Article pubs.acs.org/IECR
Characterization and Suspension Stability of Particles Recovered from Offshore Produced Water Dorota Dudásǒ vá,† Johan Sjöblom, and Gisle Øye* Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælandsvei 4, NO-7491 Trondheim, Norway ABSTRACT: Particles were recovered from offshore produced water and analyzed in terms of chemical composition, surface properties, and suspension stability. The recovered particles were identified to be iron (FeIIFeIIIOx(OH)y) aluminosilicate covered with an organic layer. Both the chemical composition and surface properties were altered upon calcination. Destabilization of suspensions occurred through sedimentation and aggregation of particles. Increased temperature enhanced the destabilization of the investigated suspensions, while the organic layer surrounding the particles and the dissolved components in the produced water slowed it down.
1. INTRODUCTION Produced water is water that is coproduced with oil and gas during petroleum production. In 2011 the total amount of produced water at the Norwegian Continental Shelf was 131 million m3,1 while the amount of produced oil was 97.3 million m3.2 It is therefore obvious that the water constitutes a significant fraction of the total amount of fluids. Most of the produced water (about 85%) is currently discharged to sea, but increasing portions are being reinjected into reservoirs. Produced water is a complex mixture of dispersed and dissolved components. The former include dispersed oil and solids, while the latter can consist of water-soluble organic compounds, dissolved salts, and production chemicals. The pollutants must be removed or minimized to allowed levels before discharge of the water to sea. The main strategy in this respect has been to reduce the content of dispersed oil. One rational for this might be that good removal of dispersed oil has been reported to give good removal of soluble hydrocarbons as well.3,4 Current regulations at the Norwegian Continental Shelf require that the concentration of oil in water must be less than 30 ppm before the water can be discharged to sea.5 Dispersed solids can cause several problems during handling of produced water. They can lead to particle stabilized emulsions during mixing with oil, which can be hard to remove by typical water treatment processes such as hydrocyclones and compact flotation units.6−8 The solids can also accumulate in various treatment units or clog filters.7 Further, if the produced water is going to be reinjected, well integrity issues can occur due to reduced injection efficiency by solids getting stuck on the walls of the injection well or plugging the reservoir pores.9−11 The extent of these difficulties can often be related to the wettability and dispersion behavior of the solids. Furthermore, the chemical composition of particles in produced water depends on the origin of the solids. Sand and clay particles follow the fluids from the reservoir, while iron oxide and iron sulfide are typical corrosion products that can be formed along the transport and processing lines. Changes in process variables such as temperature, pressure, and pH can also lead to precipitation of scale products like CaCO3 and BaSO4.12 The surface properties of the particles can be altered © 2014 American Chemical Society
by adsorption of interfacially active components from the water or crude oil.13 Such alterations will affect the tendency of particles to aggregate, sediment, and form particle stabilized emulsions.14−16 Ultimately, this may increase the separation difficulties. In the current investigation, we have analyzed particles recovered from produced water sampled at an offshore installation. The premise for the work was that improved fundamental understanding of parameters having an effect on aggregation and sedimentation of solids in produced water streams can enable more reliable and optimized operating windows for produced water treatment equipment. Furthermore, it can strengthen the basis of design when planning new produced water facilities. The aim of the investigation was to identify the chemical composition of the solids and to study how the surface properties influenced the suspension stability of the particles. The surface properties were altered by calcination of the recovered particles.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The investigated produced water was sampled at the inlet of a hydrocyclone placed downstream from the primary separator and a degasser at a North Sea platform. Particles were recovered from the produced water by filtration: Initially, the largest solids were removed by filtration through standard “589/2 white ribbon” filter paper. Two successive filtrations were carried out through 0.45 μm filter papers and the recovered produced water particles, herein denoted “PWp”, were used in the investigations. Part of the particles was calcinated at 700 °C in air for 10 h in order to remove adsorbed components and residues. This fraction is referred to as “PWp-700”. Some of the recovered particles were also calcinated at 350 °C in air for 7 h. Received: Revised: Accepted: Published: 1431
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consisted of signals collected every 40 μm along the sample height and were collected every 30 s for 75 min (i.e., 150 scans were recorded for each sample). Measurements were carried out at 25 and 50 °C and repeated two times in each case. Only the transmission scans were used in the data analysis (since the backscattering signal contains internal reflections from the measurement cell for dilute suspensions). The TLab Expert 1.13 software was used to calculate the particle migration rates from the time-dependent change in transmission signal at the top of the samples. The same software was used to determine the aggregation behavior from the time dependent change of the transmission in the middle (i.e., from 15 to 30 mm above the bottom) of the suspensions. Furthermore, the stability 20 mm above the bottom of the samples were described by the following stability index:
This fraction is denoted “PWp-350” and was only used in the XRD analyses. The water collected from the last filtration step was used in the suspension stability measurements and is denoted “PW”. 2.2. Characterization Methods. X-ray powder diffraction patterns were recorded on a Philips 1050 diffractometer using Cu Kα radiation (λ = 1.5418 Å). The diffractograms were collected from 2θ = 5° to 2θ = 70°. FT-IR spectra of the particles were collected with a Tensor 27 FT-IR spectrometer (Bruker Optics), using a MKII Golden Gate diamond ATR unit (Specac) and a N2-cooled MCT detector. Contact angles of the particles were determined at room temperature by the sessile drop method using an Optical Contact Angle Meter with a high speed camera (CAM 200, KSV Instruments). The particles were pressed into pellets with a diameter of 13 mm, and images were taken when a water drop (Millipore milli-Q system, Ω = 18.2 MΩ cm) was placed at the pellet surface by a Hamilton syringe. The contact angles were determined by fitting the Young−Laplace equation to the drop profile of the first image taken after contact between the liquid and solid phase. The reported values are the average of at least four parallel measurements. Zeta potential measurements were conducted at room temperature on a Malvern Zetasizer 3000HS connected to a titrator (Malvern Instruments, U.K.). The particles were dispersed in aqueous NaCl (0.01 M) solutions by placing the samples at an ultrasound bath for 15 min. Large particles were allowed to settle out before measurement. The electrophoretic mobility was measured when the pH was varied between 2 and 10. The pH was adjusted by 0.5 M NaOH and 0.5 M HCl solutions. The zeta potentials were calculated by the Smoluchowski approximation. Particle size distributions were determined by a Coulter Multisizer II instrument using a tube with orifice diameter of 30 μm. 0.15 M NaNO3 (aq) was used as the conductive liquid and the solution was filtrated two times through 0.22 μm filters before measurements. At least 3 parallels were run for each sample. The total hydrocarbon content of the PW sample (produced water after filtration) was determined by a GC-MS instrument (Agilent GC-MS system; GC-6890N, MS-5975 coupled with headspace autosampler Teledyne Tekmar HT3) using pentane as the standard. Three replicates were run. 2.3. Suspension Stability Measurements. Suspensions were prepared with PWp or PWp-700 as the dispersed phase and PW or synthetic brine (3.5 wt % NaCl dissolved in Milli-Q water from a Millipore milli-Q system, Ω = 18.2 MΩ·cm) as the continuous phase. Appropriate amounts of particles (0.05 wt %) and aqueous solutions were added to glass tubes. This amount of particles was within the broad range found in produced waters.17 The samples were placed in a sonication bath (Bandelin Sonorex RK102H) for 15 min. Subsequently, they were mixed using an Ultra Turrax (IKA, S25N-10G with 10 and 7.5 mm stator and rotor diameters, respectively) at 17 500 rpm for 2 min. The suspensions were transferred to a Turbiscan LAbExpert instrument (Formulaction, France) immediately after preparation. The stability was followed by recording the transmission (0° from incident beam) and backscattering (135° from the incident beam) signals of a pulsed near-infrared light source (λ = 850 nm) that moved vertically along the full height of the sample. Complete transmission and backscattering scans
n
stability index =
∑i = 1 (Xi − X T)2 n−1
(1)
where Xi is the measured transmission value, XT is the average of Xi , and n is the total number of scans.
3. RESULTS AND DISCUSSION 3.1. Characterization of Particles. Visual inspection revealed that the particles recovered from the produced water (PWp) were light orange in color and that they became brownred after calcination at 700 °C (Figure 1). The X-ray diffraction
Figure 1. Photo of the “PWp” (left) and “PWp-700” (right) samples.
patterns of the samples showed that the change in color was accompanied with phase transitions in the samples (Figure 2). The sample calcinated at 350 °C was included to facilitate the interpretation of the phase transitions. Furthermore, the peak assignments were based on high intensity peaks since low intensity maxima were not observed due to poor crystallinity of the samples. Kaolinite was present in all samples, and the unaltered intensity of the peaks indicated that the concentration was constant upon calcination.18 Three peaks observed in the diffraction patterns for the PWp and PWp-350 samples were assigned to magnetite (Fe3O4) and/or maghemite (γFe2O3). These assignments were due to the similar occurrence of the high intensity maxima in the diffractograms of magnetite and maghemite.19 The intensities of the peaks were higher for the PWp-350 sample than for the PWp sample. This difference in abundance ratio might be due to an organic layer surrounding the latter particles (see below) and which was 1432
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1379 cm−1).22 This demonstrated that hydrocarbons were present at the particles when recovered from the produced water. The organic constituents can either be water-soluble organic components adsorbed onto the particles from the water phase or components adsorbed onto the particles upon contact with the crude oil during transport/processing of the fluids. The band between 1700 and 1600 cm−1 also vanished upon calcination and can be associated with aromatic CC stretching vibrations22 and/or H−O−H bending vibrations for adsorbed water.23 Furthermore, the broad band around 3371 cm−1 partly disappeared after calcination. This band was assigned to the O−H stretching and underpinned the presence of adsorbed water. The vibrations bands that were present before and after calcination were attributed to the inorganic structure. Overlapping bands in the region from 1285 to 800 cm−1 for the calcinated sample can be assigned to Si−O stretching, typically found in silicates and clays.24 The lower intensity and shift in the bands after calcination might be due to removal of the organic material. Furthermore, the bands around 682 and 635 cm−1 can be attributed to a mixture of Si−O and Al−O vibrations in aluminosilicates.25−27 Other metal−oxygen stretching vibrations (like Fe−O stretching) are usually found in the region from 550 to 300 cm−1. Since this was outside the range of our measurements, some simple qualitative reaction tests were performed to identify the presence of iron in the samples. The color of the PWp sample changed from light orange to rust color when H2O2 (aq) were added. This was attributed to oxidation of iron(II) hydroxide to iron(III) hydroxide. A yellow liquid phase was formed when HCl (aq) were added to the sample, which likely resulted from dissolution of iron(III) hydroxide into [FeCl4−] complexes. It was evident from these observations that the PWp sample most likely contained FeIIFeIIIOx(OH)y. Similar tests were performed on the PWp-700 sample. No color changes were observed, and the sample was considered to contain α-Fe2O3.28 The surface properties of the samples were markedly altered upon calcination (Table 1). The contact angle changed from 51
Figure 2. X-ray diffraction patterns of PWp (bottom), PWp-350 (middle), and PWp-700 (top). The peak assignments are identified as kaolinite (K), magnetite (Ma), maghemite (Mh), and hematite (H).
removed during calcination. In addition, dehydration of γiron(III) oxide-hydroxide (amorphous FeOOH) will form γFe2O3 between 220 and 290 °C.20 The diffraction peaks corresponding to magnetite and maghemite disappeared for the PWp-700 sample, while two other diffraction peaks occurred. These were assigned to hematite (α-Fe2O3), since it is known that phase transitions from γ-Fe2O3 to α-Fe2O3 and from βFe2O3 to α-Fe2O3 will occur in the range of 480−540 °C20 and around 600 °C,21 respectively. Consequently, it was considered that the oxide mixture (magnetite and maghemite) identified in the PWp sample was converted to hematite upon calcination at 700 °C. The FT-IR spectra showed distinct differences between the PWp and PWp-700 samples since several absorption bands disappeared upon calcination (Figure 3). These changes were most clearly seen in the regions 3000−2800 cm−1 and 1700− 1300 cm−1. The PWp sample displayed several characteristic C−H vibration bands in these regions: C−H stretching of methyl and methylene groups (at 2956, 2921, and 2849 cm−1), C−H scissoring (at 1462 cm−1), and C−H methyl rocking (at
Table 1. Properties of the Produced Water Particles before and after Calcination PWp contact angle [deg] isoelectric point [pH] particle size distribution [μm]a a
51 ± 7 3.9 ± 0.4 0.4−3.4 (average: 1.9)
PWp-700 11 ± 4 2.5 ± 0.3 0.6−2.2 (average: 1.4)
Measured in 3.5 wt % aq. NaCl after mixing.
± 7° (PWp sample) to 11 ± 4° (PWp-700 sample) when the hydrocarbons were removed from the particles, i.e., the particles became more hydrophilic. In a previous study, contact angles of several model particles were measured before and after exposure to asphaltene solutions.14 Here, the contact angle of the PWp sample was lower than what was typically observed for asphaltene coated particles. Adsorption of organic components from the water phase or exposure to the entire range of crude oil components (not only asphaltenes) in the case of the actual produced water particles (PWp) might explain this difference. The low contact angle of the PWp-700 sample, however, agreed well with the contact angle found for inorganic particles with the pristine surface.13,29,30
Figure 3. FT-IR spectrum of the recovered produced water particles (PWp) and the calcinated particles (PWp-700). 1433
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Figure 4. Transmission profiles measured at 25 °C for the suspension containing 0.05 wt % PWp-700 dispersed in brine. Each measurement was identified by a color and the corresponding measurement times are listed to the right. Every 6th measurement is shown.
corresponds to moving from the bottom to the top of the sample (i.e., the height of the suspension was about 43 mm). The associated transmissions are shown along the y axis. The time dependency is indicated by different colors for each scan along the sample; that is, purple and red scans represent the first and last measurement, respectively. The significant increase in transmission at the top of the samples is associated with sedimentation of particles (i.e., clarification of sample),38 and this region was used to calculate the particle migration rates. Increased particle sizes can also be associated with increased transmission flux,38 and the aggregation was followed by changes in transmission in the region of sample where sedimentation could be neglected, i.e., between 15 and 30 mm above the bottom of the samples. The particle migration rates were considerably higher at 50 °C than at 25 °C (Table 2). Furthermore, calcination of the
Calcination also resulted in a shift in pH of the isoelectric point (IEP) of the particles (Table 1). The isoelectric point was found at pH 3.9 ± 0.4 for the PWp sample, while this was reduced to pH 2.5 ± 0.3 for the PWp-700 sample. Shifts of the IEP toward higher pH values have been reported to be due to coverage of silanol sites at the particle surface by asphaltenes or resins.30 Adsorption of anions at clay surfaces have also been reported to cause shifts in the IEP.31 These mechanisms could also be responsible for the shift observed here. Furthermore, the IEP for the PWp-700 sample (pH 2.5 ± 0.3) was closer to the IEPs reported for clays13,32−34 than for α-Fe2O3.35,36 Studies of soils rich in kaolinite and iron minerals have showed that soils calcinated between 772 and 1173 K had IEPs similar to that of kaolinite, while calcination at 1373 K resulted in IEP closer to that of iron oxide.37 The results were therefore considered to agree well with the compositional analyses above. The size distributions of the particles were quite broad both before and after calcination (Table 1). The polydispersity and average (number) diameter decreased somewhat upon calcination. This might be due to breakup of particle aggregates during calcination, but this requires further investigations to confirm. However, none of the samples showed any significant peaks in the distributions. 3.2. Characterization of the Aqueous Phase (PW). The PW sample and the brine solution (3.5 wt % NaCl in water) had similar transmission when measured by Turbiscan. This demonstrated that the dispersed components were removed from the produced water during the filtration procedures. The total hydrocarbon content of the PW sample was 5.8 ± 0.6 ppm and revealed that water-soluble crude oil components were present. Furthermore, pH of the water was 7, and the densities were 1.080 and 1.069 g/cm3 at 25 and 50 °C, respectively. 3.3. Suspension Stability. Sedimentation and aggregation are the kinetic phenomena that will destabilize suspensions and ultimately give rise to phase separation. Both phenomena can be followed by changes in transmission signals over time. Typical transmission data obtained by Turbiscan LAb are shown in Figure 4. Moving from left to right along the x axis
Table 2. Particle Migration Rates of the Suspensionsa sample PWp-700 in brine PWp-700 in PW PWp in brine PWp in PW
migration rate (μm/min), 25 °C
migration rate (μm/min), 50 °C
68 ± 5
727 ± 212
35 ± 4 51 ± 1 33 ± 1
235 ± 114 606 ± 39 153 ± 64
a
The values were calculated from the clarification front at the top of the samples.
particles had little influence on the migration rates when the samples were dispersed in PW. When the samples were dispersed in brine, however, the migration rate was higher for the calcinated particles (PWp-700). This trend was more obscure at 50 °C, due to larger uncertainties. The reason for this is not clear. For PWp-700 dispersed in brine the transmission in the middle of the sample started to increase shortly after the measurement was started and increased almost linearly about 10% during the measurement (Figure 5). The induction time 1434
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The enhanced aggregation was attributed to more frequent collisions between the particles due to increased thermal movement. The build-up of larger aggregates also contributed to the increased sedimentation rate. In addition, the higher density difference between the dispersed and continuous phases contributed to faster sedimentation. It was also evident that aggregation and sedimentation were slower in the samples containing recovered particles than in those with calcinated particles. This demonstrated that the organic layer adsorbed at the recovered particles provided steric stabilization in the suspensions. Similar effects were previously observed for model systems.14 Moreover, the suspensions were more stable in the produced water (PW) than in the brine. The exact composition of the PW was not known, but it might be speculated that the dissolved organic compounds can increase the viscosity of the aqueous phase and/or adsorb onto the particles and in this way enhance the suspension stability. Another contributing factor could be the ionic composition of the PW. It is known that the presence of polyvalent cations, like Ca2+ and Mg2+, can influence suspension stability39 as well as the interfacial activity of dissolved components.40,41
Figure 5. Time dependent increase of transmission in the middle of the samples for the suspensions measured at 25 °C. Every 4th data point is shown.
(i.e., the time before detectable change in transmission) was longer and the transmission increased about 4% when the calcinated particles were dispersed in brine. The longest induction times (about 45 min) and smallest change in transmission (1.5−2.5%) were seen for the PWp suspensions. The stability index values were also calculated in the middle of the samples and represented the tendency of aggregation in the samples as well. The higher the index the more readily aggregation occurred. At 25 °C, the indices decreased in the same sequence as described for the transmission profiles (Figure 6). This showed that the stability indices gave the same
4. CONCLUSIONS Particles were recovered from produced water collected at an offshore installation in the North Sea and identified to be iron rich (FeIIFeIIIOx(OH)y) aluminosilicate covered with an organic layer of crude oil components. The organic components were removed and the inorganic structure underwent phase transition into a mixture of α-Fe2O3 and aluminosilicate upon calcination at 700 °C. Both sedimentation and aggregation of particles contributed to destabilization of the suspensions. The destabilization was enhanced when the temperature increased from 25 to 50 °C, while the organic layer surrounding the particles caused steric stabilization. Also the dissolved components in the produced water slowed the separation. Furthermore, analysis protocols have been established that can be used for further produced water investigations. In terms of produced water management, the results suggested that particularly the processing temperature and surface properties of the solids are important variables to address when optimizing a produced water system. The improved knowledge of how these variables influence separation can also improve the basis of design for produced water treatment systems.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +47 73 59 41 35. Fax: +47 73 59 40 80. E-mail: gisle.
[email protected]. Present Address †
Statoil, Strandveien 4, Stjørdal, Norway.
Notes
Figure 6. Stability indices in the middle of the suspensions.
The authors declare no competing financial interest.
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overall measure of aggregation tendency as the transmission profiles. Detailed information about the time dependency, however, was only obtained by the profiles. The stability index (i.e., aggregation) increased markedly for all samples at 50 °C, but the effect was less pronounced when the particles were dispersed in PW. The Turbiscan measurements showed that both sedimentation and aggregation were promoted at higher temperature.
ACKNOWLEDGMENTS The work was done within the project “Treatment of produced Water” founded by the Research Council of Norway (Project No. 163505/S30) and industrial partners: Shell Technology Norway AS, Statoil, Total E&P Norge AS, Chevron Energy Technology Company, DNV, Champion Technologies and Aibel AS. Sumihar H. D. Silalahi and Peter Billik are 1435
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acknowledged for help with GC-MS and XRD measurements, respectively.
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