Mechanism discussion of nanofluid for enhanced oil recovery

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Mechanism Discussion of Nanofluid for Enhanced Oil Recovery: Adhesion Work Evaluation and Direct Force Measurements between Nanoparticles and Surfaces Dong Wang,*,† Baoxue Tian,† Meiwen Cao,† Yawei Sun,† Songyan Li,‡ Teng Lu,‡ and Jiqian Wang*,† State Key Laboratory of Heavy Oil Processing & Centre for Bioengineering and Biotechnology and ‡School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China

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†

S Supporting Information *

ABSTRACT: Nanofluids, which are dispersed nanoparticles (NPs) in aqueous or organic fluids, are effective in enhanced oil recovery. In this paper, different hydrophobic and hydrophilic silica NPs were dispersed in water to prepare nanofluids with the help of surfactants (CTAB, AOT, and TX-100). The contact angle of the model oil on the solid surface increased obviously when using the oil drop on a quartz plate immersed in nanofluids. Core flooding experiments showed that nanofluids displaced more oil from the core than the surfactant solution without NPs. Finally, we investigated the nanofluid effect mechanism on the separation of model oil from the solid surface by an adhesion work evaluation and the force−distance curve measurement with an atomic force microscope. We found that hydrophobic NPs were more easily adsorbed on the solid surface than hydrophilic NPs. Hydrophilic NPs were more easily adsorbed on the model oil surface than hydrophobic NPs. These results may provide a better understanding of the complex phenomena involved in the enhanced spreading of nanofluids on solid surfaces.

1. INTRODUCTION For most oilfields around the world, plenty of reserved oil remains in formations after primary (i.e., natural flow or artificial lift) and secondary (i.e., water flooding) recovery methods have been adopted.1 Therefore, techniques that can improve either micro- or macroscopic efficiency are acceptable to oilfields for further increasing oil production. This process is the so-called tertiary or enhanced oil recovery (EOR)2 process. Numerous methods have been developed for enhancing oil recovery, such as chemical and physical methods.3−8 Nanofluids are used in various industrial and biological processes, such as EOR, cleaning and cooling microchips, drug delivery, heat transfer, hydraulic fracturing to enhance well productivity, and detergency, because of their enhanced thermophysical properties.9−12 In comparison to traditional chemical methods, silica nanofluids are inexpensive, efficient, and environmentally friendly. Researchers have observed positive results for ultimate oil recovery with nanofluid injection in core samples. In the EOR process, the spreading of nanofluid on solid surfaces is important to separating oil from rock surfaces. The spreading of nanofluid composed of liquid suspensions of nanoparticles (NPs) is different from the spreading of liquids without NPs. Therefore, the interaction between NPs and solid surfaces played an important role on spreading nanofluid on solid surfaces.13−21 Understanding the complex nature of the interactions between NPs in nanofluids and solid substrates is critical to understand the enhanced spreading ability of nanofluid on solids (i.e., the ability of nanofluid wet solid substrates) under structural disjoining pressure (i.e., the excess pressure in a film relative to that in the bulk solution), and some works have already been studied. For example, Nikolov et al. investigated © XXXX American Chemical Society

the complex mechanism involved in solid−nanofluid−oil interactions by directly observing the phenomenon of NPs self-layering (i.e., stratification) as a result of the confinement of NPs in a thin film for the first time.22 Kondiparty et al. experimentally investigated the spreading dynamics of nanofluids using the advanced optical techniques. They monitored the contact line position with time and measured the rate of nanofluid spreading, which was driven by the structural disjoining pressure, by varying the size of oil drops (i.e., the capillary pressure) and the NP concentration.23 Liu et al. theoretically analyzed the movement of the advancing inner contact line and the effects of important technological parameters, such as the NP concentration, particle size, contact angle, and capillary pressure, on the contact line velocity.24 However, the nanofluid EOR mechanism is still not fully understood. The driving force of spreading (i.e., the force driving the arrangement of NPs on the substrates) is not clear. To obtain the direct force information between NPs and solid surfaces or oil surfaces, a colloidal probe technique is regarded as an effective method using atomic force microscopy (AFM).25−31 In this technique, the sharp AFM tip on the cantilever was modified with a colloidal particle. Such colloidal probes are normally prepared by gluing a particle with the diameter of a few micrometers to a tipless cantilever with a micromanipulator. This colloidal probe approached the sample by means of an AFM scanner, and the force can be measured through the deflection of the cantilever. With modern AFM, one can typically achieve a force resolution of 10−50 pN, and Received: August 14, 2018 Revised: October 23, 2018 Published: October 24, 2018 A

DOI: 10.1021/acs.energyfuels.8b02825 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Core flooding experimental apparatus. 2.4. Core Flooding Experiments. In core flooding experiments, the model oil was also prepared by dilution of the crude oil 3 times with kerosene. The density of the model oil was measured by the pycnometer method. The density of the model oil was 0.9232 g/cm3 at 20 °C. The permeability of the artificial cores used in the displacement experiment was 2600, 550, and 220 mD, respectively. The porosity of these artificial cores was about 35.02, 30.98, and 28.19%, respectively. The surface of artificial cores was hydrophilic. The cores were first washed by pure water and then dried off in an oven at 100 °C for 4 h. The core was first saturated with water for 12 h. Then, the oil was pressed into the core to displace the water from one end of the core until the oil flowed out and no water flowed out from the other end of the core. The volume of the displaced water was equivalent to the saturated oil volume (Vt). Nanofluid was injected into the oilsaturated core until no oil flowed out, and the volume of the displaced model oil (V) was measured. The rate of oil recovery can be describe as V/Vt × 100%. The experimental apparatus was illustrated in Figure 1. Nanofluids were 0.5 wt % NPs with 3.0 mmol L−1 AOT or 4.0 mmol L−1 TX-100. 2.5. Adhesion Force Measurement. Here, we prepared the colloidal probes with four NPs with different surface hydrophobicities and diameters, including hydrophobic 8 and 30 μm SiO2 NPs and hydrophilic 8 and 30 μm SiO2 NPs. Hydrophilic 8 and 30 μm SiO2 NPs were used directly as purchased. Hydrophobic 8 and 30 μm SiO2 NPs were modified from hydrophilic NPs by the silane coupling agent. Fourier transform infrared spectroscopy (FTIR) curves in Figure S2 of the Supporting Information proved that the hydrophobic chain was modified on the NPs. The adhesion force between NPs and the silica surface or oilcoated silica surface was measured by AFM through a modified spherical colloidal probe. The probe was modified through the following protocol: First, an AFM probe moved slowly close to a mica chip coated with a layer of curing glue. After the probe contacted with the curing glue, we lifted it. Then, SiO2 NPs were sprinkled uniformly on another clean mica chip. The probe coated with curing glue moved close to the mica surface. When the probe contacted with one particular NP (monitored under the microscope), we lifted the probe and irradiated it under the ultraviolet (UV) light for 15 min to make the NP be fixed on the cantilever (panels a−c of Figure 2). According to Hooke’s law, F = −kX, where k is the elastic coefficient of the cantilever (spring constant) and X is the deformation quantity of the cantilever, we can obtain the force− distance curve. Form this curve, adhesion force and elastic force between the cantilever tip and surface could be obtained. The typical force−distance curve is shown in Figure 2d. The adhesion force is the force when the colloid tip leaves the surface (black arrow shown in Figure 2d). The spring constant k needed to be adjusted first by testing the force curve between the colloidal probe and a hard surface (sapphire or silicon surface). Then, k was calculated by the thermal tune method. k values of the 30 μm hydrophilic colloidal probe, 30 μm hydrophobic colloid probe, 8 μm hydrophilic colloidal probe, and 8

by the subsequent averaging of the force profiles, the noise can be reduced by a factor of 10 or more. With the advantages of this technique, a wide range of particles and surfaces can be investigated. Therefore, we investigated the interaction of NPs and solid surfaces to understand the mechanism of nanofluid EOR theoretically. In this paper, we have prepared several types of nanofluids with different silica NP and surfactant mixtures and examined their performance in EOR. It was found that some of the nanofluids have good oil displacement ability from the solid substrate. To further understand the mechanism of nanofluid spreading, we tested the adhesion force between different NPs and solid surfaces or the model oil surface by AFM. We considered the SiO2 NPs with different hydrophilic and hydrophobic properties, which would affect the adhesion force of NPs and the solid surface. The core displacement experiments also proved that different NPs have different oil displacement efficiencies.

2. MATERIALS AND METHODS 2.1. Materials. SiO2 NPs (N20, H15, and H18) were purchased from Wacker Chemie AG. All of the NPs were spherical or ellipsoidal, with the diameter of 15−20 nm. H15 and H18 were modified by methyl chlorosilane and dimethyl polysiloxane by the producer, respectively. Chemical reagents, such as CTAB, AOT, TX-100, and nheptane, were purchased from Sinopharm. All of the chemicals were used as received. The artificial cores with different permeabilities were purchased from Zhisheng East Co., Ltd. 2.2. Nanofluid Preparation. Three different SiO2 NPs, hydrophilic NP N20, hydrophobic NP H15, and hydrophobic NP H18, were used in nanofluid preparation. The hydrophilicity and hydrophobicity of the NP surface (NPs were pressed to plates) were measured by contact angle measurements. Figure S1 of the Supporting Information showed that the hydrophilic property sequence was N20 > H15 > H18. Nanofluids were prepared by dispersing NPs in water with the help of surfactants (CTAB, AOT, and TX-100). 2.3. Contact Angle Measurements. The quartz plate was used as the model of the rock surface. The experimental quartz pieces were immersed in the prepared piranha solution (H2SO4/H2O2 = 1:4). Then, the solution was heated to 90 °C and stirred for 1 h to remove residual organic matter on the surface of the quartz plate. The quartz plate was rinsed by the pure water after ultrasound. A model oil drop was deposited on the quartz plate. The quartz plate was then turned upside down and placed on two supports inside a transparent glass cuvette. Then, nanofluid was introduced into the cuvette, and the model oil drop was immersed in nanofluid. The oil droplets were observed through a contact angle meter. The model oil was prepared by dilution of the original crude oil (from the Shengli Oilfield) 3 times with kerosene. B


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2.6. Other Measurements. Dynamic light scattering (DLS) and ζ-potential experiments were carried out on a Malvern Zetasizer Nano ZS. FTIR was carried out on a Thermal Nicolet 6700 infrared spectrometer. Surface tension was measured with a Krüss surface tension meter (EasyDyn). Turbidity was measured with a Hanna turbidity meter (Lp2000-11N).

3. RESULTS AND DISCUSSION 3.1. Stability of Nanofluids. The dispersion and stability of the SiO2 NPs in aqueous medium were investigated. Different surfactants (CTAB, AOT, and TX-100) were used as dispersing agents to disperse 0.5 wt % SiO2 NPs (N20, H15, and H18, respectively). Figure 4 shows the size distribution and ζ potential of NPs with different surfactant concentrations. N20 NPs with a hydrophilic surface had better dispersity than H15 and H18 with a hydrophobic surface without surfactants. Both hydrophilic and hydrophobic SiO2 NPs were negatively charged in water, although the ζ potential of hydrophobic NPs was slightly higher than that of hydrophilic NPs as a result of the silane modification. Cationic surfactant CTAB could adsorb on the NP surface through electrostatic interaction. The ζ potential of NPs increased with the increase of the CTAB concentration (panels a−c of Figure 4). When the ζ potential was near 0 mV, the size of NPs would be maximal, indicating the aggregation behavior of NPs. At a relatively higher ζ potential, more CTAB would be beneficial to disperse more NPs. In the turbidity measurements, the solution turbidity increased obviously at a certain concentration of CTAB, indicating that the NPs were well-dispersed (Table S1 of the Supporting Information). The turbidity decrease means that NPs aggregated to precipitate. A concentration of 2.5 mmol L−1 CTAB was suitable to disperse these three 0.5 wt % NPs.

Figure 2. Images of the modified cantilever and force−distance curve: (a) cantilever without particles, (b) cantilever with 30 μm particles, (c) cantilever with 8 μm particles, and (d) force−distance curve of the NP and solid surface. μm hydrophobic colloid probe were 0.59, 0.38, 0.66, and 0.58 N/m, respectively. The bare silicon wafers with a thin silica layer on the surface were first cleaned by ethanol in ultrasound equipment for 10 min, then immersed in pure water, and ultrasounded for 10 min. Finally, they were rinsed by pure water several times and dried with flowing nitrogen. The model oil film was spin-coated on the silicon wafer. The model oil in this section was diluted liquid paraffin instead of diluted crude oil because of its non-volatility and easy spin-coating property. The images of treated surfaces are shown in Figure 3.

Figure 3. AFM images for the (a) clean silicon wafer surface and (b) model oil coated surface. C


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Figure 4. DLS and ζ-potential measurements of different nanofluids.

3.2. Contact Angle of Model Oil on the Solid Surface in Nanofluids. The formation of the nanofluid film on the surface was beneficial for the exfoliation of the oil drop from the surface.16,22−24 The oil on the oil-wet rock surface in formation usually has a small contact angle, as shown as Figure 5a, and the oil film is hard to separate from the solid surface. When fluids was used to soak the oil film, the contact angle increases and is more easily peeled off. With the NPs in the fluids, the spreading force would be enhanced under the structural disjoining pressure action, and thus, the contact angle of oil would increase further. When the contact angle was sufficiently large, the oil drop would be separated from the surface (Figure 5b). We have measured the contact angle of oil drops in the fluids (water, surfactant solutions, and nanofluids). CTAB was apt to adsorb on the quartz plate by electrostatic interaction, which caused the surface to transform from hydrophilicity to hydrophobicity. This transformation benefitted oil spreading on the surface but not the divorce of the oil film. Figure 5c also showed that CTAB-dispersed silica nanofluids did not enhance the contact angle of oil on solid surfaces. However, both AOTand TX-100-dispersed silica nanofluids had enhanced the contact angle of oil on the solid surface (panels d and e of Figure 5). Pure water enhanced the contact angle to

The anionic surfactant AOT could also adsorb on the surface of SiO2 NPs by hydrophobic interaction (panels d−f of Figure 4). With the increasing AOT concentration, the size of hydrophilic N20 NPs changed little, indicating their well dispersion. However, for the hydrophobic H15 and H18 NPs, AOT would play a more important role in dispersion. The two hydrophobic chains of AOT adsorbed on the hydrophobic surface of H15 and H18, and the polar head stretched outward to the solution as a result of the hydrophobic interaction between the surface and the surfactant chains. Thus, the dispersion of hydrophobic H15 and H18 NPs was increased obviously. In combination with the turbidity results in Table S1 of the Supporting Information, a concentration of 3.0 mmol L−1 AOT was suitable to disperse these three 0.5 wt % NPs. The non-ionic surfactant TX-100 also adsorbed on the surface of NPs through hydrophobic interaction. According to the diameter distribution, NPs were well-dispersed when the concentration of TX-100 was increased (panels g−i of Figure 4). We considered that the spatial effect of the surfactant promoted the dispersed of NPs. In combination with the turbidity results in Table S1 of the Supporting Information, a concentration of 4.0 mmol L−1 TX-100 was suitable to disperse the three 0.5 wt % NPs. D


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Figure 5. Contact angle measurements: (a) image of the model oil on the solid surface (the quartz plate was put upside down to measure the contact angle in fluids), (b) image of the model oil in nanofluids (the quartz plate was put upside down), (c) oil contact angle on the solid surface in CTAB-dispersed nanofluids, (d) oil contact angle in AOT-dispersed nanofluids, and (e) oil contact angle in TX-100-dispersed nanofluids.

where γl1l2, γl1l2, and γl1l2 are the interface tensions of the gas−oil, gas−solid, and oil−solid, respectively. Adhesion work (W′a) between oil and the solid surface in nanofluids (or another liquid medium) can be described as follows:

approximately 60°. AOT and TX-100 solution enhanced the contact angle to 136° and 135° maximally, respectively. With regard to AOT and TX-100 silica nanofluids, the contact angle further increased to about 150°, indicating better oil-divorcing abilities (panels d and e of Figure 5). In most instances, the oil showed a larger contact angle in hydrophobic NP nanofluids than in hydrophilic NP nanofluids. 3.3. Adhesion Work Evaluation of Nanofluids. According to the contact angle measurement, nanofluids could promote the divorce of oil from the solid surface. We have analyzed the change of adhesion work of oil on solid surfaces before and after washing by nanofluids to understand the mechanism thermodynamically. When nanofluids separated the oil film from the solid surface, the oil/solid interface disappeared, while a new nanofluid/solid interface formed. Oil wets the solid surface with a small contact angle as a result of its large adhesion force on the solid surface. To peel the oil off, the adhesion force between oil and the solid surface should be decreased or the adhesion force of the fluid should be increased. We have evaluated the value of three different adhesion works (W): (i) the adhesion work between oil and the solid surface in air, (ii) the adhesion work between oil and the solid surface in nanofluids, and (iii) the adhesion work between nanofluids and the solid surface in oil. In the following equations, g represents gas, s represents the solid surface, l1 represents the oil phase, and l2 represents fluids (water, surfactant solutions, or nanofluids). Adhesion work (Wa) between oil and the solid surface in air can be described as follows: Wa = −ΔG = γgl + γgs − γl s 1

1

Wa′ = −ΔG′ = γl l + γl s − γl s 12

2

(2)

1

where γl1l2, γl2s, and γl1s are the interface tensions of the fluid− oil, fluid−solid, and oil−solid, respectively. This equation can be interpreted as the adhesion work of an oil droplet and solid surface in the medium of fluids. Adhesion work (Wa″) between fluid and the solid surface in the oil phase can be described as follows: Wa″ = −ΔG″ = γl l + γl s − γl s 12

1

(3)

2

where γl1l2, γl1s, and γl2s are interface tensions of the fluid−oil, oil−solid, and fluid−solid, respectively. This equation can also be interpreted as the ability of fluid displacement of the oil from solid surfaces. The difference of the oil−solid interface adhesion work between air and fluids (W′a − Wa) indicates the ability of fluids to change the adhesion force between oil and the solid surface. The smaller the (Wa′ − Wa), the better the separation effect that the fluid has. The difference between the oil−solid and fluid−solid interface adhesion work (W″a − W′a) indicates the ability of the fluid to displace oil from the solid surface. The larger the (Wa″ − Wa′), the easier the fluid displaces oil from the solid surface. These two differences can be described as follows: Wa′ − Wa = γl l + γl s − γgl − γgs = γl l + γl s − γl − γs 12

(1)

2

1

12

2

1

(4) E


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Energy & Fuels Table 1. Adhesion Work Parameters of Fluids fluid

γl1 (mN/m)

γl2 (mN/m)

θ (deg)

W′a − Wa (mN/m)

W″a − W′a (mN/m)

water AOT TX-100 AOT/N20 AOT/H18 TX-100/N20 TX-100/H18

20.5 20.5 20.5 20.5 20.5 20.5 20.5

72.8 30.7 38.2 26.0 28.8 30.6 33.2

110 38 45 0 0 0 0

56.6 −34.6 −29.9 −41 −41 −41 −41

−90.6 7.4 13.0 11.0 16.6 20.2 25.4

Figure 6. Adhesion force measurements between colloid probes and the silicon wafer surface or model oil surface: (a) 30 μm hydrophobic silica NP colloid probe, (b) 30 μm hydrophilic silica NP colloid probe, (c) 8 μm hydrophobic silica NP colloid probe, and (d) 8 μm hydrophilic silica NP colloid probe.

Wa″ − Wa′ = 2(γl s − γl s) 1

Wa″ − Wa′ = 2(γl cos θ − γl )

(5)

2

2

(6)

γl s = γs − γl

(7)

12

1

1

2

2

1

1

The interface tension of fluid and oil cannot be calculated by the Antonoff rule because the fluid does not wet the solid surface completely. However, according to Young’s equation, the interface tension of fluid/oil can be described as follows: γgs = γl s + γgl cos θ 2

(8)

2

γl s = γgs − γgl cos θ = γs − γl cos θ 2

2

2

(9)

Then, eqs 4 and 5 can be rewritten as follows according to eqs 6, 7, and 9: Wa′ − Wa = γl (1 − cos θ ) − 2γl 2

1

(11)

The surface tension of oil (γl1), surface tension of fluids (γl2), and contact angle of fluids on the solid surface (θ) can be measured experimentally. Then, (W′a − Wa) and (Waa″ − W′a) have been calculated and shown in Table 1. From Table 1, the following points have been concluded: (i) Water was difficult to displace the oil from the solid surface. (ii) The surfactant solution and nanofluids could displace the oil effectively. Nanofluids were more effective on the oil displacement than surfactant solutions. (iii) Nanofluids with hydrophobic NPs were more effective than those with hydrophilic NPs. 3.4. Direct Measurement of Adhesive Force. We have also used a more direct approach to evaluate the displacement ability of nanofluids and tried to further understand the mechanism of the better adhesion work on the solid surface of nanofluids. Wasan et al. regard that the structural disjoining pressure was the driving force of the nanofluid spreading.13,16,22−24 The structural disjoining pressure originates from the osmotic effect. If NPs are confined in a thin film, they tend to arrange themselves into well-ordered layers. The ordered arrangement increases the entropy of the overall

According to the Antonoff rule, the interface tension of the fluid−oil (γl1l2) and the interface tension of the oil−solid (γl1s) can be described as follows: γl l = |γl − γl | = γl − γl

1

(10) F


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Figure 7. Scheme of nanofluids spreading on the solid surface.

Table 2. Results of Model Oil Displacement Experiments core (mD)

water (%)

AOT (%)

AOT/N20 (%)

AOT/H18 (%)

TX-100 (%)

TX-100/N20 (%)

TX-100/H18 (%)

2600 550 220

58 57 55

65 66 69

80 65 57

92 68 55

66 65 67

76 64 45

88 69 49

core permeability. For the 550 mD core, the recovery ratio of AOT nanofluids was comparable to that of AOT solution. For the 220 mD core, the recovery ratio of AOT nanofluids was only similar to that of water. These results indicated that the silica NPs might partially aggregate and could not permeate into the core to displace the oil. From Table 2, we also found that the nanofluids with hydrophobic NPs had better effects on oil displacement than those with hydrophilic NPs. The TX-100 nanofluids had oil displacing results similar to AOT nanofluids. These results were corresponding to the adhesive force analysis in sections 3.3 and 3.4. In short, nanofluids with silica NPs could enhance the oil recovery of cores with high permeability, while for lower permeability cores, there is still work to do, performing worse than high-permeability cores.

suspension by permitting greater freedom for the NPs in the bulk liquid. The ordered arrangement results in an excess pressure in the film relative to the bulk dispersion. We have directly explored the adhesion force between NPs and the solid surfaces by AFM to understand the self-arrangement of NPs. According to the method in the experimental section, four different colloidal probes, 30 μm hydrophobic silica NP, 30 μm hydrophilic silica NP, 8 μm hydrophobic silica NP, and 8 μm hydrophilic silica NP, were prepared. We have measured the force curve between NP and model oil or the solid surface. As shown in Figure 6a, the adhesion force between 30 μm hydrophilic silica NP and the blank silicon wafer was roughly 55 nN, which was smaller than that between 30 μm hydrophilic silica NP and the model oil surface (88 nN). The adhesion forces between 30 μm hydrophobic silica NP and the blank silicon wafer (76 nN) and model oil surface were 76 and 36 nN, respectively (Figure 6b). Similar phenomena were also found with 8 μm hydrophilic and hydrophobic silica NPs (panels c and d of Figure 6). It was indicated that hydrophobic NPs were easily adsorbed on the wafer surface and hydrophilic NPs were easily adsorbed on the model oil surface. The possible mechanism of the nanofluid effect on oil displacement is shown as the cartoon in Figure 7. The NPs easily adsorbed on the solid surface would be arranged more orderly. Thus, the nanofluids with hydrophobic NPs would cause larger structural disjoining pressure. Because the hydrophilic NPs were apt to adsorb on the oil−fluid interface, a significant amount of NPs would be lost on the interface of the oil−fluid and less NPs participated in the film formation between oil and the solid surface. Moreover, NPs close to the oil−fluid interface were more chaotic than that those close to the solid surface, indicating a lower structural disjoining pressure. Therefore, it was concluded that the hydrophilic NPs had a worse separation effect on oil from the solid surface than hydrophobic NPs. 3.5. Core Flooding Experiments. We have assessed the oil displacement ability of these nanofluids with three cores of different permeability (Table 2). For the core of 2600 mD, pure water displaced ∼58% oil from the core and AOT solution displaced ∼65%. As discussed previously, the better spreading property would allow for nanofluids to displace more oil than surfactant solutions. The core flooding results showed that ∼80 and ∼92% oil have been recovered using AOT nanofluids with N20 and H18, respectively. However, the oil recovery ratio of nanofluids decreased with the decreasing of

4. CONCLUSION We have prepared a kind of stable SiO2 nanofluid with the help of surfactants and assessed the performance in EOR. The mechanism of nanofluids in oil displacement has been studied through contact angle measurement, adhesion force measurement, and adhesion work analysis. The core flooding results showed that anionic and non-ionic surfactant-dispersed silica nanofluids could enhance the recovery ratio more efficiently than surfactant solution. The contact angle of oil on the solid surface increased in hydrophobic nanofluids and made the oil film easily peel off. The direct adhesive force measurement and adhesive analysis showed that the hydrophobic NPs were apt to adsorb on the solid surface, while hydrophilic NPs were apt to adsorb on the model oil surface. The difference resulted in different structural disjoining pressure and, thus, relatively high displacement oil efficiency of nanofluids with hydrophobic NPs. We hope that this work would provide a better understanding of the complex phenomena of nanofluids spreading on solid and oil surfaces involved in EOR.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b02825. Contact angle of water on NP plates (Figure S1), FTIR measurements of modified particles (Figure S2), and turbidity of nanofluids (Table S1) (PDF) G


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Meiwen Cao: 0000-0002-3072-6780 Songyan Li: 0000-0001-9436-6705 Teng Lu: 0000-0001-5356-8941 Jiqian Wang: 0000-0002-7525-5943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Program on Key Basic Research Project (973 Program, Grant 2015CB250904), the National Science and Technology Major Project of China (Grant 2017ZX05009004-002), and the Fundamental Research Funds for the Central Universities (Project 18CX02127A).

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