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Unveiling the Synergistic Effect of ZnO Nanoparticles and Surfactant Colloids for Enhanced Oil Recovery
Colloid and Interface Science Communications 29 (2019) 33–39
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Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Unveiling the Synergistic Effect of ZnO Nanoparticles and Surfactant Colloids for Enhanced Oil Recovery
T
Marwan Y. Rezk, Nageh K. Allam
⁎
Energy Materials Laboratory, School of Sciences and Engineering, the American University in Cairo, New Cairo 11835, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Nanofluids stability Interfacial tension ZnO Fluid flow in porous media EOR
Although chemical flooding is widely used to enhance oil productivity at lower cost, the continuously increasing prices of chemicals is a real barrier to achieve such an aim. Hence, various ways have been explored to reduce the amount of chemicals used while yielding the same performance or even higher to make the whole process cost efficient. Herein, we investigate the use of zinc oxide nanoparticles (ZnO NPs) to improve the surfactantbased extraction of an organic fluid (n- dodecane) from a porous medium (sandstone). The main mechanism is a synergetic effect between surfactant and nanoparticles in reducing the oil/water interfacial tension (IFT), which assists in overcoming the capillary trapping of oil. In addition, the aqueous solutions of the investigated surfactant (sodium dodecyl sulphate) provide better suspension stability for ZnO nanoparticles compared to water. The presented experiments showed an increase in oil recovery efficiency of 8% when both ZnO and surfactant are used compared to the surfactant-based oil recovery processes. To further explain the dominant mechanism for the increased oil production, simulation models were constructed and showed wettability shift to more water wet characteristics.
1. Introduction Although the use of nanomaterials is promising in many fields, there are challenges that still need to be addressed to use them efficiently. Firstly, the production of some nanoparticles could have a relatively high cost. Secondly, the physical and chemical properties of the same synthesized nanomaterial could differ upon changing the fabrication method [1–3]. The use of nanoparticles appears to be an emerging technology in the oil industry [4]. Within the context of enhanced oil recovery (EOR) a number of nanoparticles have been investigated (e.g. SiO2 and Al2O3) some of which have shown potential in designing improved EOR processes [5–9]. In another application, metal oxide nanoparticles have shown great performance as drilling fluids additives in controlling fine migration and increasing plastic viscosity, yield point, and gel strength [10,11]. However, size, shape, and type of nanoparticles could be optimized further to achieve enhanced mud properties. Among those nanoparticles, silica is widely used as it is more stable than other metal oxide nanoparticles and it has a similar surface charge as quartz, which counts for most reservoir rocks [12]. In addition, silica nanoparticles have shown a synergistic effect with different surfactants reducing IFT and hence increasing oil recovery [13,14]. However, using materials other than silica can open up a set of solutions to other problems facing the field as the retention of silica NPs ⁎
in carbonates in the presence of high/low salinity [15,16]. Recently, ZnO nanoparticles have been receiving a great attention to help in oil recovery through different mechanisms, mainly due to their ability to change the wettability and reduce the oil viscosity [17–21]. However, the reported results so far are controversial. For example, while one study [22] claimed that ZnO NPs increase the interfacial tension, another report claimed that they reduce the IFT [23]. This controversy needs to be discussed considering the stability of NP suspensions. Ideally, a NP suspension needs to stay stable over the time-scale of the process it is designed for. Less stable suspensions experience particle sedimentation, which in turn impacts the physical properties such as IFT. It has been shown that surfactants can enhance the stability of NP suspensions [24–27]. In the case of ZnO NPs, anionic surfactants, such as sodium dodecyl sulfate (SDS), are shown to be promising in suspension stabilization. More importantly, surfactants such as SDS provide reduced IFT, making SDS solutions an excellent choice for EOR processes. Given the economic and environmental aspects of EOR processes, designing optimized ZnO/SDS suspensions that offer minimum IFT at lowest concentrations (NPs and surfactant) is of great significance. Herein, we have systematically studied the effect of ZnO NPs on oil recovery when used in combination with SDS surfactant. The stability of ZnO NP suspensions over time was also investigated,
Corresponding author. E-mail address: [email protected] (N.K. Allam).
https://doi.org/10.1016/j.colcom.2019.01.004 Received 25 December 2018; Received in revised form 15 January 2019; Accepted 21 January 2019 2215-0382/ © 2019 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloid and Interface Science Communications 29 (2019) 33–39
M.Y. Rezk, N.K. Allam
Fig. 1. (a) FESEM of the fabricated ZnO nanoparticles, and (b) the corresponding EDX spectrum.
dried in an oven for 24 h. The cores were assembled in Vinci core holder with a confining pressure of 2000 Psi. The procedures can be summarized as follows:
which provides an insight on the applicability of this technology at the field scale. 2. Experimental Methods & Materials
1) The cores of 1.0-inch diameter and 2.6 in. length were collected from the actual reservoir in Egypt. The core samples were saturated using vacuum saturator at a pressure of 14 bars at room temperature. The core samples were saturated overnight to fully saturate the core samples with deionized water (DIW). The test was performed against DIW to assess the effect of ZnO NPs on oil recovery before introducing mono/divalent ions. 2) Having the core samples with 100% water saturation (SW), absolute permeability was measured at room temperature and flow rates of 2, 3, 4 cc/min. 3) Oil (n-dodecane) was injected at flow rates of 0.5 and 1.0 cc/min. Until no more water was produced (SWc). 4) Water was injected to displace oil and simulate for water flooding. The water was injected at a flow rate of 0.5 cc/min for 5 pore volumes. 5) By the last phase, both core samples were injected with 6 pore volumes at a rate of 0.5 cc/min. Core sample 1 (S1) was injected with SDS as a control experiment for core sample 2 (S2), which was flooded with ZnO NPs/SDS fluid. 6) The effluent fluids were collected in 25 mL test tube to be analyzed later for oil production to calculate the produced oil and recovery factor of ZnO NPs/SDS flooding.
2.1. Synthesis and Characterization of ZnO Nanoparticles Zinc acetate (purchased from Merck 99.99% trace metals basis) and methanol (purchased from Merck anhydrous 99.8%) were mixed by stirring for 10 min at room temperature followed by vigorous stirring at 80 °C. The obtained powder was annealed at 450 °C for 6 h [28]. The obtained nanopowder was examined by field scanning electron microscope (FSEM, Zeiss SEM ultra 60 and Zeiss 1550 VP) equipped with Energy-dispersive X-ray spectroscopy (EDX). The morphology was further investigated via transmission electron microscopy (HR-TEM using JEOL-2100 microscope), and crystal structure was analyzed by glancing angle X-ray diffraction (XRD), (GAXRD, Bruker thin film) for phase identification and other useful data that can be acquired through the same technique. To test the wettability of the fabricated ZnO NPs, the contact angle between water and a thin film of the ZnO was measured using sessile drop measurement (Drop Shape Analyser (DSA 25), Kruss, Germany). 2.2. Dispersion of ZnO NPs Three ZnO concentrations (0.05, 0.1, and 0.2 wt%) were suspended in 0.2 wt% SDS surfactant which is the critical micelle concentration (CMC). The suspension was acquired by stirring for 25 min of ultrasonication in a cold water bath to avoid undesirable heating of the suspension that may lead to Ostwald ripening for the ZnO NPs. The stability of the prepared suspensions was quantitatively assessed by measuring their absorbance by JENWAY 7300 spectrophotometer at a wavelength of 720 nm and observed visually for 2 weeks. The suspensions' stability was further investigated via dynamic light scattering, (Zetasizer Nano ZS, Malvern). In addition, the interfacial tension between aqueous (ZnO suspensions) and oleic phase (n-dodecane- anhydrous, purity > 99% purchased from Merck) was measured using k9 (Kruss) tensiometer. The synthesized ZnO NPs showed better stability compared to the commercial ZnO NPs as shown in supporting documents.
3. Results and Discussion 3.1. Morphological and Structural Properties of ZnO Nanoparticles Fig. 1a shows FESEM images of the fabricated ZnO nanoparticles obtained via the reaction between zinc acetate and methanol. The images reveal the formation of highly uniform/mildly agglomerated spherically-shaped particles that range in size from 50 ± 20 nm. The synthesis procedure proved to be reproducible, yielding almost the same range of particle sizes when repeated many times. In addition, the EDX spectrum, Fig. 1b, shows the good purity of the synthesized ZnO. Furthermore, the zinc element showing an intense peak that is in agreement with the literature [29,30]. To further assess the morphology of ZnO nanoparticles, transmission electron microscopy (TEM) was performed, Fig. 2, indicating the formation of spherical nanoparticles. In addition, the sizes of the ZnO nanoparticles are in a good agreement with those obtained from the SEM results. XRD analysis was performed to investigate the crystal structure of the fabricated nanoparticles. The obtained diffraction pattern, Fig. 3,
2.3. Core Flooding of ZnO NPs/SDS A suspension of 0.05 wt% of ZnO NPs in 0.2 wt% SDS was selected for the core flooding experiment. Two sandstone core samples were used to compare the effect of SDS surfactant flooding to ZnO NPs/SDS flooding. The core samples were cleaned using Soxhlet extractor and 34
Colloid and Interface Science Communications 29 (2019) 33–39
M.Y. Rezk, N.K. Allam
Fig. 4. Contact angle measurement of the synthesized ZnO nanoparticles versus water at room temperature.
Fig. 2. HR-TEM of the fabricated ZnO NPs.
where Dp = crystallite size, K = 0.94, β = line broadening in radians, θ = Bragg's angle, and λ = x-ray wavelength.
=
Strain
tan( )
(2)
where η is the strain, βStrain = broadening due to strain, and θ = Bragg's angle. The degree of poly-dispersity (ρ) can be calculated using Eq. (3).
=
Table 1 Calculated crystallite size, strain, and polydispersity for the fabricated ZnO NPs. FWHM (degrees)
Dp (nm)
η (%)
ρ (%)
36.3
0.2952
29.59
0.39
23
3.2. Stability of ZnO NPs The stability of the ZnO NPs suspensions was assessed quantitatively and visually. The ZnO NPs suspended in SDS appeared to be visually stable over 2 weeks, Fig. 5. Fig. 6a shows the UV–Vis absorption spectra of the formulations tested in SDS with different ZnO NPs concentrations. The spectra show two regions I and II. In region I, the dominant mechanism is the coagulation and flocculation causing particles to be totally unstable. Moreover, the instability could be attributed to the acquired energy
indicated hexagonal zinc oxide phase. Note that the different intensities of the peaks may indicate the difference of preferential growth of the crystals with the highest intensity (highest area of crystal face) found for (101). The XRD calculations using Bragg's law take into account some assumptions that never happen in actual cases. The assumptions are that the beam analyzing the crystal is perfectly parallel and monochromatic and that the crystal is perfect. Thus, it is worth mentioning that such imperfections affect the broadening of the XRD peaks as well as other factors. There are mainly three factors that affect the broadening of the diffraction peaks including instrumental effects (arising from imperfect focusing), crystallite size (Dp), and lattice strain (η). Thus, it is possible to quantitatively evaluate the effect of microstrain as well as crystallite size using Scherrer equation (Eq. (1)) and the resulting data are presented in Table 1.
Dp =
1 2
K× × cos( )
(3)
where ρ is the degree of poly-dispersity, σ is the standard deviation of crystal-size, and d is the mean crystal-size. Usually, a small ρ value indicates uniform size distribution. To test the wettability of the synthesized nanoparticles with water, a thin film of the synthesized ZnO powder was made. The test was repeated three times, however, the ZnO showed super-hydrophilicity that as soon as the water drop reached the surface of the thin film it was quickly diffused into the membrane, Fig. 4. Note that the drop shape analyser did not get enough time to detect the spreading of the water droplet over the thin film before it diffuses. The contact angle results showed super-hydrophilicity that could make it a good candidate to shift the wettability of the core surface to more water wet even when ZnO NPs gets adsorbed on the rock surface.
Fig. 3. XRD pattern of the fabricated ZnO nanoparticles.
2θ (degrees)
d
Fig. 5. (A) 0.05 wt% ZnO NPs. (B) 0.1 wt% ZnO NPs. (C) 0.2 wt% ZnO NPs. suspended in 0.2 wt% SDS (a) Upon preparation, and (b) after 2 weeks.
(1) 35
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Fig. 6. (a) UV–Vis spectra showing the difference in the stability of the fabricated ZnO NPs/SDS suspensions at different NPs concentrations and (b) Effect of different concentrations of ZnO nanoparticles suspended in 0.2 wt% SDS on the interfacial tension against n-dodecan. The zeta potential of the ZnO colloid was quantitatively evaluated for further investigation of the suspension's stability using Zeta.
3.4. Two-Phase Fluid Displacement Test in Sandstone
from sonication in the first hour. Later on, the instability of the suspension for the rest of the first 25 h could be attributed to particle growth and precipitation. When Brownian motion is extremely high (after sonication), particles get coalesced and grew in size to precipitate. In region II, the effect of adding the SDS surfactant as a stabilizing agent can be seen clearly from the slope of the absorbance graph. The surfactant macromolecules get adsorbed on the colloidal particles to form a tough shell that helps to keep the particles apart. Thus, the main mechanism causing the suspension to be stable in region 2 could be attributed to solvation shell interactions [31,32]. In this regard, the suspension containing 0.2 wt% SDS in 0. 05 wt% ZnO NPs was found to be the most stable formulation over two weeks. It was reported that the zeta potential of ZnO NPs ultrapure water suspensions with a particle size 20 nm and concentration of 0.5 g/L is 17.56 ± 2.13 mV, which was changed to −27.96 ± 2.59 mV upon the addition of SDS surfactant at a concentration of 0.03% [33]. This change shows that increasing the SDS surfactant concentration, when added to ZnO NPs, decreases its zeta potential. The measured surface zeta potential of ZnO suspension in SDS by zeta analyzer was −28.0 ± 4.2 mV. The result is in a good agreement with the aforementioned work done by Li et al. [33]. The results also confirmed the effect of the surfactant macromolecules inducing repulsive forces in between ZnO nanoparticles causing them to keep suspended.
Two series of fluid displacement experiments were performed on two sandstone samples (S1 and S2) to investigate the effect of ZnO NPs on oil recovery in a surfactant-based process. The properties of these two samples are listed in Table 2 and the results of these experiments are presented in Table 3 and Fig. 7. Our results show that the SDS experiment left about 3% more residual oil saturation than that of the ZnO/SDS experiment. This is despite the fact that in the NPs/SDS experiment, the water flooding had left about 6% more oil entrapped in the core than that of SDS experiment. In other words, as shown in Fig. 7, SDS/ZnO NPs caused 19% increase in oil recovery in the tertiary phase raising the oil recovery from 16% to 35% (Fig. 8). No emulsions were observed during/after the ZnO/SDS flooding. The only drawback we can see about ZnO/SDS flooding as shown in Fig. 7 is the increase in differential pressure. The increase can be attributed to the retention of ZnO nanoparticles in the porous media. However, Adil et al. claimed low adsorption capacity of nanoparticles and hence low retention in a porous medium. In addition, the use of SDS along with ZnO NPs minimized permeability reduction caused by nanoparticles retention [37]. Table 4 summarizes some the most significant work reported in literature using ZnO nanoparticles to enhance oil recovery. To the authors' best knowledge, Table 4 shows a vital progress in using ZnO from the stability standpoint of view as well as oil recovery enhancement. To justify the core flood experiments results, we ran simulation models (using Sendra) to construct relative permeability curves using Corey and Skjæveland correlations. It was possible to construct three relative permeability curves for water flooding, SDS flooding, and ZnO/ SDS as shown in Fig. 9. The three curves from simulation prove the wettability shift from intermediate wet (water flooding) to slightly
3.3. IFT Measurement Using SDS + ZnO NPs Fig. 6b shows a significant decrease in the IFT upon adding different concentrations of ZnO NPs to SDS. This confirms the same concept proved previously using alumina nanoparticles [34]. The huge decrease in IFT is expected to yield higher oil recovery by reducing the oil entrapment within the pore space of the rock [35]. The reduced IFT could be also attributed to the uniformity/low polydispersity of the ZnO NPs that would consequently result a higher displacement of oil/water contact line, in agreement with the simulation models previously reported by Chengara et al. [36]. In this instance, ZnO nanoparticles get confined in an ordered layered thin film (wedge) that is in contact with oil. This arrangement is due to increased entropy of the overall suspension allowing more freedom for the nanoparticles movement. This process results in excessive pressure within the ZnO nanoparticles film relative to the bulk, which is reduced along with the energy as the thickness increases. Subsequently, the highest energy point of the ZnO nanoparticles wedge within the suspension in contact with oil caused a deformation in the meniscus, leading to an increased oil displacement.
Table 2 Parameters of the tested core samples (S1: SDS surfactant flooding), (S2: ZnO nanoparticles/SDS flooding).
36
Core ID
S1
S2
Diameter (cm) Length (cm) Surface Area (cm2) Volume (cm3) Pore volume (mL) Porosity (%) Absolute permeability (mD)
2.54 6.72 63.18 33.52 8.89 26.51 28.13
2.54 6.67 62.78 33.27 8.87 26.65 15.74
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Table 3 Core flood details for SDS (S1) and SDS/ZnO NPs (S2).
SDS
SDS/ZnO NPs
Process
Sw (%)
So (%)
Swc
Sor
q (cc/min)
PVinjected (mL)
Water (Kabs) Oil Water flood SDS Water Oil Water SDS/ZnO NPs
100 32.5 66.25 71.87 100 29 60.6 74.1
0 67.5 33.75 28.13 0 71 39.4 25.9
NA 32.5 32.5 32.5 NA 29 29 29
NA NA NA 28.13 NA NA NA 25.9
2, 3, 4 0.5, 1 0.5 0.5 2, 3, 4 0.5, 1, 2 0.5 0.5
8.89 4.9 PV = 44.5 5 PV = 44.45 6 PV = 53.34 8.87 4PV = 35.48 5 PV = 44.35 8 PV = 70.96
Fig. 7. Cumulative recovery factor for SDS VS SDS/ZnO NPs.
water wet (SDS flooding) then eventually to more water wet (ZnO/SDS flooding). The simulation results reveal that the oil recovery improvement can be ascribed to the increased displacement efficiency. Furthermore, it confirms the results obtained from the interfacial tension tests that have been performed prior to the core flood experiments. In addition, the shift in wettability to more water wet could also be attributed to the positive disjoining pressure. The excess pressure in the wedge film compared to the bulk is due to the arrangement of nanoparticles to increase the entropy of the suspension in the porous media by more free movement of the ZnO NPs in the bulk liquid. 4. Conclusions Highly uniform and mildly agglomerated ZnO NPs (50 ± 20 nm) have been synthesized via wet chemistry processing and characterized using FESEM and XRD techniques. The stability of the synthesized ZnO NPs in 0.2 wt% SDS surfactant was tested over time by following the variation in their absorbance. The synthesized NPs were stable for two weeks, which is higher than that of the commercial counterparts that totally precipitate after two days. The IFT test of the most stable suspension (0.05/0.2 wt% NPs/SDS) showed a reduction in the interfacial tension from 32.5 mN/m to 7.1 mN/m. Hence, the selected suspension
Fig. 8. Oil recovery percentage increase for SDS VS SDS/ZnO NPs.
Table 4 Summary of the most significant work done to enhance oil recovery using ZnO NPs. Dispersing medium
NPs concentration (wt%)
Surfactant concentration
Core type
Recovery increase %
Oil type
Ref
Ethanol Brine Water SDS Brine (3 wt%)
0.3 0.3 0.3 0.05 0.1
0 0 0 NA 0.025% - SDBS (stabilizer)
Sandstone Sandstone Sandstone Unconsolidated Glass Beads Sandpack
−4.2 −4.2 3.3 4.5 8.5–10.2
NA NA NA Arab heavy oil Crude oil (Tapis field)
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Relative permeability (frac.)
1.2
Krw (water flooding) Krw (SDS flooding) Krw (ZnO/SDS flooding)
1
Kro (water flooding) Kro (SDS flooding) Kro (ZnO/SDS flooding)
0.8 0.6 0.4 0.2 0 0.2
0.3
0.4
0.5 0.6 Water saturation (Sw)
0.7
0.8
Fig. 9. Relative permeability curves for different phases of production.
has been compared to the pristine 0.2 wt% SDS in a core flooding experiment. Using sandstone core samples (1″ × 2.6″), the residual oil saturation in the experiment with SDS alone was only 3% higher than that of ZnO/SDS experiment. Therefore, the ZnO NPs/SDS flooding showed 8% oil recovery increase compared to the conventional SDS flooding. The dominant mechanism was proved to be improving displacement efficiency by running the simulation model by Sendra. The wettability to aqueous phase has increased upon comparing models from water flooding, to SDS flooding, then eventually to ZnO NPs/SDS flooding respectively. The shift in wettability and the reduced IFT were attributed to the positive disjoining pressure exerted by ZnO NPs.
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Nomenclature FSEM EDX XRD NPs SDS IFT DIW CMC
Field scanning electron microscope Energy-dispersive X-ray spectroscopy X-ray diffraction Nanoparticles Sodium dodecyl sulfate Interfacial tension Deionized water Critical micelle concentration
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growth and characterization of one-dimensional ZnO/TiO2 hybrid nanoelectrodes for solar fuel production, J. Phys. Chem. C 117 (2013) 18502–18509, https://doi. org/10.1021/jp405515v. M. Manciu, E. Ruckenstein, Role of the hydration force in the stability of colloids at high ionic strengths, Langmuir 17 (2001) 7061–7070, https://doi.org/10.1021/ la010741t. C. Vauthier, G. Ponchel, Polymer Nanoparticles for Nanomedicines A Guide for their Design, Preparation and Development, Springer International Publishing, Cham, 2018. X. Li, M. Yoneda, Y. Shimada, Y. Matsui, Effect of surfactants on the aggregation and sedimentation of zinc oxide nanomaterial in natural water matrices, Sci. Total Environ. 581-582 (2017) 649–656, https://doi.org/10.1016/j.scitotenv.2016.12. 175. W. Al-Shatty, A.M. Lord, S. Alexander, A.R. Barron, Tunable surface properties of aluminum oxide nanoparticles from highly hydrophobic to highly hydrophilic, ACS Omega 2 (2017) 2507–2514, https://doi.org/10.1021/acsomega.7b00279. G.V. Chilingarian, T.F. Yen, E.C. Donaldson, Enhanced Oil Recovery, Elsevier, Amsterdam, 1989. A. Chengara, A.D. Nikolov, D.T. Wasan, A. Trokhymchuk, D. Henderson, Spreading of nanofluids driven by the structural disjoining pressure gradient, J. Colloid Interface Sci. 280 (2004) 192–201, https://doi.org/10.1016/j.jcis.2004.07.005. M. Adil, K. Lee, H.M. Zaid, N.R.A. Latiff, M.S. Alnarabiji, Experimental study on electromagnetic-assisted ZnO nanofluid flooding for enhanced oil recovery (EOR), PLoS One 13 (2018), https://doi.org/10.1371/journal.pone.0193518. N.A. Ogolo, O.A. Olafuyi, M.O. Onyekonwu, Enhanced oil recovery using nanoparticles, Proceedings of the SPE Saudi Arabia Section Technical Symposium and Exhibition 2012, 2012, pp. 276–284.
Contents lists available at ScienceDirect
Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Unveiling the Synergistic Effect of ZnO Nanoparticles and Surfactant Colloids for Enhanced Oil Recovery
T
Marwan Y. Rezk, Nageh K. Allam
⁎
Energy Materials Laboratory, School of Sciences and Engineering, the American University in Cairo, New Cairo 11835, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Nanofluids stability Interfacial tension ZnO Fluid flow in porous media EOR
Although chemical flooding is widely used to enhance oil productivity at lower cost, the continuously increasing prices of chemicals is a real barrier to achieve such an aim. Hence, various ways have been explored to reduce the amount of chemicals used while yielding the same performance or even higher to make the whole process cost efficient. Herein, we investigate the use of zinc oxide nanoparticles (ZnO NPs) to improve the surfactantbased extraction of an organic fluid (n- dodecane) from a porous medium (sandstone). The main mechanism is a synergetic effect between surfactant and nanoparticles in reducing the oil/water interfacial tension (IFT), which assists in overcoming the capillary trapping of oil. In addition, the aqueous solutions of the investigated surfactant (sodium dodecyl sulphate) provide better suspension stability for ZnO nanoparticles compared to water. The presented experiments showed an increase in oil recovery efficiency of 8% when both ZnO and surfactant are used compared to the surfactant-based oil recovery processes. To further explain the dominant mechanism for the increased oil production, simulation models were constructed and showed wettability shift to more water wet characteristics.
1. Introduction Although the use of nanomaterials is promising in many fields, there are challenges that still need to be addressed to use them efficiently. Firstly, the production of some nanoparticles could have a relatively high cost. Secondly, the physical and chemical properties of the same synthesized nanomaterial could differ upon changing the fabrication method [1–3]. The use of nanoparticles appears to be an emerging technology in the oil industry [4]. Within the context of enhanced oil recovery (EOR) a number of nanoparticles have been investigated (e.g. SiO2 and Al2O3) some of which have shown potential in designing improved EOR processes [5–9]. In another application, metal oxide nanoparticles have shown great performance as drilling fluids additives in controlling fine migration and increasing plastic viscosity, yield point, and gel strength [10,11]. However, size, shape, and type of nanoparticles could be optimized further to achieve enhanced mud properties. Among those nanoparticles, silica is widely used as it is more stable than other metal oxide nanoparticles and it has a similar surface charge as quartz, which counts for most reservoir rocks [12]. In addition, silica nanoparticles have shown a synergistic effect with different surfactants reducing IFT and hence increasing oil recovery [13,14]. However, using materials other than silica can open up a set of solutions to other problems facing the field as the retention of silica NPs ⁎
in carbonates in the presence of high/low salinity [15,16]. Recently, ZnO nanoparticles have been receiving a great attention to help in oil recovery through different mechanisms, mainly due to their ability to change the wettability and reduce the oil viscosity [17–21]. However, the reported results so far are controversial. For example, while one study [22] claimed that ZnO NPs increase the interfacial tension, another report claimed that they reduce the IFT [23]. This controversy needs to be discussed considering the stability of NP suspensions. Ideally, a NP suspension needs to stay stable over the time-scale of the process it is designed for. Less stable suspensions experience particle sedimentation, which in turn impacts the physical properties such as IFT. It has been shown that surfactants can enhance the stability of NP suspensions [24–27]. In the case of ZnO NPs, anionic surfactants, such as sodium dodecyl sulfate (SDS), are shown to be promising in suspension stabilization. More importantly, surfactants such as SDS provide reduced IFT, making SDS solutions an excellent choice for EOR processes. Given the economic and environmental aspects of EOR processes, designing optimized ZnO/SDS suspensions that offer minimum IFT at lowest concentrations (NPs and surfactant) is of great significance. Herein, we have systematically studied the effect of ZnO NPs on oil recovery when used in combination with SDS surfactant. The stability of ZnO NP suspensions over time was also investigated,
Corresponding author. E-mail address: [email protected] (N.K. Allam).
https://doi.org/10.1016/j.colcom.2019.01.004 Received 25 December 2018; Received in revised form 15 January 2019; Accepted 21 January 2019 2215-0382/ © 2019 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. (a) FESEM of the fabricated ZnO nanoparticles, and (b) the corresponding EDX spectrum.
dried in an oven for 24 h. The cores were assembled in Vinci core holder with a confining pressure of 2000 Psi. The procedures can be summarized as follows:
which provides an insight on the applicability of this technology at the field scale. 2. Experimental Methods & Materials
1) The cores of 1.0-inch diameter and 2.6 in. length were collected from the actual reservoir in Egypt. The core samples were saturated using vacuum saturator at a pressure of 14 bars at room temperature. The core samples were saturated overnight to fully saturate the core samples with deionized water (DIW). The test was performed against DIW to assess the effect of ZnO NPs on oil recovery before introducing mono/divalent ions. 2) Having the core samples with 100% water saturation (SW), absolute permeability was measured at room temperature and flow rates of 2, 3, 4 cc/min. 3) Oil (n-dodecane) was injected at flow rates of 0.5 and 1.0 cc/min. Until no more water was produced (SWc). 4) Water was injected to displace oil and simulate for water flooding. The water was injected at a flow rate of 0.5 cc/min for 5 pore volumes. 5) By the last phase, both core samples were injected with 6 pore volumes at a rate of 0.5 cc/min. Core sample 1 (S1) was injected with SDS as a control experiment for core sample 2 (S2), which was flooded with ZnO NPs/SDS fluid. 6) The effluent fluids were collected in 25 mL test tube to be analyzed later for oil production to calculate the produced oil and recovery factor of ZnO NPs/SDS flooding.
2.1. Synthesis and Characterization of ZnO Nanoparticles Zinc acetate (purchased from Merck 99.99% trace metals basis) and methanol (purchased from Merck anhydrous 99.8%) were mixed by stirring for 10 min at room temperature followed by vigorous stirring at 80 °C. The obtained powder was annealed at 450 °C for 6 h [28]. The obtained nanopowder was examined by field scanning electron microscope (FSEM, Zeiss SEM ultra 60 and Zeiss 1550 VP) equipped with Energy-dispersive X-ray spectroscopy (EDX). The morphology was further investigated via transmission electron microscopy (HR-TEM using JEOL-2100 microscope), and crystal structure was analyzed by glancing angle X-ray diffraction (XRD), (GAXRD, Bruker thin film) for phase identification and other useful data that can be acquired through the same technique. To test the wettability of the fabricated ZnO NPs, the contact angle between water and a thin film of the ZnO was measured using sessile drop measurement (Drop Shape Analyser (DSA 25), Kruss, Germany). 2.2. Dispersion of ZnO NPs Three ZnO concentrations (0.05, 0.1, and 0.2 wt%) were suspended in 0.2 wt% SDS surfactant which is the critical micelle concentration (CMC). The suspension was acquired by stirring for 25 min of ultrasonication in a cold water bath to avoid undesirable heating of the suspension that may lead to Ostwald ripening for the ZnO NPs. The stability of the prepared suspensions was quantitatively assessed by measuring their absorbance by JENWAY 7300 spectrophotometer at a wavelength of 720 nm and observed visually for 2 weeks. The suspensions' stability was further investigated via dynamic light scattering, (Zetasizer Nano ZS, Malvern). In addition, the interfacial tension between aqueous (ZnO suspensions) and oleic phase (n-dodecane- anhydrous, purity > 99% purchased from Merck) was measured using k9 (Kruss) tensiometer. The synthesized ZnO NPs showed better stability compared to the commercial ZnO NPs as shown in supporting documents.
3. Results and Discussion 3.1. Morphological and Structural Properties of ZnO Nanoparticles Fig. 1a shows FESEM images of the fabricated ZnO nanoparticles obtained via the reaction between zinc acetate and methanol. The images reveal the formation of highly uniform/mildly agglomerated spherically-shaped particles that range in size from 50 ± 20 nm. The synthesis procedure proved to be reproducible, yielding almost the same range of particle sizes when repeated many times. In addition, the EDX spectrum, Fig. 1b, shows the good purity of the synthesized ZnO. Furthermore, the zinc element showing an intense peak that is in agreement with the literature [29,30]. To further assess the morphology of ZnO nanoparticles, transmission electron microscopy (TEM) was performed, Fig. 2, indicating the formation of spherical nanoparticles. In addition, the sizes of the ZnO nanoparticles are in a good agreement with those obtained from the SEM results. XRD analysis was performed to investigate the crystal structure of the fabricated nanoparticles. The obtained diffraction pattern, Fig. 3,
2.3. Core Flooding of ZnO NPs/SDS A suspension of 0.05 wt% of ZnO NPs in 0.2 wt% SDS was selected for the core flooding experiment. Two sandstone core samples were used to compare the effect of SDS surfactant flooding to ZnO NPs/SDS flooding. The core samples were cleaned using Soxhlet extractor and 34
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Fig. 4. Contact angle measurement of the synthesized ZnO nanoparticles versus water at room temperature.
Fig. 2. HR-TEM of the fabricated ZnO NPs.
where Dp = crystallite size, K = 0.94, β = line broadening in radians, θ = Bragg's angle, and λ = x-ray wavelength.
=
Strain
tan( )
(2)
where η is the strain, βStrain = broadening due to strain, and θ = Bragg's angle. The degree of poly-dispersity (ρ) can be calculated using Eq. (3).
=
Table 1 Calculated crystallite size, strain, and polydispersity for the fabricated ZnO NPs. FWHM (degrees)
Dp (nm)
η (%)
ρ (%)
36.3
0.2952
29.59
0.39
23
3.2. Stability of ZnO NPs The stability of the ZnO NPs suspensions was assessed quantitatively and visually. The ZnO NPs suspended in SDS appeared to be visually stable over 2 weeks, Fig. 5. Fig. 6a shows the UV–Vis absorption spectra of the formulations tested in SDS with different ZnO NPs concentrations. The spectra show two regions I and II. In region I, the dominant mechanism is the coagulation and flocculation causing particles to be totally unstable. Moreover, the instability could be attributed to the acquired energy
indicated hexagonal zinc oxide phase. Note that the different intensities of the peaks may indicate the difference of preferential growth of the crystals with the highest intensity (highest area of crystal face) found for (101). The XRD calculations using Bragg's law take into account some assumptions that never happen in actual cases. The assumptions are that the beam analyzing the crystal is perfectly parallel and monochromatic and that the crystal is perfect. Thus, it is worth mentioning that such imperfections affect the broadening of the XRD peaks as well as other factors. There are mainly three factors that affect the broadening of the diffraction peaks including instrumental effects (arising from imperfect focusing), crystallite size (Dp), and lattice strain (η). Thus, it is possible to quantitatively evaluate the effect of microstrain as well as crystallite size using Scherrer equation (Eq. (1)) and the resulting data are presented in Table 1.
Dp =
1 2
K× × cos( )
(3)
where ρ is the degree of poly-dispersity, σ is the standard deviation of crystal-size, and d is the mean crystal-size. Usually, a small ρ value indicates uniform size distribution. To test the wettability of the synthesized nanoparticles with water, a thin film of the synthesized ZnO powder was made. The test was repeated three times, however, the ZnO showed super-hydrophilicity that as soon as the water drop reached the surface of the thin film it was quickly diffused into the membrane, Fig. 4. Note that the drop shape analyser did not get enough time to detect the spreading of the water droplet over the thin film before it diffuses. The contact angle results showed super-hydrophilicity that could make it a good candidate to shift the wettability of the core surface to more water wet even when ZnO NPs gets adsorbed on the rock surface.
Fig. 3. XRD pattern of the fabricated ZnO nanoparticles.
2θ (degrees)
d
Fig. 5. (A) 0.05 wt% ZnO NPs. (B) 0.1 wt% ZnO NPs. (C) 0.2 wt% ZnO NPs. suspended in 0.2 wt% SDS (a) Upon preparation, and (b) after 2 weeks.
(1) 35
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Fig. 6. (a) UV–Vis spectra showing the difference in the stability of the fabricated ZnO NPs/SDS suspensions at different NPs concentrations and (b) Effect of different concentrations of ZnO nanoparticles suspended in 0.2 wt% SDS on the interfacial tension against n-dodecan. The zeta potential of the ZnO colloid was quantitatively evaluated for further investigation of the suspension's stability using Zeta.
3.4. Two-Phase Fluid Displacement Test in Sandstone
from sonication in the first hour. Later on, the instability of the suspension for the rest of the first 25 h could be attributed to particle growth and precipitation. When Brownian motion is extremely high (after sonication), particles get coalesced and grew in size to precipitate. In region II, the effect of adding the SDS surfactant as a stabilizing agent can be seen clearly from the slope of the absorbance graph. The surfactant macromolecules get adsorbed on the colloidal particles to form a tough shell that helps to keep the particles apart. Thus, the main mechanism causing the suspension to be stable in region 2 could be attributed to solvation shell interactions [31,32]. In this regard, the suspension containing 0.2 wt% SDS in 0. 05 wt% ZnO NPs was found to be the most stable formulation over two weeks. It was reported that the zeta potential of ZnO NPs ultrapure water suspensions with a particle size 20 nm and concentration of 0.5 g/L is 17.56 ± 2.13 mV, which was changed to −27.96 ± 2.59 mV upon the addition of SDS surfactant at a concentration of 0.03% [33]. This change shows that increasing the SDS surfactant concentration, when added to ZnO NPs, decreases its zeta potential. The measured surface zeta potential of ZnO suspension in SDS by zeta analyzer was −28.0 ± 4.2 mV. The result is in a good agreement with the aforementioned work done by Li et al. [33]. The results also confirmed the effect of the surfactant macromolecules inducing repulsive forces in between ZnO nanoparticles causing them to keep suspended.
Two series of fluid displacement experiments were performed on two sandstone samples (S1 and S2) to investigate the effect of ZnO NPs on oil recovery in a surfactant-based process. The properties of these two samples are listed in Table 2 and the results of these experiments are presented in Table 3 and Fig. 7. Our results show that the SDS experiment left about 3% more residual oil saturation than that of the ZnO/SDS experiment. This is despite the fact that in the NPs/SDS experiment, the water flooding had left about 6% more oil entrapped in the core than that of SDS experiment. In other words, as shown in Fig. 7, SDS/ZnO NPs caused 19% increase in oil recovery in the tertiary phase raising the oil recovery from 16% to 35% (Fig. 8). No emulsions were observed during/after the ZnO/SDS flooding. The only drawback we can see about ZnO/SDS flooding as shown in Fig. 7 is the increase in differential pressure. The increase can be attributed to the retention of ZnO nanoparticles in the porous media. However, Adil et al. claimed low adsorption capacity of nanoparticles and hence low retention in a porous medium. In addition, the use of SDS along with ZnO NPs minimized permeability reduction caused by nanoparticles retention [37]. Table 4 summarizes some the most significant work reported in literature using ZnO nanoparticles to enhance oil recovery. To the authors' best knowledge, Table 4 shows a vital progress in using ZnO from the stability standpoint of view as well as oil recovery enhancement. To justify the core flood experiments results, we ran simulation models (using Sendra) to construct relative permeability curves using Corey and Skjæveland correlations. It was possible to construct three relative permeability curves for water flooding, SDS flooding, and ZnO/ SDS as shown in Fig. 9. The three curves from simulation prove the wettability shift from intermediate wet (water flooding) to slightly
3.3. IFT Measurement Using SDS + ZnO NPs Fig. 6b shows a significant decrease in the IFT upon adding different concentrations of ZnO NPs to SDS. This confirms the same concept proved previously using alumina nanoparticles [34]. The huge decrease in IFT is expected to yield higher oil recovery by reducing the oil entrapment within the pore space of the rock [35]. The reduced IFT could be also attributed to the uniformity/low polydispersity of the ZnO NPs that would consequently result a higher displacement of oil/water contact line, in agreement with the simulation models previously reported by Chengara et al. [36]. In this instance, ZnO nanoparticles get confined in an ordered layered thin film (wedge) that is in contact with oil. This arrangement is due to increased entropy of the overall suspension allowing more freedom for the nanoparticles movement. This process results in excessive pressure within the ZnO nanoparticles film relative to the bulk, which is reduced along with the energy as the thickness increases. Subsequently, the highest energy point of the ZnO nanoparticles wedge within the suspension in contact with oil caused a deformation in the meniscus, leading to an increased oil displacement.
Table 2 Parameters of the tested core samples (S1: SDS surfactant flooding), (S2: ZnO nanoparticles/SDS flooding).
36
Core ID
S1
S2
Diameter (cm) Length (cm) Surface Area (cm2) Volume (cm3) Pore volume (mL) Porosity (%) Absolute permeability (mD)
2.54 6.72 63.18 33.52 8.89 26.51 28.13
2.54 6.67 62.78 33.27 8.87 26.65 15.74
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Table 3 Core flood details for SDS (S1) and SDS/ZnO NPs (S2).
SDS
SDS/ZnO NPs
Process
Sw (%)
So (%)
Swc
Sor
q (cc/min)
PVinjected (mL)
Water (Kabs) Oil Water flood SDS Water Oil Water SDS/ZnO NPs
100 32.5 66.25 71.87 100 29 60.6 74.1
0 67.5 33.75 28.13 0 71 39.4 25.9
NA 32.5 32.5 32.5 NA 29 29 29
NA NA NA 28.13 NA NA NA 25.9
2, 3, 4 0.5, 1 0.5 0.5 2, 3, 4 0.5, 1, 2 0.5 0.5
8.89 4.9 PV = 44.5 5 PV = 44.45 6 PV = 53.34 8.87 4PV = 35.48 5 PV = 44.35 8 PV = 70.96
Fig. 7. Cumulative recovery factor for SDS VS SDS/ZnO NPs.
water wet (SDS flooding) then eventually to more water wet (ZnO/SDS flooding). The simulation results reveal that the oil recovery improvement can be ascribed to the increased displacement efficiency. Furthermore, it confirms the results obtained from the interfacial tension tests that have been performed prior to the core flood experiments. In addition, the shift in wettability to more water wet could also be attributed to the positive disjoining pressure. The excess pressure in the wedge film compared to the bulk is due to the arrangement of nanoparticles to increase the entropy of the suspension in the porous media by more free movement of the ZnO NPs in the bulk liquid. 4. Conclusions Highly uniform and mildly agglomerated ZnO NPs (50 ± 20 nm) have been synthesized via wet chemistry processing and characterized using FESEM and XRD techniques. The stability of the synthesized ZnO NPs in 0.2 wt% SDS surfactant was tested over time by following the variation in their absorbance. The synthesized NPs were stable for two weeks, which is higher than that of the commercial counterparts that totally precipitate after two days. The IFT test of the most stable suspension (0.05/0.2 wt% NPs/SDS) showed a reduction in the interfacial tension from 32.5 mN/m to 7.1 mN/m. Hence, the selected suspension
Fig. 8. Oil recovery percentage increase for SDS VS SDS/ZnO NPs.
Table 4 Summary of the most significant work done to enhance oil recovery using ZnO NPs. Dispersing medium
NPs concentration (wt%)
Surfactant concentration
Core type
Recovery increase %
Oil type
Ref
Ethanol Brine Water SDS Brine (3 wt%)
0.3 0.3 0.3 0.05 0.1
0 0 0 NA 0.025% - SDBS (stabilizer)
Sandstone Sandstone Sandstone Unconsolidated Glass Beads Sandpack
−4.2 −4.2 3.3 4.5 8.5–10.2
NA NA NA Arab heavy oil Crude oil (Tapis field)
[38] [38] [38] [23] [37]
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Relative permeability (frac.)
1.2
Krw (water flooding) Krw (SDS flooding) Krw (ZnO/SDS flooding)
1
Kro (water flooding) Kro (SDS flooding) Kro (ZnO/SDS flooding)
0.8 0.6 0.4 0.2 0 0.2
0.3
0.4
0.5 0.6 Water saturation (Sw)
0.7
0.8
Fig. 9. Relative permeability curves for different phases of production.
has been compared to the pristine 0.2 wt% SDS in a core flooding experiment. Using sandstone core samples (1″ × 2.6″), the residual oil saturation in the experiment with SDS alone was only 3% higher than that of ZnO/SDS experiment. Therefore, the ZnO NPs/SDS flooding showed 8% oil recovery increase compared to the conventional SDS flooding. The dominant mechanism was proved to be improving displacement efficiency by running the simulation model by Sendra. The wettability to aqueous phase has increased upon comparing models from water flooding, to SDS flooding, then eventually to ZnO NPs/SDS flooding respectively. The shift in wettability and the reduced IFT were attributed to the positive disjoining pressure exerted by ZnO NPs.
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Nomenclature FSEM EDX XRD NPs SDS IFT DIW CMC
Field scanning electron microscope Energy-dispersive X-ray spectroscopy X-ray diffraction Nanoparticles Sodium dodecyl sulfate Interfacial tension Deionized water Critical micelle concentration
Acknowledgment We would like to express our sincere appreciation to the American University in Cairo for funding this work and to Dr. Tannaz Pak from the school of Science, Engineering, and Design in Teesside University for supporting required tools for application. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2019.01.004. References [1] J. Gómez-Pastora, E. Bringas, I. Ortiz, Recent progress and future challenges on the use of high performance magnetic nano-adsorbents in environmental applications, Chem. Eng. J. 256 (2014) 187–204, https://doi.org/10.1016/j.cej.2014.06.119. [2] W.A. Abbas, M. Ramadan, A.Y. Faid, A.M. Abdellah, A. Ouf, N. Moustafa, N.K. Allam, Photoactive catalysts for effective water microbial purification: Morphology-activity relationship, Environ. Nanotechnol. Monit. Manag. 10 (2018) 87–93, https://doi.org/10.1016/j.enmm.2018.05.002. [3] R. Saidur, K. Leong, H. Mohammad, A review on applications and challenges of nanofluids, Renew. Sust. Energ. Rev. 15 (2011) 1646–1668, https://doi.org/10. 1016/j.rser.2010.11.035.
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[22] [23]
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