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Initial Stages of Barium Sulfate Formation at Surfaces in the Presence of Inhibitors Published as part of a virtual special issue of selected papers presented at the 2010 Annual Conference of the British Association for Crystal Growth (BACG), Manchester, U.K., September 57, 2010 Eleftheria Mavredaki,*,† Anne Neville,† and Ken S. Sorbie‡ † ‡
School of Mechanical Engineering, University of Leeds, Woodhouse Lane Leeds LS2 9JT, United Kingdom Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom ABSTRACT: The formation of barium sulfate (BaSO4) in the oilfield is known to occur as a precipitation process from the aqueous phase within oil production facilities. This barite deposition causes many problems related to flow assurance. In this paper, the initial stages of barium sulfate deposition on a metallic surface are investigated. The mass rate of deposition of barite on the surface was measured using a quartz crystal microbalance (QCM). The morphology of the deposited BaSO4 was then observed directly with an atomic force microscope (AFM) and the main crystal faces of barite were identified. Both the formation kinetics and the crystallography of the deposited barium sulfate were studied at three supersaturation ratios and in the presence and absence of 2 chemical scale inhibitors, namely, polyphosphino carboxylic acid (PPCA) and diethylene triamine penta acetic acid (DETPMP). In addition, the precipitation of the barite in the bulk phase was also monitored (by turbidity measurements) thus giving a more complete description of the overall bulk/surface barium sulfate kinetics. PPCA proved to be an effective barite inhibitor both at the surface and in the bulk phase in these measurements. DETPMP did not perform so well but there are several reasons why this may be so in such early time experiments and these are discussed in the paper. Thus, such experiments must be interpreted with some caution when we are relating the results to the oilfield scale system. In the industrial application addressed here, the brine system is very specific in that it is at relatively high ionic strength, the divalent cations (Ca2+/Mg2+/Sr2+) play an important role and the supersatuaration ratios of barite are very high. However, barite deposition is currently a very significant problem in the oil and gas industry, and this paper presents findings that will contribute to better ways of managing barite scale in the future.
’ INTRODUCTION The formation of sparingly soluble inorganic salts from aqueous supersaturated solutions is known as “scaling” and constitutes one of the major problems in oil recovery and in aqueous distillation plants. Scale precipitation, like corrosion and biofouling phenomena, can cause major operational problems.1 The scale deposits adhere to the surfaces of the producing well tubing and water handling equipment, leading to problems in pumps, valves, topside facilities, and heat transfer equipment. Therefore, the impact on flow assurance is high. The most commonly occurring hard types of inorganic oilfield scales are the sulphates (BaSO4, CaSO4, and SrSO4) and calcium carbonate (CaCO3). Of these inorganic scales, barium sulfate is considered to be particularly difficult to deal with because of its very low solubility and its hardness once formed. The bonds between the Ba2+ and SO42 ions in the barite lattice belong to the first coordination sphere and are characterized by high energy thus the solubility of BaSO4 is very low and the formation of barite is difficult to inhibit.2 Barite crystallizes in an orthorhombic form (space group Pnma), with lattice parameters a = 8.87 Å, b = 5.45 Å, and c = 7.15 Å.3 Barite scale forms due to the incompatibility of the injected seawater (high in SO42) with the formation water (high in Ba2+) in r 2011 American Chemical Society
the reservoir. Deposition of inorganic barite and other sulfate salts may occur wherever mixing of such incompatible brines takes place. The driving force for the inorganic scale to form is the supersaturation ratio (SR), which must be greater than 1, in the mixed solution for the barite crystals to form. The supersaturation ratio (SR) of the solution with respect to a crystal of Barite is related to the supersaturation index (SI) according to eq 1.4 SI ¼ logðSRÞ or SI ¼ log
a
Ba2þ a SO2 4 Ksp
ð1Þ
2+ where aBa2+, aSO2 and SO42, 4 are the activities for Ba respectively and Ksp the thermodynamic solubility product of the barite. For BaSO4, the solubility product, Ksp =9.82 1011 M2 at 25 C.5 Generally, the supersaturation ratio of the produced brine in an oilfield may vary widely because of changes in the conditions that characterize the precipitating system (ionic concentrations, temperature, ionic strength, pressure, etc.). However, the initial supersaturation ratio of a given brine is one of the
Received: November 29, 2010 Revised: August 22, 2011 Published: September 07, 2011 4751
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Table 1. Composition of Brines mixture A type of ions
formation water
mixture B
synthetic seawater
formation water
mixture C
synthetic seawater
formation water
synthetic seawater
+
K
400 ppm
350 ppm
1906 ppm
380 ppm
1906 ppm
Ca2+
200 ppm
700 ppm
2033 ppm
405 ppm
2033 ppm
405 ppm
Mg2+
700 ppm
200 ppm
547 ppm
1215 ppm
547 ppm
1215 ppm
Ba2+
100 ppm
0 ppm
80 ppm
0 ppm
3982 ppm
0 ppm
Sr2+
200 ppm
350 ppm
417 ppm
0 ppm
417 ppm
0 ppm
0 ppm
101 ppm
0 ppm
2780 ppm
0 ppm
2780 ppm
13570 ppm
17940 ppm
26535 ppm
10900 ppm
26535 ppm
10900 ppm
SO42Na+
most significant factors that affects the particle morphology of the resulting barium sulfate crystals that form.6 There are a number of computational methods for calculating the supersaturation ratio for a given brine system. In this paper, the supersaturation index of each tested brine was calculated using the ScaleSoftPitzer software, which is considered to be one of the most accurate codes for such predictions.7 This software calculates the ion activity coefficients based on the Pitzer electrolyte theory,8 and all the main parameters of the system such as the ionic concentrations, pH, temperature and pressure. Both mechanical scale removal and the application of chemical inhibition techniques have been widely applied for many years in the oil industry. From these studies, it has been widely concluded that in most cases the application of preventative antiscalant chemicals is usually the most effective treatment to control mineral scale problems. Scale inhibition technology is based upon the ability of specific molecules to inhibit or delay the nucleation or the crystal growth of inorganic scale crystals. Potentially, the application of chemical additives results in inhibition of the growth rates of the forming scale in all the developed directions of the lattice planes and loss of the anisotropy of the growth.9 The use of chemical additives containing phosphonate or poly phosphino carboxylic groups is wellknown for the inhibition of barium sulfate.1013 Although scaling as a production problem relates to the built up of inorganic salts on surfaces, most previous barium sulfate studies have focused on the barite deposition from the bulk phase.4,1416 For example, the most common method for testing the inhibition efficiency of chemical scale inhibitors for barite involves bulk static (jar) tests. A distinction between the formation kinetics of barite on the surface and in the bulk has taken place only in the past decade.17,18 This paper presents results for the formation kinetics of barium sulfate on a surface during the initial stages of scaling in the presence of other divalent cations. Very few studies studies have previously investigated BaSO4 formation from complex brines containing divalent cations.17,19,20 The interest in such a complicated brine system as the one examined here is motivated by the need to understand industrially relevant crystallization systems under thermodynamically nonideal conditions. The deposition of BaSO4 is also investigated in the presence and absence of chemical scale inhibitors. We will show that the surface growth kinetics of barite is different from the corresponding bulk kinetics. Moreover, the morphologies of the deposited barite crystals reveal the inhibition activity of the two chemical additives. Understanding the behavior of the inhibitors and their retardation mechanisms will lead to improvements in these antiscaling techniques.
380 ppm
’ EXPERIMENTAL DETAILS Brine Preparation. Barite was formed after mixing two brines consisting of a formation water and a synthetic seawater. The formation water contains, NaCl, KCl, CaCl2 3 6H2O, MgCl2 3 6H2O, BaCl2 3 6H2O, SrCl2 3 6H2O and represents the downhole brine composition found in an oilfield; the synthetic seawater was made by adding NaCl, KCl, CaCl2 3 6H2O, MgCl2 3 6H2O, SrCl2 3 6H2O, and Na2SO4 anhydrous, to distilled water. After preparation, the brines were filtered through 0.2 μm cellulose nitrate membrane filters to remove any particulate impurities. The pH of the solutions was adjusted to the range of pH ∼5.56.5. The mixing ratio of formation water to seawater was taken as 1:1 in all the tests. Three brine compositions were tested in this work as presented in Table 1. The presence of high levels of divalent cation in both types of water makes these compositions similar to those found in the offshore environment. The supersaturation index values for the three different compositions were 1.45, 2.64, and 4.32 for the mixtures A, B, and C, respectively. All the experiments were conducted under static conditions at room temperature (22 C). Inhibitors. The chemical additives tested for their inhibition effect on barium sulfate formation were two commercial scale inhibitors used frequently in the oil and gas sector: (i) a polyphosphino carboxylic acid (PPCA) with a molecular weight of 3600 g/mol and activity 42%, supplied by Biolab, and (ii) a diethylene triamine penta(methylenephosphonic acid) (DETPMP), with molecular weight 573 g/mol and activity 45%, supplied by Rhodia. For the inhibition tests, the additive was applied at concentrations of 1, 4, and 10 ppm in the synthetic seawater before mixing of the brines. Surface and Bulk Experiments. The principle of the quartz crystal microbalance (QCM) is based on Sauerbrey theory.21 Equation 2 is Kanazawa’s improvement of the original Sauerbrey equation, which is applied in the liquid environment and considers the solution viscosity and the density of the solution on the crystal resonance frequency. Therefore, the change in the resonance frequency, Δf, which accompanies the immersion of the crystal into a viscous medium, is given by !1=2 FL ηL ð2Þ Δf ¼ f ν 3=2 πFq μq where fν is the frequency of oscillation of the unloaded crystal, Fq is the density of quartz, μq is the shear modulus of quartz, FL is the density of liquid in contact with the electrode, and ηL is the viscosity of the liquid in contact with the electrode. The quartz crystal microbalance used for this work was the QCM200 model supplied by Stanford Research Systems. The electrodes used were coated with a titanium underlayer and gold (Au/Ti). The QCM electrodes were characterized by Fq= 2.648 g cm3 and μq= 2.947 1011 g cm1 s2, and the surface roughness was measured as 0.014 μm using a Taylor Hobson precision apparatus. The batch tests with the quarzt crystal microbalance were conducted at 22 C for 1 h with a 4752
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Figure 1. Uninhibited deposition of BaSO4 as measured by QCM (a) at three supersaturation indices (SI) for the first hour and (b) for the first 10 min for SI = 1.45. sample interval of 0.01s. Capacitance was adjusted to zero before every test, as soon as the quartz crystal was transferred from the air to the aqueous, supersaturated environment. The barite precipitation occurring in the bulk phase was quantified by measuring the turbidity in the bulk. A HACH DR 890 was used for measuring the turbidity of the barium sulfate formed in the bulk phase. The turbidity was measured in Formazin Attenuation Units (F.A.U). All the surface and bulk tests were repeated at least three times. Surface Observations. The topography of the adhered barite was observed using an Atomic Force Microscope (Veeco systems) after the deposition tests with the QCM. The tips used during the scanning of the samples were nonconducting silicon nitride (model MLCT-EXMT-A1) and the length of the cantilever was 0.59 μm. The samples were scanned using noncontact mode. The scan ranges varied between (100 μm 100 μm) and (5 μm 5 μm). The crystal planes of the deposited barium sulfate were characterized with in situ X-ray diffraction (details of the procedure have been presented elsewhere22). The crystallographic orientation of the sample crystals with respect to the AFM scan direction was determined from the observed etch pit morphology.
’ RESULTS AND DISCUSSION Uninhibited Deposition of Barium Sulfate. Initially, the effect of the supersaturation index (SI), as the main driving force of the scale formation, was examined. The formation of barium sulfate on the metal surface was recorded by QCM for various SI values as presented in Figure 1a. Results in Figure 1b indicate that the deposition of barite on the surface occurs as a two stage process independent of the SI. The initial part (first two minutes) of the plots are characterized by a steep slope which represents the nucleation process and the beginning of the growth of BaSO4 on the surface. The second part of the plot has a lower slope and the growth of barium sulfate in this period continues following a near-linear trend. Figure 1b shows the first 10 min of the mass deposition measurement for the case with the lowest supersaturation index (SI = 1.45). Even at these very low supersaturation conditions, the crystallization slope changes after the second minute of the deposition revealing a slower growth. Although the mass measurements of barite using the QCM reveal that the formation kinetics follow the same trend for each supersaturation index studied, the atomic force microscope images show that the BaSO4 morphologies vary as a function of the supersaturation index. The AFM morphologies of barite
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presented in Figure 2 are for crystals depositing on the QCM surface. This is one of the few reports where barium sulfate is scanned with the AFM after forming directly on a metallic substrate. Most other authors have used (001) monolayers of barite and have then studied the growth on these planes.11,20 The adhered barite on the surface for mixture A (SI =1.45) in Figure 2a shows the growth of strict, rhombohedral structures of BaSO4. Figure 2a illustrates the dominant crystal faces (001) and (210) of BaSO4 as well as the flat (010) crystal plane. These rhombohedral morphologies have been observed previously by Bromley et al.23 and they are expected to be formed when SI < 3.24 The barite morphologies illustrated in Figure 2a may suggest that there is the start of a spiral growth mechanism where the upper and lower surfaces of the crystal are not the same size, as in the perfect rhombohedral system in Figure 2d, but instead the upper surface is smaller in area. This might suggest that the spiral growth is restricted and not able to proceed. This is possibly linked to the nature of the barite system studied where the supersaturation ratio decreases within time and hence the growth becomes very slow. By increasing the supersaturation index to 2.6, the rhombohedral structure of barium sulfate crystals is lost, as illustrated in Figure 2b. The morphologies of the surface-formed barite are less well-defined, presumably because of a different process dominating the formation of barium sulfate at this SI. The growth of barite occurs on the (001) crystal plane of BaSO4 and follows the main directions of [120] and [100]. The characteristic circular sector (as shown in the box of Figure 2b) and the barite growth following as main directions the [120] and [100] provide evidence for two-dimensional nucleation occurring. The twodimensional islands are half a unit cell high and they show sectors with straight edges parallel to Æ120æ directions.11 According to the birth and spread model, the birth of nuclei occurs anywhere on complete layers as well as islands formed by already developed nuclei.25 The barite islands formed at supersaturation index 2.64 satisfy the criteria of the two-dimensional nucleation and the island illustrated in the indicated area of Figure 2b is similar to the well-known, two-dimensional morphology of barium sulfate published by Pina et al.11 The excessive growth of the barite islands shown in Figure 2b can be linked to the high supersaturation index and to the presence of the divalent cations, which are expected to interact or coprecipitate within the barite lattice. By further increasing the SI to 4.32 (mixture C), the observed BaSO4 crystals are largest, compared with the crystals obtained at the lower SIs, as shown in Figure 2c. The morphologies are elongated on the b axis because of the high anisotropy along the [120] direction at this high SI. Hence the growth of the (210) crystal face of barium sulfate is extended. Deposition of Barium Sulfate in the Presence of Inhibitors. In the presence of the scale inhibitor PPCA, the growth trend of barium sulfate remains similar although the final mass of deposited barite recorded with the QCM after 1 h of precipitation is smaller. Figure 3 shows the effect of PPCA at 1, 4, and 10 ppm on the mass of barite depositing from mixture B. As expected, the increasing concentration of PPCA resulted in higher inhibition of barite. At the highest concentration of PPCA tested (10 ppm), 89% inhibition of barite was observed. The high performance of PPCA at SI = 2.64 is confirmed by AFM showing the small number of the round-shaped barite particles formed on the surface, as illustrated in Figure 5b and c for the concentrations of 4 ppm and 10 ppm PPCA, respectively. The smaller number of 4753
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Figure 2. AFM images of BaSO4 morphologies deposited from (a) mixture A (20 20 μm), (b) mixture B (20 20 μm), (c) mixture C (20 20 μm), and (d) diagram of the barite rhombohedral structures as presented by Bromley et al.23 (e) Two dimensional islands by Pina et al.11
Figure 3. PPCA effect on the deposited BaSO4 from mixture B. Figure 4. DETPMP effect on the deposited BaSO4 from mixture B.
barite particles on the surface compared to the number of particles in the absence of chemical additive demonstrates that PPCA inhibits the formation of barium sulfate from the early nucleation stage. Furthermore, the round shaped particles of barite deposited in the presence of PPCA have no crystal faces that can be clearly determined, and this suggests that the inhibitor is also quite efficient during the crystal growth stage. Figure 4 shows the mass of barite deposited for uninhibited mixture B (SI = 2.64) and in the presence of 1, 4, and 10 ppm DETPMP. In this experiment, none of the concentrations of DETPMP examined was able to inhibit the formation of barium sulfate. This apparently poor performance of DETPMP is contrary to the work of other researchers, who concluded that DETPMP is effective in blocking the formation of barite.10,26,27 There are several reasons why we find this poor performance of DETPMP in early time deposition experiments of this type. The inhibition action of DETPMP is known to be dependent on a range of factors such as (i) the presence of divalent cations (Ca2+ and Mg2+) in solution, (ii) the temperature, and (iii) the
pH during precipitation.28 It may be the low temperature in this study which is reducing the effectiveness of DETPMP.29 Also, it is known that the type of scale inhibitor consumption that occurs as the barite is formed in the presence of DETPMP shows a sudden sharp fall at early time,28 and this may promote an early time deposition of barite. However, once the barite crystals form, DETPMP is expected to be an effective crystal growth inhibitor and this is consistent with the AFM observations below. The barium sulfate morphologies shown in Figure 5d and e for the concentrations of 4 and 10 ppm DETPMP show strong modifications of the uninhibited BaSO4 crystallography. In these figures, the barite particles show star-like or needle-like shapes. DETPMP may appear not to be interfering in the growth rate (i.e., the mass) of barium sulfate, but it strongly affects the barite islands shape. This indicates how DETPMP manages to interact with the forming barite at the crystal growth stage. The formation of needle-shape particles of barium sulfate can be related to a prolonged growth along the [001] where the (011) crystal face 4754
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Figure 5. AFM images (100 100 μm) of deposited Barite after treatment with (a) no inhibitor, (b) 4 ppm PPCA, (c) 10 ppm PPCA, (d) 4 ppm DETPMP, and (e) 10 ppm DETPMP.
Figure 6. PPCA effect on the deposited BaSO4 from mixture C.
becomes the main surface. Needle-shaped morphologies have been published elsewhere as the result of high concentration of Sr2+ being present in the system,30 where the high concentration of Sr2+ was able to stabilize the (011) crystal face. In the present work, a further analysis is needed to determine whether or not strontium cations are responsible for stabilizing the recorded (011) surface of barite. However, strontium cations easily interact with the crystal faces of barite even at high test temperatures and easily coprecipitate within the barium sulfate lattice, as recorded previously with the in situ SXRD measurements.22 Figure 6 shows the effect of PPCA on the mass of barium sulfate precipitating from mixture C (SI = 4.32). The best inhibition performance was recorded when 4 ppm of PPCA was added to the synthetic seawater, and the 10 ppm PPCA
case resulted in an increase in the deposited mass of barite on the surface. Some care must be taken in interpreting this result since such “optimum values” in scale inhibitor concentration are rarely seen in bulk solution inhibition experiments. However, such an optimum PPCA inhibition efficiency behavior has been observed for particular combinations of divalent ion (Ca2+/Mg2+) concentrations,31 although this may not be the correct interpretations here, and this matter does require further study. The adhering crystals resulting from barite inhibition with PPCA at a high supersaturation index (SI = 4.32) is shown in Figure 7. A very interesting point is the illustration of the spiral growth mechanism which is taking place on the (001) monolayer (see Figure 7a). The presence of PPCA in the system controls the growth of barite on the surface and the amount of the actual depositing barium sulfate is low revealing the same growth mechanism that occurs at low supersaturation ratio. Figure 7a shows the presence of symmetrical growth steps when 1 ppm of PPCA is present. At concentration of 4 ppm PPCA (Figure 7b), the steps are less obvious as the PPCA has retarded their formation. At the highest PPCA concentration of 10 ppm, the growth steps of the barite are again present (Figure 7c), but they are less symmetrical than for the 1 ppm PPCA case. At this severe supersaturation index, an increase in the concentration of the PPCA to 10 ppm, does not contribute very significantly to reduction of the growth rate since this is well below the MIC (minimum inhibitor concentration) for this 4755
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Figure 7. AFM images of deposited barite: (a) 1 ppm PPCA (10 10 μm), (b) 4 ppm PPCA (10 10 μm), and (c) 10 ppm PPCA (20 20 μm).
Table 2. BaSO4 Linear Growth Trend Slope (μg cm2 min1) inhibitor none 1 ppm PPCA 4 ppm PPCA
mixture A 0.0011
mixture B
mixture C
0.0015
0.0155
0.0013 0.0009
0.0054 0.0012
10 ppm PPCA
0.0002
1 ppm DETPMP
0.0029
4 ppm DETPMP
0.0023
10 ppm DETPMP
0.0025
Figure 8. Initial stage of BaSO4 deposition after precipitating from mixture A.
system. To reach clearer conclusions on this very severe SI case, further investigations are necessary. Barite Formation Kinetics at the Surface. The early time nonlinear part of the barite kinetics during deposition is illustrated in Figure 8 for the mixture A (SI = 1.45) case. The model, which fits the experimental data, is a simple quadratic at these low supersaturation index conditions, which is consistent with previous kinetic studies of Nancollas et al.32 Similarly a polynomial model characterizes the formation kinetics of barite when depositing from mixture B (SI = 2.64). For the highest supersaturation index conditions (mixture C), the model fitted on the recorded data is a polynomial of fourth order. The high order of the polynomial model reveals that the formation of barium sulfate on the surface is quicker at this high supersaturation index, as expected. Fourth-order equations have been reported previously when the growth and the dissolution process of barite formation were examined in seeded growth tests; however, no further investigation on these kinetic rates took place.32 For the second, linear stage of the recorded QCM measurements (Figure 1), the slopes of the BaSO4 deposition between the 30th and the 60th minute were determined and these are listed in Table 2 for the various cases studied. Mixtures A and B gave similar slopes, despite the difference in the SI, implying that in this period the barium sulfate growth occurs at a similar rate for SI between 1.45 and 2.64. However, for the highest SI = 4.32, the crystallization rate of barium sulfate was higher by a factor of 10 compared to that recorded for mixture B.
The slopes of barite growth rates at the surface in the presence of inhibitors are also presented in Table 2. They show the effect of (i) inhibitor type and (ii) concentration of the inhibitor on the rate of BaSO4 deposition. The main focus is on the decrease in the crystallization rates of the deposited barite in the presence of PPCA compared to the rates in uninhibited conditions. The smaller slope during the linear growth of barium sulfate on the surface after treatment with PPCA reveals the effectiveness of the specific inhibitor on barium sulfate. On the other hand the slopes of the barite growth in the presence of DETPMP seem to be higher than in the uninhibited test. This appears to indicate that DETPMP has no inhibition effect on the growth rate of the barium sulfate in this experiment and this correlates well with the QCM results where the amount of barite mass was similar in the presence and absence of DETPMP. However, this again relates to the previous comments on the style of DETPMP consumption and the early time enhanced barite precipitation observed in bulk inhibition efficiency tests.31 This again indicates that we need to interpret experiments of the type presented here very carefully in the light of other such corresponding bulk and surface deposition experiments to arrive at a clear picture of the barite depositional process at the various size and time scales. The effectiveness of the two inhibitors may be related to their chemistry. PPCA is a polyphosphinacrylate inhibitor having carboxylic groups as functional groups. DETPMP is a phosphonate chemical with 5 phosphonate groups as the functional units for inhibition. Besides the inhibitor’s chemistry, the size of these molecules and how they approach and interact with the forming barite particles are important. In general, big molecules like DETPMP tend to be effective since many of their functional 4756
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Table 3. Induction Time of Barite in the Bulk Phase type of brines
supersaturation index
tinduction (minutes)
mixture A
1.45
15.5
mixture B
2.64
0.7
mixture C
4.32
0.3
Figure 10. Comparison of bulk and surface measurements for mixture B after treatment with PPCA.
Figure 9. Comparison of bulk and surface kinetics of BaSO4 at three supersaturation indices.
groups attach and absorb on the formed particles. Both the chemistry and the size of the inhibitors in terms of length of the chain, are crucial parameters for the inhibitor’s performance during the nucleation and growth. Thus, when it comes to scale precipitation, PPCA has proved to be a good nucleation and early time growth retarder of barite.29 For later times (>24 h), the effectiveness of PPCA tends to decrease since, like many polymers, it is a less good long time crystal growth inhibitor under more severe barite formation conditions. DETPMP has been shown to act as a much better longer time (>24 h) crystal growth retarder of barium sulfate formation. From the work presented here, PPCA remains a good nucleation and early time growth retarder of the deposited barite however. On the other hand DETPMP appears to be a poorer early time nucleation (or growth) retarder of the barium sulfate formation, at least at the specific conditions. Regarding the efficiency of DETPMP on inhibiting the deposited barite is low, compared to the efficiency of PPCA. The poor performance of DETPMP in these experiments can be linked to the factors discussed above, for example, the low temperature, as DETPMP has proved to be a better inhibitor at temperatures above 90 C. However, in both bulk inhibition efficiency experiments and surface deposition inhibition under oilfield conditions, DETPMP is known to be a very good barite inhibitor, and therefore, we must interpret the observations of experiments such as those presented here with some caution. Bulk and Surface Kinetics. Turbidity measurements were performed to quantify the rate of appearance of BaSO4 in the bulk phase. The recorded induction times from these turbidity tests for all three mixtures are presented in Table 3. The induction time for barite formation in the bulk clearly decreases as the supersaturation index increases. The formation kinetics of the BaSO4 in the bulk phase (by turbidity) and on the surface (by QCM) for the three brines are compared in Figure 9. The mass and the turbidity measurements of the BaSO4 confirm that an increase in the supersaturation index is followed by (i) increase in the bulk appearance of barite and (ii) increase in the deposited mass of barite on the surface. For the three supersaturation indices studied, the results in Figure 9 indicate that the scaling
Figure 11. Comparison of bulk and surface measurements for mixture B after treatment with DETPMP.
activity on the surface continues although the kinetics in the bulk phase has already reached a plateau. Figure 10 shows a comparison of the formation kinetics of barite deposited on the surface and in the bulk for brine B for various levels of PPCA (1, 4, and 10 ppm). These results show that, for the first hour of precipitation the addition of PPCA at any concentration, results in limitation of the scale formation activity in the bulk phase. The turbidity values of the barite formed in the bulk when PPCA was present are zero at all inhibitor concentration studied, and therefore, the plots referring to the PPCA_bulk tests coincide. However, the deposition of barite on the surface continues despite the presence of PPCA, although the deposited mass is successively reduced over the 60 min period as the PPCA concentration increases. Figure 11 shows the effect of DETPMP on the formation kinetics of barite in both the bulk and on the surface. The presence of DETPMP at these concentrations (1, 4, and 10 ppm) does not have a strong effect on the mass deposition of the barium sulfate as the final recorded mass of barite on the surface is similar to that recorded in the absence of the inhibitor. However, as shown in Figure 11, DETPMP does to some extent affect the barite formation in the bulk phase. As presented in Figure 11, the lowest turbidity level (barite bulk formation) is at 1 ppm DETPMP and the highest is for 4 ppm. This is not a general finding in the literature and must be interpreted in terms of the various other interactions that can be observed in such a complex system. For example, in this brine mixture (B) DETPMP may interact with the divalent cations present in the system resulting in formation of nonbarite particles, which also may account for 4757
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’ SUMMARY Studies of BaSO4 formation kinetics on a surface are uncommon and much of the focus in previous literature has been on investigating the bulk phase of the precipitating systems. The quartz crystal microbalance was used to assess the mass rate of barium sulfate formation on the surface. The resulting barium sulfate crystals on the surface were then observed using an atomic force microscope. It was shown that different barite crystal morphologies were observed depending on the supersaturation index of the solution. The formation of barite on the solid substrate occurred as a two stages process, and the crystallization rate of barite may be controlled (or at least affected) by the presence of chemical scale inhibitors. Two commonly applied oilfield scale inhibitors were studied in this work, namely, PPCA and DETPMP. A more complete interpretation of the barite formation process is made possible in this work by comparing the kinetics in the bulk phase and at the surface. The evaluation of the total kinetics revealed that the growth of barite on the surface continues even when the activity in the bulk phase has stopped. Thus, the formation of barium sulfate needs to be considered as two simultaneous stages, each of which may be affected in a different manner and to a different extent by the various scale inhibitors. PPCA has proved to be an effective retarder of the barite both at the surface and in the bulk state. On the contrary, DETPMP was not very effective in inhibiting the formation of barite at these specific conditions. However, it cannot be concluded from these type of experiment that DETPMP is ineffective in general as there are several processes occurring in such complex brines which can explain this behavior. Thus, we must be cautious in our interpretation of these experiments since they may be probing aspects of the bulk/surface barite inhibition mechanisms which may not be the main contributors in the prevention of barite scale by DETPMP in real oilfield conditions. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge the FAST III joint industrial project and the University of Leeds for the financial support. ’ REFERENCES
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dx.doi.org/10.1021/cg101584f |Cryst. Growth Des. 2011, 11, 4751–4758
