Experimental Evaluation of Common Sulfate Mineral Scale

Jun 18, 2019 - Experimental Evaluation of Common Sulfate Mineral Scale Coprecipitation Kinetics in Oilfield Operating Conditions ...
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Cite This: Energy Fuels 2019, 33, 6177−6186

Experimental Evaluation of Common Sulfate Mineral Scale Coprecipitation Kinetics in Oilfield Operating Conditions Ping Zhang,*,† Zhang Zhang,‡,∥ Jiayao Zhu,† Amy T. Kan,‡,§ and Mason B. Tomson‡,§ †

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Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau 999078, China ‡ Department of Civil and Environmental Engineering and §Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, Rice University, Houston, Texas 77251-1892, United States S Supporting Information *

ABSTRACT: As one of the top three water-related production chemistry threats, mineral scale deposition can result in severe oilfield operational issues. Existing studies related to mineral scale deposition and control focus predominately on the thermodynamic aspect of scale deposition with little consideration of scale deposition kinetics. Coprecipitation of different types of mineral scales is a common phenomenon in both the natural environment and industrial applications. However, there are limited studies on coprecipitation kinetics of different scales, especially sulfate scales. In this study, coprecipitation kinetics of BaSO4/SrSO4 mineral scales in oilfield operational conditions was experimentally evaluated in a systematic manner. The impacts of barium to strontium molar ratio, scale saturation index, and electrolyte medium were investigated. It shows that the molar ratio and calcium electrolyte medium have the most significant impacts on sulfate scale deposition kinetics. This study can expand our capability in evaluating scale threats with complicated water produced compositions at operating conditions. The conclusions from this study can be profoundly insightful in developing an oilfield scale control strategy. The methodology elaborated in this study has the potential to be adopted as the standard procedure to evaluate scale threats when more than one type of scale are expected to deposit from the produced water.

1. INTRODUCTION Oilfield production chemistry is the subject to study the impact on production fluid flow because of physicochemical changes in the production system.1,2 The scope of production chemistry encompasses the overall oil and gas production and processing systems from reservoirs to wells and further to processing facilities. Depending on the nature of the field and the location of the production system of interest, the production chemistry threats can include material corrosion, gas hydrate blockage, mineral scale deposition, asphaltene formation, emulsion and foaming, and so forth. Among them, material corrosion, gas hydrate blockage, and mineral scale deposition are the top three water-related production chemistry threats.3 Mineral scale (hereafter referred to as “scale”) is the hard crystalline inorganic solid precipitated from the water phase because of exceeding the local saturation limit.4,5 The formation and deposition of scale solids can considerably reduce the inner diameter of production tubing and flowlines, leading to a significant reduction in the fluid flow rate and, in extreme cases, a complete throughput blockage. Scale can also form in wellbore reservoirs, leading to damage of reservoir formation. Particularly during a seawater injection campaign, a considerable amount of seawater will be injected into a reservoir in order to enhance oil production and to maintain the reservoir pressure. This practice can result in the commingling of seawater, which is high in sulfate species with formation water which contains barium species, leading to formation of sparingly soluble barium sulfate (barite). In addition to barium sulfate, other types of sulfate scales, such as calcium sulfate (gypsum) and strontium sulfate (celestite), can © 2019 American Chemical Society

also be formed in a similar manner because of mixing of incompatible waters.3−5 Considering the severity of scale formation especially in upstream production systems, a significant effort has been made in the industry to control scale deposition by use of both physical methods and chemical inhibition.3 Various types of scale inhibitor chemicals including environmentally friendly inhibitors have been formulated and produced in the past several decades to control scale threat at different operating conditions.6−9 However, the first line of defense in oilfield scale control relies on the correct assessment of scale threat, which is still an ongoing challenge in the oil and gas industry. This requires the knowledge and deep technical insights of the thermodynamics and kinetics aspects of the scale deposition process given the operating conditions.10,11 Thermodynamic studies on the scale formation process can identify the scale threat severity level and predict the amount of scale particles to be precipitated from the produced water. Existing studies and academic publications on the topic of scale control, particularly oilfield scale control, focus predominately on the thermodynamics of scale formation.12−14 Extensive efforts have been made to calculate the supersaturation level of the produced water with respect to different types of inorganic scales, which is the chemical driving force for scale precipitation and deposition.15,16 Some key impact factors include water chemistry, solution temperature, and system pressure, to name a few.17 A number of software packages have Received: April 3, 2019 Revised: June 11, 2019 Published: June 18, 2019 6177

DOI: 10.1021/acs.energyfuels.9b01030 Energy Fuels 2019, 33, 6177−6186

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Energy & Fuels been produced to facilitate scale threat assessment.1,3 However, there are limited studies on mineral scale deposition kinetics.13,14 Studies on the kinetics aspect of scale formation can reveal the expected time frame for scale deposition to occur, that is, how fast scale will precipitate. This piece of information is of paramount importance to scale threat evaluation and is lacking in thermodynamic studies. It is possible that the scale deposition process is kinetically slow, which requires a longer time duration than the duration for the production fluids to flow out of the production system of concern.17 Although extensive studies on the subject of scale deposition kinetics are required, a number of recent publications are dedicated to investigate such a process by adopting different instruments and experimental approaches.13,18,19 For instance, Zhang et al. adopted a plugflow type reactor to study carbonate scale deposition kinetics onto the inner surface of the tubing device and also the process of scale deposition onto the surface of precoated carbonate solids.13,14 These authors suggested that the overall scale deposition process can be segmented into a number of stages and water chemistry together with system conditions can play a significant role in influencing scale deposition kinetics. In another study, Sanni et al. reported the kinetic study of carbonate scale by use of once-through flow visualization technique. This developed apparatus allows in situ observation of scale precipitation and deposition and assessment of the impact of water chemistry and system conditions.18 These authors argued that other than the adhesion of the formed scale particles crystals, heterogeneous surface crystallization can also considerably impact the surface scaling process. Setta and Neville (2011) evaluated the inhibition efficiency of common scale inhibitors on calcium carbonate scale precipitation kinetics on the surface of stainless steel.19 It is found that a higher inhibitor concentration is required to achieve full inhibition of carbonate scale than for scale precipitation. Coprecipitation of different types of scale solids is a wellobserved and -documented phenomenon in natural environments and in industrial applications. As a matter of fact, it is rare to find pure solids occurring in nature and isomorphous replacement in the crystalline lattice by a foreign constituent is one of the most important pathways to form mixed compounds.15 In material science, coprecipitation is one of the most commonly used methods for material preparation.20 With regard to sulfate scales, it has been reported that barite solid normally precipitates together with celestite solid.3,4 Therefore, the presence of strontium and barium element will inevitably impact the physical and chemical properties of the formed sulfate scale solids. In addition, it can be speculated that the presence of barium or strontium in produced water will also impact the kinetics of the celestite or barite scale deposition process. In other words, the coprecipitation of barite and celestite might have a different deposition kinetics from the barite or celestite solid deposition process alone. If the presence of barium can expedite the deposition kinetics of strontium sulfate or vice versa, conclusions from laboratory and computational studies on barite or celestite scale deposition alone will become inadequate and true scale threat might be underestimated as a result of the expedited scale deposition because of the coprecipitation phenomenon. However, there are limited studies on the topic of the coprecipitation kinetics of barite and celestite, especially in oilfield operational conditions.

In this study, a systematic laboratory investigation was carried out for the purpose of evaluating the coprecipitation kinetics of barium sulfate (BaSO4) and strontium sulfate (SrSO4) mineral scales in oilfield operational conditions. The focus was given to study the change in aqueous species concentration and also the solid scale supersaturation level. It is found that the presence of barium species can have a considerable impact on the strontium scale deposition process. This impact includes the change in scale deposition kinetics and also the extent of barite solid deposition. To the best of the authors’ knowledge, this paper is the first to systemically describe the coprecipitation kinetics of BaSO4/SrSO4 mineral scales in oilfield operational conditions. This study can expand our capability in evaluating scale threats with complicated produced water compositions at operating conditions. The conclusions from this study can be profoundly insightful in developing scale control strategy, particularly the strategy to control sulfate scale formation. Consideration of coprecipitation kinetics of different types of scales can prevent the possibility of underestimating scale threat because of the coprecipitation phenomenon. The methodology elaborated in this study has the potential to be adopted as the standard procedure to evaluate scale threats when more than one type of scales are expected to deposit from the produced water.

2. MATERIALS AND METHODS 2.1. Chemicals. Solids of sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), barium chloride (BaCl2), strontium chloride (SrCl2), and sodium sulfate (Na2SO4) were reagent grade and purchased from Fisher Scientific. Commercial grade bis-hexamethylenetriamine-penta (methylenephosphonic)acid (BHPMP) was employed as the scale inhibitor to inhibit sulfate scale deposition. Figure S1 in the Supporting Information illustrates the BHPMP molecular structure. Deionized water (DI water) was prepared by reverse osmosis and ion exchange water purification processes. 2.2. Precipitation Kinetics in the NaCl Medium. BaCl2 stock solution was prepared by adding a calculated amount of BaCl2 salt and NaCl salt into DI water with a NaCl concentration of 1 M. Similarly, stock solutions of SrCl2 and Na2SO4 were also prepared in 1 M NaCl medium. To investigate the coprecipitation kinetics of BaSO4/SrSO4, a glass beaker setup was adopted by mixing the stock solutions of BaCl2, SrCl2, and Na2SO4. Figure S2 in the Supporting Information presents the schematics of the experimental setup. To begin with, a known amount of BaCl2 and SrCl2 stock solutions and DI water were premixed inside the glass beaker and heated to 80 °C. After the Na2SO4 stock solution was heated to 80 °C in another glass container, a calculated amount of the Na2SO4 stock solution was added into the glass beaker. The Na2SO4 solution was delivered by a syringe propelled by a syringe pump and the solution addition was generally completed within 45 s. A magnetic stirrer was placed inside the glass beaker prior to the addition of Na2SO4 solution to facilitate solution mixing. The temperature of the mixture solution inside the glass beaker during the coprecipitation experiment was controlled by a circulation water bath at 80 °C. A plastic film was used to cover the opening of the glass beaker to prevent water evaporation. The moment when Na2SO4 stock solution delivery was completed was regarded as the onset of the coprecipitation experiment. Multiple samples with a volume of ca. 2.5 mL were taken from the glass beaker to measure the aqueous Sr and Ba concentrations during the course of the precipitation experiment. Samples were taken by use of a plastic syringe with a 0.2 μm nylon filter (Fisher Scientific) attached. In order to inhibit continuing solid precipitation from the sampled solution, about 0.1 mL of the BHPMP inhibitor solution was pre-added inside the plastic syringe body before attaching the filter to the syringe. The active BHPMP concentration of the inhibitor solution was ca. 4000 mg L−1. The filtered sample solution upon collection was 6178

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Energy & Fuels Table 1. Conditions of Coprecipitation Experiments with SI(SrSO4) = 0.6 in a 1 M NaCl System Ba/Sr MRa parameter

unit

initial Ba concentration initial Sr concentration initial SO4 concentration initial SI(BaSO4) final Xf(SrSO4)

−1

mg L mg L−1 mg L−1

0:1

1:100

1:20

1:10

1:5

1:2

0 564 601 N.A. N.A.

13 564 610 1.2 93%

42 559 614 1.9 89%

82 551 606 2.2 80%

164 561 616 2.5 61%

390 555 607 2.9 43%

a

These MR values are the initial Ba to Sr MRs at the beginning of the experiment.

instantaneously mixed with the concentrated inhibitor solution to yield a BHPMP inhibitor concentration of ca. 160 mg L−1. The exact amounts of the added inhibitor solution and the collected sample solution were measured by weighting the syringe before and after inhibitor solution addition and sample collection. 2.3. Precipitation Kinetics in KCl and CaCl2 Media. Similar to the experimental procedure described above, kinetics of coprecipitation of BaSO4/SrSO4 was also investigated in KCl and CaCl2 media, respectively. The electrolyte medium of NaCl salt was replaced by either KCl salt or CaCl2 salt. In order to maintain an equivalent solution ionic strength of 1 M, the concentration of KCl salt was 1 M and the concentration of CaCl2 salt was 0.33 M. 2.4. Instrumental Analysis and Brine Chemistry Calculation. The inhibitor-containing sample solution after collection was immediately measured by inductively coupled plasma−optical emission spectrometer (ICP−OES) (Optima 4300 Dv, PerkinElmer) for aqueous Sr and Ba concentrations. ICP−OES was also used to determine the BHPMP concentration of the concentrated inhibitor solution by measuring the phosphorus concentration. Surface morphologies of the deposited BaSO4/SrSO4 solids were characterized by scanning electron microscopy (SEM) (FEI Quanta 400, Hillsboro, Oregon). The SEM instrument was equipped with an energy-dispersive X-ray spectrometer for elemental analysis. Brine chemistry and scale saturation index (SI) were calculated using ScaleSoftPitzer software version SSP2017 (hereafter abbreviated as SSP).4 In addition, Visual MINTEQ version 3.0 (hereafter abbreviated as VM) was also adopted for brine chemistry and scale SI calculations.

Another commonly used term, SI, is the 10-base logarithm of the saturation ratio. Therefore, the SI value of a brine with respect to BaSO4 (hereafter referred to as SI(BaSO4)) can be calculated as ÄÅ É ÅÅ (Ba 2 +)(SO 2 −) ÑÑÑ ÅÅ ÑÑ 4 Ñ SI = log10ÅÅ ÅÅ K sp(BaSO4 ) ÑÑÑ ÅÇ ÑÖ where the parentheses denotes the activity of ionic species. Ksp corresponds to the conditional solubility product. Both Ksp and SI are highly dependent upon the system conditions, such as temperature. If the calculated SI is higher than zero under a certain condition, the solution of interest will be supersaturated with the solid of interest and solid precipitation is expected, leading to a reduction in the measured concentrations of the aqueous scaling cation species. 3.2. Coprecipitation Kinetics of BaSO4/SrSO4 in the NaCl Medium. In this study, the kinetic characteristics of coprecipitation of BaSO4/SrSO4 at a representative oilfield condition was experimentally investigated by mixing a Ba/Srcontaining brine solution with another SO4-containing brine solution. The solution mixing and subsequent precipitation experiment took place in a glass beaker under constant stirring. To begin with, the experiments were carried out by employing NaCl as the electrolyte medium, which is also the background electrolyte of standard oilfield brine.2 Sample solutions were collected intermittently from the glass beaker to measure the variation of aqueous Sr and Ba species concentration during the course of the precipitation experiment. In this study, an effort was made to inhibit continuous solid precipitation after sample collection so as to accurately measure the aqueous phase Sr and Ba concentrations at the moment of sampling. A BHPMP inhibitor was utilized to inhibit solid precipitation from the sampled solution. BHPMP is a commonly used scale inhibitor in oilfields to control sulfate scales.1 Calculation by SSP suggests that the required maximum BHPMP concentration to inhibit sulfate scale formation considering the ionic species concentrations and the experimental condition in this study is about 60 mg L−1. Thus, the presence of ca. 160 mg L−1 BHPMP inhibitor in the sampled solution is expected to effectively inhibit SrSO4 and BaSO4 scale precipitation. Moreover, once collected, the sampled solution was immediately measured for aqueous Ba and Sr concentrations by ICP− OES to minimize the time gap between sampling and instrumental analysis. As shown in Table 1, the first set of the experiments were conducted at the same initial SI(SrSO4) of 0.6. Figure 1a illustrates the change of the measured aqueous Sr concentration during the precipitation experiment of 100 min at SI(SrSO4) of 0.6 in 1 M NaCl medium. Different Ba to Sr molar ratios (MRs) were considered in this study. It shows that in the absence of the Ba species, that is, MR of 0:1, the final Sr

3. RESULTS AND DISCUSSION 3.1. Understanding of Mineral Scale Solid Precipitation. This study adopts a solution ionic strength of 1 M and a system temperature of 80 °C in order to evaluate the sulfate scale precipitation kinetics at representative oilfield operational conditions. Oilfield brine water is typically of high salinity under an elevated temperature condition in the subsurface.2 Both solution ionic strength and system temperature can considerably impact the kinetics of the scale precipitation process. As for the mineral scale of SrSO4 and BaSO4, ionic strength can influence the activity coefficients of dissolved ionic species, such as Sr2+ and Ba2+. Solution temperature can profoundly impact the aqueous solubility of sulfate scale solids.4,17 As one of the most important concepts related to solid precipitation, saturation ratio reflects the extent of solution supersaturation with respect to a solid. Saturation ratio dictates the solid precipitation kinetics as the chemical driving force to promote precipitation. As a matter of fact, in oilfield operations, mineral scale threat is commonly characterized by the calculated saturation ratio values of different solids.11,17 Saturation ratio is calculated as: Saturation ratio =

ion activity product solubility product 6179

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Figure 2. SrSO4/BaSO4 coprecipitation experimental results obtained at SI(BaSO4) of 0.8 at various MRs. (a) Aqueous Ba concentrations. (b) Calculated SI(BaSO4) values.

Figure 1. SrSO4/BaSO4 coprecipitation experimental results obtained at SI(SrSO4) of 0.6 at various MRs. (a) Aqueous Sr concentrations. (b) Calculated SI(SrSO4) values.

SI(BaSO4) of 2.1 occurring at the MR of 1:2 within the first 5 min of the experiment. For most scenarios, more than 80% of the initial aqueous Ba species formed BaSO4 solid precipitates within 5 min of the experiment. Thus, it can be speculated that the aforementioned expedited precipitation kinetics of SrSO4 in the presence of Ba is due to the formation of BaSO4 solids which served as the nuclei to promote SrSO4 solid precipitation, which kinetics is faster than SrSO4 homogeneous nucleation without Ba species presence. As shown in Figure 2, except for the scenario of MR of 1:2, once SI(BaSO4) dropped to zero, the aqueous Ba concentration continued to decline leading to a negative final SI(BaSO4) value. The average final SI(BaSO4) value at 100 min was approximately −0.6 for experiments of MRs from 1:100 to 1:5. The phenomenon of continuous BaSO4 solid precipitation after the solution became undersaturated with BaSO4 can be explained by the reduction in solubility of the minor constituent in solid solution. As explained by the theory of solid solution, the solubility of the minor constituent in the solid solution (BaSO4 solid in this case) is reduced relative to the solubility of the pure BaSO4 solid.16,21 Table 1 presents the calculated final mole fraction of the SrSO4 solid in the SrSO4/ BaSO4 solid solution, referred to as Xf(SrSO4). It shows that Xf(SrSO4) values for MRs from 1:100 to 1:5 were calculated to be above 50%, suggesting that BaSO4 was the minor constituent in the solid solution. The shown SI(BaSO4) values in Figure 2b are the SI values of pure BaSO4 solid at the experimental condition of interest. Therefore, because of the solid solution effect on minor constituent solubility, the solubility of BaSO4 was reduced and BaSO4 continued to precipitate although the calculated SI(BaSO4) for pure solid became negative. In a similar manner, the presence of a negative SI(SrSO4) and the continuous decline of SI(SrSO4)

concentration was close to the initial Sr concentration, suggesting that SrSO4 precipitation was insignificant. The presence of the Ba species can considerably promote the precipitation kinetics of the SrSO4 scale solid. It shows that Sr concentration reduction and hence SrSO4 solid precipitation were more rapid within the first 40 min of the experiment, followed by a more gradual decline of the Sr concentration. Generally speaking, the increase in the MRs lead to an expedited precipitation kinetics of SrSO4, except for the scenario of MR of 1:5. For instance, the aqueous Sr concentration by the end of the experiment drops by 20 and 37% when the MRs are 1:100 and 1:10, respectively. Figure 1b presents the calculated corresponding SI(SrSO4) values during the course of the experiment. As SI(SrSO4) is positively related to the Sr concentration, the shapes of the SI(SrSO4) curves are similar to those of the Sr concentration curves as in Figure 1a. It should be noted that other than the scenario of MR of 1:2, SI(SrSO4) was maintained positive throughout the duration of the precipitation experiment for all other scenarios. At MR of 1:2, SI(SrSO4) drops below zero within 5 min of the precipitation experiment. Obviously, the precipitation kinetics of SrSO 4 was substantially impacted by the presence of the Ba species. The variation of the measured Ba concentration as well as the corresponding SI(BaSO4) values are shown in Figure 2. In all the experiments with Ba presence, the initial SI(BaSO4) values are above 1, suggesting that the solution is highly supersaturated with respect to BaSO4. Thus, a rapid BaSO4 precipitation occurred within 5 min of the experiments and the aqueous Ba concentration dropped by 20% at an MR of 1:100 to over 95% at MR of 1:2 with a maximum reduction of 6180

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Energy & Fuels Table 2. Conditions of Coprecipitation Experiments with SI(SrSO4) = 0.8 in 1 M NaCl Medium Ba/Sr MRa parameter initial Ba concentration initial Sr concentration initial SO4 concentration initial SI(BaSO4) final Xf(SrSO4)

unit −1

mg L mg L−1 mg L−1

0:1

1:100

1:20

1:10

1:5

1:2

0 720 767 N.A. N.A.

14 684 765 1.4 97%

54 702 772 2.1 91%

112 703 772 2.4 83%

208 699 763 2.7 69%

493 703 765 3.1 42%

a

These MR values are the initial Ba to Sr MRs at the beginning of the experiment.

the surface of the bulk precipitate. As discussed above, the Ba concentration dropped rapidly within the first few minutes of the precipitation experiment. Some of the Ba species can reversibly adsorb onto the surface of the bulk precipitate. These adsorbed Ba species can desorb from the precipitate surface and diffuse back into the aqueous solution, resulting in a temporary increase in aqueous Ba concentration, that is, the rebound phenomenon. Such a desorption phenomenon can occur multiple times as shown in the experiment with MR of 1:20. However, because of the chemical driving force of supersaturation, these desorbed Ba species would gradually precipitate with SO4 species forming the BaSO4 solid, as long as the solution was supersaturated with BaSO4. The less obvious rebound phenomenon at higher MRs of 1:5 and 1:2 might be attributed to the fact that SI(BaSO4) values are higher at these two experiments, subjecting the majority of the aqueous Ba species to form precipitation as the BaSO4 solid instead of being adsorbed to the bulk precipitate surface. As for the experiment with MR of 1:100, within the first 25 min of the experiment, the reduction of both Sr concentration and Ba concentration was low, suggesting that a limited amount of solid precipitate was formed. Therefore, adsorption of Ba species onto the surface of the precipitate was insignificant because of the limited amount of bulk precipitate surface in the aqueous phase at MR of 1:100. Similar to the BaSO4/SrSO4 co-precipitation experiment carried out at SI(SrSO4) of 0.6, the coprecipitation experiment was also conducted at SI(SrSO4) of 0.8. The experimental condition was maintained the same at 80 °C and 1 M NaCl as the electrolyte medium and the same MRs were evaluated (Table 2). As shown in Figure 3a, although SI(SrSO4) increased by only 0.2 unit, the increase in SI has a profound impact on SrSO4 precipitation kinetics in the absence of the Ba species. Without the Ba presence (MR of 0:1), the measured Sr concentration continuously dropped from initially 720 to 370 mg L−1 by the end of the experiment. In the presence of the Ba species, it shows that the increase in the Ba concentration generally results in an expedited precipitation of SrSO4 except for the scenario of MR of 1:100. Similar to the experimental results with SI(SrSO4) of 0.6, the precipitation of SrSO4 was more rapid within the first 40 min, followed by a more gradual reduction in the aqueous Sr concentration. The calculated SI(SrSO4) curves (Figure 3b) followed the same shapes as those of the Sr concentration curves. Similar to the experimental results with SI(SrSO4) of 0.6, the calculated SI(SrSO4) values were maintained positive during the course of the precipitation experiment, except for the scenario of MR of 1:2. At MR of 1:2, the SI(SrSO4) dropped below zero within 6 min of the experiment because of the solid solution impact on SrSO4 solubility as a minor constituent with Xf(SrSO4) being 42% in Table 2. With regard to the Ba species concentration and the calculated SI(BaSO4) shown in Figure

after SI(SrSO4) became negative at MR of 1:2, as shown in Figure 1b, can be well explained. When MR was increased to 1:2, SrSO4 was no longer the major constituent in the solid solution with a calculated Xf(SrSO4) of 43%. As a minor constituent, the solubility of SrSO4 was reduced, leading to precipitation of the SrSO4 solid and reduction of the aqueous Sr concentration, whereas the calculated SI(SrSO4) for pure solid was negative. It should be noted that there are a number of fluctuations in the obtained scale precipitation experimental data, such as the lines of Ba/Sr of 1:100 in both graphs of Figure 1. This fluctuation can be partially attributed to experimental errors including measurement errors. One can recall that the instrumental error of ICP−OES can be as high as 10%.22 Another factor which can be responsible for the observed data fluctuation is the continuous change in brine solution composition. As elaborated in a previous study,13 solution SI and ionic strength can substantially impact the precipitation of mineral solids. Thus, in this study, with the continuous precipitation of sulfate scale solids from the aqueous phase, solid precipitation kinetics in the local aqueous environment and/or the bulk solution can be constantly impacted, leading to either expedited or delayed scale precipitation. Another interesting aspect of the experimental results is the variation characteristics of the Ba concentration as well as the calculated SI(BaSO4). As shown in Figure 2a, instead of being a monotonic decline for aqueous Ba concentration, the measured Ba concentration exhibited a “rebound phenomenon” especially within the first 40 min of the precipitation experiment. As for the scenario of MR of 1:10, the Ba concentration was initially 81.6 mg L−1 and rapidly dropped to 1.5 mg L−1 within 5 min, indicative of 98% reduction in Ba concentration. Between 5 and 10 min, the Ba concentration rose up to as high as 10.7 mg L−1, followed by a continuous reduction to below 0.3 mg L−1 toward the end of the experiment. Precipitation experiment with MR of 1:20 showed that the rebound phenomena occurred twice at 10 and 35 min, respectively. Correspondingly, the calculated SI(BaSO4) curves followed the same shapes as those of the Ba concentration curves with rebound phenomenon occurring at MRs from 1:20 to 1:2. Among the various scenarios with different MRs, the rebound phenomenon is particularly pronounced at intermediate MRs of 1:20 and 1:10 and is less obvious at higher MRs of 1:5 and 1:2. When MR drops to 1:100, this phenomenon is almost unnoticeable. Evidently, there seems to be an optimal MR range for the occurrence of such rebound phenomenon at SI(SrSO4) of 0.6. Given the fact that the total amount of Ba species inside the glass beaker was unchanged during the precipitation experiment and that Ba species occurred either in dissolved form or as a BaSO4 precipitated solid, one plausible explanation for this rebound phenomenon can be attributed to the reversible adsorption of Ba species on 6181

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SI(BaSO4) became negative. As shown in Table 2, at MR of 1:100−1:5, Xf(SrSO4) was calculated to be above 50% and the BaSO4 solid was the minor constituent in the solid solution, which results in a reduction of BaSO4 solubility. In terms of the rebound phenomenon of the Ba concentration, such a phenomenon was also observed at SI(SrSO4) of 0.8. Different from the experimental results obtained at SI(SrSO4) of 0.6, when SI(SrSO4) was elevated to 0.8, the rebound phenomenon of Ba concentration occurred at a wider range of MRs from 1:100 to 1:5 and no multiple rebound phenomenon was observed at any MR. As shown in Figure 4a, except for the scenario of MR of 1:2, the rebound phenomenon took place between 5 and 25 min of the precipitation experiment. This can be explained by the proposed Ba species adsorption onto the surface of bulk precipitate. At an elevated SI(SrSO4), a higher chemical driving force of supersaturation existed to promote the formation of the SrSO4 precipitate, evidenced by the expedited precipitation of SrSO4 at MR of 0:1. Within the same time duration after the onset of the experiment, SrSO4 solids precipitated more rapidly, creating more available precipitate surface area for Ba species to adsorb onto, compared with SI(SrSO4) of 0.6. Thus, Ba species can adsorb onto the formed bulk precipitate surface, leading to the rebound phenomenon occurring at a wider range of MRs. The fact that there is no multiple rebound phenomenon observed at SI(SrSO4) of 0.8 might be attributed to the higher SI(BaSO4) values in these experiments. It can be speculated that after the occurrence of the rebound phenomenon, the aqueous Ba species preferably formed the BaSO4 solid instead of being re-adsorbed onto the precipitate surface. SEM characterization was carried out on the resultant solid precipitates after the completion of a few selected experiments. Solid samples from the experiments conducted at both SI(SrSO4) values with Ba/Sr of 1:2 and 1:100 were selected for SEM and associated EDXS characterization. Figure 5a presents the SEM image of the solid sample collected by the end of the experiment with SI(SrSO4) of 0.6 and Ba/Sr of 1:100. From a morphological standpoint, the majority of the solids are orthorhombic crystals which are typical of celestite. Sporadic smaller octahedral-shaped crystals of barite can be seen as well. From the SEM image, the celestite and barite solids seem to co-exist and barite was distributed uniformly with celestite. Additional SEM images were available in the Supporting Information. These SEM characterization studies suggest that the SrSO4 solid and the BaSO4 solid co-existed in these images, indicative of mutual precipitation of these two scale solids. Figure 5b illustrates a section-based EDXS analysis result on the precipitated solid sample following the SEM analysis. A point-based EDXS analysis result of the same sample is available in the Supporting Information. The similarity of the point-based and section-based EDXS results in terms of the signals of Sr and Ba elements suggests that the distribution of Sr and Ba elements is uniform over the area of the examined sample. Similar EDXS analyses were also conducted following the completion of other SEM characterizations. Collectively, these EDXS results confirm a uniform distribution of Sr and Ba elements in the tested samples. 3.3. Coprecipitation Kinetics of BaSO4/SrSO4 in the KCl Medium and the CaCl2 Medium. Potassium and calcium are also common ionic species in oilfield-produced waters. It has been reported that in a sandstone reservoir located in Eastern Ohio, the concentrations of potassium and

Figure 3. SrSO4/BaSO4 coprecipitation experimental results obtained at SI(SrSO4) of 0.8 at various MRs. (a) Aqueous Sr concentrations. (b) Calculated SI(SrSO4) values.

4, it shows that other than the scenario of MR of 1:2, Ba concentration and SI(BaSO4) continued to decline after

Figure 4. SrSO4/BaSO4 coprecipitation experimental results obtained at SI(BaSO4) of 0.8 at various MRs. (a) Aqueous Ba concentrations. (b) Calculated SI(BaSO4) values. 6182

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Figure 5. (a) SEM characterization of the precipitated solid sample collected from the experiment with SI(SrSO4) of 0.6 and Sr/Ba MR of 1:100. The section with the red rectangle is the area selected for EDXS analysis. (b) Section-based EDXS analysis result.

tration shown in Figure 6a, Ba species concentrations were measured to be higher in the KCl medium during the course of the experiment. Obviously, it suggests that both SrSO4 and BaSO4 precipitation kinetics were delayed in the KCl medium. In a previous study, it was reported that an ionic medium can play a marked role in impacting the growth kinetics of the calcium phosphate solid.24 The precipitation rate of calcium phosphate was enhanced in the NaCl than in the KCl medium. The reason was believed to be related to the replacement of Ca by Na in the calcium phosphonate crystal lattice. Potassium, on the other hand, can only reversibly adsorb onto the calcium phosphate surface instead of replacing the calcium element. In this experiment, the expedited deposition of both BaSO4 and SrSO4 solids in the NaCl medium might be attributed to cation replacement of Ba/Sr with Na in the crystal lattice. Experimental results obtained at MR of 1:5 as shown in Figure 7 suggest that at a higher MR, the difference of aqueous Sr concentrations in both media is considerably reduced and so is the difference of the aqueous Ba concentrations. This

calcium species in the brine water can be as high as 10 000 and 40 000 mg L−1, respectively.23 In this study, the impact of an electrolyte medium on SrSO4/BaSO4 precipitation was evaluated by replacing NaCl with either KCl or CaCl2 as the electrolyte medium. In order to maintain the same solution ionic strength, the concentrations of KCl and CaCl2 were chosen as 1 and 0.33 M, respectively. The experimental condition of SI(SrSO4) of 0.8 and two MRs of 1:10 and 1:5 were selected to examine the KCl and CaCl2 impact. As shown in Figure 6a, with MR of 1:10, the shape of the Sr

Figure 6. Comparison of coprecipitation experimental results obtained at KCl and NaCl media at SI(SrSO4) of 0.8 and Ba/Sr MR of 1:10. (a) Sr concentration and SI(SrSO4) curves. (b) Ba concentration and SI(BaSO4) curves.

concentration curve in the KCl medium generally follows that in the NaCl medium during the course of the precipitation experiment. Throughout the experimental duration, the measured Sr concentration was about 60−100 mg L−1 higher in the KCl medium than in the NaCl medium. Correspondingly, the calculated SI(SrSO4) curves in both media shared a similar shape. As illustrated in Figure 6b, the measured Ba species concentration curves in both media followed a similar shape with the rebound phenomenon occurring in both at approximately 10 min. Similar to the measured Sr concen-

Figure 7. Comparison of coprecipitation experimental results obtained at KCl and NaCl media at SI(SrSO4) of 0.8 and Ba/Sr MR of 1:5. (a) Sr concentration and SI(SrSO4) curves. (b) Ba concentration and SI(BaSO4) curves. 6183

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tration and a delayed rebound phenomenon of Ba concentration can be observed at MR of 1:5 as shown in Figure 9. The

suggests that the impact of the increased Ba species concentration outweighs the impact of difference in the electrolyte medium. Because of the similarity of aqueous Sr and Ba concentrations in both media at MR of 1:5, the differences in the calculated SI(SrSO4) and SI(BaSO4) values in both media are mainly due to the differences in SO4 concentration and species activity coefficients. Compared with the experimental results in the KCl medium, the results obtained in the CaCl2 electrolyte medium are considerably different despite the same equivalent solution ionic strength. As shown in Figure 8a, the Sr concentration in

Figure 9. Comparison of coprecipitation experimental results obtained at CaCl2 and NaCl media at SI(SrSO4) of 0.8 and Ba/Sr MR of 1:5. (a) Sr concentration and SI(SrSO4) curves. (b) Ba concentration and SI(BaSO4) curves.

impact of the CaCl2 medium on SrSO4 and BaSO4 solid precipitation can be explained by the formation of calcium sulfate scale. An SSP calculation reveals that at the experimental condition of this study in the presence of 0.33 M CaCl2 electrolyte, gypsum (CaSO4·2H2O) and anhydrite (CaSO4) scale solids are oversaturated with a calculated SI of 0.32 and 0.53, respectively. Thus, the formation of gypsum and anhydrite solids consumed a fraction of the aqueous SO4 species, leading to a reduction in aqueous SO4 species concentration and a reduced precipitation kinetics of SrSO4 and BaSO4. It is worthwhile to mention that the VM calculation suggests that in the presence of 0.33 M CaCl2, brine solutions in this study are not supersaturated with respect to either gypsum or anhydrite with the VM calculated SI values being negative. However, VM calculation indicates that 70% of the total SO4 species would form aqueous CaSO4 complex and this value was only 5 and 10% for the BaSO4 complex and SrSO4 complex, respectively. Therefore, regardless of the nature of calcium sulfate as either a precipitated solid or an aqueous complex, SO4 species was consumed in forming calcium sulfate, which considerably delayed the precipitation kinetics of both SrSO4 and BaSO4. Conventionally, the oilfield scale control strategy is developed based upon each individual type of mineral scale without considering the scenario of coprecipitation of different types of scales.11 The results from this study suggest that in the presence of the Ba species, despite a low MR of 1:100, the deposition kinetics of SrSO4 can be substantially enhanced. Therefore, a scale control strategy based on consideration of Sr and SO4 species

Figure 8. Comparison of coprecipitation experimental results obtained at CaCl2 and NaCl media at SI(SrSO4) of 0.8 and Ba/Sr MR of 1:10. (a) Sr concentration and SI(SrSO4) curves. (b) Ba concentration and SI(BaSO4) curves.

the CaCl2 medium stayed relatively stable at ca. 610 mg L−1 throughout the experimental duration when MR was 1:10. Thus, the calculated SI(SrSO4) was maintained at a relative constant level of ca. 0.57. It appears that substituting CaCl2 for NaCl as the electrolyte medium inhibits the precipitation of SrSO4. As for the aqueous Ba concentration at MR of 1:10, the rebound phenomenon occurred in both NaCl and CaCl2 media except that the phenomenon was postponed to 35 min in the CaCl2 medium compared with 5 min in the NaCl medium. In addition, the Ba concentration post the rebound declined more gradually in the CaCl2 medium and the final Ba concentration was 8.4 mg L−1, which is much higher than the final Ba concentration of only 0.5 mg L−1 in the NaCl medium. Moreover, the calculated final SI(BaSO4) value in the CaCl2 medium was above 1, where SI(BaSO4) dropped to below zero in the NaCl medium. Obviously, the precipitation of both SrSO4 and BaSO4 was substantially hindered in the CaCl2 medium. Similar characteristics of the change in the aqueous phase Sr and Ba concentrations as well as the calculated SI values can also be observed when MR was increased to 1:5 in the CaCl2 medium. A relatively stable aqueous Sr concen6184

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Energy & Fuels concentrations alone can lead to an underestimate of the scale threat severity when a considerable amount of Ba species is present in the produced water. Thus, the conclusions of this study call for a comprehensive scale threat assessment with a holistic view of various contributing factors, such as water chemistry, operational conditions, and so forth. The impact of coprecipitation of different types of scales should also be included in the scale threat assessment so as to capture the likelihood of expedited scale deposition kinetics because of coprecipitation. 3.4. Future Work. 1. For each mineral scale precipitation study, duplicate measurements will be carried out and reported in terms of error bars to indicate the experimental error range and significance of the obtained precipitation data; 2. Carry out X-ray diffraction (XRD) characterization of the precipitated mineral solids. XRD patterns can provide direct mineralogy evidence of the obtained mineral scale solids.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+853) 8822 4917. ORCID

Ping Zhang: 0000-0003-0820-2056 Present Address ∥

Fulcrum Resources Inc. San Francisco, CA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the sponsorship of the Science and Technology Development Fund, Macao SAR (FDCT) (0063/ 2018/A2). This work was also financially supported by the 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).

4. CONCLUSIONS In this study, the kinetic characteristics of coprecipitation of BaSO4/SrSO4 at a representative oilfield condition was experimentally investigated by mixing a Ba/Sr-containing brine solution with another SO4-containing brine solution in the NaCl medium. The impacts of Ba to Sr MR, SI and solution electrolyte were systematically investigated. It was found that the presence of the Ba species can considerably promote the precipitation kinetics of the SrSO4 scale solid, which is likely due to the formation of BaSO4 solids serving as the nuclei. This phenomenon of continuous BaSO4 solid precipitation after the solution became undersaturated with BaSO4 can be explained by the reduction in solubility of the minor constituent in solid solution. Moreover, the measured aqueous Ba concentration exhibited a rebound phenomenon which can be explained by the reversible adsorption of the Ba species on the surface of the bulk precipitate. Different from the experimental results obtained at a lower SI(SrSO4) of 0.6, when SI(SrSO4) was elevated to 0.8, the rebound phenomenon of the Ba concentration occurred at a wider range of MRs. SEM and EDXS characterization results confirm a uniform distribution of Sr and Ba elements in the tested samples. The impact of replacing NaCl with KCl as the electrolyte medium on sulfate scale deposition kinetics is insignificant except that SrSO4 and BaSO4 deposition kinetics were delayed in the KCl medium at low MRs because of the absence of replacement of Ba/Sr with Na in the crystal lattice. On the other hand, substituting CaCl2 for NaCl as the electrolyte medium will result in a substantial hindrance of SrSO 4 and BaSO 4 deposition kinetics, which can be explained by the formation of calcium sulfate scale. The conclusions from this study can be profoundly insightful in developing an oilfield scale control strategy. The methodology elaborated in this study has the potential to be adopted as the standard procedure to evaluate scale threats when more than one type of scale are expected to deposit from the produced water.

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Molecular structure of the BHPMP inhibitor; schematic representation of the experimental setup; SEM characterization; and EDXS characterization (PDF)

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b01030. 6185

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