USE OF ION CHROMATOGRAPHY TO DETERMINE SIMULTANEOUSLY HIGH CHLORIDE AND LOW IONS CONCENTRATIONS IN PRODUCED WATER

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1 USE OF ION CHROMATOGRAPHY TO DETERMINE SIMULTANEOUSLY HIGH CHLORIDE AND LOW IONS CONCENTRATIONS IN PRODUCED WATER a,b Costa, E. C. T. A. 1 ; a,b Frota, T. M. P.; a,b Silva, D. R. a Universidade Federal do Rio Grande do Norte b Núcleo de Estudo em Petróleo e Gás Natural ABSTRACT Waters normally produced with petroleum are rich in salt, mainly chlorides, which, in contact with metal surfaces, may induce corrosive processes. The determination of the concentration of chlorides and other ions when the chloride content is high is very difficult due to many interfering parameters. In this study, we report on the optimization of a new application of ion chromatography to analyze produced water containing high concentration of chloride, which can be quantified along with the concentrations of different ions also present in the water but in lower concentrations, such as fluoride, bromide, nitrate, sulfate, acetate and formate. Sample preparation and conditions of elution and separation were adapted to the nature of this kind of sample. Analytical curves were drawn for chloride and at reduced concentrations for the other ions. Using this technique, it was possible to determine chloride and the remaining ions simultaneously and free from interfering agents. KEYWORDS chloride in high concentration; ion chromatography; produced water; interfering agents; optimization 1 To whom all correspondence should be addressed. Address: Av. Senador Salgado Filho, S/N, Lagoa Nova, CEP , Natal-RN, Brazil emily.tossi@gmail.com 29

2 1. INTRODUCTION Ion chromatography was originally described by Small and coworkers in 1975 as a new technique for the characterization of a variety of solutions and their respective anions and cations (Small et al., 1975). As with all chromatography techniques, there must be precise analytical parameters to ensure accuracy of retention times and of the effects that these parameters may have on the retention times of each species. Ion chromatography (IC) remains one of the most powerful instrumental techniques for the determination of inorganic anions and organic acids in different samples (Krataa et al., 2009) such as natural waters, snow, ice, rain, and air (Neal et al., 2007). Oil production water, which is called connate water, may exist in oil reservoirs ever since its formation or as a mixture with underground water, called injection water, which may be used in secondary recovery processes. Injection water can have a widely varied composition (Xiaoyan et al., 2009). By examining oil produced water specifications, one can find a limit of up to 100,000 mg.l -1 of total dissolved solids (Gabardo, 2007), that is, inorganic compounds. However, this composition can vary as a function of a number of factors, such as the use of fresh water captured in production wells for injection, seawater or the oil production water itself. The latter may contain salt and emulsified oil and, during oil separation processes, may receive chemical products such as demulsifiers and anti-foaming agents. Drilling and completion fluids can also be added (Fakhru l-razi et al., 2009). Completion fluids are used in oil production operations to hold the reservoirs using the hydrostatic pressure that is imposed by them, causing a minimum of damage to the producing formation. An inorganic salt is used in these fluids to avoid hydration of the clay formation, which causes swelling and subsequent damage to the formation. These fluids generally use sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl 2 ) as inorganic salts and the choice is made according to the specific weight of the fluid to be used. These salts contain high concentrations of chloride and are aggressive to the environment, causing corrosion in metallic materials (Costa et al., 2002). Renowned and widely studied classical methods for determining chlorides in water, such as argentometric methods, are generally used (Pereira, 2007). Another technique consists in using a selective ion electrode (potentiometric titration) but, in both techniques, some components in the samples may cause interference, impairing the determination of the real concentration of different species present (Harris, 2006). The effect of interfering agents is even more significant in complex matrices (Hu et al., 1999), such as the oil production waters containing these fluids. This means that before identifying and determining the components of the mixture, they must be separated or removed. Since IC is a separation technique, it can be used as an alternative to determine chloride in samples with high concentrations of other ions such as bromide, iodine, sulfide, etc. In this study, IC was used as an alternative technique to determine anions in oil production water, a matrix considered problematic due to its complexity. We propose to test the efficiency of IC to determine high chloride concentrations (> 100,000 mg.l -1 ). 2. MATERIALS AND METHODS The anions were determined using the ion chromatography technique, with detection and suppression of conductivity, according to the procedure 4110-C of the APHA Standard Methods (1998) Single-Column Ion Chromatography with Electronic Suppression of Eluent Conductivity and Conductimetric Detection. Due to being considered a complex matrix, a number of precautions were taken during the preparation of the samples, such as prior filtration in activated carbon filters to retain organic materials that could damage the structure of the analytical column and filtration with cellulose membranes, with a 0.45 µm porosity, to retain any suspended solids. After this cleaning stage, the samples were diluted 100 and 500 times to fit the concentrations to the analytical curves (Table 1). The samples were analyzed in a DIONEX ICS

3 Table 1. Conditions of the analytical curves obtained in the simultaneous determination of anions. STANDARD CONCENTRATION (mg.l -1 ) R 2 P1 P2 P3 P4 PCl1 PCl2 PCl3 PCl4 PCl5 Fluoride Acetate Formate Chloride Bromide Nitrate Sulfate P1, P2, P3 and P4 are multi-elementary standards. PCl1, PCl2, PCl3, PCl4 and PCl5 are chloride standards in high concentrations; R 2 = linear correlation coefficient. ion chromatograph, at the chromatographic conditions described in Table 2. Analytical curves were drawn for fluoride, acetate, formate, chloride, nitrate, bromide, nitrite, sulfate and phosphate, simultaneously, and at different concentrations, using standards manufactured by DIONEX, traceable to NIST (National Institute of Standards and Technology). Five samples from a single well were analyzed, collected on consecutive days, each one with two different dilutions and analyzed in triplicate. 3. RESULTS AND DISCUSSION Seven anions have been detected and quantified. Table 3 shows the results obtained in the analyses performed in this study. It can be observed that chloride and acetate were the anions with the highest concentrations, and chloride ions corresponded to approximately 95% of the sample anions. Also, high concentrations of acetate may indicate that the well from which the samples were collected can be undergoing acid chemical cleaning to dissolve incrusted salts (Xiaoyan, 2009), which tend to decrease local production. Another anion that showed a significant concentration was bromide, a potentially interfering agent in the determination of chloride using titrimetric methods. Sulfate ions, in the presence of sulfatereducing bacteria (SRB), are reduced to sulfide ions, which must be monitored due to their potential to promote corrosion (Penna et al., 2003). These anions also react with silver in potentiometric titration, which may give a false result in the determination of chloride by this technique. This type of interference does not occur in ion chromatography, taking into account that it Table 2. Conditions used in the chromatographic method. CHROMATOGRAPHIC CONDITIONS Column IONPAC AS19, mm Flow 1.0 ml/min Eluent 10 mm KOH (0-10min), 45 mm KOH (10-30min) Loop 25 µl Detector Condutivity with electrochemical suppression Temperature 30 C Table 3. Results of concentration media of anion analyses by ion chromatography. SAMPLE CONCENTRATION (mg.l -1 ) Chloride Fluoride Bromide Nitrate Sulfate Acetate Formate 1 168, , , , ,

4 is a separation method (Weiss, 2004). It is interesting to observe that the high chloride concentration, surpassing values of 150,000 mg.l -1, was determined simultaneously with anions at lower concentrations, as fluoride and formate, as shown in Figure 1, where the signal for the conductivity of chloride was approximately 400 µs whilst formate ions showed the smallest signal, less than 0.4 µs. This is more easily observed in the chromatogram in enhanced scale shown in Figure 2. Gabardo (2007) determined anions and organic acids in the Brazilian coast by IC, but chose to remove chloride ions from the sample with silver cartridges for injection in the chromatograph, considering their interference in this technique and quantifying them by potentiometry. In this work, all organic and inorganic anions were determined simultaneously by IC, with the prior dilution and 600 µs chloride - 9, fluoride - 4, acetate - 5, formiate - 6, bromide - 14, nitrate - 16, , sulphate - 20, , , min 0,0 5,0 10,0 15,0 20,0 25,0 30,0 Figure 1. Chromatogram of sample 3. 5,00 µs 4,00 3,00 2,00 1, fluoride - 4, acetate - 5, formiate - 6, chloride - 9, bromide - 14, nitrate - 16, , sulphate - 20, , ,783-0,50 min 0,0 5,0 10,0 15,0 20,0 25,0 30,0 Figure 2. Chromatogram of sample 3 in enhanced scale. 32

5 removal of suspended solids and undesired organic material, with no loss in the final result. Chloride ions have a high affinity for the mobile phase and elute in only 10 minutes of run, avoiding saturation of the chromatographic column or enlargement of peaks. In addition, no peak overlapping or displacement of retention times, owing to the effects of modifying the ionic force of the medium, was observed. Although it has been already reported that high ion concentrations may cause these effects on retention times and the resolution of other species (Bynum, 1981; Jenke, 1981), for some samples considered as problematic, gradient dilutions and elutions are commonly used to solve these problems when determining high anion concentrations jointly with trace anions (Weiss, 2004). The ion chromatography technique, therefore, is able to accurately determinate different ions at low concentrations in the presence of chloride in high concentration without interference. 4. CONCLUSIONS With the procedures adopted and described in this work for the determination of anions in produced water, no significant interference was observed, despite the complexity of the matrix. The determination of high chloride concentrations by chromatography was reproducible, without causing the usual chromatographic disturbances, such as peak overlapping or changes in the retention times of components for other anions. The methodology used was suitable to detect anions in produced water containing high concentrations of dissolved salts (chloride) and different ions in lower concentrations without interferences. This new application of ion chromatography is more efficient then current methods like potentiometric and titrimetric argentometric techniques. 5. REFERENCES Fakhru l-razi, A.; Pendashteh, A.; Abdullah, L.C.; Biak, D.R.A.; Madaeni, S.S.; Abidin, Z.Z. Review of technologies for oil and gas produced water treatment. Journal of Hazardous Materials, 2009, doi: /j.jhazmat (article in press) APHA. Standard Methods for Examination of Water and Wasterwater. Washington, D.C.: 21 st Ed., Bynum, M.A.O.; Tyree Jr, S.Y.; Weiser, W.E. Effects of major ions on the determination of trace ions by ion chromatography. Analytical Chemistry, v.53, p , Costa, A.K.S.; Mattos, O.R.; Joia, C.J.B.M. Seleção de Inibidores para Fluidos de Completação ( Packer Fluids ) 2002, 6 th COTEQ. Salvador, Brazil. (in Portuguese) Gabardo, I. T. Caracterização química e toxicológica da água produzida descartadas em plataformas de óleo e gás na costa brasileira e seu comportamento dispersivo no mar. 250f. Tese de Doutorado. Programa de Pós- Graduação em Química, Universidade Federal do Rio Grande do Norte, (in Portuguese). Harris, D. C. Análise Química Quantitativa. Rio de Janeiro: LTC, 6ª Ed., (in Portuguese). Hu, W.; Haddad, P. R.; Hasebe, K.; Tanaka, K.; Tong, P.; Khoo, C. Direct Determination of Bromide, Nitrate, and Iodide in Saline Matrixes Using Electrostatic Ion Chromatography with an Electrolyte as Eluent. Analytical Chemistry, v.71, p , Jenke, D. Anion Peak Migration in Ion Chromatography. Analytical Chemistry, v.53, p , Krataa, A.; Kontozova-Deutscha, V.; Bencsa, L.; Deutsch, F.; van Grieken, R. Single-run ion chromatographic separation of inorganic and low-molecular-mass organic anions under isocratic elution: Application to environmental samples. Talanta, v.79, p , Neal, M.; Neal, C.; Wickham, H.; Harman, S. Determination of bromide, chloride, fluoride, nitrate and sulphate by chromatography: comparisons of methodologies for rainfall, cloud water and river water at the Plynlimon catchments of mid-wales. Hydrology and Earth System Sciences, v.11, p , Penna, M. O.; Oliveira, H. B.; Silva. E. D. Avaliação da atividade metabólica (produção de H2S) de culturas mistas de bactérias redutoras de 33

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