to ensure that the properties of nano-fluids have not been changed during the tests, stability analysis was performed. Then, low concentration nano-fluids were utilized and transient as well as equilibrium behavior of wettability alteration process were analyzed through contact angle measurement. Moreover, a mechanism for wettability alteration process was proposed and verified with different tools. Results showed that the SNP suspensions could effectively change the wetness of strongly oil-wet calcite to water wet (e.g. from 156° to 41.7° at 2000 mg/L nano-fluid). This ability was enhanced by increasing concentration, time and salinity. Two equations were proposed to

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predict the equilibrium and transient contact angles with a good agreement. Analyzing the transient behavior of the wettability alteration indicated that the rate constant increased from 0.0019 to 0.0021 h-1 with the increase in nano-fluid concentration from 500 to 1000 mg/L. It was further increased to 0.0026 h-1 for 1000 mg/L in 0.05 M electrolyte solution. The partial release of carboxylate groups from the oil-wet calcite surface and their replacement with SNP was suggested to be the responsible mechanism for wettability alteration. Surface equilibria and interaction studies, Fourier transform infrared spectroscopy and scanning electron microscopy provided verifications in support of proposed mechanism. The enhanced wettability alteration in the electrolyte media was attributed to the role of Na ions facilitating the adsorption and release of SNP and stearates respectively. Also, the presence of electrolyte favorably affected the position of system’s equilibria. Keywords: oil-wet carbonate, wettability alteration, Silica nanoparticles, equilibrium, transient behavior, mechanism 1. INTRODUCTION To meet the high energy demand around the world, enhancing the production from oil reservoirs, as the main source of energy, is vital. More than half of the oil reserves are found to be located in fractured carbonate reservoirs. 1 Recovery from carbonate reservoirs by water flooding, as a common enhanced oil recovery (EOR) method, is inefficient. Initially, injected water sweeps the fractures. Because the wettability of the carbonate reservoirs is mainly regarded as neutral to preferential oil-wet, the imbibition of water into matrix blocks is prohibited due to negative effect of capillary forces.2, 3 Therefore, the performance of water flooding is related to the wettability of the reservoir rocks; i.e. changing the wettability of the carbonate reservoir rocks toward water-wet condition can improve the water flood efficiency.


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Different techniques have been addressed to alter the wetting state of the reservoir rocks. In addition to their ability to reduce the oil-water interfacial tension, some types of surfactants were considered to be wettability alteration agents. Several researchers have discussed the applications and mechanisms of surfactants for changing the wettability of the carbonate/sandstone reservoir rocks.4-8 In this area, two main mechanisms have been proposed: (a) interaction of the surfactant molecules with adsorbed crude oil component and stripping them off, (b) adsorption of the surfactant molecules onto oil-wet surface via hydrophobic interactions. However, the first mechanism was thought to be more effective.

6

Introduction of alkaline along with surfactants was also reported to increase their wettability alteration potential.

9

Moreover, studies revealed that the presence of potential determining

ions (i.e. Ca , SO

, Mg ) in the sea water had a dominant impact on the wettability

alteration and increasing oil recovery from the carbonate reservoirs. 10-12 Recently, it has been experimentally shown that nanoparticles (NP) could also be used as an effective wettability alteration agent. Lim et al. explored the effect of NP on altering the solid wettability.13 Kondiparty et al. performed experimental studies considering the role of structural disjoining pressure on the wetting and spreading of nano-fluids on solid surfaces.14 From EOR point of view, titania, zirconium dioxide, alumina and SNP were found to be effective for changing the wetting state of the reservoir rocks. Ehtesabi et al. used anatase and amorphous titanium dioxide NP to improve heavy oil recovery from sandstone cores. They reported that the wettability alteration toward water-wet condition was the underlying EOR mechanism.

15

Karimi et al. designed nano-fluids composed of zirconium dioxide NP and

mixtures of a nonionic surfactant and proved their effectiveness in the wettability alteration of the carbonate rocks.

16

Application of alumina-based nano-fluids as potential wettability

modifiers have been introduced by Giraldo et al.17 They utilized imbibition test and contact angle measurement to study the effect of nano-fluids on wettability. It was shown that the 3 ACS Paragon Plus Environment

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applied nano-fluids could beneficially change the wetting condition of the sandstone rocks from oil-wet to water-wet state.17 Compare to the other types of NP, the experimental studies on wettability alteration using SNP were more observed in the available literature. This can be due to some inherent advantages of silica such as their ability to be easily surface functionalized and some other benefits as noted by Miranda. 18 Onyekonwu and Ogolo tested the ability of three different polysilicon nanoparticles (PN); lipophobic-hydrophilic PN (LHPN), hydrophobic-lipophilic PN (HLPN) and neutral wet PN (NWPN) for EOR in sandstone. They reported that the HLPN and NWPN improved the oil recovery in water-wet rocks by wettability alteration and interfacial tension reduction mechanisms.19 Ju et al. presented theoretical and experimental investigations on change in wettability and permeability due to the adsorption of LHPN onto the sandstone rocks. They found that the LHPN suspension could enhance the oil recovery during water injection.

20, 21

In a series of works, Hendraningrat et al. studied some parameters (e.g. NP concentrations and core permeability and wettability) affecting the performance of hydrophilic SNP to enhance oil recovery from the sandstone rocks.22-24 More recently, Zhang et al. explored the role of a special type of silica nano-fluids for displacing crude oil from Berea sandstone and single-glass capillaries through imbibition tests. 25 In spite of several studies focused on the performance of SNP for altering the wettability of the sandstone rocks, only a limited literature is available reporting their application in carbonates. Also, the mechanism involved in such a system has not been discussed yet. The main scope of this work, in continue of our previous investigation 26, is therefore to present a mechanistic study on the potential application of low concentration silica nano-fluids for wettability alteration of strongly oil-wet calcite toward water-wet state.


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For this purpose, the initially water-wet calcite surface is rendered to oil-wet by a carboxylic acid solution. Then, the wettability reversal potential of SNP suspensions is tested using equilibrium and transient wettability alteration studies at different levels of ionic strength. To ensure that the properties of nano-fluids are remained unchanged during the experiments, the stability analysis is applied. In the next step, two equations are developed which can predict the contact angle measurement data with a good agreement. Finally, a mechanism is proposed to interpret the observed wettability alteration process. The suggested mechanism is verified by surface equilibria and interaction studies, Fourier transform infrared spectroscopy and scanning electron microscopy visualizations. 2. MATERIALS AND METHODS 2.1 Materials 2.1.1 Nanoparticles The utilized NP were ultrapure (99.999%) SNP provided from TECNAN (Navarrean Nanoproducts Technology, Spain). The surface of SNP was unmodified and received in the form of white powder. The morphology of NP was spherical with the nominal average size of 10-15 nm. Density and specific surface area of particles were 2.2 g/cm3 and 180-270 m2/g respectively. A TEM image of SNP is shown in Figure 1. 2.1.2 Oil Phase In order to have control on the oil phase properties, model oil comprised of stearic acid (SA) dissolved in n-heptane was used for all experiments. The concentration of SA in the model oil was 0.018 M.

4

The n-heptane (99.9% purity) and SA (>99% purity) were purchased from

Merck.


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2.1.3 Electrolyte The solution of sodium chloride (NaCl) in deionized water was used as electrolyte. The sodium chloride (99.5% purity) was supplied by Merck. 2.1.4 Carbonate Rock Sample The carbonate rock sample was received from Aligoodarz Mine located in south west of Iran. The main constituent of the rock sample was calcite and its purity was known to be as high as approximately 99%.26 The X-ray Diffraction (XRD) pattern of the rock sample, shown in Figure 2, was demonstrated to be in accordance with the pure calcite pattern. 2.2 Methods Figure 3 shows a schematic of the methodology applied in this study. The details of the procedures are described in the following sections. 2.2.1 Nano-fluid Preparation SNP suspensions were made by adding the desired amount of NP powder to deionized water. Magnetic stirring followed by the sonication process were utilized to prepare a homogenous and uniform colloidal suspensions. If only magnetic stirring was applied, the NP settled down in couple of seconds because it was not able to break the SNP aggregations and reduce their size in an efficient manner. Thus, the sonication process must be used thereafter. As a preliminary step four nano-fluids at different sonication times of 5, 10, 20 and 40 min were prepared. Then, the stability of suspensions were studied (see next section) to find an efficient time for sonication. Analysis revealed that the stability of nano-fluids was improved by increasing the sonication time. However, no significant difference was observed between the 20 and 40 min. Therefore, for confidence in the process, the efficient sonication time was selected to be 40 min. A stock solution of 2500 mg/L was made and then diluted with 6 ACS Paragon Plus Environment

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deionized water or electrolyte to the designed concentrations. The stock solution was used within an hour to ensure the stability. However, the results of stability analysis revealed that it can be stable for much more time (see section 3.1). Furthermore, to provide a homogeneous distribution in the prepared samples, magnetic stirring was used during the dilution process. 2.2.2 Nano-fluid Stability Analysis To ensure that the properties of SNP suspensions have not been changed during the tests, the stability of the prepared nano-fluids was studied through two different methods: (i) visual observation, (ii) optical absorbance analysis using an ultraviolet-visible (UV-Vis) spectrophotometer. The effect of SNP concentration, ionic strength and time were investigated. These methods were also utilized by other researchers24, 27. 2.2.3 Surface Modification of Calcite In the first step, a number of substrates were prepared from the calcite sample. The unmodified calcite surfaces were water-wet. To render the substrates to oil-wet state, they were aged in the model oil for 24 h at the room temperature. Before that, the substrates were wetted with deionized water for improving the adsorption of SA from the model oil.

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Having aged the substrates, they were washed with n-heptane and deionized water and then dried. 2.2.4 Wettability Alteration Using Nano-fluids To testify the efficiency of SNP for alteration the wettability of calcite, the oil-wet substrates were aged in the nano-fluids and then the contact angles were measured. For this purpose, two methods of equilibrium and transient analysis were applied. In the equilibrium tests, the substrates were immersed in beakers containing different concentrations (250-2000 mg/L) of SNP for about 50 h. To investigate the effect of salinity, the equilibrium tests were conducted 7 ACS Paragon Plus Environment

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for ionic strength of 0.05 M NaCl. In a distinct scenario, the equilibrium experiments designed for 250 and 500 mg/L were also repeated in the ionic strength range of 0-0.2 M. Transient analysis was performed in the concentrations of 500 and 1000 mg/L. To do so, six oil-wet substrates were immersed in nano-fluids. Then, at suitable time intervals a substrate was pulled out for analysis. To see the effect of salinity, the transient tests were also conducted in concentration of 1000 mg/L and 0.05 M. All the tests were performed at the ambient condition. The intervals of the different properties such as salinity and nano-fluid concentrations were selected based on the experience provided by our pervious reported work investigating the adsorption behavior of SNP onto calcite surface.26 2.2.5 Contact Angle Measurement Contact angle measurement was utilized to evaluate the potential of SNP suspensions for wettability alteration of calcite. The following procedure was used: (a) the oil-wet substrates were aged in the nano-fluids. Then, they were pulled out and washed with deionized water. (b) Substrates were then placed in a horizontal position and submerged in a water filled container. (c) An oil droplet was released from a needle and captured below the substrate. (d) The image of the droplet was captured using a high resolution camera and thereafter the contact angle was estimated using drop-shape image analysis. The difference between left and right contact angles; measured at the both sides of the drops; was less 0.1 degree. The contact angle measurements were double checked and the maximum change in the second sets of experiments was less than ±2.5 degrees. 2.2.6 Scanning Electron Microscopy The surface modification processes were visualized by scanning electron microscopy (SEM). The characterization was performed using a field emission scanning electron microscope (FESEM; S-4160 Hitachi instrument). 8 ACS Paragon Plus Environment

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2.2.7 Fourier Transform Infrared Spectroscopy Fourier transform infrared (FTIR) spectra of the samples were recorded in the wave number range of 400-4000 cm-1 using an ABB-Bomem MB-100 FTIR spectrophotometer. The scan resolution was 4 cm-1. This technique utilizes the chemical bond analysis to qualitatively identify the type and structure of chemical species. The scanning was performed on the solid samples through potassium bromide (KBr) pellet technique. The KBr is the most commonly used alkali halide for FTIR because it is completely transparent in the mid-infrared region.29 To prepare a pellet, about 2-3 mg of the sample was mixed with approximately 200 mg of KBr and grinded thoroughly.

29

A Mini-Press accessory; comprised of two bolts and a nut;

was then applied to press the mixture. To do so, a bolt was placed in the nut and the mixture was put in the nut on top of the bolt. The second bolt was then screwed tightly to pressurize the mixture. When two bolts were removed, a pellet was formed in the nut that can be used for FTIR spectroscopy. It must be noted that in this technique the bands due to moisture (water) often appear in the spectra. The bands in the 3200-3600 cm-1 region 30 and also 16001700 cm-1 region

29

are attributed to the absorbed water. Contribution due to moisture is

difficult to avoid, and so KBr should be kept dried.29 However, it can only minimize the water effect.

29

In the other hand, it must be pointed out that in all experiments, the samples

were in contact with water and the water molecules can be remained in their structures even after drying. As a result, the presence of such bands is unavoidable. Therefore, we will consider this issue in the FTIR interpretations. 3. RESULTS AND DISCUSSIONS 3.1 Nano-fluids Stability The effectiveness of nano-fluids is directly related to their stability because in an unstable condition the NP interact to form agglomerations with aggregate size in the range of several 9 ACS Paragon Plus Environment

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microns and even larger. Having lost their nano-sized feature, NP advantages are dramatically reduced. Thus, in this work, a preliminary step was design to check the stability of nano-fluids prior to main wettability alteration tests. The time dependent stability tests were carried out for three SNP concentrations of 500, 1000 and 2500 mg/L at five different levels of ionic strength, i.e. 0.086, 0.172, 0.346, 0.522, 0.699 M (corresponding to 0.5, 1, 2, 3, 4 wt% respectively). Figure 4 illustrates the direct visualization of nano-fluids stability over 6 days. No detectable particle agglomeration and sedimentation was seen for 500 mg/L SNP suspensions over the tested range of ionic strength. However, in the case of 1000 mg/L nano-fluids, a small amount of sedimentation was observed for ionic strength of 0.699 M after 6 days. The instability was enhanced for 2500 mg/L suspensions. The first evidence of visual destabilization was seen for 0.699 M in the 4th day. After 6 days, the suspensions at ionic strength of 0.086 and 0.172 M were still stable. Although the visual observation gives some insight about the stability, the aggregation of NP may occur in the nano-sized scale which cannot be visually seen. Thus, beside this technique, the optical absorbance measurements using an UV-Vis spectrophotometer were utilized. Using light absorption analysis of nano-fluids, one can accurately detect the particle agglomeration at early stage of instability. Figure 5 shows the resulted absorbance profile for different nano-fluids over the time (the absorption was conducted at the wavelength of 400 nm as used by Metin et al.27). The stability of 500 mg/L nano-fluids was confirmed for all ionic strengths as there was no significant change in the light absorption (Figure 5 (a)). However, Figure 5 (b) illustrates that the critical value of salt concentration for instability of 1000 mg/L suspensions (determine at the point of sharp increase in absorbance) does reduce with time. This value decreased to 0.522 M in the 3rd day and reached to 0.172 M after 6 days. Figure 5 (c) shows the variation of critical salt concentration for 2500 mg/L. As 10 ACS Paragon Plus Environment

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expected, the critical value decreased at smaller time intervals. This value was reduced to 0.172 M and 0.086 M after 2 and 3 days respectively. The intensified instability of nanofluids with the increase in salt concentration was attributed to the compression of the double layer

31, 32

. At the higher ionic strengths, the extent of repulsive electrostatic interaction

reduces and the energy barrier is lowered. Thus, some particles that have enough kinetic energy can overcome the barrier and aggregate. Among different types of NP used for EOR, SNP suspensions were reported to be more stable even without addition of any dispersant agents while the other types of untreated nano-fluids (e.g. titanium and aluminum oxides) were stable only for a few hours. 24 The results of this analysis prove the stability of SNP suspensions used for subsequent experiments in the utilized range of NP concentration, ionic strength and time. 3.2 Adsorption of SA on Calcite Surface Thomas et al. investigated the role of organic compound on the carbonate reservoir properties in two successive issues.

33, 34

In the first paper, the adsorption characteristics of organic

materials and their impact on the carbonate mineral wettability was explored. Based on their studies, fatty acids (carboxylic materials), e.g. SA, were found to be adsorbed strongly onto the carbonate surfaces and had a dramatic effect on changing the surface wettability toward oil-wet condition.33 These compounds may provide “anchors” for other material adsorbing the surface.4 If the crude oil contains enough surface-active constituents the carbonate surface can be completely covered by the organic matters and a strong oil-wet state is achieved.4 In this study, the concentration of carboxylic acid in sample oil (0.018 M solution of SA in nheptane) ensures the whole coverage of carbonate substrates that result in the preparation of strongly oil-wet surfaces. Although the fluid system utilized in the current study is different from that of oil reservoirs, it can mimic the process involved in the presence of crude oil


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because the carboxylic materials are the main compounds that affect the wettability of carbonates. SA dissolved in n-heptane was also used as a model oil to change the wetting state of chalk by Standnes and Austad.

4

Hansen et al. reported that the presence of water film had

improved the adsorption of dissolved SA on the calcite; likely pertains to the dissociation of SA and the calcite dissolution in water.

35

The governing equilibria that control the calcite-

water system are relatively complex. However, it can be briefly described as follow:36, 37 CaCO s ⇋ Ca aq + CO

aq

(1)

CO

aq + H Ol ⇋ HCO aq + OH aq

(2)

HCO

aq + H Ol ⇋ H CO aq + OH aq

(3)

H CO aq ⇋ H Ol + CO g

(4)

H CO aq ⇋ HCO aq + H aq

(5)

Ca aq + HCO aq ⇋ CaHCO aq

(6)

Ca aq + H Ol ⇋ CaOH aq + H aq

(7)

Ca aq + 2H Ol ⇋ CaOH s + 2H aq

(8)

SA is also dissociated in solvent/water interface: R − COOH ⇋ R − COO aq + H aq

(9)

The anionic groups of dissociated stearic acid (stearate ions; R − COO ) in the presence of water film, by partitioning, reach the calcite and adsorb onto the surface from which a strongly oil-wet condition is achieved. The details of process are discussed later.


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3.3 Wettability Alteration Using SNP Suspension 3.3.1 Equilibrium Studies Experiments: The ability of SNP in changing the wettability of oil-wet calcite was evaluated based on the contact angle values measured before and after aging the substrates in the designed NP suspensions. To measure the initial state of wettability, following the procedure described for surface modification of calcite, a number of oil-wet substrates were immersed in deionized water for a couple of days. Then, the contact angle tests were done on the substrates. The values of measured contact angles, around 156°, demonstrated an oil-wet initial state; based on Anderson’s classification, measured contact angles through the water in range of 0-75° indicated water-wet, from 75-105° intermediate wet, and from 105-180° oilwet state. 38 The effect of SNP concentration on wettability of calcite surfaces is illustrated in Figure 6; the performance of nano-fluid for wettability reversal was improved as the concentration of NP increased. A considerable decrease in the contact angle value was achieved for 2000 mg/L nano-fluid (from 156° to 41.7°). However, more than half of this reduction was just occurred for 500 mg/L suspension. Therefore, the SNP can alter the wettability of the oil-wet calcite to water-wet even at a relatively low concentration. This makes the application of low concentration SNP economically attractive for changing the wettability of oil-wet carbonates. Equation fit: The resulted equilibrium wettability profile was evaluated by a suitable equation proposed based on Langmuir isotherm.

39

The mathematical form of this equation

can be expressed as:

= −

1+

(10)

!


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where is the equilibrium contact angle, is the contact angle at the initial state of wettability, "#$ is the SNP concentration and A and B are constants that should be obtained experimentally. A is related to the maximum change in contact angle and 1/B is similar to Langmuir constant in Langmuir isotherm. Linearizing this equation, the plot of "#$ / − against "#$ yielded a straight line. The values of constants were then calculated using corresponding slope and intercept. The fit of this equation to experimental data is shown in Figure 7(a). To check the quality of regression process, three statistical indicators, i.e. maximum and minimum error percentage, analysis of variance (ANOVA) tables and residual analysis (difference between observed and predicted values), were utilized.40-42 The information for fitted linear equation is listed in Table 1. The correlation coefficient (R2) was calculated to be 0.9910 which demonstrated that the proposed equation had successfully described the experimental data. The ANOVA table for equilibrium contact angles is shown in Table 2. The first column provides the source of deviation including regression, residuals and total (sum of regression and residuals). The other four columns list the different measures of variance for these deviation sources: Degrees of freedom (DF), Sum of the squares (SS), Mean squares (MS) (i.e. SS divided by DF) and F test (i.e. MS of regression divided by MS of residuals). The detailed description of these parameters can be found elsewhere 40, 41. F test gives information for evaluating the model that best fits the data (examining the null hypothesis that & = 0). If the value of “F” is higher than a critical value of “F (DF of regression, DF of residuals)”, the null hypothesis is rejected. Considering the significance level of 0.05, ()*+ = (1,4 = 7.71. The observed value of F (=329.5356) was much greater than 7.71 and provided strong evidence against the null hypothesis. Residual plot (plot of residuals versus independent variable) was also used to evaluate the regression process. If the residual plot featured by randomly distributed points around the horizontal axis, the linear regression model could appropriately describe the data. As shown in


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Figure 7(b), the residual data for equilibrium contact angles were randomly dispersed along the xaxis. Thus, all the analysis confirmed the suitability of proposed equation. The values of A and B

were found to be 136.9863 and 384.2740 respectively. The resulted equation is compared to the measured contact angles in Figure 6. 3.3.2 Transient Analysis Experiments: The transient behavior of SNP for altering the wettability of oil-wet substrates was evaluated at two different concentrations of 500 and 1000 mg/L. Figure 8 illustrates the effect of aging time on the wettability alteration. The curve is featured by an earlier relatively fast decrease in contact angle followed by a slower decline rate. Therefore, the main change in wettability of oil-wet calcite toward water wet state occurs at the initial stage of nanofluids contact (more than two-third of change in contact angle values is occurred after approximately 12 h). Equation fit: to describe the data using a mathematical relation, an equation was proposed based on pseudo-second order equation of adsorption kinetics. 43 For this goal, the change in the rate of wettability alteration (contact angle) was supposed to be related to a second order change in the wettability. Thus, the governing differential equation can be written as: 0 − + = 2" − − + 01

(11)

Solving this differential equation for boundary conditions of + = at 1 = 0 and + = + at 1 = 1 , after some manipulation of constants gives: + = − +

1

(12)

+3

where + is the contact angle at a specified time t and C and D are constants. C is the maximum change in contact angle and D is related to the rate constant (k; in h-1) through: 2 = 1/3" . 15 ACS Paragon Plus Environment

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This equation was used to describe the time dependent behavior of the wettability alteration. The constants were determined based on experimental data. To evaluate this equation, a linear plot of 1/ − + versus 1 was constructed. The linearization of proposed equation for transient data at two different SNP concentrations is shown in Figure 9(a). The transient behavior in the presence of salt is also illustrated. However, the discussion will be presented later. The Regression analysis results are listed in Table 3. R2 values; 0.9978 and 0.9989 for 500 and 1000 mg/L respectively, indicated the good fit of the suggested equation. The corresponding ANOVA table for transient contact angle data is shown in Table 4. The value of F for 500 and 1000 mg/L data were found to be 1843.1853 and 3592.3878 respectively. The observed values were much greater than critical F. Therefore, the null hypothesis (& = 0) was rejected and the data could be properly explained by the proposed equation. Moreover, the residual plots, Figure 9 (b) and (c), show the random distribution of data points around the horizontal axis and in turn confirm appropriate linear models. The resulted transient equation parameters are listed in Table 5. The calculated values for k revealed that the constant rate of wettability alteration increased from 0.0019 to 0.0021 h-1 for an increase in SNP concentration from 500 to 1000 mg/L. A comparison between proposed equation and experimental data is shown in Figure 8. 3.3.3 Impact of Salinity on Wettability Alteration To investigate the effect of salinity (or ionic strength) on the wettability alteration, three distinct scenarios were designed. In the first scenario, the equilibrium study for a range of SNP concentrations (250-2000 mg/L) was performed at ionic strength of 0.05 M. The effect of added salt on the ability of SNP suspension for changing the wettability of oil-wet calcite is illustrated in Figure 10. To have a comparison, the data for equilibrium study in the absence of salt is also shown. As can be seen from this figure, the addition of salt to the


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suspensions intensified the wettability alteration process. For the second scenario, the transient analysis in 1000 mg/L SNP suspension was re-conducted for ionic strength of 0.05 M. Figure 11 compares the transient behavior of wettability alteration in the presence and absence of salts. The transient data can be also fitted by the proposed transient equation. Regression analysis results and ANOVA table are shown in Table 3 and Table 4 respectively. R2 value of 0.9975 and a large observed F (1621.1802 > 7.71) indicated a good fit of data. This goodness of fit is also supported by residual plot (Figure 9(d)). Analysis showed that the rate constant of wettability alteration was increased from 0.0021 to 0.0026 h-1 by addition of electrolyte (Table 5). In the last scenario, the effect of salinity increase on the wettability alteration was evaluated using tests performed at a constant SNP concentration in the ionic strength range of 0-0.2 M. To achieve this, two SNP concentrations of 250 and 500 mg/L were selected. Experimental data, shown in Figure 12, indicated that the increase in salinity resulted in lower contact angles. However, the reduction in the contact angle values was leveled off as salinity increased. In all scenarios, it was proved that the wettability alteration process was enhanced in the presence of electrolyte. The mechanisms will be discussed in the following sections. 3.4 Mechanistic Study 3.4.1 Suggested Mechanism for Wettability Alteration by SNP Suspensions The wettability alteration of oil-wet calcite by SNP is likely due to the adsorption of hydrophilic NP on the substrate surfaces. Recently, we have demonstrated that the SNP can be adsorbed onto the unmodified calcite surfaces.26 However, the present situation is different because the surface of calcite is modified by SA. In order to prove the supposed mechanism, the state of SNP adsorption onto the SA modified calcite surface must be studied.


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As mentioned before, the hydrophobicity of the modified substrates was due to the adsorption of stearate anions to the positive site on the calcite surfaces. Regarding equation (1) and (2),

Ca , HCO

and CO (i.e. potential determining ions

44

) dictate the calcite surface charge.

However, presence of H and OH (which also referred as potential determining ions by Somasundaran and Agar 44) in the next equations reminds that they can also contribute in the process via making complex with some species. CaHCO

and CaOH are some common

complexes. Thus, positive sites on the calcite surface are provided by either the main potential determining ions or complexation process. Geffroy et al. concluded that the ionic sites on the calcite surface are −Ca and−CO that can be further hydrated. 45 Based on literature, two types of stearate salts on the SA modified calcite surface can be formed: a chemisorbed stearate monolayer and calcium stearate.46,

47

The formation of

chemisorbed monolayer can be described as:

−CaOH + HOOC − R → −CaOOC − R + H O

(13)

Beside this reaction, adsorption of CaOH (equation (7)) onto −CO can also provide −CO Ca sites for adsorption of stearate as follow: 48 −CO

+ CaOH → −CO CaOH

(14)

−CO CaOH + HOOC − R → −CO CaOOC − R + H O

(15)

Figure 13 (a) shows the initial state of oil-wet calcite surface. Stearate anions are adsorbed onto the primary and induced positive charge sites. In Figure13 (b), the oil-wet substrate is immersed in the SNP suspension. To alter the wettability toward water-wet, NP must be adsorbed to the rock surface. But the surface is initially coated with SA (i.e. stearate anions adsorbed to the positive sites on the surface). Therefore, wettability alteration can only occur by removal and replacement of the stearates with SNP. But how is this possible? To answer


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this question, we begin with a discussion about the state of electrostatic interaction between the species presented in the abovementioned system. At the first stage, the distribution of different species in the calcite-water equilibria should be determined in order to identify which of the produced species in the mentioned reactions for this system are dominant. Such a calculation requires the information about the equilibrium constants of reactions. If the activity coefficients of the species are assumed to be 1, the equilibrium constants of reactions (1) to (8) are:

K7 =

8Ca 98CO

9 = 10 :. : 8CaCO 9

from Plummer and Busenberg 49

(16)

K =

8HCO

98OH 9 = 10

.; 8CO

9

from Garrels and Christ 50

(17)

K =

8H CO 98OH 9 = 10 ;.; 8HCO

9

from Garrels and Christ 50

(18)

K =

=

8HCO

98H 9 = 10 ?. > 8H CO 9

from Stumm 51

(20)

K? =

8CaHCO

9 = 107.7 8Ca 98HCO

9

from Plummer and Busenberg 49

(21)

K; =

8CaOH 98H 9 = 10 7.@ 8Ca 9

from Somasundaran et al. 36

(22)

K: =

8CaOH 98H 9 = 10 .: 8Ca 9

from Somasundaran et al. 36

(23)

The low value of K8 reminds that the concentration of CaOH can be ignored. Based on these equilibrium constants, the concentration of different species as a function of solution pH can be determined. The starting point of calculation is equation (19). As the 19 ACS Paragon Plus Environment

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equilibrium system of calcite-water used in this work is open to atmosphere, the corresponding partial pressure of CO2 (PCO2) is 10 3.5.51 Incorporation of this value in the logarithmic expression of equation (19) gives: log 8H CO 9 = log K + log pCD = −3.5 − 1.47 ≈ −5 Thus, the concentration of H CO in an open system is independent of pH and can be represented by a horizontal line in the double logarithmic plot of concentration versus pH. In the second step, the concentration of HCO

can be found from equation (20): log 8HCO

9 = log K > + log 8H CO 9 − log 8H 9 = −6.35 − 5 + pH

= −11.35 + pH Which gives a straight line with slope = 1 and intercept = −11.35 on the double logarithmic plot. All the species concentrations can be determined in the same manner. The resulting concentration profile in the equilibrium calcite-water system is shown in Figure 14. In this study, the equilibrium pH of calcite-water system was measured to be about 8.3. As illustrated in Figure 14 at this condition, the positive species contribution is mainly coming

from Ca and to lesser extent from CaHCO

. The HCO is the dominant negative specie at

this pH. In the other hand, in contact with water, the surface of SNP has been negatively charged. It is attributed to the dissociation of silanol groups (SiOH) and the formation of −SiO sites on the silica surfaces. When an oil-wet substrate is immersed in nano-fluid, the negatively charged NP approach to the calcite surface and disturb the charge balance and result in a more negatively charged surface. A competition is then emerged between SNP and stearate anions while the positive species of solution make complex with stearate anions. Thus, the contribution of positive species helps to release the stearate anions and their replacement with 20 ACS Paragon Plus Environment

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anionic surface groups of SNP. It should be pointed out that the concentration of positive species increased as the calcite sample brought in contact with nano-fluids. The reason is that the presence of SNP in suspensions produced some additional H ions due to dissociation of silanol groups and this in turn lowered the equilibrium pH. The measured initial pH of nano-fluids was in the range of 5-6 depending on SNP concentrations. When the systems came into contact with the calcite, the pH increased and reached an equilibrium value between 7-7.8. The change in equilibrium pH was therefore measured to be about 0.5-1.3. According to Figure 14, the concentration of positive species raises with the decrease in pH. Thus, their contribution in the proposed mechanism is further pronounced. In addition to electrostatic interaction, attractive London-van der Waals interaction energy 26 and the formation of hydrogen bond between hydroxyl groups of silica and oxygen on the calcite surface can also aid the adsorption of SNP. As can be inferred from the contact angle data, the replacement process depends on concentration of NP in the nano-fluids and aging time. This implies that the release of SA is partially and enhances with the increase in the contact time and concentration. Therefore, replacement and adsorption of hydrophilic SNP on the calcite surface is responsible for altering the wettability toward water-wet state. The final state of process is shown in Figure 13 (c). It is worth mentioning to note that, the first evidence for such adsorption mechanism for change in wettability is the agreement between “adsorption” and “wettability alteration” behavior of SNP. The equilibrium adsorption of SNP onto calcite26 and equilibrium wettability alteration data are both described by Langmuir type equations. Moreover, both of adsorption kinetics

26

and transient behavior of wettability alteration followed a pseudo-


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second order model. In the following sections, the suggested mechanism is further confirmed by FTIR and SEM analysis. 3.4.2 FTIR Spectroscopic Analysis To demonstrate the interaction of SA and SNP with calcite, FTIR spectra of unmodified calcite, SA modified calcite and oil-wet calcite aged in the SNP suspension were studied. Figure 15 illustrates the mentioned spectra in transmittance mode. The FTIR spectrum of fresh calcite, shown in Figure 15 (a), depicts a strong adsorption band which is the characteristic adsorption band of carbonate anions. This relatively smooth, symmetrical and broad band, centered at 1421.89 cm-1, is resulted from C‒O stretching (vs CO2). Carbonate 3 bending vibrations also produce sharp band in the region of 900-650 cm-1. 30 The adsorption bands at 875.363 cm-1 and 712.629 cm-1 are attributed to out-of-plane (ωO‒C‒O) and inplane bending (δs O‒C‒O) vibration respectively. Adsorbed water produces a band in the 3200-3600 cm-1 region due to O‒H stretching vibration (vs OH).30 The resulted FTIR spectrum for calcite is in good agreement with literature.30 Figure 15 (b) shows the FTIR spectrum of SA modified calcite. The characteristic bands of carbonate groups are clearly depicted. However, a slight change in the shape of the band centered at 1420.73 can be seen. Deviation in band symmetry, such as slight shoulder or an unusual tail, is an evidence for presence of overlapping band.30 Inspection of carboxylic acids spectrum reveals that they show a characteristic in-plane bending band at 1430 cm-1

29

that

can overlap with mentioned carbonate band. Moreover, the emergence of some other new bands is another criterion for interaction of SA with calcite. The bands emerged at 2955.82, 2916.82, 2874.61 and 2849.57 cm-1 are attributed to methyl symmetric (vs CH3), methylene asymmetric (vas CH2 ), methyl asymmetric (vas CH3 ) and methylene symmetric (vs CH2 ) C‒H


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stretching vibration

29

of SA. The small window in the Figure 15 provides a better

comparison between the two spectra of fresh and SA coated calcite. Carboxylic acids salts, such as stearate, usually produce one or two sharp adsorption bands near the 1585-1545 cm-1.30 The two small sharp adsorption bands at 1577.51 and 1544.5 cm-1, due to asymmetric C‒O stretching, is an evidence for presence of carboxylate salt. Due to smallness of these bands, the FTIR spectroscopy was repeated and again such small bands were observed. Derrick et al. pointed out that the relative intensity of a band, in comparison to the other bands, provides information about the amount of its corresponding functional group

30

. Thus, these low intensity bands can be due to presence of a small amount of

carboxylate salt. However, the presence of methylene and methyl bands was sufficient for confirming the interaction of SA with calcite. Bending vibration of O‒H in absorbed water molecules, occurs in 1700-1600 cm-1 range 29, can be assigned to the sharp band observed at 1617.81 cm-1.The obtained FTIR spectrum for SA modified calcite was in close agreement with the spectra reported by Zhang et al. 52 and also by Osman and Suter.53 FTIR spectrum of SA modified calcite aged in SNP suspension is illustrated in Figure 15 (c). The evident change in the spectrum is the emergence of a new band centered at 1113.82 cm-1. This band is attributed to asymmetric Si‒O stretching vibration (vas O‒Si‒O)

29, 30

and

provides a confirmation in support of proposed mechanism for the interaction of SNP with SA modified calcite. The SA stretching bands are also presented but their intensity is reduced which in turn verifies the partial removal of SA from the calcite surface as the wettability alteration mechanism. 3.4.3 FESEM Visualization Surface modification of calcite substrate using SA and SNP was also characterized by FESEM technique. Before imaging the samples were coated with a gold layer to reduce 23 ACS Paragon Plus Environment

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charging. FESEM image of unmodified calcite is shown in Figure 16 (a) and (b) at two different magnifications. The picture depicts a relatively smooth surface with some small roughness (pore and cracks) and dust particles. In Figure 16 (c) and (d) FESEM observations of SA modified calcite surface are presented. Compare to images of fresh calcite, the figures obviously indicate the presence of a SA layer on the substrate surface. This layer is responsible for strong hydrophobicity of the SA modified samples. The interaction of SNP with oil-wet calcite surface is also visualized by FESEM images of oil-wet substrate aged in the nano-fluid (Figure 16 (e) and (f)). Two different magnifications of FESEM pictures confirm the adsorption of SNP with a spherical morphology that replace the stearates on the surface. Figure 16 (g) and (h) reveal that there exits some sites on the surface in which the removal of stearate is partial. The images are captured from two different points on the SNP treated oilwet substrate. As illustrated in these figures, the surface density of SNP at some location is low and traces of stearate are still observed on the surface (the circled areas depict some examples of regions in which the SA layer can still be seen and the red arrows show the adsorbed SNP). The results of FESEM visualization are in accordance with FTIR and contact angle measurements. 3.4.4 Salinity Effect Mechanism The role of NaCl on the adsorption of SNP onto the calcite surface was studied before. It was shown that the amount of SNP adsorption increased in the presence of Na ions.26 Similarly, the results of contact angle measurements revealed that the addition of NaCl to the SNP suspensions improve their wettability alteration potential. This can be also attributed to the Na ions. Ricci et al. visualize single ions close to surface of calcite (through its stern layer) in the solution employing amplitude modulation atomic force microscopy.54 Their 24 ACS Paragon Plus Environment

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observation indicated that, in comparison with Ca and Cl , Na ions can come closer to the calcite surface.54 The presence of Na ions near the calcite surface has two important merits in wettability alterations of oil-wet calcite by SNP: (i) Na ions could neutralize some negative sites on the calcite surface and facilitated the adsorption of negatively charged SNP onto calcite surface; (ii) Na ions help other positive species in complexation process and removal of stearate from the surface. Utilizing these mechanisms, Na ions improve the wettability alteration of oil-wet calcite by SNP. (iii) Furthermore, the presence of electrolyte affects the position of chemical equilibria. Activity (or effective concentration), a, is used to investigate this effect. For species X the activity depends on the concentration of electrolyte in the system and is defined as:55

GH = 8X9JK

(24)

where GH is the activity, 8X9 is the molar concentration and JK is the activity coefficient of the species X. Electrolyte can affect the activity coefficient and thus the activity. For calculation of equilibrium constants given by expressions (16) to (23), all the activity coefficients are assumed to be unity (as is the case for pure water) and so the activity and the molar concentration will be equal. However, for electrolyte solutions this assumption is valid only for uncharged species.55 Introducing the activity coefficients in the expressions allows the accurate determination of ionic strength dependent equilibrium constants, namely, K′. K is called the “thermodynamic equilibrium constant” and K′ is the “concentration constant”. The concentration of different species can now be determined in terms of ionic strength using the corresponding values of Ks and K′s. To illustrate the relationship between K′ and K, the calculation of K′> (as an example) is presented below:

K> =

GMCDON GMP GMQ CDN

JMCDON JMP 8HCO 98H 9 JMCDON JMP = . = K′> . 8H CO 9 JMQCDN JMQ CDN


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The Debye-Huckle equation was used to calculate the ionic strength dependent activity coefficients of ions:55

− log JK =

0.51 ST √V

(25)

1 + 3.3αT √V

where ST is the species charge, V is the ionic strength (M) and αT is the effective diameter of the hydrated ion X (nm). To investigate the effect of electrolyte on the position of equilibrium in calcite-water system, the calculation was performed for ionic strength of 0.1 M. Table 6 lists the activity coefficients of ions along with the obtained values of K′s for 0.1 M electrolyte medium. The values of YT were taken from Skoog et al.55 The distribution of the different species in the system were then calculated and plotted in Figure 17. The dashed lines represent the ion distribution in pure water and the corresponding parallel solid lines indicate the concentration of species at 0.1 M. Figure 17 illustrates that, as in the pure water, Ca and HCO

are again the dominant positive and negative species respectively. However, the presence of electrolyte does increase the concentrations of species. This increase is more pronounced by Ca that results in enhancement of proposed mechanism. As the ionic strength increases up to 0.1 M, the activity coefficient of ions decreases and the state of equilibrium changes to raise the concentration of Ca. However, at higher salinity (i.e. I > 0.1), the activity coefficients increase with the increase in ionic strength.55 Therefore, as observed previously (see Figure 12), the effect of salinity on the wettability alteration was leveled off at the higher ionic strength. It is worth to note that the divalent ions (such as Ca , Mg and SO

) by themselves were proved to be able to change the wetting state of reservoir rocks

10-12, 56, 57

while neither Na

nor Cl could alter the wettability 58, 59. Therefore, utilizing NaCl as salt gave the opportunity 26 ACS Paragon Plus Environment

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for investigating the sole effect of SNP on the wettability alteration process in the presence of electrolyte. The results of this study proved the potential application of SNP for EOR from carbonate reservoirs through the wettability alteration toward water-wet state. In oil field application, at the reservoir condition, the main issue is to prepare a stable nano-fluid, and so more care should be considered in this way. To improve the stability of NP suspensions, some remediation including the addition of surfactant

17

and polymers

24

to the base fluid were

suggested. Recently, Zhang et al. formulated a type of silica-based nano-fluids that could preserve their stability in the harsh reservoir environment.25 However, in this study, the main objective was to mechanistically investigate the individual role of SNP on the wettability alteration of the carbonate rocks. Therefore, to provide a well-defined environment no dispersant agent was applied. Although the wettability alteration behavior in the reservoir rocks (at the harsh reservoir conditions) could not be quantitatively represented by that observed in this study, the results of this work could be a spotlight to better understanding the mechanism involved in this process for silica nano-fluids –carbonate rocks systems.

4. CONCLUSIONS In this work, a mechanistic study on the potential application of SNP for the wettability alteration of oil-wet calcite was presented. For this purpose, equilibrium and transient contact angle measurements, equation fitting approaches, surface equilibria and interaction studies, FTIR analysis and FESEM visualization were utilized. Based on the results of this study, the following concluding remarks can be drown:


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•

The effect of SNP concentration and ionic strength on the stability of nano-fluids was studied. The critical salt concentration for SNP aggregation was found to be reduced with the increasing in nano-fluid concentration over the tested time intervals. The 500 mg/L nano-fluid was stable for the all utilized ionic strengths in a period of 6 days. However, the critical salt concentration for 1000 mg/L was decreased to 0.172 after 6 days. For 2500 mg/L nano-fluids, this value was reduced to 0.086 M just in 3 days. Based on these results, the stability of the utilized SNP suspensions in the experiments was proved in the corresponding range of NP concentration, ionic strength and time.

•

The behavior of SNP suspensions for altering the wettability of oil-wet calcite was explored through the equilibrium and transient contact angle measurements. Results showed that the effectiveness of SNP nano-fluids for wettability alteration was improved by increasing concentration, contact time and addition of electrolyte.

•

The equilibrium and transient contact angle measurements were described using a Langmuir type and a pseudo-second order equation respectively. Statistical analysis (i.e. error analysis, ANOVA tables and residual plots) confirmed the goodness of fit of the utilized equations. Rate constant calculations showed that this parameter increased from 0.0019 to 0.0021 h-1 for an increase in nano-fluid concentration from 500 to 1000 mg/L. The rate constant was further increased to 0.0026 h-1 for 1000 mg/L in 0.05 M electrolyte media.

•

It has been found that the responsible mechanism for wettability alteration is the partial release of stearate from the oil-wet calcite surface and their replacement with SNP. The mechanism was verified by surface equilibria and interactions studies of different species presented in the system. Equilibrium calculations allowed determining the distributions of different species and their concentrations in the calcite-water/nano-fluids systems. Thus, the contribution of dominant species in the 28 ACS Paragon Plus Environment

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mechanism can be proved. Moreover, FTIR spectroscopy and FESEM visualization provide some objective verification in support of suggested mechanism. Agreement between the adsorption and wettability alteration behavior of SNP was also another evidence for this wettability alteration mechanism. •

The enhanced wettability alteration in the presence of salt was attributed to the role of Na ions in the process. Na ions can help in the adsorption and release of SNP and stearate respectively. Furthermore, analysis showed that the presence of electrolyte, favorably change the position of system’s equilibria such that the contribution of Ca as the dominant positive species was increased. However, this effect was leveled off at higher ionic strength (i.e. I > 0.1).

•

Low concentration of NP suspensions were used in this study make it economically attractive for EOR applications in carbonate reservoirs.

AUTHOR INFORMATION Corresponding Author *

E-mail:[email protected], Sharif University of Technology, Chemical and Petroleum

Engineering Department, Azadi Ave., Tel: +98-21-66166413, Fax: +98-21-6622853.

NOMENCLATURE Acronyms ANOVA = analysis of variance DF = degree of freedom EOR = enhanced oil recovery HLPN = hydrophobic-lipophilic polysilicon nanoparticles 29 ACS Paragon Plus Environment

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LHPN = lipophobic-hydrophilic polysilicon nanoparticles MS = mean squares NP = nanoparticles NWPN = neutral wet polysilicon nanoparticles PN = polysilicon nanoparticles SA = stearic acid SNP = silica nanoparticles SS = sum of squares

Latin Letters A = constant related to the maximum change in contact angle a = activity B = constant whose reciprocal is indicative of Langmuir constant C = maximum change in contact angle "#$ = silica nanoparticles concentration (mg/L) D = constant related to the rate constant, k V = ionic strength (M) K = equilibrium constant k = rate constant (h-1) K′ = concentration constant t = time (h) ST = species charge

Greek Letters αT = effective diameter of the hydrated ion X (nm) J = activity coefficient = equilibrium contact angle 30 ACS Paragon Plus Environment

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= contact angle at the initial state of wettability + = contact angle at a specified time

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Table 1. Linear regression analysis information (Z = [ + &\) for equilibrium equation Coefficient

Coefficient value

Standard error

Lower 95%

Upper 95%

R2

[ &

2.8052

0.4969

1.2239

4.3866

0.9910

0.0073

0.0004

0.0060

0.0086


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Table 2. ANOVA table for regression analysis of equilibrium contact angle data

Source regression residual total

DF

SS

MS

F

1 3 4

110.2775 1.0039 111.2814

110.2775 0.3346

329.5356


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Table 3. Linear regression analysis information (Z = [ + &\) for transient equation Concentration (mg/L), Ionic Strength (M)

Coefficient

Coefficient value

Standard error

Lower 95%

Upper 95%

R2

500, 0

[ &

0.0606

0.0059

0.0442

0.0770

0.9978

0.0106

0.0002

0.0099

0.0113

0.0391

0.0036

0.0290

0.04921

0.0091

0.0002

0.0087

0.0096

0.0275

0.0050

0.0136

0.0415

0.0085

0.0002

0.0079

0.0091

1000, 0

1000, 0.05

[ & [ &


0.9989

0.9975

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Table 4. ANOVA table for regression analysis of transient contact angle data

Concentration (mg/L), Ionic Strength (M) 500, 0

Source

DF

SS

MS

F

regression residual total

1 4 5

0.1917 0.0004 0.1921

0.1917 0.0001

1843.1853

1000, 0


1 4 5

0.1474 0.0002 0.1476

0.1474 0.00004

3592.3878

1000, 0.05


1 4 5

0.1274 0.0003 0.1277

0.1274 0.00008

1621.1802


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Energy & Fuels

Table 5. Transient equation parameter values. Concentration (mg/L), Ionic Strength (M)

Parameters Values

500, 0

C D k (h-1)

94.3396 0.0606 0.0019

1000, 0

C D k (h-1)

109.8901 0.0391 0.0021

1000, 0.05

C D k (h-1)

117.6471 0.0275 0.0026


Energy & Fuels

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Table 6. Activity coefficient of ions and calculated concentration constants at 0.1 M

Ion

Activity coefficient of ion at 0.1 M

Ca CO

HCO

OH

H CaHCO

CaOH

0.40 0.36 0.77 0.76 0.83 0.75 0.75

Equilibrium constant Concentration constant at 0.0 M at 0.1 M

:. : K7 = 10 K′7 = 10 ;.? K = 10

.; K′ = 10

.@

;.; K = 10 K′ = 10 ;.; K = 10 7. ; K′ = 10 7. ;

?. > K > = 10 K′> = 10 ?.7? K′? = 10].;7 K ? = 107.7

7.@ K ; = 10 K′; = 10 7 .7


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Figure 1. TEM image of SNP.26


Energy & Fuels

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Figure 2. XRD pattern of utilized calcite sample.26


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Figure 3. Schematic of methodology utilized in this study.


Energy & Fuels

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Figure 4. Visual stability of SNP nano-fluids at the different NP concentrations and ionic strengths after 1 h, 1 day, 4 days and 6 days.


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Figure 5. UV-Vis absorption at 400 nm wavelength of SNP nano-fluids in terms of ionic strength and time; (a) 500 mg/L, (b) 1000 mg/L and (c) 2500 mg/L.


Energy & Fuels

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Figure 6. Wettability profile of the initially oil-wet calcite aged in the SNP suspensions (contact angles were measured in water/oil/calcite system).


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Figure 7. (a) Evaluation of proposed equation for wettability alteration profile. (b) Residual plot for the equilibrium contact angles.


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Figure 8. Transient behavior of SNP in the wettability alteration of oil-wet calcite.


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Energy & Fuels


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Figure 9. (a) Evaluation of the suggested equation for fitting the transient wettability alteration data. (b) Residual plot for transient contact angle at 500 mg/L. (c) Residual plot for transient contact angle at 1000 mg/L. (d) Residual plot for transient contact angle at 1000 mg/L and 0.05 M electrolyte.


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Figure 10. Effect of salinity on the equilibrium wettability alteration.


Energy & Fuels

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Figure 11. Effect of salinity on the transient behavior of the wettability alteration for 1000 mg/L nano-fluid.


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Figure 12. Effect of salinity on the contact angles in a constant SNP concentration.


Energy & Fuels

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Figure 13. Suggested mechanism for the wettability alteration of oil-wet calcite using SNP suspension; (a) initial sate of oil-wet substrate, (b) SNP could interact with the positive sites on the calcite surface while the positive species of solution help to remove the stearates through complexation process, (c) the stearate anions was partially removed and replaced with hydrophilic SNP which resulted in wettability alteration to a more water-wet state.


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Figure 14. Distribution of different species in the open equilibrium carbonate-water system.


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Figure 15. FTIR spectra (transmittance mode) of (a) fresh calcite, (b) SA modified calcite and (c) SNP treated oil-wet calcite.


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Figure 16. FESEM images of (a and b) unmodified calcite surface, (c and d) SA modified calcite surface, (e and f) SA modified calcite treated with SNP suspension and (g and h) partial removal of SA in some sites; circled areas: regions in which the SA layer can still be seen; Red arrows: adsorbed SNP.


Energy & Fuels

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Figure 17. Effect of the presence of electrolyte (0.1 M) on the distribution of different species in the open equilibrium carbonate-water system (the solid lines show the concentration of species at 0.1 M while the dashed lines are for pure water).


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Potential Application of Silica Nanoparticles for Wettability Alteration of

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