Impact of Metal Oxide Nanoparticles on Enhanced Oil Recovery from

Article pubs.acs.org/EF

Impact of Metal Oxide Nanoparticles on Enhanced Oil Recovery from Limestone Media at Several Temperatures Ali Esfandyari Bayat,* Radzuan Junin, Ariffin Samsuri, Ali Piroozian, and Mehrdad Hokmabadi Department of Petroleum Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia S Supporting Information *

ABSTRACT: Recently, researchers have proved the application of nanoparticles (NPs) for enhanced oil recovery (EOR) in ambient temperature. However, to our knowledge no attempt has been undertaken experimentally to investigate the influence of NPs on EOR at higher temperatures. In this study, aluminum oxide (Al2O3), titanium dioxide (TiO2), and silicon dioxide (SiO2) were selected for EOR purposes from an intermediate-wet limestone sample at 26, 40, 50, and 60 °C. These metal NPs were dispersed in deionized water at concentration of 0.005 wt %. First, transportation of the NPs through limestone was evaluated. It was found that Al2O3 (8.2%) had the lowest and TiO2 (27.8%) and SiO2 (43.4%) had the highest adsorption on the limestone. Consequently, wettability of the limestone was changed into water-wet through NPs adsorption. The contact angle in the presence of Al2O3, TiO2, and SiO2 nanofluids was measured as 71° ± 2°, 57° ± 2°, and 26° ± 2°, respectively. Interfacial tension was also noticeably reduced with these nanofluids at all temperatures. Al2O3 and TiO2 nanofluids respectively demonstrated better results in EOR compared to SiO2 at all temperatures. Reduction of capillary force was the main reason for EOR via the nanofluids. Moreover, a considerable reduction in oil viscosity was observed after Al2O3 and TiO2 nanofluids flooding at 50 and 60 °C.

1. INTRODUCTION With the growth in energy demand and decline in oil production from conventional oil reservoirs, attention has turned to enhanced oil recovery (EOR) techniques due to their lower investment cost and higher profit opportunities. Since 1865, water flooding has been the oldest and the most used method among EOR techniques. This method is cheap due to the availability of water resources in nature, and no environmental pollution is involved. However, water cannot completely sweep the oil reservoir due to its lower viscosity compared to the oil phase.1 In addition, the capillary number obtained in this flooding is remarkably small; thereby, the sweep efficiency is low, and significant amounts of oil are left in the form of small spherical globules in the center of the larger pores in porous media.2 The water flooding sweep efficiency has been enhanced by adding chemical agents such as polymers, surfactants, and alkalines. Thus, the capillary number has greatly improved through changing effective parameters such as viscosity (μ), interfacial tension (IFT, σ), and contact angle (θ). However, application of chemicals is limited by the temperature and salinity of porous media. The mechanical and shear properties of chemicals are degraded at high temperatures and in saline reservoirs. Moreover, the tendency for chemicals to be adsorbed on the rock surfaces is another problem that increases the amount of chemicals required for injection. Chemicals are precious components, and losing them in porous media incurs extra operational expenditures.3−5 Recently, the application of nanoparticles (NPs) suspension flooding for EOR purposes has been proved.5−11 NPs can improve fluid-rock interaction characteristics such as wettability and heat transfer coefficient. Some of the fluid properties, such © XXXX American Chemical Society

as density and viscosity of the displacing phase (water), IFT, and oil viscosity, are enhanced by NPs. Moreover, NPs have been accepted to be more stable than surfactants and polymers at higher temperatures and/or in saline environments.12 The most common type of NPs utilized for EOR is silicon dioxide (SiO2) (Table 1). SiO2 is basically known as an IFT and θ depressant.11,13 Using other metal NPs can also be a good option in EOR. For example, the effects of some metal particles, such as iron oxide (Fe2O3), nickel oxide (Ni2O3), and aluminum oxide (Al2O3), as catalyst and support catalyst on changing the rheology of oil at high temperatures were determined within reactors.14−20 The results demonstrated that the viscosities of produced oils were reduced using metal particles through the aquathermolysis reactions. The presence of metal particles during aquathermolysis reactions can transfer the available hydrogen atoms to the free radicals, preventing polymeration and condensation reactions and preventing coke formation. In addition, Hascakir21 and Hamedi Shokrlu and Babadagli22 found that metal NPs are efficient to reduce oil viscosity not only at high temperatures but also at low temperatures. The authors believed that the exothermic chemical reactions between metal NPs and oil molecules, especially with asphaltene groups, provide the required energy for breaking carbon−sulfur bonds. Generally, nanoscale metals in the presence of metal oxides have the potential to be used as energetics since high energy is released during oxidation.23 However, in all the mentioned studies, metal NPs were applied out of porous media. Received: June 18, 2014 Revised: September 15, 2014

A

dx.doi.org/10.1021/ef5013616 | Energy Fuels XXXX, XXX, XXX−XXX

0.05 wt % 10 vol %

titanium dioxide

aluminum oxide, titanium dioxide, and silica

silica and IIT nanofluid

Ehtesabi et al.5 (2014)

Hendraningrat and Torsæter25 (2014) Zhang et al.41 (2014)

dispersion medium

mixture of sodium dodecyl sulfate and DIW ethanol

mixture of nonionic surfactant (Tween 80 + Span 85 + glycerin) and DIW brine (NaCl 3 wt %)

distilled water, brine, ethanol, and diesel

ethanol

sulfanole alkyl aryl sodium sulfonate

ethanol and water

DIW and brine

brine (NaCl 3 wt %)

0.01 and 1 wt % brine (NaCl 0.5 wt %)

0.05 wt % 0.1−0.4 wt %

zinc oxide and aluminum oxide hydrophobic silica

Zaid et al.24 (2013) Roustaei et al.39 (2013)

0.01, 0.05, and 0.1 wt %

hydrophilic silica

5 and 10 wt %

0.3 wt %

0.1−0.4 wt %

hydrophobic silica

aluminum, magnesium, iron, nickel, tin, zinc, and zirconium oxides; hydrophilic and hydrophobic silica zirconium oxide

0.001 wt %

non-ferrous metal nanoparticles

NP concn 0.2−0.3 wt %

NP type

hydrophilic, neutralized, and hydrophobic silica

Hendraningrat et al.11 (2013)

Karimi et al. (2012)

6

Ogolo et al.7 (2012)

Onyekonwu and Ogolo8 (2010) Suleimanov et al.40 (2011) Shahrabadi et al.2 (2012)

ref

Table 1. List of Studies Carried out on EOR via Nanofluids porous media

sandstone cores sandstone cores carbonate cores Berea sandstone cores glass bead sandstone cores sandstone cores sandstone cores sandstone cores

sandstone cores quartz sand

T (°C)

main variable(s) for EOR

IFT reduction IFT and contact angle reduction contact angle reduction

IFT and contact angle reduction

contact angle reduction

IFT reduction and varying contact angle surface tension and contact angle reduction IFT and contact angle reduction dispersion media

IFT and contact angle reduction 25 and 55 disjoining pressure

22

75

ambient ambient

22

70

ambient

ambient

25

ambient

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B


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shaken prior to use for the ζ-potential measurements. The ζ-potential values were obtained by averaging three ζ-potential measurements for all NPs suspensions. All characterization techniques were conducted at room temperature (26 °C). 2.4. Sedimentation Experiments. The suspension stability of Al2O3, TiO2, and SiO2 NPs in DIW was determined through sedimentation experiments. The procedure of nanofluids preparation was described in section 2.2. The dynamic aggregation process was investigated via an ultraviolet−visible (UV−vis) spectrophotometer (model 105 from Buck Scientific., Inc.). The sedimentation processes of Al2O3, TiO2, and SiO2 NPs were measured via time-resolved optical absorbance. The absorbance of the samples was measured at a wavelength of 400 nm. Optical absorbency was recorded every 5 min for 180 min. The experiments were repeated twice, and the presented data are the average of the records. 2.5. Porous Media. A sample of limestone from a surface outcrop nearby the Ipoh oil field (located in the west of Malaysia) was applied as porous media. The large blocks were broken off into the smaller pieces and then crushed to powder form using a crusher machine (Pulverizer Type from BICO Inc.). Limestone powders were sieved in the range of 125−180 μm and were used in this study. The limestone fractions were pre-treated using a sequential water rinse, ultrasonication, and oven-drying (at 80 °C) procedure to eliminate impurities. The average specific gravity of limestone grains was measured as 2.52 g/cm3. X-ray diffraction (XRD) analysis was carried out to determine the limestone components using an X-ray diffractometer (Siemens D5000) with monochromatic Cu Kα radiation (λ = 1.5418 Å) operated at 40 kV and 30 mA. Energy dispersive X-ray (EDX) analysis was also applied to detect the presence of limestone elements via a field emission scanning electron microscope (FESEM, Hitachi SU8020). In addition, scanning electron microscopy (SEM, Philips XL40) images were prepared from the limestone to determine the morphology of grains before performing experiments. The ζ-potential of limestone grains in DIW was measured through the same method explained by Tufenkj and Elimelech.26 2.6. NPs Transportation through Limestone. A stainless steel tube with 0.81 cm inner diameter (i.d.) and 30.5 cm length was applied as limestone holder. The limestone grains were slowly loaded into the tube. An air compressor was used to push the grains to the end of holder while the holder was simultaneously shaken. A 50 μm filter cloth was placed at both ends of the tube to prevent limestone grains migration during displacement tests. The holder was evacuated by a vacuum pump for 2 h. Then, it was saturated via DIW from the bottom using a syringe pump (model PSK-01, Nikkiso Co., Ltd.). Ten pore volumes (PVs) of DIW were first passed through the column before passing NPs suspensions to ensure a homogeneous compaction of the column. The flow direction was selected vertically upward to equilibrate the influent solution and enhance packing homogeneity. The measured average porosity of the porous media was reported as 43%. The permeability coefficient of the porous media was calculated as 3.12 Darcy according to the ASTM D2434-68 method.27 In addition, 2 PVs of each nanofluid with flow velocity (i.e., Darcy velocity) of 7.52 × 10−4 m/s (flow rate of 1 mL/min) were injected to the column. Again, DIW was injected to the column until no NP is observed in the outlet. At the same time, the effluents were collected using the fraction collector in samples of 2 mL each. A total of 150 effluent samples were collected for each conducted experiment. UV− vis absorbance spectroscopy of each collected sample was measured using the spectrophotometer over a wavelength range of 200−800 nm. Calibration was based on a maximum absorbance wavelength of 400 nm. Finally, the concentrations of NPs dispersions entering the porous medium, CO, and in the outlet, C, were applied to generate breakthrough curves of C/CO as a function of PVs passing through the porous medium. The areas under drawn curves represent the value of recovered NPs from the columns. It should be noticed that all experiments in this section were performed at 26 °C without presence of any ionic strength in the porous media. Moreover, each test was carried out in duplicate, and data are expressed as a standard error of the mean.

Recently, some researchers, including Ehtesabi et al.,5 Ogolo et al.,7 Zaid et al.,24 and Hendraningrat and Torsæter,25 used metal oxide NPs such as Al2O3, titanium dioxide (TiO2), and zinc oxide (ZnO) for EOR purposes. However, they did not report any changes in quality of their produced oils. Most of these studies were carried out at ambient temperature which may explain why they did not mention the rheology of their produced oils. Thus, it is believed that a comprehensive study is required to determine the role of temperature on EOR while using metal NPs. In this study, the effects of three metal NPs namely Al2O3, TiO2, and SiO2 on the quantity and quality of the produced oil from an intermediate limestone sample at 26, 40, 50, and 60 °C were evaluated. For this aim, transportation of these NPs through limestone was evaluated to determine the amount of NPs adsorption on the surface of the limestone. The mentioned NPs were then applied in displacement tests for EOR. The IFT, θ, and displacing and displaced phase viscosities are parameters which were evaluated in this study.

2. MATERIALS AND METHODOLOGY 2.1. Materials. Three commercial types of NPs, namely SiO2 (20 nm, purity 99.5%, and specific surface area 160 m2/g), Al2O3 (40 nm, purity 99%, and specific surface area 60 m2/g), and TiO2 (10−30 nm, purity 99.5%, and specific surface area 50−100 m2/g), were purchased from SkySpring Nanomaterials, Inc. (Houston, TX). Sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2) from QRëC Chemical Co., Ltd. (Thailand) were used as salts. All experiments were conducted with deionized water (DIW). A sample of Malaysian crude oil (density of 0.863 g/cm3 and viscosity of 21.7 ± 0.02 cP at 26 °C) was also applied in this study. 2.2. Fluids Preparation. Al2O3, TiO2, and SiO2 NPs in concentrations of 0.005 wt % (50 mg/L) were dispersed in DIW to make nanofluids. DIW was selected as dispersion medium in this study. The suspensions were agitated for 1 h using an orbital shaker at 220 rpm and ultrasonicated via an ultrasonic bath for a period of 1 h to obtain homogeneous suspensions prior to each test. The suspensions were prepared 15 min before introduction to porous media. Moreover, two synthetic reservoir brines were prepared. The first featured a combination of NaCl 2 wt %, KCl 0.2 wt %, CaCl2 0.2 wt %, MgCl2 0.1 wt % (approximately 25,000 ppm), and DIW to simulate an original reservoir brine. In the second, NaCl 0.3 wt % (3000 ppm) was used for water flooding stage. The density and viscosity of prepared fluids were measured using a pycnometer and rheometer (Brookfield, model MLVT115), respectively. The pH was measured using a digital pH meter (Eutech Instruments, model pH700). The viscosity, density, and pH of the applied fluids at 26 °C are reported in Table 2.

Table 2. Properties of the Fluids Applied in the Experiments at 26 °C fluid

density (g/cm3)

degassed crude oil synthetic brine, 2.5 wt % brine, NaCl 0.3 wt % Al2O3 nanofluid, 0.005 wt % TiO2 nanofluid, 0.005 wt % SiO2 nanofluid, 0.005 wt %

0.863 1.0161 0.9933 0.9901 0.9908 0.9919

viscosity (cP)

pH

± ± ± ± ± ±

− 6.90 6.95 6.75 6.35 6.65

21.7 0.98 0.94 1.44 1.65 1.28

0.02 0.02 0.02 0.02 0.02 0.02

2.3. Characterization of NPs. Transmission electron microscopy (TEM, model JEM-2100/HR, Acc.200.00 kV) was utilized to determine the Al2O3, TiO2, and SiO2 NPs morphology and check their sizes. The average zeta potential values of Al2O3, TiO2, and SiO2 NPs in DIW, NaCl 0.3 wt %, and the synthetic brine were measured via a zeta potential analyzer instrument (ZEECOM Microtec Co., Ltd.). The NPs suspensions were prepared, sonicated for 1 h, and C


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2.7. Displacement Tests. A stainless steel tube with 2.54 cm i.d. and 30 cm length was applied as limestone holder for displacement test. A 50 μm filter cloth was placed at both ends of the tube to prevent limestone grain migration during displacement tests. Porous media characterization was also carried out prior to each test. Porosity and permeability of packed porous media are reported in Table 3. The

media. A schematic diagram of the displacement tests apparatus is shown in Figure 1. The viscosity of oils produced after each round of nanofluid flooding was measured via the rheometer. In addition, the IFT was measured between each nanofluid and crude oil were conducted using an EasyDyne tensiometer at predetermined temperatures. The method used to calculate IFT was the Du Noüy Ring method.28

Table 3. Summary of Displacement Tests Results

3. RESULTS AND DISCUSSION 3.1. Nanoparticle Characterization. According to the TEM images and manufacturers’ reports, all the NPs can be considered to have a spherical morphology, as shown in Figure 2. In addition, the average ζ-potentials of Al2O3, TiO2, and SiO2 NPs in the different dispersions are presented in Figure 3. As shown, the presence of salts in dispersion media significantly declined the NPs’ ζ-potential. NPs’ ζ-potential values have a direct relation with stability in such a way that NPs with higher ζ-potential values (either positive or negative) are electrically more stabilized compared to those having lower values, which usually flocculate and precipitate.29 Instability due to formation of salt bridges among NPs has been evidenced when salts are present in dispersion media.30,31 Saleh et al.32 and He et al.33 found that the stability of NPs in aqueous phases is proportional to the amount of ionic strength (salt), where higher ionic strength leads to higher NP suspension instability. Although the presence of salts in hydrocarbon reservoirs is common, the authors propose that DIW or solutions with lower salinities be used as dispersion media for EOR purposes. 3.2. Sedimentation Analysis. The results from sedimentation experiments indicated that there is no considerable sedimentation of NPs during injection to the porous media. According to the optical absorbency results (Figure 4), for the first 60 min of the experiments, sedimentation can be considered negligible for the nanofluids. By considering a 1 mL/min injection rate of nanofluids into the porous media during the adsorption and displacement tests, 19 and 60 min periods are required, respectively. Therefore, it is justifiable that the transport theory for steady-state systems is applied, and the results can be supported by the theory. 3.3. Evaluation of NPs Transport through Limestone. One of the main reasons for using NPs for EOR purposes is to deliberately change the wettability (refer to Table 1). The wettability alteration occurs if NPs are adsorbed on the surfaces of the grains. Therefore, NPs transportation tests should be performed prior to the NPs being applied for EOR purposes in order to determine the amount of NPs adsorption on the rocks’

oil recovery (%) porous media

a

ϕ (%)

k (Darcy)a

SWC (%)

T (°C)

via brine

via NPs

total

47.4 49.8 52.7 55.8

4.5 5.4 7 9.9

51.9 55.2 59.7 65.7

1 2 3 4

41.8 42.4 43.1 42.7

2.05 2.3 2.19 2.08

Al2O3 Nanofluid 14.6 26 12.8 40 13.5 50 11.8 60

5 6 7 8

43.3 43.1 42.8 42.3

2.03 2.25 2.12 2.36

TiO2 Nanofluid 12.6 26 13.8 40 16.1 50 15.1 60

47.8 49.5 52.1 55.3

3 4.1 5.2 6.6

50.8 53.6 57.3 61.9

9 10 11 12

41.1 42.1 43.2 42.7

2.4 2.35 2.2 2.34

SiO2 Nanofluid 13.7 26 12.6 40 14.6 50 15.3 60

46.7 48.8 51.5 54.8

2 2.5 2.8 2.9

48.7 51.3 54.3 57.7

Permeability coefficient of the porous media.

holder was located vertically, and the flow injection direction was from downward to upward. The procedure of displacement tests included air evacuation, initial saturation of column with synthetic reservoir brine (2.5 wt%), and then oil flooding until connate water saturation (SWC) is reached. The process of oil flooding was conducted with flow rate of 0.2 mL/min until 2 PVs to ensure the whole porous media was saturated with oil. The amount of SWC for each experiment is also reported in Table 3. Thereafter, the water flooding process with the brine (NaCl 0.3 wt %) was performed at a constant flow rate of 1 mL/ min (Darcy velocity 8 × 10−5 m/s) until no oil was observed in the outlet. This was followed by flooding of 2 PVs of each nanofluid at the same flow rate into the porous media. The temperature of the holder for each run was controlled by running the experiments within a water bath. The water bath was set at four different temperatures: 26, 40, 50, and 60 °C. However, the injection operation was conducted within an oven to preheat the injection nanofluids before reaching the porous

Figure 1. Schematic diagram of displacement test. D


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Figure 3. Zeta potential of the NPs in different dispersion media at 26 °C.

Figure 4. Sedimentation of Al2O3, TiO2, and SiO2 nanofluids in DIW.

appeared in the outlet right after 1 PV of injection (Figure 5). The NPs concentration in the effluent then noticeably

Figure 2. TEM images from NPs (a) SiO2, (b) Al2O3, and (c) TiO2.

surfaces. Particle straining and adsorption on the grain surface are the two main factors which govern colloidal particle transport through a porous medium. As particles flow through a porous medium, some may be trapped behind subminiature pore throats, known as “straining”. Filtration is defined as irreversible adsorption of the particles onto the porous medium grain surface as a result of attractive interactions between particles and the grain surface. Straining during NPs transportation is not common since the size of the NPs is much smaller than the pore throat dimensions.34 Therefore, NPs transportation tests were carried out to determine the tendency of the NPs to adsorb on the surface of the limestone due to filtration. The first transportation test was performed to study Al2O3 nanofluid on the limestone porous medium. The Al2O3 NPs

Figure 5. Transportation of Al2O3, TiO2, and SiO2 NPs suspensions through limestone column at 26 °C.

increased until 15 PVs. Thereafter, the trend showed a moderate decrease until 27.5 PVs. Above this PV, Al2O3 was not observed in the effluent, even after addition of an extra 10 PVs of DIW. The final results show that 91.8% of injected Al2O3 has been recovered. As a result, low-affinity adsorption on the limestone is attributed to Al2O3 NPs. The main reason is the electric surface charges of the Al2O3 NPs and limestone grains, which are both positive. The ζ-potential of limestone E


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Figure 6. (a) SEM image of limestone grains before flooding test. (b) XRD result from limestone sample.

adsorption: first, the time extension of TiO2 NPs recovery compared to previous tests, and second, FESEM plus EDX measurements which were conducted on both sides of the column from limestone after the experiment was finished. According to Figure 7, which was taken from the entrance of the column, TiO2 NPs were adsorbed on the surface of the limestone. The XRD and EDX analyses from limestone before injection were performed to verify the source of Ti. The XRD result demonstrates that limestone is mainly comprised of calcite, CaCO3 (Figure 6b). Furthermore, Ti was not detected as an element in the limestone before the injection test via EDX measurements. The last test was run with SiO2 NPs. These NPs were observed in the effluent after injection of 1.5 PVs. The concentration of SiO2 NPs in the outlet fluctuated widely, but the trend was upward (Figure 5). In all, 56.6% of SiO2 was recovered from the porous medium. The huge difference between electric surface charges of SiO2 and limestone causes the SiO2 to adsorb strongly on the surface of the limestone. This difference is 19.4 mV greater than the difference between TiO2 and limestone, which explains the stronger adsorption of SiO2 on the limestone than TiO2. Consequently, FESEM plus EDX measurements were also conducted on both sides of the column from the limestone after the experiment was finished (Figure 8). As shown, SiO2 NPs were adsorbed on the surface of the limestone. In conclusion, Al2O3 and SiO2 NPs with 8.2% and 43.4%, respectively, show the highest and lowest extent of adsorption on the limestone surface.

was determined to be +23.6 mV (ζ-potential is assumed equal to surface charge in low ionic strength conditions35). Dunphy Guzman et al.36 declared that the surface charge of NPs is a primary factor in their transport through soil. Darlington et al.23 evaluated Al2O3 transportation through soil and sand. They found that Al2O3 adsorbed on surfaces of soil and sand due to the charge difference between Al2O3 and these media. Furthermore, Bradford et al.37 found that grain surface roughness is another parameter that affects colloidal transport. The SEM images of limestone grains demonstrate that the grains’ surfaces are full of irregular dents and bumps, which make it rough, as shown in Figure 6a. Therefore, a small amount of Al2O3 (8.2%) was trapped in the porous media due to the morphology of the limestone, which has been determined to have a rough surface. The next NP to undergo the adsorption test was TiO2. The TiO2 NPs were observed in the effluent after 1.3 PVs injection. The concentration of TiO2 NPs in the outlet increased until 13 PVs. This amount then plateaued for the next 11.5 PVs. Finally, it began to decrease significantly, and after 30 PVs, there were no NPs in the outlet (Figure 5). In all, 72.2% of injected TiO2 NPs were recovered, and 27.8% of NPs remained in the porous media after 28 PVs DIW injection. The main reason for lower recovery of TiO2 compared to Al2O3 is the existing difference between electric surface charge signs of TiO2 NPs and limestone grains. As a result, TiO2 has affinity to adsorb on the surface of the limestone. There are also two pieces of evidence in this test which confirm the affinity of TiO2 for F


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Figure 7. (a) FESEM image from limestone grains after the flooding of TiO2 nanofluid. (b) EDX measurement after the flooding of TiO2 nanofluid from the entrance side of the column.

Figure 9. At 60 °C, the ratio of oil recovery achieved via Al2O3 and TiO2 nanofluids remained constant, while this ratio increased by a factor of 3.4 between Al2O3 and SiO2. In addition, the difference among oil recoveries at various temperatures by SiO2 was not significant, and increasing the temperature from 26 to 60 °C only increased the recovery by 1.45 times, while the recoveries via Al2O3 and TiO2 were noticeably promoted by increasing the temperature up to 60 °C, by a factor of 2.1 and 2.2, respectively. 3.5. Displacement Mechanisms. Different mechanisms to produce oil via nanofluids can be drawn. Capillary number, which is defined as the ratio of viscous force to capillary force, was selected as a means to determine the effective parameters for EOR via nanofluids. Capillary number (Nc) is calculated as follows:38

3.4. Displacement Tests Results. The displacement test results show that the use of brine (NaCl 0.3 wt %) for water flooding resulted in an average of 47.3% oil recovery at 26 °C. At the same temperature, Al2O3, TiO2, and SiO2 nanofluids improved the recovery up to 52.6, 50.9, and 48.7%, respectively. The temperature increment from 26 to 40 °C enhanced the oil recovery to 49.3% for the brine, 55.2 for Al2O3, 53.6 for TiO2, and 51.3% for SiO2. At a temperature of 50 °C, the oil recovery factor reached 52.1% after water flooding when brine was used, while 59.7, 57.3, and 54.3% were reported for Al2O3, TiO2, and SiO2 nanofluids flooding, respectively. Eventually, the highest recoveries were obtained after the second recovery (55.3%) at 60 °C. Again the lowest recovery was obtained through SiO2 nanofluid flooding (57.7%), and higher recoveries were reached via Al2O3 (65.7%) and TiO2 (61.9%), respectively. A summary of these test data is given in Table 3. Furthermore, the oil recovery performance and profile of differential pressure versus PV injected in displacement tests at 26, 40, 50, and 60 °C are shown in Figures S1−S4 (see Supporting Information). It was observed that the highest differential pressure was obtained by SiO2, followed by TiO2 and Al2O3. The reason can be explained on the basis of the high adsorption of SiO2 and TiO2 NPs on the grains’ surfaces, resulting in clogging in the porous media. According to the experimental results, oil recovery at ambient temperature (26 °C) via Al2O3 nanofluid is 1.5 and 2.25 times greater than that with TiO2 and SiO2, respectively, as shown in

Nc =

μInj × v viscous force = capillary force σ × cos θ

(1)

where μInj is dynamic viscosity of injected fluid, v is Darcy velocity, σ is interfacial tension, and θ is contact angle. All the parameters in eq 1 should be surveyed individually to fully investigate any changes in capillary number via nanofluids. 3.5.1. Viscous Force. As mentioned earlier, μInj and v are two parameters which affect the viscous force. The viscosities of Al2O3, TiO2, and SiO2 nanofluids at 26, 40, 50, and 60 °C were G


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Figure 8. (a) FESEM image from limestone grains after the flooding of SiO2 nanofluid. (b) EDX measurement after the flooding of SiO2 nanofluid from the entrance side of the column.

viscosities are not strong enough to explain the noticeable changes in EOR. Ehtesabi et al.5 also declared that the viscosity of injected nanofluids up to 2 wt % cannot be the reason for EOR. In addition, v influence was the same in all the experiments since v was considered to be constant. As an overall result, the viscous force change cannot be considered as an important factor for EOR via nanofluids. 3.5.2. Capillary Force. 3.5.2.1. Effect of Wettability Alteration via NPs. Quantitative measurement of θ can define the wettability of a rock. For this aim, four polished limestone cores with 0.4 cm thickness and 5 cm diameter were prepared. These cores were submerged in different nanofluids for 48 h at different temperatures. The cores were laid vertically in suspensions to avoid deposition of NPs on the surface of the limestone cores. Therefore, wettability alteration could only occur due to adsorption of NPs on the surface of the cores. Side images of oil drops on the limestone cores were then taken using a microscope camera, and the contact angles were measured. Contact angle measurement of the oil droplet at 26 °C showed that the original θ of limestone is 90° ± 2°, which indicates an intermediate wettability. In addition, θ values in the presence of Al2O3, TiO2, and SiO2 nanofluids at 26 °C were measured as 71° ± 2°, 57° ± 2°, and 26° ± 2°, respectively, as demonstrated in Figure 10. θ values in the presence of nanofluids were also measured at 40, 50, and 60 °C. The results

Figure 9. Oil recovery via Al2O3, TiO2, and SiO2 nanofluids at different temperatures after brine flooding stage.

measured. The nanofluids behave as Newtonian fluids at all temperatures. The viscosities of the nanofluids at different temperatures are shown in Figure S5. For example, the viscosities of Al2O3, TiO2, and SiO2 nanofluids at 26 °C are 1.44 ± 0.02, 1.65 ± 0.02, and 1.28 ± 0.02 cP, respectively (Table 2). In comparison with the brine viscosity (0.94 ± 0.02 cP), the nanofluids are more viscous. Thus, the oil sweep efficiency can be higher in the nanofluids flooding compared to the brine. However, the authors believe that the nanofluid H


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Figure 10. Microscopic image from oil droplet (flipped image) at 26 °C in the presence of (a) brine (NaCl 0.3 wt %), (b) Al2O3 nanofluid, (c) TiO2 nanofluid, and (d) SiO2 nanofluid.

demonstrate that θ declined as temperature increased (Figure 11). As shown, the nanofluids altered the wettability of the

example, Onyekonwu and Ogolo7 found that hydrophilic SiO2 was not a good agent for EOR due to changing wettability of sandstone from water-wet (36°) to strongly water-wet (10°), while significant oil recovery was achieved using hydrophobic SiO2 due to changing the wettability to intermediate (around 90°). As a result, wettability alteration is one of the dominant parameters for EOR via nanofluids. The use of those nanofluids which change wettability toward intermediate-wet results in capillary force reduction and consequently increasing EOR. 3.5.2.2. Effect of IFT Reduction via Nanofluids. The experimental results from measuring IFT demonstrated that using Al2O3, TiO2, and SiO2 nanofluids noticeably reduced the IFT by 33, 37, and 42%, respectively, compared to the brine (NaCl 0.3 wt %) at 26 °C. As shown in Figure 12, this reduction trend was observed at all temperatures. Therefore,

Figure 11. Contact angle of oil/aqueous phases on the limestone surface at different temperatures and at ambient pressure.

limestone cores from intermediate to water-wet at all temperatures. The highest and lowest changes in θ are related to SiO2 and Al2O3 at all temperatures. These results confirmed that Al2O3 and TiO2 have lower affinity to adsorb on the surface of the limestone compared to SiO2 (refer to section 3.2). It can be concluded that lower NPs adsorption on the surface of the limestone caused lower changes in θ, and low reduction in θ resulted in more oil recovery due to the vicinity to the intermediate stage. Several studies have demonstrated that changing the wettability of rocks from water or oil-wet to intermediate-wet yields greater oil recovery.2,5,6,8,39 For

Figure 12. IFT of oil/aqueous phases at different temperatures and at ambient pressure. I


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of metal NPs on oil viscosity reduction, this task has many unknowns and requires more evaluation. We believe that the main mechanism for oil viscosity reduction via metal NPs is still unknown and requires more evaluation.

IFT reduction is another parameter which significantly influences oil recovery via the nanofluids. IFT reduction decreases the capillary force, thereby increasing the oil recovery. There are several studies showing the IFT reduction between oil and aqueous phases after using nanofluids which result in oil recovery improvement.2,8,24,25,39 When the IFT between the oil and the aqueous phase is reduced, the trapped oil droplets are deformed. Thus, they can easily pass the pore throats.2,39 In conclusion, capillary force reduction is the dominant factor in EOR via nanofluids. Accordingly, the capillary number for Al2O3 nanofluid on average was calculated to be 1.4 and 2.5 times greater than those for TiO2 and SiO2 nanofluids, respectively, at all temperatures. This explains why using Al2O3 nanofluid results in higher oil recovery as compared to using TiO2 and SiO2 nanofluids. 3.5.3. Effect of NPs on Produced Oil Viscosity. The viscosity of the oil produced after each nanofluid flooding was also measured to detect any changes. It was observed that the viscosity of oils produced after the nanofluids flooding was indeed changed. The rates of these changes were low at 26 °C, while they became considerable at higher temperatures. Al2O3 and SiO2 nanofluids caused the highest and lowest amount of viscosity reduction at all temperatures (Figure 13). For

4. CONCLUSION In the current study, the impact of three metal oxide NPs, namely, Al2O3, TiO2, and SiO2, on EOR through a limestone sample at different temperatures was evaluated. Before displacement tests, transportation of the NPs through limestone was determined. It was found that Al2O3 with 8.2% adsorption had the lowest tendency, and TiO2 with 27.8% and SiO2 with 43.4% adsorption had higher tendency to adsorb on the limestone grains’ surfaces. The main reason for NPs adsorption was found to be the difference in the surface charges of NPs and limestone. In addition, the displacement tests at different temperatures demonstrated that using Al2O3, TiO2, and SiO2 nanofluids provides the potential for oil recovery enhancement through capillary force reduction. Al2O3 was the best NP in terms of EOR through limestone porous media at all temperatures. TiO2 and SiO2 in order were the next best options for EOR. When the nanofluids were introduced to porous media, NPs influenced the immobile oil droplets through reduction of oil viscosity and IFT and swept them toward the producer. Alteration of wettability via NPs was found to be another important mechanism in EOR. The θ reduction from intermediate to water-wet highly depends on the extent of NPs adsorption on the limestone surface. Thereby, the lowest and highest θ reductions were obtained via Al2O3 and SiO2 NPs for all temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4, showing the oil recovery performance and profile of differential pressure versus pore volume injected in displacement experiments at 26, 40, 50 and 60 °C, and Figure S5, showing the dynamic viscosity of nanofluids at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 13. Oil viscosity before and after the nanofluids flooding at different temperatures and at ambient pressure.

AUTHOR INFORMATION

Corresponding Author

*Tel: (+60) 147 15 25 95. E-mail: ali.esfandiari.bayat@gmail. com.

example, Al2O3, TiO2, and SiO2 nanofluids flooding at 60 °C caused 34, 24, and 8% viscosity reduction compared to the original oil viscosity at the same temperature, respectively. Furthermore, the rate of viscosity reduction via SiO2 at different temperatures was not considerable compared to the original crude oil. The reduction of oil viscosity via the metal oxide NPs has been also reported by previous researchers. For example, Hamedi Shokrlu and Babadagli22 mixed Fe2O3, Ni, and CuO NPs with a heavy oil sample and measured oil viscosity at 25, 50, and 80 °C. They found that the percentage of oil viscosity reduction depends on the applied metal type and the oil sample composition, especially asphaltene content. Those NPs with high thermal conductivity are more suitable to reduce oil viscosity. These authors believed that different types of metal particles reduce the oil viscosity through different series of exothermic chemical reactions. According to Hamedi Shokrlu and Babadagli,22 Al2O3 and TiO2 nanofluids must reduce oil viscosity more than SiO2 due to their higher thermal conductivities. Although there are some studies on the effect

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Universiti Teknologi Malaysia which provided materials and equipment. The authors also thank Mr. Eskandar and Mr. Adnan for providing FESEM images and XRD results.

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REFERENCES

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