Laboratory evaluation of synergistic effect of transition metals with mineral scale inhibitor in controlling halite scale deposition

Journal of Petroleum Science and Engineering 175 (2019) 120–128

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Laboratory evaluation of synergistic effect of transition metals with mineral scale inhibitor in controlling halite scale deposition

T

Ping Zhanga,∗, Yuan Liua, Amy T. Kanb,c, Mason B. Tomsonb,c a

Department of Civil and Environmental Engineering, Faculty of Science and Technology University of Macau, Macau, China Department of Civil and Environmental Engineering, Rice University, Houston, TX, USA c Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, Rice University, Houston, TX, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Halite Lead ion (Pb2+) Scale inhibitor Inhibition efficiency

Sodium chloride (halite) has been traditionally regarded as an unconventional oilfield mineral scale, compared with more prevalent carbonate and sulfate scales. However, with the increasing productions from offshore deepwater and shale gas fields, halite scale occurrence becomes more frequent, leading to significant technical and financial challenges to productions. Although low-salinity water dilution remains the primary approach in controlling halite scale threat, chemical inhibition has been evaluated as an alternative halite control strategy due to the intrinsic issues associated with low-salinity water availability and water quality. There are limited studies concerning the combined effect of metals, especially transition metals with scale inhibitors in halite control. In this study, the combined effect of a number of transition metal ions with two common halite scale inhibitors was evaluated in a laboratory setup. It was found that among the transition metals studied, Pb2+ ion demonstrates a synergistic effect with the scale inhibitors evaluated by considerably extending the halite induction time. The calculated inhibition efficiency also indicates that the presence of Pb2+ improves inhibitor performance. Although Pb2+ is an environmental pollutant, an aqueous Pb2+ concentration of up to 10 mg L−1 can naturally occur in both natural gas produced water and oilfield produced water. The operators can take advantage of the reported synergistic effect of Pb2+ with scale inhibitors in this study while designing field halite control strategy. The enhanced inhibition efficiency elaborated in this study can be useful in designing the halite scale control strategy, if a decent amount of Pb2+ is present in the produced water.

1. Introduction

functionality (Bukuaghangin et al., 2016; Vazirian et al., 2016). Moreover, scale can form in the wellbore reservoir immediately surrounding the well, leading to formation damage (Chen et al., 2005; Vazquez et al., 2016). From a corrosion behavior standpoint, the formed scale particles can impact both the uniform and general corrosion behavior of carbon steel pipe (Mansoori et al., 2017, 2018). The most commonly encountered oilfield scales are calcium carbonate (calcite) and barium sulfate (barite) (Frenier and Ziauddin, 2008). Calcite formation is mainly due to physicochemical condition changes, such as temperature increase. Barite formation is typically a result of mixing incompatible waters, especially mixing seawater and formation water during offshore water injection campaigns in the reservoir as an enhanced oil recovery method. Another common oilfield scale is calcium sulfate. Similar to barite, calcium sulfate formation is also water mixing associated. Compared with these common oilfield scales,

Mineral scales (hereafter referred to as “scales”) are hard crystalline inorganic precipitates from aqueous solution (Fink, 2011; Frenier and Ziauddin, 2008). Scale formation in oilfields is a result of exceeding local saturation with respect to a mineral salt due to changes in operational conditions, such as temperature, pressure and solution compositions (Kan and Tomson, 2012; Zhang et al., 2015). Although scale formation is a common household phenomenon, the major financial loss associated with scale formation stems from oil and gas industry, particularly the upstream sector (Zhang et al., 2017). Deposited scale particles can considerably reduce the throughput of oil production facilities and tubing, leading to a substantial reduction in fluid flow rate. Scale deposition can also modify the surface properties of the facilities, such as heat transfer coefficient, resulting in a reduction in facility

Abbreviations: DI water, deionized water; FCN, ferrocyanide; HPHT, high pressure high temperature; ICP-OES, inductively coupled plasma-optical emission spectrometer; PAA, poly(acrylic acid); PIPES, piperazine-1,4-bis(2-ethanesulfonic acid) sodium salt; SI, saturation index; THPS, tetrakis(hydroxymethyl)phosphonium sulfate ∗ Corresponding author. E-mail address: [email protected] (P. Zhang). https://doi.org/10.1016/j.petrol.2018.12.036 Received 11 September 2018; Received in revised form 18 November 2018; Accepted 13 December 2018 Available online 14 December 2018 0920-4105/ © 2018 Elsevier B.V. All rights reserved.

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sodium chloride (halite or NaCl) is a less common or “exotic” type of scale formed in high salinity water, especially in high pressure high temperature (HPHT) wells and low water cut wells (Ho et al., 2013; Wylde and Slayer, 2013). A major difference between halite scale and the “more common” oilfield scales is the difference in solid aqueous solubility. For instance, the values of aqueous solubility at 20 °C are only 2.4 × 10−3 and 6.2 × 10−3 g per liter of water for barite and calcite, respectively. While the solubility of halite is as high as 360 g per liter of water (Haynes, 2014). In offshore deepwater operations, the HPHT nature of the offshore fields can cause solution supersaturation with respect to halite due to considerable temperature and/or pressure reduction by lifting the produced fluids from reservoir (Chen et al., 2009; Ho et al., 2014). In shale gas field, hydraulic fracturing treatment involves injection of fracturing fluid into reservoir of a very high reservoir temperature. The rapid temperature elevation during fracturing fluid injection will produce an enormous water evaporation, subjecting the fluid supersaturated with halite and other mineral salts (Vankeuren et al., 2017). Another reason for water evaporation in the gas field is due to the substantial drawdown between the reservoir pressure and the flowing bottom hole pressure (Goodwin et al., 2016). Halite scale can deposit in the reservoir or on the interior surfaces of production tubing or processing facilities, leading to operational issues, such as equipment failure and system shutdown and in extreme cases, field abandonment (Ho et al., 2013). In order to control halite deposition threat, the primary method is water treatment by injecting fresh water or low-salinity water to the upstream of where halite deposition occurs in a batch mode or continuously. Although water dilution can effectively prevent and mitigate halite scale threat, a large quantity of water is typically required to readily reduce scaling ion concentrations and/or to dissolve the precipitated halite (Wylde and Slayer, 2013). Because of this requirement, it is often difficult or impossible to obtain a large amount of low-salinity water. Moreover, the dilution water can often contain a large amount of constituent ionic species, such as calcium, sulfate and bicarbonate. It has been reported by many authors that water dilution treatment can result in mixing of incompatible waters and subsequent carbonate and sulfate scale deposition (Goodwin et al., 2012; Ho et al., 2013, Ho et al., 2014; Lu et al., 2015). Therefore, the source water to be injected typically requires extensive treatment for the sake of controlling corrosion, scale and clay swelling (Goodwin et al., 2012). In light of these challenges associated with water dilution treatment, chemical inhibition has been evaluated as an alternative approach to manage halite threat (Wylde and Slayer, 2013). Scale inhibitors are a class of specialty chemicals capable of delaying scale deposition kinetics (Frenier and Ziauddin, 2008). According to classical nucleation theory, the initially formed crystal nuclei need to overcome an energy barrier in order to aggregate into larger crystals so as to precipitate from aqueous phase (Dave and Garside, 2000; Mullin, 2001). The time duration from the onset of supersaturation to the appearance of scale particles is called induction time (Stamatakis et al., 2005). The presence of scale inhibitors can effectively prolong the induction time so that scale deposition threat can be deferred into downstream facilities where scale threat is easier to manage (Zhang et al., 2017). Different from chelating chemicals, scale inhibitors belong to the category of threshold inhibitors in that scale inhibitors can effectively inhibit scale formation with a concentration level significantly lower than those of the scaling ions (Liu et al., 2016). In oilfield operations, halite scale control is routinely managed by combining the practices of dilution with low-salinity water, injection of scale inhibitors and also limiting drawdown (Goodwin et al., 2012). According to Kelland (2014) on Page 56, there are mainly two classes of halite scale inhibitors, i.e., hexacyanoferrate and nitrilotrialkanamides salts. Moreover, compared with inhibitors for common sulfate and carbonate scales, halite inhibitors are typically dosed at a much higher concentration. Chen et al. (2009) reported a new halite inhibitor product which is able to inhibit halite at a much lower concentration based upon laboratory testing. More recently, an

environmentally friendly polymeric multifunctional halite inhibitor has been reported and compared with traditional inhibitors (Spicka et al., 2012). Another important aspect related to halite scale inhibition is the testing method to evaluate inhibitor performance. A number of studies have reported several laboratory testing methods to evaluate the performance of halite inhibitors. Static jar method is the most commonly used in oilfield to screen and rank the performance of different inhibitor chemicals. This method is easy and fast, suitable for field trials and testing. The drawback of the static jar method is that hydrodynamic conditions, such as shaking, can considerably influence halite deposition (Ho et al., 2013). Another halite inhibitor testing method is based upon a temperature-driven approach by deliberately cooling a heated solution to force precipitate supersaturated halite from the aqueous solution. Although this method is also easy and fast to carry out, the limitation of this method is that only a relatively small scaling tendency of NaCl can be established, limited by the solution boiling point (Wylde and Slayer, 2013). The third method is dynamic loop test by mixing highly concentrated NaCl solution with another chloride-containing solution (e.g., CaCl2 and MgCl2). Compared with the other two methods, this method can best represent the flowing and hydrodynamic conditions in the field. However, this method requires extensive experimental setup and is more time-consuming in conditioning experimental apparatus (Chen et al., 2009). Most of the reported studies on halite inhibitor performances focus on the system with only one inhibitor chemical, without considering the impact of the presence of metals, especially transition metals. Although transition metals are present with a much lower concentration in produced waters than alkaline earth metals, such as Ca2+ and Mg2+, they can have a noticeable impact on scale inhibitor performance (Kan et al., 2009; Smith and Przybylinski, 2006). Kan et al. (2009) reported that the presence of transition metal Zn2+ can function synergistically with two types of common phosphonate scale inhibitors in barite inhibition with a Zn2+ concentration of less than 1.4 mg L−1. Moreover, these authors observed a substantially enhanced retention of scale inhibitor in the presence of Zn2+. The enhancement in inhibitor performance in the presence of Zn2+ is speculated as a result of the formation of strong solution complex of phosphonate scale inhibitor with Zn2+ ions, which affects the inhibition properties of scale inhibitors. So far, there are limited studies evaluating halite inhibitor performance considering the synergistic effect of scale inhibitors with metal ions, especially transition metals. Smith and Przybylinkski (2006) studied the effect of common brine constitutes of magnesium and zinc on halite inhibitor efficacy. These authors reported that magnesium has a negligible impact on inhibitor efficacy. On the other hand, zinc was found to have a considerable impact on halite scale deposition by itself but showed an antagonistic effect to both inorganic and organic inhibitors. However, these authors did not disclose the chemical compositions of the tested inhibitor packages. In the present study, the combined effect of a number of transition metals with common halite inhibitors were evaluated experimentally in a laboratory setup. The tested scale inhibitors include potassium ferrocyanide and poly(acrylic acid). Supersaturation of halite was achieved by cooling a heated brine solution from an elevated temperature to the target temperature. Initially, the inhibition efficiency of each scale inhibitor was evaluated individually. Subsequently, a screening test was performed to select the metal-inhibitor combination with noticeable synergistic effect for halite control. Based upon the screening test result, the synergistic effect of lead ion with scale inhibitors was systematically evaluated. To the best of the authors’ knowledge, the present study is the first to report a systematic investigation of the combined effect of transition metals with halite inhibitors and the synergistic effect of lead ion with both ferrocyanide and poly(acrylic acid) halite inhibitors. The synergistic effect elaborated in the present study can be useful in designing the field halite scale control strategy in terms of reducing inhibitor minimum effective dosage and optimizing halite inhibition operations. The present study provides the experimental approach and technical insights for 121

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bath was reduced to 87 °C for 3 h with vials under constant stirring. The solution pH was measured to be ca. 6.3 at 87 °C. Next, all three glass vials were transferred to another identical oil bath setup with a lower system temperature. The tested system temperature of the oil bath range from 62 °C to 42 °C and the corresponding SI(NaCl) values vary from 0.051 to 0.079 (Fig. 1b). Under constant stirring, the temperature of solution inside Vial C is monitored. It is assumed that the temperature of the solutions in Vial A and Vial B is the same as in Vial C throughout the experiment. The induction time measurement starts to count when Vial C temperature was reduced to the target temperature of the oil bath. Due to the rapid oil circulation rate of the second oil bath and magnetic stirring of the brine solutions, it typically took 1–4 min for the brine solution inside the glass vial to drop to the target temperature. The precipitation of NaCl salt due to the temperaturedriven supersaturation was detected by a laser-scattering method described previously (Fan et al., 2011). Briefly, solution turbidity was measured by a green laser of 532 nm wavelength. The induction time is the time duration between the solution reaching the target temperature and the first noticeable change in the measured turbidity can be observed. The impact of the presence of other chemical species, such as metals and scale inhibitors, was evaluated by adding the stock solution (s) of the chemical(s) into Vial B immediately after transferring the vials from 87 °C oil bath to the second oil bath with a lower temperature. Since the stirring velocity can have an impact on halite scale deposition, in this study the stirring velocities during 87 °C staging in the first oil bath and during the experiments in the second oil bath were exactly the same so as to eliminate stirring velocity impact on experimental results.

evaluation of halite inhibitor performance in the presence of metals ions. 2. Materials and methods 2.1. Chemicals Stock solutions of ferrocyanide ([Fe(CN)6]4-) (hereafter abbreviated as FCN) and poly(acrylic acid) (PAA) were purchased from SigmaAldrich™ and used as scale inhibitors. The molecular weight of PAA is approximately 3500. Chemicals such as sodium chloride, lead chloride, zinc chloride, sodium hydroxide standard solution and piperazine-1,4bis(2-ethanesulfonic acid) sodium salt (PIPES) were reagent grade and purchased from Fisher Scientific™. PIPES was used as a chemical buffer to control solution pH. Deionized water (DI water) was prepared by reverse osmosis followed by a four-stage ion exchange water purification process, including a cation/anion column, two ion exchange column and an organics removal column (Barnstead Internationals™). 2.2. Experimental setup and procedure A temperature-driven bottle test apparatus was adopted to determine the halite scale induction time and to study the impact of presence of metal ions and scale inhibitors. Fig. 1a illustrates the schematic of the experimental setup, which includes a magnetic stirrer, oil bath, glass vial and a thermometer. To begin with, a weighted amount of NaCl salt, CaCl2 salt and PIPES together with a Teflon-coated magnetic stir bar were placed into each of the three 20 mL glass vials. After adding in a calculated amount of DI water, NaOH standard solution was added into the vial to adjust the solution pH to 6.6 at 25 °C. Subsequently, the glass vials were capped and placed into an oil bath with the system temperature carefully controlled. The three glass vials (Vial A, B and C) were initially heated up to 90 °C under constant stirring to dissolve the added NaCl and CaCl2 salts. In each of the glass vial, the brine compositions are NaCl of 6.316 mol kg H2O−1, Ca(NO3)2 of 0.1 mol kg H2O−1 and PIPES of 2.5 mmol kg H2O−1. Since it is technically difficult to directly measure the temperature of the solution inside the glass vial while observing scale deposition, Vial C is employed for solution temperature measurement purpose only (Fig. 1a). After staging the vials at 90 °C for 40 min, the temperature of the oil

2.3. Analytical methods and brine chemistry calculation Concentrations of cationic species in metal stock solutions were measured by inductively coupled plasma-optical emission spectrometer (ICP-OES) (Optima 4300, Perkin Elmer). FCN inhibitor concentration in the stock solution was measured based on the iron concentration measurement by ICP-OES. PAA concentration in the stock solution was measured via total dissolved organic carbon measurement (TOCVCSH, Shimadzu Corp.). Brine chemistry and halite saturation index were calculated using ScaleSoftPitzer™ software (Version SSP2018). 2.4. Scale-related equations In this study, the quantification of mineral scale threat was achieved by calculating the saturation index (SI) of halite, i.e., SI(halite), at different temperature conditions. At a certain temperature, SI can be calculated by Eqn. (1):

IAP ⎞ SIT = log10 ⎛⎜ ⎟ ⎝ K sp ⎠

(1)

where SIT represents the saturation index at a certain temperature. Ksp corresponds to the solubility product of halite at a given condition. IAP denotes ion activity product. If SI(halite) is calculated to be zero at a given temperature, the solution is in equilibrium with halite scale. A positive SI(halite) value suggests the solution is supersaturated with respect to halite and halite scale deposition is expected. FCN and PAA scale inhibitors can kinetically delay scale deposition by extending the induction time. The relation between the inhibitor concentration and the induction time can be calculated via Eqn. (2):

t log10 ⎛ ind ⎞ = binh × Cinh ⎝ t0 ⎠ ⎜

⎟

(2)

where t0 (s) denotes the induction time without inhibitor presence. tinh (s) represents the induction time with scale inhibitor presence. Cinh (mg L−1) is the inhibitor concentration. The term b (L mg −1) is a constant characterizing inhibition efficiency. For a given inhibitor, a higher b value indicates of a higher inhibition efficiency. Hence, the term tinh

Fig. 1. (a) Schematic of the experimental setup for barite induction time measurement. (b) Relation between the halite saturation index and target solution temperature.

t0

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PAA and FCN inhibitors were tested for their abilities in controlling halite scale deposition. As shown in Fig. 2, in the presence of 50 mg kg H2O−1 PAA has little effect in controlling the halite scale with no noticeable increase in induction time. For instance, at the highest SI value of 0.079, the presence of PAA only marginally increased the induction time by only 0.5 min. On the other hand, the presence of FCN inhibitor can considerably increase the halite induction time for all the SI(halite) values evaluated, especially at higher FCN inhibitor concentrations. For instance, with the presence of a FCN concentration of 4.7 mg kg H2O−1, the halite induction time was greatly prolonged to 1110 min and 208 min at SI(halite) values of 0.051 and 0.058, respectively. According to Eqn. (2), the inhibition efficiency (the b value) can be calculated for each SI value studied by comparing the measured induction time with and without inhibitor and fitting the experimental data via least square method. Based upon the calculated FCN inhibition efficiency values as shown in Fig. 3, obviously the increase in SI(halite) leads to a deterioration of FCN inhibition efficiency. This result is in line with the nucleation theory and experimental observation as reported previously (He et al., 1996; Mullin, 2011).

signifies the prolonged induction time because of halite scale inhibitor presence. With regard to the b value, He et al. (1996) indicated that it is a function of SI, pH and molar ratio of lattice anions. 3. Results and discussion 3.1. Inhibition efficiency of FCN and PAA inhibitors on halite scale control In this study, all experiments were carried out at ambient pressure of 1 atm. NaCl supersaturation was promoted by a reduction in system temperature of an otherwise saturated saline brine at an elevated temperature of 87 °C. Other than NaCl salt, Ca was also added into the brine solution due to the prevalence of Ca species in oilfield produced waters. The Ca concentration was selected as 0.1 mol kg H2O−1 (4000 mg kg H2O−1) to represent the commonly observed Ca content in the field (Fink, 2011). Since the inhibition reactions of scale inhibitors typically involve acid/base reactions leading to a change in solution pH. Thus, PIPES buffer of 2.5 mmol kg H2O−1 was also added into the saline brine to control solution pH at ca. 6.3 at elevated temperature. Otherwise, the change in solution pH associated with aqueous acid/ base chemical reactions might lead to a change in activity coefficient of ionic species. Brine chemistry calculation suggests that at the testing condition, the prepared saline brine will be in an exact equilibrium with respect to NaCl at 87 °C. During the experiment, the prepared brine solutions were initially conditioned at 90 °C under constant stirring for the purpose of dissolving all the added salts. Subsequently, the saline solution was conditioned at 87 °C for 3 h, in order to set the solution in equilibrium with respect to NaCl. Any further reduction in system temperature will subject the solution supersaturated with NaCl, resulting in NaCl deposition. Since solution temperature measurement by inserting the thermometer into the glass vial will interfere with scale deposition and observation. In this study, Vial C is included for temperature measurement purpose. Since all the glass vials were under the same physiochemical and hydrodynamic conditions and experimental treatment, it is thus assumed that the brine solutions inside these vials should have the same temperature during the test. In this laboratory study, halite scale deposition is naturally occurring, driven by temperature reduction from 87 °C. As shown in Fig. 1b, the range of the tested temperature is from 62 °C to 42 °C with a corresponding SI(halite) value of 0.051–0.079. Based on SSP2018 calculation, the reduction in temperature from 87 °C to 62 °C (SI(halite) = 0.051) can result in as much as 8.8 g of NaCl precipitated per kg H2O. Similarly, the reduction in temperature from 87 °C to 42 °C (SI(halite) = 0.079) can lead to 12.9 g of NaCl precipitated per kg H2O. The counting of the induction time started from the moment when the test solution temperature was reduced to the target temperature. The induction time is registered as the time elapse from brine solution reaching the target temperature till the first noticeable change in the measured turbidity can be observed. The inhibition effect of PAA and FCN inhibitors were individually investigated across the range of SI(halite) values considered in this study. PAA inhibitor was studied at 50 mg kg H2O−1, while FCN inhibitor was studied from 0.2 to 4.7 mg kg H2O−1. As shown in Fig. 2, the induction time in the absence of scale inhibitor (far-left column group) is between 4 and 11.5 min. Generally, the increase in SI value (a higher temperature differential) will result in an enhanced halite deposition with a shorter induction time. PAA is a commonly used scale inhibitor to control deposition of a variety of scales, including calcite, barite and calcium sulfate. FCN has been reported to be effective in controlling halite scale formation (Kelland, 2014) on Page 70. Rodriguez-Navarro et al. (2002) reported that the presence of FCN additive can increase the critical supersaturation and also modify the NaCl crystal morphology while the saline fluid was percolating through and evaporating from a saturated porous body. In another study of the inhibition effect of FCN species on NaCl, FCN was observed to adsorb onto the surface of NaCl crystals and the inhibitory effect of FCN was explained by charge mismatch (Bode et al., 2012). In this study, both

3.2. Screening of metal-inhibitor combination for enhanced halite scale control It has been reported by a number of studies that the combination of a scale inhibitor chemical with another production chemical can exceed the performance of the scale inhibitor alone (Kelland, 2014) on Page 87. Such synergistic effect due to the presence of another production chemical is believed to be related to the interaction of the added chemical with inhibitor forming a new complex and/or modifying the incumbent inhibitor chemical structure. For instance, it has been reported that the blends of iminodisuccinic acid with tetrakis(hydroxymethyl) phosphonium sulfate (THPS) can result in a synergistic effect in removing iron sulfide scale, exceeding the performance of THPS alone (Trahan, 2010). In this study, an effort has been made to investigate the combination of different metal ions with FCN and PAA inhibitors. Since the inhibition efficiency of the inhibitors studied, especially FCN, is the highest at the lowest SI evaluated, i.e., 0.051, synergistic effect of metal ions with inhibitor was examined at an SI(halite) of 0.051. The metal ions evaluated in this study include Al3+, Bi3+, Pb2+, Zn2+ and Sn2+. One of the criteria of the metal ions to be selected in this study is that these metals should have a decent aqueous solubility. Thus, Fe(III) species was excluded due to its extremely low aqueous solubility and rapid formation of ferric hydroxide solid once placed in water (Benjamin, 2014). Although Fe(II) is also commonly observed in oilfield water with a decent solubility, Fe(II) was also excluded since it can be readily oxidized to Fe(III) by atmospheric oxygen and it requires a strictly controlled experimental setup to maintain an anoxic condition (Chen and Thompson, 2018). Preliminary studies showed that the presence of the alkaline earth metals, such as Ca2+, Mg2+ and Ba2+, has little synergistic impact on either PAA or FCN inhibitor performance. The metal ions are thus selected from the transition metals since transition metals generally have a stronger interaction with the functional groups of scale inhibitors. This can be illustrated by the higher stability constants of transition metals with inhibitor functional groups than with alkaline earth metals (Stumm and Morgan, 1996) on Page 252. Among the five metal cations presented in Table 1, only lead (Pb2+) ion can produce an apparent synergistic effect with both PAA and FCN inhibitors. The other four metals either produced no enhancement to halite induction time or could only marginally increase the induction time beyond 11.5 min, which is the induction time without any inhibitor. At shown above, 50 mg kg H2O−1 PAA alone results in an induction time of only 10 min. The addition of 5.1 mg kg H2O−1 Pb2+ can considerably enhance the induction time to as long as 120 min. In a similar manner, the presence of 5.1 mg kg H2O−1 Pb2+ can prolong the induction time to over 1260 min with 5.0 mg kg H2O−1 FCN, compared with 1110 min without Pb2+ presence. This suggest 123

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Fig. 2. Impact of the presence of PAA and FCN inhibitors on halite induction time.

3.3. Synergistic effect of Pb2+ with FCN and PAA inhibitors for halite deposition control Although Pb2+ is an environmental pollutant, Pb element can be often found in field produced waters. According to the review study carried out by Ahmadun et al. (2009), the concentration ranges of Pb2+ in natural gas produced water and oilfield produced water are 0.2–10.2 mg L−1 and 0.01–8.8 mg L−1, respectively. The source of Pb in the produced water is from natural origin involving migration and transformation of lead element in the ore source layer and associated physical and chemical reactions of lead (Jiménez et al., 2018). Fig. 4 illustrates the halite induction time in the presence of up to 5.1 mg kg H2O−1 Pb2+, without the presence of any scale inhibitor. The selected Pb2+ concentrations in this study were within the range of the naturally occurring Pb2+ concentrations in the field produced waters. Obviously, the increase in Pb2+ aqueous concentration results in a prolonged induction time. This phenomenon is most apparent at a lower SI(halite). For instance, 5.1 mg kg H2O−1 Pb2+ at SI(halite) of 0.051 generates an induction time of 90 min, which is over 9 times longer than that without Pb2+ presence. Additional research is required to understand the mechanism of Pb2+ impact on halite deposition. Fig. 5 presents the synergistic effect of Pb2+ with PAA inhibitor on halite deposition control. As also indicated in Table 1, 5.1 mg kg H2O−1 Pb2+ at SI(halite) of 0.051 produces an induction time of 120 min. However, compared with the measured induction time in the presence of FCN inhibitor along, the synergistic effect of Pb2+ with PAA is less significant. The inhibition efficiency values for the scenario of Pb2+ along and Pb2+ with PAA can be calculated based upon Eqn. (2) and compared against those of FCN inhibitor along. As shown in Fig. 6, for all the SI values considered in this study, the sequence of the calculated inhibition efficiency follows the order of Pb2+ along < Pb2+ with PAA < FCN along (from low to high). For instance, at SI(halite) of 0.051, the inhibition efficiency calculated for these three scenarios are 0.19, 0.21 and 0.41, respectively. The difference of the inhibition efficiency between Pb2+ alone and Pb2+ with PAA is not remarkable and both are lower than those of FCN alone. This suggests that although the presence of Pb2+ provides noticeable enhancement to PAA performance, the synergistic effect of Pb2+ with PAA in terms of inhibiting halite scale is still inferior to that of FCN inhibitor alone.

Fig. 3. Calculated inhibition efficiency in the presence of FCN inhibitor only.

Table 1 Halite induction time at different metal-inhibitor combinations with SI(halite) of 0.051. Exp. #

Metal Ion

Metal Conc. (mg kg H2O−1)

Scale Inhibitor

Inhibitor Conc. (mg kg H2O−1)

Induction Time (min)

1 2 3 4 5 6 7 8 9 10

Al3+ Bi3+ Pb2+ Sn2+ Zn2+ Al3+ Bi3+ Pb2+ Sn2+ Zn2+

5.0 5.0 5.1 5.3 5.3 5.0 5.0 5.1 5.3 5.3

FCN

5.0

PAA

50.0

12.5 10.5 > 1260 13 16 15 25 120 13 11

that the presence of Pb2+ can lead to a synergistic inhibition effect to control halite deposition in the presence of both FCN and PAA inhibitors. The next section will systematically evaluate the synergistic effect of Pb2+ with PAA and FCN and compare the inhibition efficiency of different combinations of Pb2+ ion with two types of scale inhibitors.

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Fig. 4. Impact of the presence of Pb2+ on halite induction time.

Finally, the synergistic effect of Pb2+ with FCN inhibitor was evaluated and presented in Fig. 7 a and b. Fig. 7a shows that at a fixed Pb2+ concentration of 5.1 mg kg H2O−1, the presence of Pb2+ and FCN inhibitor can substantially increase the induction time compared with the scenario without Pb2+ and FCN. Furthermore, the increase in FCN concentration from 1.0 to 5.0 mg kg H2O−1 leads to a pronounced increase in the induction time, especially at higher SI(halite) values. For instance, at SI(halite) of 0.079 and in the presence of 5.1 mg Pb2+ kg H2O−1, the increase in FCN concentration from 1.0 to 3.0 mg kg H2O−1 leads to a noticeable increase in halite induction time from 11 min to 459 min. Fig. 7b illustrates that at a fixed FCN inhibitor concentration of 5.0 mg kg H2O−1, the increase of Pb2+ concentration from 1.0 to 6.3 mg kg H2O−1 does not lead to an obvious increase in induction time at lower SI(halite) from 0.051 to 0.066, with an induction time between 1000 min and 1260 min. The effect of the increase in Pb2+ concentration becomes more pronounced at higher SI of 0.073 and 0.079. For

instance, when SI(halite) is 0.079, the induction time increased from 20 to 480 min when Pb2+ concentration increases from 1.0 to 6.3 mg kg H2O−1. Referring to the aforementioned discussion of the inhibition efficiency shown in Fig. 6, the inhibition efficiency values in the presence of both Pb2+ and FCN are substantially higher than all other scenarios evaluated. Particularly at low SI values, the synergistic effect of Pb2+ with FCN is the most pronounced with a calculated inhibition efficiency higher than 0.5. At the highest SI(halite) of 0.079, the inhibition efficiency of Pb2+ with FCN is still maintained at 0.46 while the inhibition efficacy of FCN alone is reduced to 0.13. Kan et al. (2009) previously reported a similar phenomenon by studying the impact of the addition of Zn2+ on scale inhibitor performance in controlling barite scale deposition via turbidity measurement. These authors indicated that the presence of a trace level of Zn2+ can significantly improve the performance of phosphonate scale inhibitors. In addition, the presence of Zn2+ also shows a minor synergistic effect

Fig. 5. Synergistic effect of the presence of Pb2+ and PAA inhibitor on halite deposition control. 125

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Fig. 6. Calculated inhibition efficiency of different scenarios.

Fig. 7. Synergistic effect of the presence of Pb2+ and FCN inhibitor on halite deposition control. (a) At a fixed Pb2+ concentration of 5.1 mg kg H2O−1; (b) at a fixed FCN concentration of 5.0 mg kg H2O−1. 126

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efficiency elaborated in this study can be useful in designing the halite scale control strategy, if a decent amount of Pb2+ is present in the production fluids.

with polymeric inhibitors. These authors argued that the observed enhancement in inhibitor performance in the presence of transition metal might be due to complexation of transition metal with functional groups of the inhibitors. In light of this, the observed synergistic effect of Pb2+ with inhibitors especially FCN in this study might be due to a similar reason as the complexation of Pb2+ with FCN. As a matter of fact, FCN, also called Prussian blue, is widely used as a sequestering agent to treat heavy metal poisoning (Crisponi and Nurchi, 2016). Due to its multidentate molecular structure, FCN can readily sequester Pb2+ forming PbeFCN complex. The formed metal-inhibitor complex can have a high stability constant, contributing to the strong interaction between metal and inhibitor, affecting the inhibition properties of the inhibitor. The significance of this study for oilfield halite scale control is that although Pb2+ is an environmental pollutant to be treated before water discharge, the operators can take advantage of the reported synergistic effect of Pb2+ with PAA and especially FCN inhibitors for halite control while designing field halite control strategy. Particularly, for shale operations, halite threat is mainly induced by water evaporating as a result of rising temperature during fracturing fluid injection (Vankeuren et al., 2017). Typically, scale inhibitor is added into the fracturing fluid chemical package. The enhanced inhibition efficiency elaborated in this study can be useful in designing the halite scale control strategy in terms of reducing inhibitor minimum effective dosage and optimizing halite inhibition operations, if a decent amount of Pb2+ is present in the fracturing fluid and/or formation water. With regard to the future work of experimental investigation of the mechanism of Pb2+ on halite inhibitor performance, based upon a number of recent studies of iron species on scale inhibitor performance (Zhang et al., 2018, 2019), it is speculated that such mechanism might be due to (1) the formation of Pb-inhibitor complex which interferes with the effectiveness of inhibitor performance; (2) the formation of lead-containing solid, such as lead oxide-hydroxide, followed by adsorption of inhibitor onto the solid surface. Laboratory studies can be carried out to explore the mechanism by carrying out a series of water chemistry experiments with a representative aqueous Pb2+ concentration.

Acknowledgement The authors appreciate the sponsorship of Science and Technology Development Fund, Macao S.A.R (FDCT) (0063/2018/A2). This work was also financially supported by Brine Chemistry Consortium companies of Rice University, including Aegis, Apache, BHGE, BWA, Chevron, ConocoPhillips, Coastal Chemical, EOG Resources, ExxonMobil, Flotek Industries, Halliburton, Hess, Italmatch, JACAM, Kemira, Kinder Morgan, Nalco, Oasis, Occidental Oil and Gas, Range Resources, RSI, Saudi Aramco, Schlumberger, Shell, SNF, Statoil, Suez, Total and the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2018.12.036. 5. Declarations of interest None. References Ahmadun, F.-R., Pendashteha, A., Abdullaha, L.C., Biaka, D.R.A., Madaenic, S.S., Abidina, Z.Z., 2009. Review of technologies for oil and gas produced water treatment. J. Hazard Mater. 170, 530–551. Benjamin, M.M., 2014. Water Chemistry, second ed. Waveland Pr Inc. Bode, A.A.C., Vonk, V., van den Bruele, F.J., Kok, D.J., Kerkenaar, A.M., Mantilla, M.F., Jiang, S., Meijer, J.A.M., van Enckevort, W.J.P., Vlieg, E., 2012. Anticaking activity of ferrocyanide on sodium chloride explained by charge mismatch. Cryst. Growth Des. 12, 1919–1924. Bukuaghangin, O., Sanni, O., Kapur, N., Huggan, M., Neville, A., Charpentier, T., 2016. Kinetics study of barium sulphate surface scaling and inhibition with a once-through flow system. J. Petrol. Sci. Eng. 147, 699–706. Chen, C., Thompson, A., 2018. Ferrous iron oxidation under varying pO2 Levels: the effect of Fe(III)/Al(III) oxide minerals and organic matter. Environ. Sci. Technol. 52, 597–606. Chen, T., Montgomerie, H., Chen, P., Vikane, O., Jackson, T., 2009. Understanding the mechanisms of halite inhibition and evaluation of halite scale inhibitor by static and dynamic tests. SPE-121458. In: Proceeding of SPE International Symposium on Oilfield Chemistry, 20-22 April. The Woodlands, Texas. Chen, T., Neville, A., Yuan, M., 2005. Calcium carbonate scale formation—assessing the initial stages of precipitation and deposition. J. Petrol. Sci. Eng. 46, 185–194. Crisponi, G., Nurchi, V.M., 2016. Chelating agents as therapeutic compounds—basic principles. In: Aaseth, J., Crisponi, G., Anderson, O. (Eds.), Chelation Therapy in the Treatment of Metal Intoxication. Academic Press. Davey, R.J., Garside, J., 2000. From Molecules to Crystallizers: an Introduction to Crystallization. Oxford University Press. Fan, C., Kan, A.T., Zhang, P., Tomson, M.B., 2011. Barite nucleation and inhibition at 0 to 200oC with and without thermodynamic hydrate inhibitors. Soc. Petrol. Eng. J. 16, 440–450. Fink, J., 2011. Petroleum Engineer's Guide to Oil Field Chemicals and Fluids. Gulf Professional Publishing. Frenier, W.W., Ziauddin, M., 2008. Formation, Removal, and Inhibition of Inorganic Scale in the Oilfield Environment. Society of Petroleum Engineers. Goodwin, N., Graham, G.M., Frigo, D.M., Kremer, E., 2016. Halite deposition – prediction and laboratory evaluation. SPE-179861. In: Proceeding of SPE International Oilfield Scale Conference and Exhibition, 11-12 May. Aberdeen, Scotland. Haynes, W.M., 2014. CRC Handbook of Chemistry and Physics, 95th edn. CRC Press. He, S., Kan, A.T., Tomson, M.B., 1996. Mathematical inhibitor model for barium sulfate scale control. Langmuir 12, 1901–1905. Ho, K., Chen, T., Chen, P., Hagen, T., Montgomerie, H., Benvie, R., 2014. Development of novel test methodology to understand the mechanisms of halite inhibition and environmentally acceptable halite scale inhibitors for high temperature application. SPE-169803. In: Proceeding of SPE International Oilfield Scale Conference and Exhibition, 14-15 May. Aberdeen, Scotland. Ho, K., Chen, T., Chen, P., Hagen, T., Montgomerie, H., Benvie, R., Lewis, J., 2013. Development of test methods and inhibitors for halite deposition in oilfield water treatment. NACE-2013-2659. In: Proceeding of NACE International. 17-21 March, Orlando, Florida. Jiménez, S., Micó, M.M., Arnaldos, M., Medina, F., Contreras, S., 2018. State of the art of produced water treatment. Chemosphere 192, 186–208.

4. Conclusion In this study, the synergistic effect of transition metals with PAA and FCN halite inhibitors was evaluated in a laboratory setup. Halite supersaturation was achieved by cooling a heated solution to a target temperature by maintaining other experimental conditions unchanged, such as solution pH, brine chemistry, stirring velocity. A higher temperature differential leads to an increase in SI(halite) and a reduction in halite induction time. The presence of PAA inhibitor along has a negligible impact on halite deposition. FCN inhibitor, on the other hand, can effectively inhibit halite with a considerably increased induction time. A number of transition metals, including Al3+, Bi3+, Pb2+, Zn2+ and Sn2+, were examined for their possible synergistic effect with PAA and FCN inhibitors for halite control. Among them, only Pb2+ ion was able to substantially promote the inhibition effect of both PAA and FCN. Further detailed studies suggested that Pb2+ along with a concentration of up to 5.1 mg Pb2+ kg H2O−1 exhibits inhibitory effect against halite deposition. Combining Pb2+ with PAA inhibitor can extend the halite induction time to exceed the induction time of PAA along. A more pronounced synergistic effect was observed by combining Pb2+ with FCN inhibitor, particularly at higher SI(halite) where FCN along shows a reduced effectiveness. Moreover, the inhibition efficiency at different scenarios can be calculated and ranked. It was found that the sequence of inhibition efficiency follows the order of Pb2+ along < Pb2+ with PAA < FCN along < Pb2+ with FCN (from low to high). Although Pb2+ is an environmental pollutant, an aqueous Pb2+ concentration of up to 10 mg L−1 can naturally occur in both natural gas produced water and oilfield produced water. The operators can take advantage of the reported synergistic effect of Pb2+ with scale inhibitors in this study while designing field halite control strategy. The enhanced inhibition 127

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