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Shear Rheology Using De Noüy Ring To Evaluate Formation and Inhibition of Calcium Naphthenate at the Water/Oil Interface Juliana N. Bertelli,*,†,‡ Rocío M. M. Dip,† Renata V. Pires,† Flávio C. Albuquerque,§ and Elizabete F. Lucas† †

Laboratory of Macromolecules and Colloids for Petroleum Industry, Institute of Macromolecules, Federal University of Rio de Janeiro, Avenida Horácio Macedo, 2030, Cidade Universitária, 21941598, Rio de Janeiro, Rio de Janeiro, Brazil ‡ Operações de Exploraçaõ e Produçaõ do Espírito Santo (UO-ES), Petrobras, Avenida Nossa Senhora da Penha, 1688, Barro Vermelho, 29057570, Vitória, Espírito Santo, Brazil § Cenpes, Petrobras, Avenida Horácio Macedo, 950, Cidade Universitária, 21941915, Rio de Janeiro, Rio de Janeiro, Brazil ABSTRACT: The extraction of crude oils rich in tetraprotic naphthenic acids along with formation water under pH conditions above 6.5 favors the formation of naphthenic acid salts that can generate deposits with high consistency and very low solubility in the oil and water phases. There is still much uncertainty regarding the behavior of naphthenates, and only a small number of tests exist to evaluate it. The aim of this study was to obtain the parameters necessary to assess the formation of calcium naphthenate film through shear rheology (Du Noüy ring method) and to assess the rheological behavior of this film at the water/oil interface as a function of the ARN acid concentration, calcium concentration, presence of magnesium ions, and addition of commercial chemical inhibitors. We observed an increase in the elastic modulus (G′) and a reduction of the viscous modulus (G″) with the formation of the calcium naphthenate film and that the viscous deformation is restricted, which is coincident with the formation of a cross-linked network between the calcium ions and ARN acids. We also found that G′ increases with an increasing calcium ion concentration and salinity. The addition of the commercial inhibitors tested caused a reduction of G′ in relation to the control formulation (no inhibitor), with it being possible to distinguish their different performances in film formation prevention. However, we did not observe a correlation between the results of the biphasic mixture test and rheological assay throughout the concentration range tested.

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INTRODUCTION The carboxylic acids present in crude oils are mainly found in heavy, immature, and biodegradable petroleums. These acids are called naphthenic acids and are composed of complex mixtures of cyclic, acyclic, and aromatic carboxylic acids, with the general formula CnH2n+zO2. The index z is equal to zero for saturated aliphatic carboxylic acids and increases with the formation of rings or dual molecular bonds.1−4 The depressurization resulting from oil processing reduces the solubility of CO2 and, consequently, increases the pH of the produced water.5 As the pH increases, the concentration of dissociated acid groups in the water/oil interface rises. When these acid groups interact with the metal ions dissolved in the water, naphthenates are formed.6 Metallic naphthenates can stabilize emulsions or accumulate in surface equipment, such as production separators, electrostatic treaters, and heat exchangers, but can also block production strings and pipelines, causing production shutdowns and problems in maintaining flow.7 There are two main types of naphthenates that form during oil processing: calcium naphthenates, which are hard or sticky solid deposits, that form at the water/oil interface and sodium naphthenates, which cause stabilization of the emulsion, severely impairing removal of water from the oil during treatment. The main difference between the two types of naphthenates is the type of naphthenic acid that generates the salts.8 The formation of solid deposits depends upon the © 2014 American Chemical Society

presence of tetracarboxylic naphthenic acids (ARN) in the organic phase.9 ARN acids are formed of aliphatic molecules with four branches and a carboxylic terminal group in each branch. The aliphatic chain contains 4−8 cyclopentane rings. The molar mass of the most abundant component, with 6 rings, is 1230 g/ mol.7 Various characterization studies, such as infrared (IR), nuclear magnetic resonance (NMR), and Fourier transform cyclotron resonance mass spectrometry (FT-ICR MS), have been carried out to ascertain the structure of ARN acids.9−14 Some researchers have investigated the conditions that favor the formation of naphthenate deposits and the characteristics of these deposits.15,16 The first reports about the problems caused by calcium naphthenate deposition date from 2003.17 The naphthenic tetra-acids (ARN), present in parts per million (ppm) in crude oil, are known to cause organic deposits together with the calcium ions present in the formation water, which has high pH. In the past decade, several cases of calcium naphthenate deposition have been reported.10,18−20 The first cases of calcium naphthenate deposition registered in Brazil were in 2011, in two different production fields.21 Special Issue: 14th International Conference on Petroleum Phase Behavior and Fouling Received: October 9, 2013 Revised: February 4, 2014 Published: February 4, 2014 1726

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acid, sodium bicarbonate P.A., sodium borate decahydrate, calcium carbonate, dihydrated calcium chloride P.A., magnesium chloride hexahydrate P.A., sodium chloride P.A., and anhydrous sodium sulfate ́ P.A. were purchased from Vetec Quimica Fina Ltda, Duque de Caxias, Brazil, and used without additional purification. Chloroform with highperformance liquid chromatography (HPLC)-purity grade was purchased from Fisher Scientific, Belo Horizonte, Brazil. Hydrochloric acid (37%) and deuterated chloroform where acquired from Merck, Rio de Janeiro, Brazil. Dichloromethane, methanol, n-decane, and toluene, all with HPLC purity, were purchased from Honeywell Burdick & Jackon, Cotia, Brazil. Finally, tetrahydrofuran (THF) with HPLC purity was purchased from Tedia Brazil, Rio de Janeiro, Brazil. Purification of the Deposit and Obtaining the Naphthenic Acids. Various papers have suggested methods for extracting naphthenic acids from calcium naphthenate deposits,27−30 but the method used here followed the procedure established by Albuquerque31 and Albuquerque and Lopes,32 which uses Soxhlet extraction to maximize the removal of chemicals, chloroform as the solvent to provide a better defined water/oil interface, and a high concentration of HCl to facilitate phase separation. Initially, 20 g of field deposit sample were washed several times with toluene until the solvent becomes colorless. In this step, it was possible to separate, almost completely, the oil, obtaining a light beige powder. Traces of oil and other compounds are still presents in the deposit; therefore; they were removed by Soxhlet extraction using toluene, dichloromethane, and methanol, with an extraction time of 56 h for each solvent. The obtained clean powder was dried at room temperature. The naphthenic acids were obtained by solid/liquid extraction, mixing 100 mL of chloroform and 20 mL of HCl aqueous solution (6 M) per 1 g of clean deposit. This mixture was kept under stirring until the acids become well-dissolved in the organic solvent. Then, the phases were separated. To avoid the presence of water in the acid solution, we used dehydrated sodium sulfate, which was separated by filtration. Finally, to recover the naphthenic acids, chloroform was evaporated at room temperature. Characterization of the Clean Calcium Naphthenate Deposit and the Naphthenic Acids. The clean calcium naphthenate deposit was characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The naphthenic acids obtained from the clean calcium naphthenate deposit were characterized by TGA, DSC, Fourier transform infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance (1H NMR), and size-exclusion chromatography (SEC). The XRD analyses were performed with a Rigaku Miniflex diffractometer, at room temperature, with a power difference of 30 kV and current of 15 mA. The scanning was carried out in the 2θ range of 2−60°, with a goniometer speed of 0.05°/min and Cu Kα radiation (λ = 1.5418 Å). A TA Instruments Q500 thermogravimetric analyzer was used to obtain the mass loss curves as a function of the temperature increase. The analyses were carried out from 30 to 700 °C, at a heating rate of 10 °C/min, under a nitrogen flow of 60 mL/ min. The DSC analyses were performed with a TA Instruments Q200 calorimeter in the range from −50 to 300 °C, at a heating and cooling rate of 10 °C/min, under a nitrogen atmosphere. The glass transition temperature (Tg) was determined on the second heating. To obtain the FTIR spectra, the samples were dissolved in chloroform, poured over a KBr window, and analyzed in a Varian Excalibur 3100 spectrophotometer in a wavenumber range of 4000−400 cm−1, with a resolution of 4 cm−1. The ARN acid concentration in the naphthenic acid samples was calculated28 from the 1H NMR spectra obtained in a Bruker Avance III device at 800 MHz, with a 5 mm probe at 27 °C, with the sample mixed with benzoic acid in the proportion of ∼2:1, dispersed in CDCl3. The SEC analysis was performed at room temperature in a Waters 2707 chromatograph from Agilent Technologies, with a light scattering detector and three-column Styragel HR 1 THF (two of 500 Å and one of 100 Å). THF was used as the solvent at a flow of 0.6 mL/min, and the instrument was calibrated using polystyrene standards with number average molar masses (⟨Mn⟩) of 580, 980, 1680, 2450, 3250, 7000, 10 100, 11 600,

The most frequent method to mitigate calcium naphthenate deposition is treatment with acids, such as acetic acid. The injection of this acid reduces the pH of the produced water, causing the naphthenic acids to take on a protonated form and, therefore, remain adsorbed in the interface. The main disadvantages of this method are increased corrosion of processing equipment, reduction of the value of the oil (more acidic), and the large volumes necessary to reduce the pH to the desired levels.22 Another method is to complex the divalent cations, making them unavailable for reaction with tetra-acid. However, the concentration of ions in produced water can be so large that it requires huge quantities of chemical products, rendering the logistics to supply the product impractical.7 The ideal way to deal with the problem is to block the reaction through the use of chemical inhibitors/additives. Naphthenate inhibitors are just starting to be developed,22 but some patents have already been published containing details of the base chemical structures, such as phosphonates and polyoxides.23−25 Naphthenate inhibitors exhibit surfactant properties originating competition by the oil/water interface, thus preventing interactions between the organic acids (in the oil phase) and the cations (in the water phase). Therefore, according to the literature, the naphthenate inhibitor should be more active at the interface than the naphthenic acids. Before carrying out field tests with a given chemical product, it is crucial to perform tests in the laboratory to verify the efficiency and possible incompatibilities with other products used in processing as well as to estimate the initial dosage for the field tests. The simplest method used to assess the efficiency of naphthenate inhibitors is the biphasic mixture test, which involves mixing an organic phase, generally toluene containing ARN acids extracted from the oil or the deposit, with a synthetic water solution with high pH containing calcium ions, to observe the naphthenate deposition behavior. The inhibitor is added to the phase in which it is soluble.22 Gravimetric methods to analyze the interfacial layer7 and the interfacial stress26 of naphthenic acid systems with and without the addition of chemical inhibitors have also been proposed in the literature. The extraction and characterization of naphthenic acids in crude oil and their deposits as well as the knowledge of the kinetics and thermodynamics of the formation of these organic encrustations are necessary to understand the process by which naphthenates are formed. This understanding can make it easier to predict and mitigate or prevent this type of deposition. In this study, we established the parameters necessary to analyze naphthenate interfacial films using oscillatory rheometry with Du Noüy ring, to assess the influence of the ARN acid concentration, calcium concentration, presence of magnesium ions, and addition of commercial chemical inhibitors on the characteristics of the film formed. The influence of adding the commercial inhibitors was also evaluated by the biphasic mixture test.

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EXPERIMENTAL SECTION

Materials. The calcium naphthenate deposit came from an oilfield in South America. Nine commercial naphthenate inhibitors were used in this study: five products, supplied by NALCO Champion, An Ecolab Company, and identified in this paper by the letters A, B, C, D, and E, are oil-soluble based on polyether; three products, supplied by Dorf Ketal Chemicals and identified in this paper by the letters F, G, and H, are water-soluble based on aliphatic aldehyde acid. Benzoic 1727

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22 000, 28 500, and 66 000 g/mol. The sample concentration was 10 mg/mL. Biphasic Mixture Test. Because of the relatively small quantity of naphthenic acids available, the biphasic mixture test was performed through contact of only 2 mL of the oil phase and 2 mL of the water phase, both in a small test tube at room temperature (25 °C). The system was shaken manually for 1 min, and photographs were taken 24 h later, to allow time for agglomeration of the solids formed. The oil phase consisted of a mixture of n-decane and toluene in a proportion of 2:8 (v/v) with a concentration of 1000 mg/L naphthenic acids (ARN acid). The aqueous phase was prepared using deionized water mixed with 110 000 mg/L sodium chloride, 4000 mg/L dihydrated calcium chloride, 650 mg/L magnesium chloride hexahydrate, and 700 mg/L sodium bicarbonate. The pH level was raised to 9 by adding borax. The composition of the aqueous phase was typical of formation water. The inhibitor concentrations were achieved by mixing naphthenic acid solutions with and without each of the inhibitors tested. Table 1 shows the volumes of the samples without and with

L. The naphthenate deposition inhibitors were evaluated in the same inhibitor/ARN acid ratios employed in the biphasic mixture test: 1:8, 1:5, 1:4, 1:1, 3:2, and 1:2 (and also the 3:4 ratio for inhibitor E). The volumes of the oil phase with and without inhibitor used to prepare the oil phase with varied inhibitor concentrations are presented in Table 2.

Table 2. Volumes of Oil Solutions with and without Inhibitor Mixed To Prepare the Oil Phase Used in the Rheological Tests concentration of inhibitor (μL/L)

inhibitor/ARN acid ratio (both in oil phase)

volume of solution containing 25 mg/L of ARN acid without inhibitor (mL)

volume of solution containing 25 mg/L of ARN acid and 100 μL/L of inhibitor (mL)

3.125 5.000 6.250 12.500 18.750 25.000 37.500 50.000

1:8 1:5 1:4 1:2 3:4 1:1 3:2 2:1

9.688 9.500 9.375 8.750 8.125 7.500 6.250 5.000

0.312 0.500 0.625 1.250 1.875 2.500 3.750 5.000

Table 1. Volumes of the Same Phase, with and without Inhibitor, Used To Prepare Each Phase (Aqueous and Oil Phases), for the Biphasic Mixture Test concentration of inhibitor (μL/L)

inhibitor/ARN acid ratio

volume of the phase without inhibitor (μL)

volume of the phase containing inhibitor (μL)

0 125 200 250 500 1000 1500 2000

1:8 1:5 1:4 1:2 1:1 3:2 2:1

2000 1875 1800 1750 1500 1000 500 0

0 125 200 250 500 1000 1500 2000

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RESULTS AND DISCUSSION Extraction of ARN Acid. The yield of the process to obtain the clean calcium naphthenate deposit was 28.7%. With respect to the initial mass, the yield of the process to obtain the naphthenic acids (including ARN acid) was 18.4%. Before purification, the deposit was dark brown and solid, while after cleaning, it was light beige and finely powdered (clean calcium naphthenate deposit). The naphthenic acids extracted from this clean material were light brown and sticky. Because the naphthenic acids obtained were extracted from a calcium naphthenate deposit, they will be called ARN acid in this paper. Tetraprotic naphthenic acids (ARN) are not commercially available. The only way to obtain them is from a calcium naphthenate deposit from an oilfield.28 Therefore, characterization and determination of purity are essential after the extraction process. Characterization of the Clean Calcium Naphthenate Deposit and the ARN Acid. The clean calcium naphthenate powder was characterized by XRD. To verify the existence of calcium carbonate in the sample, analysis of calcium carbonate P.A. was also carried out. The results are shown in Figure 1. The diffractogram of the clean calcium naphthenate deposit presented typical peaks of calcium carbonate (2θ = 24°, 30°, 37°, 40°, 44°, 48°, 48°, and 58°). However, the quantity of these components in the deposit was small, as indicated by the relatively low intensity of the signals when compared to the result obtained from the calcium carbonate standard. Besides this, an amorphous halo was observed in the 2θ range from 12° to 23°, suggesting the presence of non-crystalline compounds, which can be attributed to calcium naphthenate. This result shows that the deposit taken from the platform equipment was mixed; i.e., there was precipitation of both calcium carbonate and naphthenate. The literature10 mentions the possibilty of this happening, attributing the loss of CO2 during oil processing to an increase in the pH caused by the produced water, followed by competition between the formation of calcium naphthenate and the precitipation kinetics of the calcium carbonate.

inhibitor. For the inhibitors soluble in the oil phase, the samples without inhibitor contained 1000 mg/L ARN acid and the samples with the inhibitor contained 1000 mg/L ARN acid and 2000 μL/L of the inhibitor. For the inhibitors soluble in the aqueous phase, the samples without inhibitor contained only the salts described above, while the samples with inhibitor contained, besides the salts, 2000 μL/ L of the inhibitor. To assess only the effect of adding the inhibitors to the systems, the biphasic mixture test was also conducted without the addition of ARN acid in the oil phase and with the addition of inhibitors at concentrations of 125, 200, 250, 500, 1000, 1500, and 2000 μL/L. Evaluation of the Water/Oil Interface Using Oscillatory Rheometry (Du Noü y Ring). A Haake Mars II rheometer equipped with a platinum Du Noüy ring sensor with a diameter of 20 mm was used for the oscilatory rheometry tests, at a temperature of 25 °C, controlled by a Peltier plate, operating with the Rheowin program. A total of 10 mL of the water phase was placed in a glass cuvette (5 cm diameter and 3 cm high). Then, the ring was positioned just below the air/water surface; 10 mL of the oil phase was added on top of the water phase; and the ring was positioned at the water/oil interface. To evaluate the frequency and stress sweep and stabilization time, the water phase was prepared with the same composition described for the biphasic mixture test, also maintaining the pH at 9. To assess the influence of the magnesium ion concentration, a test was conducted without the addition of calcium chloride dihydrate. To evaluate the influence of the calcium ion concentration, three dihydrated calcium chloride concentrations were studied (1000, 4000, and 14 500 mg/L). Sodium chloride concentrations of 100 000, 120 000, and 220 000 mg/ L were used to assess the influence of salinity. The organic phase was prepared with a mixture of n-decane and toluene (2:8), with different ARN acid concentrations depending upon the test: frequency, stress, and time sweep, 100 mg/L; time sweep in the test with the control sample, 0 mg/L; influence of calcium and magnesium concentrations, 10 mg/L; and influence of salinity and inhibitor concentration, 25 mg/ 1728

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sample was free of water contamination. We observed bands with wavelengths of 2927 and 2859 cm−1, related to the symmetric axial deformation of the CH2 and CH3 groups corresponding to the aliphatic skeleton of the ARN acid. A band observed at 1708 cm−1 confirmed the presence of carboxylic acid groups, corresponding to the axial deformation of the −CO dimer. Furthermore, this signal was strong, suggesting the abundance of CO2H groups. Symmetric deformation of the plane of the CH2 groups of the ring was observed at 1461 cm−1, while the deformation of the COH group appeared at 1410 cm−1. The peak at 1377 cm−1 corresponds to the symmetric angular deformation of the CH3 group. The band between 1280 and 1100 cm−1 is characteristic of the axial deformation of the CO group, and the band at 943 cm−1 is related to the out-of-plane deformation of OH.35 The absorption bands observed are compatible with the structure of ARN acid presented in the literature.9,13 The DSC results showed a well-defined glass transition temperature (Tg) of the naphthenic acid (ARN) starting at −4 °C. This Tg occurrence in ARN acid samples has often been cited in the literature, and this behavior is generally attributed to the intramolecular hydrogen bond between the acid groups, forming a branched network.15,16,33,36 In the clean calcium naphthenate deposit, a Tg was observed at 103 °C, which is in accordance with the hard solid aspect of the material at room temperature. This higher Tg can be attributed to the more rigid bonds of the calcium naphthenate network formed by the strong ionic bonds between the carboxylate groups and the calcium ions. It is known that deposits from different origins present different Tg values.15 The 1H NMR spectrum presented chemical shifts characteristic of the structure of naphthenic acids. The spectrum also revealed the presence of benzoic acid (internal standard for quantification). The resonance signals at 0.0 and ∼7.2 ppm refer to tetramethylsilane (TMS) and residual CHCl3 in CDCl3 used in the analysis. The signals between 0.7 and 2.7 ppm are typical of tetra-acid, referring to the hydrogen molecules connected to the aliphatic and cyclic hydrocarbon chains. Signals related to the ortho-, meta-, and para-hydrogen of the aromatic ring of the benzoic acid were observed at approximately 8.0, 7.4, and 7.5 ppm, respectively.9,28,35,37,38 The acid contents28 of the samples were quantified by the intensities of the peaks at approximately 2.05 and 2.22 ppm for ARN acid and 8.05 ppm for benzoic acid. The ARN acid content in the sample of naphthenic acids isolated was 81.9%. Other authors16 have reported purity of the extracted ARN acid varying between 80 and 94%. According to those authors, the samples were contaminated by other constituents, mainly monoacids. In this paper, we consider the naphthenic acid sample to be ARN acid, because the purity fell in the above range. Besides this, the presence of monoacids better reflects the conditions found in the field. The values of the number average molar mass (⟨Mn⟩), weight average molar mass (⟨Mw⟩), and polidispersity (⟨Mw⟩/⟨Mn⟩) found by SEC for the ARN acid were 1594 g/mol, 1911 g/mol, and 1.19, respectively. The literature13 cites a range of ⟨Mn⟩ values between 1.227 and 1.235 g/mol, obtained by other techniques. It can be considered that the results are in good agreement with those already cited in the literature because SEC is a technique that requires calibration with standard molecules and the results depend upon the interaction between solute and solvent; that is, the result is not absolute. On the other hand, SEC is the only technique that provides

Figure 1. X-ray diffractograms of calcium carbonate P.A. (gray line) and clean naphthenate deposit (black line).

The TGA of the clean calcium naphthenate deposit and the ARN acid showed that the ARN acid presents a discrete and gradual mass loss starting at a temperature of approximately 340 °C (∼8% mass loss). The mass loss is more significant in the range from 340 to 480 °C (∼88% mass loss), resulting in a total mass loss of 96%. The degradation up to 340 °C can be attributed to the decomposition of the naphthenic acids with lower molar mass. The high degradation temperature of the naphthenic acids (ARN) obtained in this study is in agreement with the structure proposed in the literature,13 indicating that it contains four carboxyl groups able to interact through strong hydrogen bonds.33 The result obtained for the clean calcium naphthenate deposit was very similar to that for ARN acid, with the main difference being greater stability of the clean deposit and a second mass loss stage starting at 620 °C. This greater stability is due to the fact that, in carboxylic acid salts, the electrostatic forces hold the ions in the network formed, requiring higher temperatures for the material to start degrading. The next mass loss stage can be attributed to the start of decomposition of the calcium carbonate present in the sample, as identified in the XRD analysis.34 For both samples, the thermal degradation temperature was relatively high. According to the literature,16 this is mainly due to the presence of the rings of five members of the aliphatic chain. Although we used a hydrochloric acid solution in the extraction process, the FTIR spectrum of the naphthenic acids (ARN acid) sample, shown in Figure 2, did not exhibited a water vibration band at 3450 cm−1, providing evidence that the

Figure 2. FTIR spectrum of ARN extracted from the calcium naphthenate deposit. 1729

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relatively high concentrations (inhibitor/ARN acid ratio of 2:1), and at this dose, inhibitor F performed a little better than inhibitor G. Inhibitor H presented excellent efficiency at high doses (inhibitor/ARN acid ratios of 3:2 and 2:1), where there was the total absence of solids or precipitates. The use of the biphasic mixture test for selection, despite the simplicity of the technique, can cause disagreement in evaluation of the results obtained by different operators because of the fact that this test is based mainly on visual observation. This justifies the search for more precise and sensible evaluation methods. The products selected for performance assessment by oscillatory rheometry using the Du Noüy ring were inhibitors D, E, and H, because they performed the best, i.e., caused the smallest quantify of solids and best defined water/oil interface, taking into consideration the doses necessary to reach their performance levels. Evaluation of the Rheological Behavior of the Interfacial Film Using Oscillatory Rheometry with Du Noüy Ring. In this technique, the rotor turns the ring, located at the interface, alternately to the right and to the left, allowing for separate analysis of the contributions of the viscous and elastic parts of the film. Because we did not find in the literature any report of the use of this type of test to assess the formation of calcium naphthenate films at the interface between two immiscible phases, it was necessary first to investigate the most suitable parameters (frequency, oscillation stress, and stabilization time), to establish a reliable analytic method. Investigation of the Analytic Parameters and Test Procedures. We initially used a stress sweep from 10−4 to 10−2 Pa m, with a constant frequency of 0.1 Hz. To assess the optimal stabilization time (complete film formation), the stress sweep test was conducted with individual samples of the same system after stabilization for 30, 60, and 90 min, where the stabilization time was defined as the interval during which the oil/water phases were kept in contact at rest before performing the analysis. For these tests, the oil phase was composed of 100 mg/L ARN in n-decane/toluene (2:8, v/v) and the aqueous phase was 110 000 mg/L sodium chloride, 4000 mg/L dihydrated calcium chloride, 650 mg/L magnesium chloride hexahydrate, and 700 mg/L sodium bicarbonate, with pH 9. G′ and G″ values in the range of 0.03−0.07 Pa m were initially registered, which immediately declined to values around 0.005 Pa m (G′) and 0.002 Pa m (G″) for oscillation stresses between 0.001 and 0.002 Pa m. The values of G′ were higher than those of G″ for the three assays. In the tests after 30 min of stabilization, after the first sharp drop in the value of G′, it continued to fall with increasing oscillation stress. In the tests after stabilization for 60 and 90 min, G′ remained constant with increasing oscillation stress starting at ∼0.0015 Pa m. Therefore, we chose 60 min of stabilization and oscillation stress of 0.003 Pa m. The frequency sweep test was carried out in the range from 0.01 to 10 Hz and a constant stress of 3 × 10−3 Pa m, taking into consideration that (i) the stress should be in the linear viscoelastic region, because in this region, the rheological properties vary little with the levels of deformation applied,39 and (ii) the instruction manual of the RheoWin software40 recommends that the stress value chosen be at least 5 times greater than the minimum torque of the device, which is 5 × 10−8 N m. At first, G′ was greater than G″, but at the frequency of ∼0.072 Hz, the curves crossed and G″ became greater than G′. This result indicates that the film formed at the interface after 60 min of stabilization presents elastic behavior for

polydispersity, and therefore, it is possible to know how different the molecule sizes are. Selection of Inhibitors by the Biphasic Mixture Test. Biphasic mixture tests were used to pre-select the products to be evaluated by interfacial rheology as well as to observe the degree of inhibition as a function of the dosage, to obtain parameters for correlation with the results of the rheological analysis. This is a simple test, in which the product performance is evaluated according to the smallest quantity of solid formed, best water separation, and best-defined interface, visually observed. The solid particles tend to appear with time.22,29 Tests performed without the presence of ARN acid in the oil phase showed that none of the eight inhibitors evaluated formed solids or emulsions in the system, at all of the concentrations studied. We first present the results obtained with the inhibitors soluble in the oil phase (A, B, C, D, and E). Inhibitor A did not perform well at the inhibitor/ARN acid ratios of 1:8, 1:5, and 1:4. The action of the additive was only observed starting at the ratio of 1:2, after which its performance improved progressively with an increased concentration, indicated mainly by the reduction in the quantity of solids formed and also the better defined aqueous phase, without the presence of an emulsion. Inhibitor B was more effective than inhibitor A, but the continued increase of the concentration did not produce an equal enhancement of performance. This inhibitor exhibited an optimal concentration at an inhibitor/ARN acid ratio of 1:2; that is, at this concentration, the lowest quantity of solids was observed. Stronger doses enhanced the formation of the solid, although reducing the formation of the emulsion. Inhibitor C did not have any effect at the concentrations tested; all of the aqueous phases were opaque, and the interfaces were poorly defined. The performance of inhibitor D improved with an increased concentration. The optimal point occurred at an inhibitor/ARN acid ratio of 1:2, because doses stronger than this did not cause significant improvements with respect to the quantity of calcium naphthenate formed. Even though the optimum concentration of inhibitor D was the same as inhibitor B, inhibitor D presented a lower quantity of solids at lower concentrations. A low dose of inhibitor E (inhibitor/ARN acid ratio of 1:5) led to a good efficiency, that is, very low quantity of solids. The performance of inhibitor E as a function of the concentration is shown in Figure 3; the concentrations are identified by the inhibitor/ARN acid ratio, as presented in Table 1. With respect to the inhibitors soluble in the water phase (F, G, and H), inhibitors F and G only had some effect at

Figure 3. Photography of the biphasic mixture test using inhibitor E (soluble in the oil phase), 24 h after manual agitation. The first tube on the left refers to the test without adding inhibitor. The other tubes are identified with the inhibitor/ARN acid ratio used, as described in Table 1. 1730

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than G″ after about 220 s. In turn, at the concentration of 10 mg/L, it took longer (about 1250 s) for this to happen. Additionally, for these lower concentrations, no break of the film was observed during the testing period (up to about 3500 s). The higher G′ observed for the system containing 10 mg/L ARN acid in relation to the other concentrations can be attributed to the formation of a greater number of bonds in the film because of the longer formation time. On the basis of the results of this test, we started to perform the analyses immediately after the contact between the phases and the positioning of the ring at the interface. To confirm that the variations observed in G′ and G″ came from the formation of the film, we performed a test with the same synthetic aqueous phase (4000 mg/L Ca2+), placing it in contact with an oil phase without the addition of ARN acid. We then compared the result to that obtained with the system containing 10 mg/L ARN acid. Figure 5 shows the results for

frequencies smaller than 0.07 Hz. Above this frequency, there is dominance of viscous behavior, which can be attributed to the rupture of the film. The use of the term gel with a solid-like characteristic is accepted for materials for which the storage modulus (G′) is higher than the viscous modulus (G″) for a period of at least 1 order of magnitude.41 The material analyzed can be considered to behave similar to this because the period after which the curves crossed was near 1 order of magnitude. The frequency chosen for the tests was 0.04 Hz, for meeting the main requirements: (i) assurance of integrity of the film formed, to avoid its rupture, and (ii) achievement of a reasonable analysis time. Because the viscoelastic region varies as a function of the stress, we performed a new stress sweep with a constant frequency of 0.04 Hz, to confirm that the parameters chosen were within the linear viscoelastic region after the formation of the film. The results showed virtually constant values of G′ and G″ up to an oscillation stress of ∼0.006 Pa m, confirming the adequacy of the parameters established. During the tests of frequency and oscillation stress sweep, it was possible to observe the formation of a film visually (assumed to be calcium naphthenate). Because the concentration of ARN acid used was relatively high (100 mg/L), as was the pH of the system (pH 9), the calcium naphthenate film began to form almost instantaneously after contact of the phases. Therefore, although rheological evaluation as a function of time usually supplies information on the change of the system with time, it was not possible to monitor the change in the rheological behavior of the film at the water/oil interface at the beginning of its formation. Therefore, to retard the formation of the film, we reduced the ARN acid concentration. Figure 4 shows the rheological test results for the three ARN

Figure 5. Storage modulus (G′) and loss modulus (G″) of the interface as a function of time for two different systems: (a) with 10 mg/L ARN acid and (b) without adding ARN acid.

G′ and G″ as a function of time for these two systems. The G′ value in the blank sample was so small, on the order of 10−6 Pa m, that it could be considered that G′ → 0, so that by this definition, this interfacial system was classified as perfectly viscous during the entire test interval (1 h). This behavior is as expected for a system with two pure phases, without a surfactant. In turn, in the system propitious for the formation of calcium naphthenate, there was a greater increase of G′ in relation to G″ as a function of time. After approximately 1300 s, this behavior inverted, with G′ becoming greater than G″. After about 2000 s, G′ was significantly greater than G″, which is characteristic behavior of a viscoelastic gel. As proposed, this confirmed the possibility of studying the behavior of the interface of these systems by means of interfacial rheology by the oscillatory shear method, using a Du Noüy ring. However, other details of the experimental procedure still required development, as performed below. To verify the reproducibility of the method, we performed the rheological assay in duplicate for the system with 10 mg/L ARN acid in the oil phase and observed that, although the behavior was practically the same (increase of G′ over G″ with time), it took longer for G′ to become greater than G″. We believe this difference is due to the difference in time elapsed to start the test: the faster the start of the test, the lower the contact time of the phases, and thus, it will take longer for G′ to surpass G″. Therefore, it was necessary to standardize the protocol of the rheological test, composed of the following steps: (i) add the aqueous phase in the recipient for the test, position the ring as near as possible to the water/air interface,

Figure 4. Storage modulus (G′) and loss modulus (G″) of the interface as a function of time for three different concentrations of ARN acid, at a constant frequency (0.04 Hz) and shear stress (0.003 Pa m). The ARN acid concentrations studied were (a) 10 mg/L, (b) 25 mg/L, and (c) 100 mg/L.

acid concentrations used (10, 25, and 100 mg/L) as a function of time. The G′ and G″ readings confirmed the visual observation of the practically instantaneous formation of the film for the ARN acid concentration of 100 mg/L, because from the very start, G′ was greater than G″. The film formed was probably still very fragile, and after approximately 1500 s, the stress provided by the oscillation of the ring ruptured the film and G″ became greater than G′. We also visually observed the rupture of the film. For ARN acid concentrations below 100 mg/L (25 and 10 mg/L), the test started with G″ > G′, indicating that the film had not yet formed or was just starting to form. At the concentration of 25 mg/L, G′ became greater 1731

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the increase in the calcium ion concentration from 1000 to 4000 mg/L. In turn, the increase in the calcium level from 4000 to 14 500 mg/L did not significantly influence the time for formation of the viscoelastic gel, although the film formed with 14 500 mg/L calcium (not shown) had a more gel-like appearance. After 3300 s, the systems with all of the concentrations presented the same elastic modulus (G′) value, which was near that of G″. This can be attributed to the rupture of the film formed. The influence of the increase in ionic force from the addition of sodium chloride on the properties of the water/oil interface is shown in Table 4. An increase in salinity of 20% (from 100

but keep it totally submersed in the aqueous phase; (ii) input the necessary data for the software of the device (description of the analysis, stress, frequency, and type of test); (iii) with the aid of a pipet, add the oil phase over the aqueous phase; and (iv) as fast as possible, position the ring at the water/oil interface and start the analysis. By following these steps, we carried out three tests with the system containing 25 mg/L ARN acid in the oil phase and 4000 mg/L Ca2+ ions in the aqueous phase, obtaining times of 226, 200, and 175 s for the curves of G′ and G″ to cross. In the deposits found in oil fields, calcium naphthenate is predominant, although other bivalent cations, such as Ba2+ and Mg2+, are part of the composition of the produced water. Therefore, we performed a test to verify if magnesium ions influence the formation of the naphthenate film, by comparing two systems containing the same oil phase [mixture of n-decane and toluene in a ratio of 2:8 (v/v) and 10 mg/L ARN acid] and two different aqueous phases: one containing 650 mg/L Mg2+ and the other without any ions. The magnesium ion concentration was established on the basis of the average found in the produced water in the oil field from which the calcium naphthenate had been obtained. The variation in G′ was very small between these two systems (on the order of 3 × 10−6 Pa m); thus, there was no formation of a viscoelastic gel just because of the influence of Mg2+ ions. This result is in accordance with that observed by potentiometric titration, metal ion depletion, and characterization of the precipitates, showing that the affinity of ions for a model ARN acid molecule decreases in the following order: Ca2+ > Ba2+ ≈ Sr2+ > Mg2+.42,43 Because the presence of magnesium ions did not contribute to the formation of the interfacial film, this metal was added to the composition of the aqueous phase, so that the synthetic brine would be as near as possible to the real composition of produced water in the oil field. Effect of Calcium and Salinity (Ionic Force) Contents on the Interfacial Film Formation. Besides the type of bivalent cation, the formation of the naphthenate film depends upon the metal2+/ARN acid ratio.36 To verify the influence of the calcium ion concentration on the formation of this film by oscillatory rheometry, we performed tests with differing calcium ion concentrations in the aqueous phase (with pH 9) and with the oil phase containing 10 mg/L ARN acid. The increase in the concentration of calcium ions led to an increase in the difference between G′ and G″ after the formation of the film, i.e., after G′ becomes higher than G″ (not shown). The increase in the concentration from 1000 to 4000 mg/L raised the strength of the gel formed (Table 3), as observed in the results obtained by shear force with a biconical accessory.44 This agreement between the results shows that the method presented in this paper is suitable to assess the rheological properties of this type of interfacial film. Furthermore, the method allows for the evaluation of the time for the formation of a viscoelastic film, which declined from 1630 to 1300 s with

Table 4. G′ and G″ at the Crossover and the Respective Time in Which the Crossover Takes Place as a Function of the Sodium Chloride Concentration

G′ and G″ at the crossover (Pa m)

time for the crossover (s)

1000 4000 14500

0.50 × 10−2 2.18 × 10−2 3.20 × 10−2

1620 1300 1200

G′ and G″ at the crossover (Pa m)

time for the crossover (s)

100000 120000 220000

0.75 × 10−2 0.93 × 10−2 1.73 × 10−2

593 633 712

000 to 120 000 mg/L) did not cause significant changes in the behavior. However, a further increase to 220 000 mg/L caused the formation of an interfacial film with a higher elastic modulus, which took longer to form a viscoelastic gel. In a recent publication,45 it was verified by NMR measurements that the kinetic of partitioning and dissociation is much slower in the presence of sodium ion compared to calcium; therefore, a small increasing in the sodium ion does contribute significantly to the elastic modulus increasing. Nevertheless, it was also observed45 that the concentration of naphthenic acid in the aqueous phase increases with the increasing ionic strength of brine, which is in agreement with the increase of the elastic modulus with the increasing salinity (Table 4). Effect of the Type and Concentration of Chemical Inhibitor on the Formation of the Interfacial Film. For the rheological tests, the inhibitor/ARN acid ratios were maintained the same as in the biphasic mixture tests to evaluate the efficiency of the inhibitors, to facilitate correlation of the results obtained by these two methods. In the evaluation of naphthenate inhibitors by rheology, a delay in the development of a strong film and a reduction in its resistance (low elastic modulus) are desired actions of a good inhibitor.46 Figure 6 depicts the results of G′ as a function of the time when inhibitor D was added to the oil phase at varied cocentrations while keeping the ARN acid concentration equal (25 mg/L) in the oil phase. At the concentrations of 3.0 and 5.0 ppm (Figure 6a), there was a delay in the increase of G′, which became more accentuated with an increasing concentration. After 500 and 1500 s, the systems containing 3.0 and 5.0 ppm of inhibitor resumed having a high G′ value, respectively. Because oilprocessing plants have a residence time of around 40 min (2400 s), this delay to increase G′ would be insufficient to prevent the deposition of calcium naphthenate. The value of G′ reached was higher by an order of magnitude than that observed in the blank test. In these cases, inhibitor D helped increase the rigidity of the calcium naphthenate film. Figure 6b shows that higher concentrations of inhibitor D (6.25, 12.5, 25.0, 37.5, and 50.0 ppm) were able to keep the G′ value of the interface at low levels (G′ → 0) within the period analyzed (up to 3600 s). This interfacial behavior is expected of

Table 3. G′ and G″ at the Crossover and the Respective Time in Which the Crossover Takes Place as a Function of the Calcium Ion Concentration Ca2+ content (mg/L)

salinity (mg/L)

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Figure 6. Elastic modulus (G′) as a function of time to evaluate the formation of the calcium naphthenate film at the interface as a function of the concentration of inhibitor D in the oil phase: (a) 0.0, 3.0, and 5.0 ppm and (b) 0.0, 6.25, 12.5, 25.0, 37.5, and 50.0 ppm.

Figure 7. Elastic modulus (G′) as a function of time to evaluate the formation of the calcium naphthenate film at the interface as a function of the concentration of inhibitor E in the oil phase: (a) 0.0, 3.0, 6.5, and 12.5 ppm and (b) 0.0, 18.75, 25.0, 37.5, and 50.0 ppm.

a system having two pure phases without the formation of films at the interface. The behavior of inhibitor E (Figure 7) was similar to that of inhibitor D, i.e., an increase in the concentration enhanced the efficiency of the additive within the concentration range analyzed. However, inhibitor E only performed well at higher concentrations (starting at 18.75 ppm), when G′ remained at low values as a function of time. The behavior of inhibitor H (Figure 8), soluble in the aqueous phase, was very different from that of inhibitors D and E, soluble in the oil phase. For the entire concentration range analyzed (from 3.0 to 50.0 ppm), the value of G′ varied similarly to that in the blank test, i.e., at first, G′ increased with time and then decreased with a tendency toward stabilization. The addition of inhibitor H led to a reduction in G′ in relation to the blank test but at values significantly higher than those attained with the addition of inhibitors D and E, indicating greater rigidity at the interface. Besides this, there was no coherent relation between the value of G′ attained and the concentration of the inhibitor. This difference in rheological behavior of the interface with time suggests that the mechanism of action of inhibitor H is different from that of inhibitors D and E. Indeed, according to information from the supplier, inhibitor H works by sequestering calcium ions, thus reducing the availability of these ions to react with the ARN acids at the water/oil interface. However, according to this mechanism, a

Figure 8. Elastic modulus (G′) as a function of time to evaluate the formation of the calcium naphthenate film at the interface as a function of the concentration of inhibitor H in the aqueous phase: 0.0, 3.0, 5.0, 6.25, 12.5, 25.0, 37.5, and 50.0 ppm.

reduction in the value of G′ would be expected with an increasing concentration of the product. On the other hand, inhibitors D and E, possibly polyethoxylated with low molar mass, are highly active at the interface, and because they have a 1733

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much greater affinity for the water/oil interface than ARN acids, they hinder the contact of these acid molecules with the calcium ions at the interface.26 Comparison between the Results Obtained in the Rheological Tests and the Biphasic Mixture Tests. For inhibitor D, the results obtained from the two techniques were broadly similar, differing only in the concentration above which no further performance improvement was noted, which were inhibitor/ARN acid ratios of 1:2 (12.5 ppm of inhibitor) and 1:4 (6.25 ppm of inhibitor) for the biphasic mixture test and the rheological test, respectively. For inhibitor E, the results were discordant: in the biphasic mixture test, the visual observation of the aqueous phase induced the conclusion that an increased inhibitor concentration aggravates the deposition, while the rheological test revealed that an increased concentration favored a significant reduction in the value of G′ of the interface for doses ≥18.75 ppm (inhibitor/ARN acid ratio of 3:4). Discordant results were also observed for inhibitor H, for which the biphasic mixture test showed a limpid aqueous phases for inhibitor/ARN acid ratios of 3:2 (37.5 ppm) and 2:1 (50.0 ppm), while the rheological test provided evidence of high resistance of the interfacial film throughout the entire concentration range analyzed, with a slightly lower G′ value being observed for the inhibitor concentration of 3 ppm (inhibitor/ARN acid ratio of 1:8). It is important to note that the evaluation of the biphasic mixture test is based only on visual inspection of the quantity of residue formed and/or the quality of the water/oil interface, with no inference regarding the “quality” of the film formed at the water/oil interface. Because the viscoelastic characteristic of the film is responsible for altering the behavior of G′, the formation of naphthenate and the action of the inhibitors can be evaluated more accurately through rheological analysis of the interface.

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slower separation of the carboxylic acid because of the increase in the ionic force in the aqueous medium.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Conselho Nacional de Desenvolvimento ́ Cientifico e Tecnológico (CNPq, Brazil), Coordenaçaõ de ́ Aperfeiçoamento de Pessoal de Nivel Superior (CAPES, Brazil), and Petrobras for the financial support and NALCO Champion, An Ecolab Company, and Dorf Ketal Chemicals for donating the commercial naphthenate inhibitors.

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CONCLUSION

The biphasic mixture test results for assessment of the performance of the calcium naphthenate inhibitors did not present correlation with the viscoelastic behavior of the film observed in the oscilatory rheology with Du Noüy ring. This can be attributed to the fact that the biphasic mixture test is based only on visual inspection of the quantity of residue formed and/or the quality of the water/oil interface. The evaluation of the calcium naphthenate inhibitors by oscillatory rheology with Du Noüy ring, besides being more precise, has the advantage of requiring smaller quantities of ARN acid than the biphasic mixture test. This is an important advantage because the tests are conducted with acid extracted, with a low yield through a laborious method, from deposits formed during petroleum production. The rheological method enables assessment of the action of naphthenate inhibitors on the start of film formation, the viscoelastic characteristic of the film, and its rupture. Besides this, in the present study, it revealed that the increase in the concentration of calcium ions in the aqueous phase, with the ARN acid concentration in the oil phase kept constant, caused faster formation of the calcium naphthenate film at the water/oil interface as well as a higher value of G′, which can be attributed to an increase in the number of ion bonds of the film. With respect to the influence of salinity, an increase in the concentration of sodium chloride in the aqueous phase led to the formation of an interfacial film with a higher elastic modulus and an increase in the time necessary for the formation of a viscoelastic gel, which can be attributed to the 1734

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