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Influence of the wettability of particles on the morphology and stability of crude oil–particle aggregates in synthetic produced water
Journal of Petroleum Science and Engineering 139 (2016) 198–204
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Influence of the wettability of particles on the morphology and stability of crude oil–particle aggregates in synthetic produced water Bartłomiej Gaweł, Meysam Nourani, Thomas Tichelkamp, Gisle Øye n Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway
art ic l e i nf o
a b s t r a c t
Article history: Received 28 August 2015 Received in revised form 12 November 2015 Accepted 16 December 2015 Available online 21 December 2015
Handling and treatment of produced water is becoming an increasing part of oil and gas production. A major challenge in this respect is removal of oil drops stabilised by particles. Understanding the influence of interfacial and wetting properties of crude oil–particle þ systems on the formation of particle stabilised emulsions is therefore essential. Elucidating these relationships was the aim of this investigation. Synthetic produced water containing different particles and crude oil were studied. The morphology of aggregates formed by crude oil drops and particles was strongly related to the contact angle (θwo) at the water–oil–particle interfaces. High θwo resulted in formation of particle stabilised emulsions, while less defined aggregates were formed at lower θwo Furthermore, coalescence of drops was related to the coverage of particles at the interface. Stable drops covered by particles were only seen when the amount of particles was significant. Otherwise drops tended to coalesce, likely due to particle bridging. & 2015 Elsevier B.V. All rights reserved.
Keywords: Produced water Particle stabilised emulsions Wettability o/w Emulsion stability Separation
1. Introduction Vast amounts of water are produced during oil and gas recovery (Lee et al., 2011). The produced water contains a complex mixture of dissolved and dispersed components. The latter includes crude oil drops as well as various inorganic particles originating from the reservoir or appearing as scale and corrosion products. Removal of dispersed oil is considered to be crucial in order to reduce the environmental impact of produced water. Therefore current regulations in the North Sea require that that the amount of oil in water must be less than 30 ppm prior to discharge to sea (OSPAR Commission, 2010). Pressure drops and shear forces during transport from the reservoirs to production facilities can lead to extensive mixing of oil drops and particles. The properties of both the oil phase and water phase can alter the surface properties, wettability and stability of the particles (Dudášová et al., 2014, 2009a, 2009b, 2008; Marczewski and Szymula, 2002; Sullivan and Kilpatrick, 2002). Furthermore, if particles become trapped at oil–water interfaces, particle stabilized emulsions can form. Oil drops stabilised by particles can be very difficult to remove by typical water treatment processes such as hydrocyclones and compact flotation units due to hindered coalescence and reduced Stokes velocity (Finborud, 2010; Markoff, 2009; Yan and Masliyah, 1997). n
Corresponding author. E-mail address: [email protected] (G. Øye).
http://dx.doi.org/10.1016/j.petrol.2015.12.019 0920-4105/& 2015 Elsevier B.V. All rights reserved.
Effective coalescence of oil drops is beneficial for ensuring good quality of the produced water. It is well known that indigenous interfacially active components in the crude oil, like asphaltene class, can cause formation of mechanically strong, viscoelastic films at the interface (Horváth-Szabó et al., 2005; Pawar et al., 2011; Poteau et al., 2005; Spiecker et al., 2003). High elasticity of such films can slow down coalescence of oil drops (Gaweł et al., 2014b). Furthermore, emulsions containing inorganic particles can be even more stable than those stabilized by asphaltenes only (Hannisdal et al., 2006; Nianxi and Masliyah, 1996; Sullivan and Kilpatrick, 2002; Yan and Masliyah, 1994, 1997; Yan et al., 1999). The ability of particles to stabilise emulsions is attributed to the large free energy of adsorption of solids which are partially wetted by both the oil and water phases (Aveyard et al., 2003; Binks, 2002). This means that particle layers adsorbed at drop interfaces can result in extremely stable emulsions (Binks and Lumsdon, 1999; Binks and Whitby, 2004; Dong et al., 2014). The wettability is a key factor affecting the properties of the particle layers and subsequently the emulsion behaviour (Gu et al., 2003; Yan et al., 2001). The contact angle for oil–water systems is related to interfacial tensions (γ) according to the Young equation:
γSO = γSW + γOW cos θ OW
(1)
where the subscripts s, o, and w represent solid, oil and water, respectively. The oil–water contact angle (θow) is measured through the water phase. The γow is related to the chemical composition of the water phase and crude oil, while γso is related
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to the composition of the crude oil and the adsorption of components at the solid surface (Abdallah et al., 2007). The free energy of adsorption of particles is dependent on the contact angle by the following expression:
ΔGd = πr 2 (1 − cos (θ ) )2
Table 2 Physical properties of the particles. Particle Shape
(2)
where r is the diameter of the particles. This expression states that adsorption of particles is strongest when θow is 90°. It is clear from this that the wettability of particles is a very important factor for emulsions stability. Another central parameter for particle stabilization emulsions is the surface coverage of the particles (LealCalderon and Schmitt, 2008; Pawar et al., 2011). However, there are extensive variations in the literature with respect to how many particles are required at the interface in order to constrain coalescence (Binks and Lumsdon, 1999; Dong et al., 2014; Frelichowska et al., 2010; Pawar et al., 2011; Tambe and Sharma, 1993; Vignati et al., 2003). Many studies related to particle stabilisation of emulsions have been focused on relatively simple model systems. During crude oil production, however, the fluid stream consists of crude oil, water and particles which become mixed during transport and can form various types of agglomerates like particles stabilised emulsions, schmoo etc. In this work the produced water stream was mimicked by mixing emulsions and particles under mild conditions. The main objective was to investigate the relations between wetting properties of particles in crude oil – synthetic produced water systems and the stability of the corresponding aggregates. Five different types of particles, typically present in produced water streams were used. The wetting properties of the particles, morphology of particle–oil aggregates and the corresponding stability in synthetic produced water were followed by contact angle, microscopy and Turbiscan measurements.
2. Materials and methods 2.1. Crude oil The crude oil used in this study has previously been characterized with respect to its physicochemical and interfacial properties. Details about the results and experimental procedures can be found elsewhere (Gaweł et al., 2014a, 2014b). Some of the properties are presented in Table 1 for convenience. 2.2. Particles The inorganic particles were chosen to represent minerals typically present in produced water (i.e. clay, scale and corrosion products): kaolin (Aldrich, USA/Germany); CaCO3 (98.2%) (Specialty Minerals Inc., USA); FeS (99.7%) (DLFTZ, Chang Hing, China); BaSO4 (99%) and Fe3O4 (98 þ %) (Nanoamor, USA). Physical properties of the particles are listed in Table 2. The particle shape, Table 1 Physicochemical properties of the crude oil (Gaweł et al., 2014a). Property
Value
Density at 20 °C (g/cm3) Viscosity at 20 °C (cP) Saturates (wt%) Aromatics (wt%) Resins (wt%) Asphaltenes (wt%) TAN (mg/g) TBN (mg/g)
0.81 3 80.0 18.0 1.9 0.1 0.4 0.6
199
Kaolin CaCO3 BaSO4 Fe3O4 FeS *
Density (g/cm3) Particle Specific surface size(from area (BET) suppliers) (m2/g)
Plate like 19.7 7 3 Rhombic 19.9 7 2 Prismatic 6.5 7 1 Spherical 45.2 7 3 Spherical 6.9 7 1
0.1–4 μm 70 nm 80–450 nm 20–30 nm 2 μm
2.65 2.71 4.5 4.8–5.1 4.6–4.8
Average aggregate size (μm)*
2.0 1.3 1.1 1.7 2.1
Measured by Coulter Counter in 3.5 NaCl solutions.
densities and particle size/size distributions were provided by the suppliers, while surface area and average aggregate sizes were measured previously (Dudášová et al., 2009a, 2009b). 2.3. Brine composition The brine solution was made by dissolving appropriate amounts of NaCl (99.5%, Merck, Germany), CaCl2 2H2O (99.5%, Merck, Germany), MgCl2 6H2O (99.5%, Merck, Germany), NaHCO3 (99.5%, Merck, Germany) and Na2SO4 (98.5%, Acros Chemicals) in MQ water from a Millipore Simplicity System. The ionic composition is listed in Table 3 2.4. Contact angles measurements The particles were pressed into flat pellets (13 mm in diameter) under high pressure using a hydraulic press (Compac, Denmark). The pellets were put in a custom made holder and immersed in the brine solution before an oil drop was placed on the pellet surface by a syringe. Images were taken of the oil drop immediately after attachment using an Optical Contact Angle Metre equipped with a computer-controlled high-speed camera (CAM200, KSV Instruments). Subsequently, the contact angles were determined by fitting the Young Laplace equation to the drop profiles. 2.5. Microscopy imaging Crude oil was weighted into 50 ml Schott bottles and the brine was added to give oil concentration of 10,000 ppm. The samples were emulsified with an Ultra Turrax mixer (IKA, S25N-10G with 10 and 7.5 mm stator and rotor diameter respectively) at 20,000 rpm for 2 min. Subsequently, particles were added to the o/ w emulsion to give a concentration of 5000 ppm. The system was dispersed using an automatic shaker (HS 501 digital IKA) at 300 rpm for 1 h. The shaking was used to mimic mixing under mild shearing conditions. Images of the dispersions were taken by a Nikon LV 100D microscope. To avoid any influence of glass slides on the oil drops, the emulsions were poured into rectangular glass capillaries Table 3 Ionic composition of the brine solution. Ions
Concentration (ppm)
Cl Na þ Ca2 þ Mg2 þ HCO3 SO42
62,810 35,393 3253 909 218 49
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(VITROTUBES, 0.4 4 mm2) and sealed. Several images from different parts of the samples were taken using conventional light microscopy and fluorescence microscopy techniques. All images with particles were taken in UV light with fluorescence filter. This allowed visualisation of fluorescent crude oil species. 2.6. Dispersion stability measurements Lower concentration of particles and crude oil were used for the Turbiscan experiments in order to reduce turbidity of the samples. Here, crude oil was weighted into glass vials and the brine was added to give oil concentration of 1000 ppm. The samples were emulsified in the same way as above. Subsequently, particles were added to the o/w emulsion to give a concentration of 1000 ppm and the system was dispersed by shaking the tubes by hand for 5 s. The dispersions 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-infra-red light source (λ ¼ 850 nm) that moved vertically along the full height of the sample. Complete transmission and backscattering scans consisted of signals collected every 40 μm along the sample height (about 40 mm) and were collected every minute for the first hour, every 10 min for the next 2 h and every 30 min for the last 10 h. The measurements were carried out at 50 °C. Since the samples studied here were dilute enough to allow considerable transmission, the detected backscattering signals also contained internal reflections from the measurement cell (Mengual et al., 1999a, 1999b). Consequently, only the transmission signals were considered in the data analysis
3. Results and discussion 3.1. Wetting behaviour
considered to be true contact angles, but rather used to evaluate the trends. The contact angles when placing a drop of water onto pellets of the same particles were reported previously (Dudášová et al., 2009a, 2009b). Also in this case all the particles were partially water wet, but with lower contact angles (r 25°). Furthermore, the particles with highest contact angles were CaCO3 and FeS with angles of 25° and 16°, respectively. These are the particles that gave the lowest contact angles in the oil–water-solid system presented here. An interesting point is that most studies related to contact angles report water–air contact angles, even though a lot of applications involve oil–water-solid phases. However, a few attempts have been made to investigate if there is any relationship between the water–air and oil–water contact angles. For a range of silanized surfaces a linear relationship between the two contact angles were found, and the oil–water contact angles were 1.4–1.5 times larger than the air–water contact angle (Grate et al., 2012). No such relationship was found for the samples studied here, as the oil– water contact angles were 1.9–6.4 times higher than the corresponding air–water contact angles, Table 3. This might be due to adsorption of interfacially active components in the crude oil (i.e. asphaltenes and resins) onto the particles, resulting in lowering of the solid-oil interfacial tension (γso). This implies that the wetting properties also could be influenced by the crude oil composition. Here, asphaltenes likely adsorbed onto the solid-oil interface and thus lowered the interfacial tension. 3.2. Dispersion properties Fig. 1 shows the dispersions containing crude oil and particles just after shaking at 300 rpm for 1 h. The separation started immediately after the shaking was terminated and the o/w emulsion (without any particles present), shown as a reference on the left, quickly got a thin oil layer at the top, indicating that not all the oil became dispersed. The samples with the particles generally
Table 4 shows the contact angles for a drop of the crude oil placed on the different particles surrounded by water. All the particles are partially water wetting with contact angles less than 90°. The lowest contact angles were seen for CaCO3 and FeS. According to Eq. (2), this corresponds to low free energy for detachment of particles from oil–water interfaces. The contact angles were somewhat higher for BaSO4 and Fe3O4 and highest for kaolinite. It is obvious that surface roughness will influence the contact angle measurements by the current method. Particle size, shape and specific surface area are parameters that are expected to have relatively large influence on the surface roughness. Considering CaCO3 and FeS, however, these resulted in similar contact angles, while the particle shape, particle size and specific surface were markedly different (Table 2). It was therefore considered that the chemical properties had larger influence on the observed contact angle than the differences in surface roughness of the pellets from the various powders. Furthermore, the measured values were not Table 4 Oil–water (θow) and water–air (θow) contact angles for the various particles. Particle
θow(°)
θow(°)
θow/θwa
Kaolinite CaCO3 BaSO4 Fe3O4 FeS
707 10 48 7 10 587 10 577 10 477 10
12 72 25 72 9 72 10 72 16 72
5,8 1,9 6,4 5,7 2,9
Fig. 1. The dispersions immediately after shaking. From left to right the following particles were added to the o/w emulsions: Pure oil, kaolinite, CaCO3, BaSO4, Fe3O4 and FeS. Bottom image shows enlarged view of kaolinite, CaCO3 and BaSO4 sample.
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Fig. 2. Crude oil drops (1 wt%) covered by BaSO4 particles (0.5 wt%).
separated into three layers; a top layer that included mixture of crude oil and particles, a middle layer with different degrees of turbidity and a bottom layer which primarily contained particles, but also some oil remains. Furthermore, it could be distinguished between two groups of behaviour. The following observations were made for the samples containing kaolinite, BaSO4 and Fe3O4: 1. Initially, drops covered with particles rose to the top of the sample. The attachment of particles to the oil water interface occurred during the shaking of the samples. This is illustrated for the BaSO4 system in Fig. 2, where the drops have diameters around 100 μm and the particles are about 1.1 μm. Particle aggregates can also be seen. 2. Dense-packed emulsion layers formed at the top of the samples. This is shown for the BaSO4 system in Fig. 3A. Coalescence within the dense-packed layer resulted in a thin oil film at the top and formation of particle-rich agglomerates, most likely containing remains of trapped oil, which gradually sank to the bottom of the sample. The sediment of the BaSO4 system is shown in the image in Fig. 3B. The samples containing CaCO3 and FeS behaved differently from the above description in the way that the particles/aggregates sank directly to the bottom of the sample, i.e. without initially being creamed together with the oil. However, interactions of CaCO3 particles with the water–air surface resulted in a top layer of particles in this case. Microscopy imaging also showed that the differences in dispersion behaviour were associated with different morphologies of particle-crude oil agglomerates in the samples, Fig. 4. The top left image shows oil drops in the emulsion before addition of particles and the subsequent shaking. The average drop size was 5.5 μm. No spherical oil drops were observed in the samples containing CaCO3 and FeS particles. Instead, irregularly shaped structures were seen, and particularly in the sample with CaCO3 particles extended network structures were apparent. Notably, these were the particles with the lowest water–oil contact angles, and consequently the lowest affinity for the oil–water interface. Spherical drops were, on the other hand, observed in the samples containing BaSO4, Fe3O4 and kaolinite particles. These were the samples with the highest water–oil contact angles (θwo) and thereby highest affinity for the oil–water interface. The average drop sizes for the samples containing BaSO4, Fe3O4 and kaolinite particles were 73 μm, 52 μm and 42 μm, respectively. This was significantly larger than the average drop size of 5.5 μm seen in the o/w emulsion prior to the addition of particles. This means that considerable coalescence took place during shaking of the samples. Flocculation of larger drops was also observed (Figs. 2 and 4). Furthermore, it was clear that the particles influenced the coalescence process. The smallest average drop size (42 μm) was observed with kaolinite present. The higher affinity of kaolinite to the interface might provide a more efficient barrier towards further coalescence than for the samples with BaSO4 and Fe3O4 present. However, the particle shape might also be a factor. The extent of particle coverage at an oil–water interface
Fig. 3. The top layer (A) and bottom layer (B) of the dispersion prepared with crude oil (1 wt%) and BaSO4 particles (0.5 wt%).
required to provide stability towards coalescence vary widely between various systems reported in the literature. Some show that even small amounts of interfacial particles will constrain coalescence (Binks and Lumsdon, 1999; Dong et al., 2014; Vignati et al., 2003), while others show that coalescence can take place until full coverage with particles is reached (Pawar et al., 2011). The results here suggest that coalescence continued until particles were densely packed at the oil drops (Fig. 2). The mechanism might be particle bridging between neighbouring oil drops at low particle coverage. This would result in increased contact time between the drops and promote coalescence (and flocculation) in the samples. Such bridging of fluid interfaces by particles has been reported by others (Nixon et al., 1999). 3.3. Stability measurements Sedimentation, creaming, aggregation, flocculation and coalescence are the kinetic phenomena that can destabilize the investigated dispersions and ultimately give rise to phase separation. Information about these phenomena was obtained by following the changes in transmission signals over time. Typical transmission data obtained by the Turbiscan LAb are shown in Fig. 5. Going from left to right along the x axis corresponds to moving from the bottom to the top of the sample (i.e. the height of the dispersions was about 40 mm). The corresponding transmissions are shown along the y axis, while the time dependency is indicated by the
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Fig. 4. The o/w emulsion without added particles (oil drops were added colour to increase the contrast) and the dispersions were and the dispersions were various particles (0.5 wt%) were added to the emulsion (1 wt%).
Fig. 5. Turbiscan transmission profiles for (A) the o/w emulsion (1000 ppm oil) without particles, (B) the suspension of BaSO4 particles (1000 ppm) and (C) the dispersion where BaSO4 particles (1000 ppm) were added to the emulsion. Each measurement is identified by a colour and the corresponding measurement times are listed to the right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
different colours for each scan along the sample where the purple and red scans represent the first and last measurement, respectively. The increase in the transmission at the bottom of the sample in Fig. 5A was associated with creaming of drops in the o/w emulsion for the initial measurements, while the creaming became less dominant at longer times. Flocculation and coalescence of drops was followed by changes in transmission in the region of the sample where creaming could be neglected, i.e. between 20 and 30 mm above the bottom of the sample. The increase in transmission caused by flocculation/coalescence is in agreement with Mie theory, predicting larger scattering in the forward directions by particles larger than the wavelength of the light source (Mengual et al., 1999a, 1999b). Fig. 5B shows the transmission data for the suspended BaSO4 particles. The increase in transmission at the top of the sample corresponds to sedimentation, due to reduced concentration of particles. In this case the sedimentation could not be neglected between 20 and 30 mm above the bottom of the sample (the sedimentation front dominates also in this region). Similar behaviour was observed for FeS and kaolinite particles. For CaCO3 and Fe3O4 particles, however, the sedimentation was preceded by flocculation (see Fig. S1 in Supplementary material) and the change in transmission in this range was associated with aggregation of particles. The transmission data for the sample where BaSO4 particles and crude oil drops were mixed are shown in Fig. 5C. Some sedimentation was observed for this sample. Similar observations were made when kaolinite and FeS particles were mixed with the emulsion, while creaming was seen for the samples mixed with CaCO3 and Fe3O4 particles (See Fig. S1 in Supplementary material). The sedimentation or creaming could be neglected between 20 and 30 mm above the bottom of the sample for all the dispersions. The change in transmission in this range could then be due to flocculation and coalescence of drops, in agreement with microscopy observations, as well as aggregation of particles. Fig. 6 shows the transmission in the middle of the sample for the suspensions with the various particles. For the suspensions with BaSO4, FeS and kaolinite there was an induction time of about
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microscopy.
4. Conclusions The behaviour and stability of crude oil drops dispersed together with various inorganic particles in synthetic produced water were investigated. It was found that the three phase contact angle (θwo) at the water–oil–particles interface was important for the morphology of crude oil–particle aggregates. High θwo corresponded to formation of particle stabilised emulsions, while low θwo resulted in less well-defined types of aggregates. It was also suggested that the extent of coalescence of oil drops was dependent on the particle coverage at the oil–water interface.
Author contributions Fig. 6. Time dependent increase of transmission in the middle (i.e. between 20 and 30 mm above the bottom) of the samples with the various suspended particles. The concentration of the particles and the crude oil was 1000 ppm.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgements The authors are grateful to the industrial sponsors (ConocoPhillips Skandinavia, ENI Norge, Schlumberger Norge PWMS, Statoil Petroleum and Total E&P Norge) (Project number 40114800) of the joint industrial programme “Produced Water Management: Fundamental Understanding of the Fluids” for financial support.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.petrol.2015.12.019.
Fig. 7. Time dependent increase of transmission in the middle (i.e. between 20 and 30 mm above the bottom) of the o/w emulsion and the dispersions where the various particles were added to the emulsion. The concentration of crude oil and particles was 1000 ppm.
10 min before any detectable change in the transmission. This suggested that sedimentation was the more dominant process in these samples, which also was in agreement with the overall transmission data. Furthermore, the rapid increase in transmission for the BaSO4 sample was due to an overlap with the sedimentation front of the sample. In the CaCO3 and Fe3O4 suspensions on the other hand, aggregation of particles were initially the dominant process. The size of the calcium carbonate and iron oxide particles were smaller compared to the other types of particles (Dudášová et al., 2009a, 2009b), and the aggregates of a certain size was required before sedimentation became significant. The transmission profiles in the middle of the samples where particles were mixed with the o/w emulsion, are shown in Fig. 7. Overall, the transmission increased faster when the emulsion was mixed with particles. The profile of the sample containing CaCO3 was most similar to that of the o/w emulsions. These particles had low affinity for the oil–water interface and irregular aggregates were observed in the microscope. Fast increase and overall high transmission were seen for the samples containing BaSO4 and kaolinite. For these particles, particle stabilised drops were observed in the microscope. The increased transmission was also consistent with the coalescence and flocculation observed by
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Influence of the wettability of particles on the morphology and stability of crude oil–particle aggregates in synthetic produced water Bartłomiej Gaweł, Meysam Nourani, Thomas Tichelkamp, Gisle Øye n Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway
art ic l e i nf o
a b s t r a c t
Article history: Received 28 August 2015 Received in revised form 12 November 2015 Accepted 16 December 2015 Available online 21 December 2015
Handling and treatment of produced water is becoming an increasing part of oil and gas production. A major challenge in this respect is removal of oil drops stabilised by particles. Understanding the influence of interfacial and wetting properties of crude oil–particle þ systems on the formation of particle stabilised emulsions is therefore essential. Elucidating these relationships was the aim of this investigation. Synthetic produced water containing different particles and crude oil were studied. The morphology of aggregates formed by crude oil drops and particles was strongly related to the contact angle (θwo) at the water–oil–particle interfaces. High θwo resulted in formation of particle stabilised emulsions, while less defined aggregates were formed at lower θwo Furthermore, coalescence of drops was related to the coverage of particles at the interface. Stable drops covered by particles were only seen when the amount of particles was significant. Otherwise drops tended to coalesce, likely due to particle bridging. & 2015 Elsevier B.V. All rights reserved.
Keywords: Produced water Particle stabilised emulsions Wettability o/w Emulsion stability Separation
1. Introduction Vast amounts of water are produced during oil and gas recovery (Lee et al., 2011). The produced water contains a complex mixture of dissolved and dispersed components. The latter includes crude oil drops as well as various inorganic particles originating from the reservoir or appearing as scale and corrosion products. Removal of dispersed oil is considered to be crucial in order to reduce the environmental impact of produced water. Therefore current regulations in the North Sea require that that the amount of oil in water must be less than 30 ppm prior to discharge to sea (OSPAR Commission, 2010). Pressure drops and shear forces during transport from the reservoirs to production facilities can lead to extensive mixing of oil drops and particles. The properties of both the oil phase and water phase can alter the surface properties, wettability and stability of the particles (Dudášová et al., 2014, 2009a, 2009b, 2008; Marczewski and Szymula, 2002; Sullivan and Kilpatrick, 2002). Furthermore, if particles become trapped at oil–water interfaces, particle stabilized emulsions can form. Oil drops stabilised by particles can be very difficult to remove by typical water treatment processes such as hydrocyclones and compact flotation units due to hindered coalescence and reduced Stokes velocity (Finborud, 2010; Markoff, 2009; Yan and Masliyah, 1997). n
Corresponding author. E-mail address: [email protected] (G. Øye).
http://dx.doi.org/10.1016/j.petrol.2015.12.019 0920-4105/& 2015 Elsevier B.V. All rights reserved.
Effective coalescence of oil drops is beneficial for ensuring good quality of the produced water. It is well known that indigenous interfacially active components in the crude oil, like asphaltene class, can cause formation of mechanically strong, viscoelastic films at the interface (Horváth-Szabó et al., 2005; Pawar et al., 2011; Poteau et al., 2005; Spiecker et al., 2003). High elasticity of such films can slow down coalescence of oil drops (Gaweł et al., 2014b). Furthermore, emulsions containing inorganic particles can be even more stable than those stabilized by asphaltenes only (Hannisdal et al., 2006; Nianxi and Masliyah, 1996; Sullivan and Kilpatrick, 2002; Yan and Masliyah, 1994, 1997; Yan et al., 1999). The ability of particles to stabilise emulsions is attributed to the large free energy of adsorption of solids which are partially wetted by both the oil and water phases (Aveyard et al., 2003; Binks, 2002). This means that particle layers adsorbed at drop interfaces can result in extremely stable emulsions (Binks and Lumsdon, 1999; Binks and Whitby, 2004; Dong et al., 2014). The wettability is a key factor affecting the properties of the particle layers and subsequently the emulsion behaviour (Gu et al., 2003; Yan et al., 2001). The contact angle for oil–water systems is related to interfacial tensions (γ) according to the Young equation:
γSO = γSW + γOW cos θ OW
(1)
where the subscripts s, o, and w represent solid, oil and water, respectively. The oil–water contact angle (θow) is measured through the water phase. The γow is related to the chemical composition of the water phase and crude oil, while γso is related
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to the composition of the crude oil and the adsorption of components at the solid surface (Abdallah et al., 2007). The free energy of adsorption of particles is dependent on the contact angle by the following expression:
ΔGd = πr 2 (1 − cos (θ ) )2
Table 2 Physical properties of the particles. Particle Shape
(2)
where r is the diameter of the particles. This expression states that adsorption of particles is strongest when θow is 90°. It is clear from this that the wettability of particles is a very important factor for emulsions stability. Another central parameter for particle stabilization emulsions is the surface coverage of the particles (LealCalderon and Schmitt, 2008; Pawar et al., 2011). However, there are extensive variations in the literature with respect to how many particles are required at the interface in order to constrain coalescence (Binks and Lumsdon, 1999; Dong et al., 2014; Frelichowska et al., 2010; Pawar et al., 2011; Tambe and Sharma, 1993; Vignati et al., 2003). Many studies related to particle stabilisation of emulsions have been focused on relatively simple model systems. During crude oil production, however, the fluid stream consists of crude oil, water and particles which become mixed during transport and can form various types of agglomerates like particles stabilised emulsions, schmoo etc. In this work the produced water stream was mimicked by mixing emulsions and particles under mild conditions. The main objective was to investigate the relations between wetting properties of particles in crude oil – synthetic produced water systems and the stability of the corresponding aggregates. Five different types of particles, typically present in produced water streams were used. The wetting properties of the particles, morphology of particle–oil aggregates and the corresponding stability in synthetic produced water were followed by contact angle, microscopy and Turbiscan measurements.
2. Materials and methods 2.1. Crude oil The crude oil used in this study has previously been characterized with respect to its physicochemical and interfacial properties. Details about the results and experimental procedures can be found elsewhere (Gaweł et al., 2014a, 2014b). Some of the properties are presented in Table 1 for convenience. 2.2. Particles The inorganic particles were chosen to represent minerals typically present in produced water (i.e. clay, scale and corrosion products): kaolin (Aldrich, USA/Germany); CaCO3 (98.2%) (Specialty Minerals Inc., USA); FeS (99.7%) (DLFTZ, Chang Hing, China); BaSO4 (99%) and Fe3O4 (98 þ %) (Nanoamor, USA). Physical properties of the particles are listed in Table 2. The particle shape, Table 1 Physicochemical properties of the crude oil (Gaweł et al., 2014a). Property
Value
Density at 20 °C (g/cm3) Viscosity at 20 °C (cP) Saturates (wt%) Aromatics (wt%) Resins (wt%) Asphaltenes (wt%) TAN (mg/g) TBN (mg/g)
0.81 3 80.0 18.0 1.9 0.1 0.4 0.6
199
Kaolin CaCO3 BaSO4 Fe3O4 FeS *
Density (g/cm3) Particle Specific surface size(from area (BET) suppliers) (m2/g)
Plate like 19.7 7 3 Rhombic 19.9 7 2 Prismatic 6.5 7 1 Spherical 45.2 7 3 Spherical 6.9 7 1
0.1–4 μm 70 nm 80–450 nm 20–30 nm 2 μm
2.65 2.71 4.5 4.8–5.1 4.6–4.8
Average aggregate size (μm)*
2.0 1.3 1.1 1.7 2.1
Measured by Coulter Counter in 3.5 NaCl solutions.
densities and particle size/size distributions were provided by the suppliers, while surface area and average aggregate sizes were measured previously (Dudášová et al., 2009a, 2009b). 2.3. Brine composition The brine solution was made by dissolving appropriate amounts of NaCl (99.5%, Merck, Germany), CaCl2 2H2O (99.5%, Merck, Germany), MgCl2 6H2O (99.5%, Merck, Germany), NaHCO3 (99.5%, Merck, Germany) and Na2SO4 (98.5%, Acros Chemicals) in MQ water from a Millipore Simplicity System. The ionic composition is listed in Table 3 2.4. Contact angles measurements The particles were pressed into flat pellets (13 mm in diameter) under high pressure using a hydraulic press (Compac, Denmark). The pellets were put in a custom made holder and immersed in the brine solution before an oil drop was placed on the pellet surface by a syringe. Images were taken of the oil drop immediately after attachment using an Optical Contact Angle Metre equipped with a computer-controlled high-speed camera (CAM200, KSV Instruments). Subsequently, the contact angles were determined by fitting the Young Laplace equation to the drop profiles. 2.5. Microscopy imaging Crude oil was weighted into 50 ml Schott bottles and the brine was added to give oil concentration of 10,000 ppm. The samples were emulsified with an Ultra Turrax mixer (IKA, S25N-10G with 10 and 7.5 mm stator and rotor diameter respectively) at 20,000 rpm for 2 min. Subsequently, particles were added to the o/ w emulsion to give a concentration of 5000 ppm. The system was dispersed using an automatic shaker (HS 501 digital IKA) at 300 rpm for 1 h. The shaking was used to mimic mixing under mild shearing conditions. Images of the dispersions were taken by a Nikon LV 100D microscope. To avoid any influence of glass slides on the oil drops, the emulsions were poured into rectangular glass capillaries Table 3 Ionic composition of the brine solution. Ions
Concentration (ppm)
Cl Na þ Ca2 þ Mg2 þ HCO3 SO42
62,810 35,393 3253 909 218 49
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(VITROTUBES, 0.4 4 mm2) and sealed. Several images from different parts of the samples were taken using conventional light microscopy and fluorescence microscopy techniques. All images with particles were taken in UV light with fluorescence filter. This allowed visualisation of fluorescent crude oil species. 2.6. Dispersion stability measurements Lower concentration of particles and crude oil were used for the Turbiscan experiments in order to reduce turbidity of the samples. Here, crude oil was weighted into glass vials and the brine was added to give oil concentration of 1000 ppm. The samples were emulsified in the same way as above. Subsequently, particles were added to the o/w emulsion to give a concentration of 1000 ppm and the system was dispersed by shaking the tubes by hand for 5 s. The dispersions 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-infra-red light source (λ ¼ 850 nm) that moved vertically along the full height of the sample. Complete transmission and backscattering scans consisted of signals collected every 40 μm along the sample height (about 40 mm) and were collected every minute for the first hour, every 10 min for the next 2 h and every 30 min for the last 10 h. The measurements were carried out at 50 °C. Since the samples studied here were dilute enough to allow considerable transmission, the detected backscattering signals also contained internal reflections from the measurement cell (Mengual et al., 1999a, 1999b). Consequently, only the transmission signals were considered in the data analysis
3. Results and discussion 3.1. Wetting behaviour
considered to be true contact angles, but rather used to evaluate the trends. The contact angles when placing a drop of water onto pellets of the same particles were reported previously (Dudášová et al., 2009a, 2009b). Also in this case all the particles were partially water wet, but with lower contact angles (r 25°). Furthermore, the particles with highest contact angles were CaCO3 and FeS with angles of 25° and 16°, respectively. These are the particles that gave the lowest contact angles in the oil–water-solid system presented here. An interesting point is that most studies related to contact angles report water–air contact angles, even though a lot of applications involve oil–water-solid phases. However, a few attempts have been made to investigate if there is any relationship between the water–air and oil–water contact angles. For a range of silanized surfaces a linear relationship between the two contact angles were found, and the oil–water contact angles were 1.4–1.5 times larger than the air–water contact angle (Grate et al., 2012). No such relationship was found for the samples studied here, as the oil– water contact angles were 1.9–6.4 times higher than the corresponding air–water contact angles, Table 3. This might be due to adsorption of interfacially active components in the crude oil (i.e. asphaltenes and resins) onto the particles, resulting in lowering of the solid-oil interfacial tension (γso). This implies that the wetting properties also could be influenced by the crude oil composition. Here, asphaltenes likely adsorbed onto the solid-oil interface and thus lowered the interfacial tension. 3.2. Dispersion properties Fig. 1 shows the dispersions containing crude oil and particles just after shaking at 300 rpm for 1 h. The separation started immediately after the shaking was terminated and the o/w emulsion (without any particles present), shown as a reference on the left, quickly got a thin oil layer at the top, indicating that not all the oil became dispersed. The samples with the particles generally
Table 4 shows the contact angles for a drop of the crude oil placed on the different particles surrounded by water. All the particles are partially water wetting with contact angles less than 90°. The lowest contact angles were seen for CaCO3 and FeS. According to Eq. (2), this corresponds to low free energy for detachment of particles from oil–water interfaces. The contact angles were somewhat higher for BaSO4 and Fe3O4 and highest for kaolinite. It is obvious that surface roughness will influence the contact angle measurements by the current method. Particle size, shape and specific surface area are parameters that are expected to have relatively large influence on the surface roughness. Considering CaCO3 and FeS, however, these resulted in similar contact angles, while the particle shape, particle size and specific surface were markedly different (Table 2). It was therefore considered that the chemical properties had larger influence on the observed contact angle than the differences in surface roughness of the pellets from the various powders. Furthermore, the measured values were not Table 4 Oil–water (θow) and water–air (θow) contact angles for the various particles. Particle
θow(°)
θow(°)
θow/θwa
Kaolinite CaCO3 BaSO4 Fe3O4 FeS
707 10 48 7 10 587 10 577 10 477 10
12 72 25 72 9 72 10 72 16 72
5,8 1,9 6,4 5,7 2,9
Fig. 1. The dispersions immediately after shaking. From left to right the following particles were added to the o/w emulsions: Pure oil, kaolinite, CaCO3, BaSO4, Fe3O4 and FeS. Bottom image shows enlarged view of kaolinite, CaCO3 and BaSO4 sample.
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Fig. 2. Crude oil drops (1 wt%) covered by BaSO4 particles (0.5 wt%).
separated into three layers; a top layer that included mixture of crude oil and particles, a middle layer with different degrees of turbidity and a bottom layer which primarily contained particles, but also some oil remains. Furthermore, it could be distinguished between two groups of behaviour. The following observations were made for the samples containing kaolinite, BaSO4 and Fe3O4: 1. Initially, drops covered with particles rose to the top of the sample. The attachment of particles to the oil water interface occurred during the shaking of the samples. This is illustrated for the BaSO4 system in Fig. 2, where the drops have diameters around 100 μm and the particles are about 1.1 μm. Particle aggregates can also be seen. 2. Dense-packed emulsion layers formed at the top of the samples. This is shown for the BaSO4 system in Fig. 3A. Coalescence within the dense-packed layer resulted in a thin oil film at the top and formation of particle-rich agglomerates, most likely containing remains of trapped oil, which gradually sank to the bottom of the sample. The sediment of the BaSO4 system is shown in the image in Fig. 3B. The samples containing CaCO3 and FeS behaved differently from the above description in the way that the particles/aggregates sank directly to the bottom of the sample, i.e. without initially being creamed together with the oil. However, interactions of CaCO3 particles with the water–air surface resulted in a top layer of particles in this case. Microscopy imaging also showed that the differences in dispersion behaviour were associated with different morphologies of particle-crude oil agglomerates in the samples, Fig. 4. The top left image shows oil drops in the emulsion before addition of particles and the subsequent shaking. The average drop size was 5.5 μm. No spherical oil drops were observed in the samples containing CaCO3 and FeS particles. Instead, irregularly shaped structures were seen, and particularly in the sample with CaCO3 particles extended network structures were apparent. Notably, these were the particles with the lowest water–oil contact angles, and consequently the lowest affinity for the oil–water interface. Spherical drops were, on the other hand, observed in the samples containing BaSO4, Fe3O4 and kaolinite particles. These were the samples with the highest water–oil contact angles (θwo) and thereby highest affinity for the oil–water interface. The average drop sizes for the samples containing BaSO4, Fe3O4 and kaolinite particles were 73 μm, 52 μm and 42 μm, respectively. This was significantly larger than the average drop size of 5.5 μm seen in the o/w emulsion prior to the addition of particles. This means that considerable coalescence took place during shaking of the samples. Flocculation of larger drops was also observed (Figs. 2 and 4). Furthermore, it was clear that the particles influenced the coalescence process. The smallest average drop size (42 μm) was observed with kaolinite present. The higher affinity of kaolinite to the interface might provide a more efficient barrier towards further coalescence than for the samples with BaSO4 and Fe3O4 present. However, the particle shape might also be a factor. The extent of particle coverage at an oil–water interface
Fig. 3. The top layer (A) and bottom layer (B) of the dispersion prepared with crude oil (1 wt%) and BaSO4 particles (0.5 wt%).
required to provide stability towards coalescence vary widely between various systems reported in the literature. Some show that even small amounts of interfacial particles will constrain coalescence (Binks and Lumsdon, 1999; Dong et al., 2014; Vignati et al., 2003), while others show that coalescence can take place until full coverage with particles is reached (Pawar et al., 2011). The results here suggest that coalescence continued until particles were densely packed at the oil drops (Fig. 2). The mechanism might be particle bridging between neighbouring oil drops at low particle coverage. This would result in increased contact time between the drops and promote coalescence (and flocculation) in the samples. Such bridging of fluid interfaces by particles has been reported by others (Nixon et al., 1999). 3.3. Stability measurements Sedimentation, creaming, aggregation, flocculation and coalescence are the kinetic phenomena that can destabilize the investigated dispersions and ultimately give rise to phase separation. Information about these phenomena was obtained by following the changes in transmission signals over time. Typical transmission data obtained by the Turbiscan LAb are shown in Fig. 5. Going from left to right along the x axis corresponds to moving from the bottom to the top of the sample (i.e. the height of the dispersions was about 40 mm). The corresponding transmissions are shown along the y axis, while the time dependency is indicated by the
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Fig. 4. The o/w emulsion without added particles (oil drops were added colour to increase the contrast) and the dispersions were and the dispersions were various particles (0.5 wt%) were added to the emulsion (1 wt%).
Fig. 5. Turbiscan transmission profiles for (A) the o/w emulsion (1000 ppm oil) without particles, (B) the suspension of BaSO4 particles (1000 ppm) and (C) the dispersion where BaSO4 particles (1000 ppm) were added to the emulsion. Each measurement is identified by a colour and the corresponding measurement times are listed to the right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
different colours for each scan along the sample where the purple and red scans represent the first and last measurement, respectively. The increase in the transmission at the bottom of the sample in Fig. 5A was associated with creaming of drops in the o/w emulsion for the initial measurements, while the creaming became less dominant at longer times. Flocculation and coalescence of drops was followed by changes in transmission in the region of the sample where creaming could be neglected, i.e. between 20 and 30 mm above the bottom of the sample. The increase in transmission caused by flocculation/coalescence is in agreement with Mie theory, predicting larger scattering in the forward directions by particles larger than the wavelength of the light source (Mengual et al., 1999a, 1999b). Fig. 5B shows the transmission data for the suspended BaSO4 particles. The increase in transmission at the top of the sample corresponds to sedimentation, due to reduced concentration of particles. In this case the sedimentation could not be neglected between 20 and 30 mm above the bottom of the sample (the sedimentation front dominates also in this region). Similar behaviour was observed for FeS and kaolinite particles. For CaCO3 and Fe3O4 particles, however, the sedimentation was preceded by flocculation (see Fig. S1 in Supplementary material) and the change in transmission in this range was associated with aggregation of particles. The transmission data for the sample where BaSO4 particles and crude oil drops were mixed are shown in Fig. 5C. Some sedimentation was observed for this sample. Similar observations were made when kaolinite and FeS particles were mixed with the emulsion, while creaming was seen for the samples mixed with CaCO3 and Fe3O4 particles (See Fig. S1 in Supplementary material). The sedimentation or creaming could be neglected between 20 and 30 mm above the bottom of the sample for all the dispersions. The change in transmission in this range could then be due to flocculation and coalescence of drops, in agreement with microscopy observations, as well as aggregation of particles. Fig. 6 shows the transmission in the middle of the sample for the suspensions with the various particles. For the suspensions with BaSO4, FeS and kaolinite there was an induction time of about
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microscopy.
4. Conclusions The behaviour and stability of crude oil drops dispersed together with various inorganic particles in synthetic produced water were investigated. It was found that the three phase contact angle (θwo) at the water–oil–particles interface was important for the morphology of crude oil–particle aggregates. High θwo corresponded to formation of particle stabilised emulsions, while low θwo resulted in less well-defined types of aggregates. It was also suggested that the extent of coalescence of oil drops was dependent on the particle coverage at the oil–water interface.
Author contributions Fig. 6. Time dependent increase of transmission in the middle (i.e. between 20 and 30 mm above the bottom) of the samples with the various suspended particles. The concentration of the particles and the crude oil was 1000 ppm.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgements The authors are grateful to the industrial sponsors (ConocoPhillips Skandinavia, ENI Norge, Schlumberger Norge PWMS, Statoil Petroleum and Total E&P Norge) (Project number 40114800) of the joint industrial programme “Produced Water Management: Fundamental Understanding of the Fluids” for financial support.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.petrol.2015.12.019.
Fig. 7. Time dependent increase of transmission in the middle (i.e. between 20 and 30 mm above the bottom) of the o/w emulsion and the dispersions where the various particles were added to the emulsion. The concentration of crude oil and particles was 1000 ppm.
10 min before any detectable change in the transmission. This suggested that sedimentation was the more dominant process in these samples, which also was in agreement with the overall transmission data. Furthermore, the rapid increase in transmission for the BaSO4 sample was due to an overlap with the sedimentation front of the sample. In the CaCO3 and Fe3O4 suspensions on the other hand, aggregation of particles were initially the dominant process. The size of the calcium carbonate and iron oxide particles were smaller compared to the other types of particles (Dudášová et al., 2009a, 2009b), and the aggregates of a certain size was required before sedimentation became significant. The transmission profiles in the middle of the samples where particles were mixed with the o/w emulsion, are shown in Fig. 7. Overall, the transmission increased faster when the emulsion was mixed with particles. The profile of the sample containing CaCO3 was most similar to that of the o/w emulsions. These particles had low affinity for the oil–water interface and irregular aggregates were observed in the microscope. Fast increase and overall high transmission were seen for the samples containing BaSO4 and kaolinite. For these particles, particle stabilised drops were observed in the microscope. The increased transmission was also consistent with the coalescence and flocculation observed by
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