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A spontaneous imbibition investigation of self-dispersing silica nanofluids for enhanced oil recovery in low-permeability cores Caili Dai, Xinke Wang, Yuyang Li, Wenjiao Lv, Chenwei Zou, Mingwei Gao, and Mingwei Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03244 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017
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A spontaneous imbibition investigation of self-dispersing silica nanofluids for enhanced oil recovery in low-permeability cores
4
Caili Dai*, Xinke Wang, Yuyang Li, Wenjiao Lv, Chenwei Zou, Mingwei Gao,
5
Mingwei Zhao*
6
School of Petroleum Engineering, State Key Laboratory of Heavy Oil Processing,
7
China University of Petroleum (East China), Qingdao 266580, P. R. China.
1 2
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ABSTRACT:
2
A new kind of self-dispersing silica nanoparticles were prepared and used to enhance
3
oil recovery in spontaneous imbibition tests of low-permeability cores. To avoid the
4
aggregation of silica nanoparticles, a new kind of silica nanoparticle was prepared
5
through
6
2-mercaptobenzimidazole as modified agents. Transmission electron microscopy
7
(TEM), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering
8
(DLS) and zeta potential measurements were employed to characterize the modified
9
silica nanoparticles. Dispersing experiments indicated that modified silica
10
nanoparticles had superior dispersity and stability in alkaline water. To evaluate the
11
performance of silica nanofluids for enhanced oil recovery, compared with pH 10
12
alkaline water and 5 wt% NaCl solution, spontaneous imbibition tests in sandstone
13
cores were conducted, respectively. The results indicated that silica nanofluids can
14
evidently improve oil recovery. To investigate the mechanism of nanoparticles for
15
enhanced oil recovery, the contact angle and interfacial tension were measured. The
16
results showed that the adsorption of silica nanoparticles can change the surface
17
wettability from oil-wet to water-wet and silica nanoparticles show a little influence
18
on oil/water interfacial tension. In addition, the change of oil droplet shape on the
19
hydrophobic surface was monitored through dynamic contact angle measurement. It
20
was shown that silica nanoparticles can gradually detach oil droplet from the
21
hydrophobic surface, which is consistent with the structural disjoining pressure
22
mechanism.
the
surface
modification
with
vinyltriethoxysilane
2
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(VTES)
and
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KEYWORDS: modified silica nanoparticles, benzimidazole, nanofluids, enhanced oil
2
recovery, structure disjoining pressure
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1. INTRODUCTION
2
At present, as most of conventional major oilfields have entered into the middle-late
3
period of the development, oil production becomes increasingly difficult. Future oil
4
supplies will be provided from enhanced oil recovery (EOR) technology and huge
5
unconventional oil resources. It’s inevitable for a transition from conventional oil
6
resources to unconventional oil resources, especially for low-permeability reservoirs.1
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Low-permeability reservoirs are always characterized by thin pore throat structure,
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lower porosity and permeability, which cause the difficulty of oil flowing in
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low-permeability matrix and low ultimate oil recovery.2-3 Recently, due to unique
10
combinations of thermal, mechanical, chemical and rheological properties at 1~100
11
nm length scale,4 the possible applications of nanomaterials in oil and gas industry
12
have become a matter of great concern.5-10 Nanofluids refer to traditional heat transfer
13
fluids, such as water, oil, and ethylene glycol, in which nanoparticles with average
14
size below 100 nm are suspending.11 Nanofluids might be a choice to enhance oil
15
recovery of low-permeability reservoirs as they can penetrate into the small pores of
16
reservoir rock and take active effects on the surface of reservoir rocks, injection water
17
and crude oil.12-13
18
Many nanoparticles, such as zirconium dioxide (ZrO2), titanium dioxide (TiO2),
19
aluminum oxide (Al2O3) and calcium carbonate (CaCO3), have been reported the
20
potential for enhanced oil recovery in free imbibition tests or coreflood
21
experiments.14-18 When it comes to silica nanoparticles, for the sake of excellent
22
dynamic and rheological properties, many researches have revealed the strong 4
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capability of silica nanoparticles to enhance oil recovery.19-24 Ju et al. studied the
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effect of lipophobic and hydrophilic polysilicon (LHP) nanoparticles on the
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wettability of porous media. They concluded that LHP nanoparticles can be adsorbed
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onto the surface of porous media and change the wettability of rock from oil-wet to
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water-wet.19-20 Onyekonwu et al. conducted coreflood experiments in water-wet
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sandstone cores to study the performance of three types of polysilicon nanoparticles,
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including hydrophobic and lipophilic polysilicon (HLP), LHP and naturally wet
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polysilicon (NWP). Results indicated that HLP and NWP dispersed in ethanol can
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improve oil recovery efficiency though changing rock wettability and reducing
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interfacial tension between oil and water.21 Hendraningrat et al. investigated the effect
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parameters of silica nanofluids, including initial rock wettability, temperature,
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nanofluids concentration, on the ultimate oil recovery. They found that silica
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nanoparticles can increase the displacement efficiency in all wettability states, and
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optimal oil recovery was reached in the intermediate-wet rock at reservoir
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temperature.22 Maghzi et al. and Hendraningrat et al. found that with the increase of
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silica nanoparticles concentration, ultimate oil recovery increased at the beginning
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and then declined when nanoparticles concentration was over a specific
18
concentration.23-24
19
However, because of high surface energy and plenty of hydroxyl groups on the
20
surface, silica nanoparticles can aggregate easily and are difficult to disperse in water.
21
In order to disperse silica nanoparticles in water, surfactants and ethanol were
22
generally used as dispersing agent and stabilizing agent.21,25,26 However, the addition 5
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of dispersing agents bring new problems. On the one hand, dispersing agents
2
continually adsorb onto the surface of porous media in the injection process and
3
nanoparticles
4
impairment.21,27 On the other hand, surfactants have an effect on the wettability of
5
rock surface and oil/water interfacial properties. Therefore, underlying mechanisms of
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nanofluids for enhanced oil recovery, including wettability alternation,28-30 interfacial
7
tension reduction,31-33 cannot be clearly identified whether surfactants or nanoparticles
8
worked in EOR process. Hence, it’s a challenging and valuable job for the preparation
9
of stable self-dispersing silica nanofluids to enhance oil recovery and to study the
10
precipitate,
which
easily
causes
porosity
and
permeability
underlying mechanisms.
11
In this study, a self-dispersing silica nanoparticle was prepared through surface
12
modification. Modified silica nanoparticles can be dispersed well in water by
13
adjusting the pH of nanofluids. In order to testify the modified nanoparticles, a series
14
of characterizations were conducted. The performance of silica nanofluids was
15
examined by spontaneous imbibition tests. Moreover, the potential mechanism of
16
silica nanofluids for enhanced oil recovery was studied and explained by interfacial
17
tension measurement, contact angle measurement and structure disjoining pressure
18
mechanism.
19
2. EXPERIMENTAL SECTION
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2.1 Materials
21
Silica nanoparticles were bought from Aladdin Reagent Co., LTd. China. Average
22
diameter of primary particles was about 7 nm and the specific surface area was 380 ± 6
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30 m2/g. Surface modification agents, including triethoxyvinylsilane (VTES),
2
2-mercaptobenzimidazole and 2,2-dimethoxy-2-phenylacetophenone (DMPA), were
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bought from Aladdin Reagent Co., LTd. China. HCl, NaOH, ethanol, dimethyl
4
formamide (DMF) were bought from Xilong Chemical Co., Ltd. The oil phase was
5
the mixture of dehydrated crude oil and kerosene with a volume ratio of 17:1. The
6
density of oil was measured as 0.804 g/cm3 and its dynamic viscosity was about 5
7
mPa·s at 25 °C. The synthetic brine (5 wt% NaCl solution) was used as reservoir
8
brine with a density of 1.025 g/cm3 and a dynamic viscosity of 0.90 mPa·s at 25 °C.
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The cores with gas permeability of 54 mD and porosity of 20% were bought from
10
Haian Oil Scientific Research Apparatus Co., Ltd. The detailed parameters of cores
11
were supplied in Supporting Information.
12
2.2 Synthesis of benzimidazole modified silica nanoparticles
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First, 2.0 g of silica nanoparticles (SNPs) were dispersed in 300 mL water by
14
ultrasonic vibration. The dispersion in a three-necked flask was stirred,
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heated and back-flowed in an oil bath at 100 °C for 0.5 hours. Second, pH was
16
adjusted to 1 with 1 mol/L HCl solution. After 0.5 hours, 1.5 g VTES was added into
17
the dispersion drop by drop. After 1.5 hours, pH was adjusted to 9 with 1 mol/L
18
NaOH solution. After stirring for 1 hour and cooling down to ambient temperature,
19
product was washed by water and ethanol three times, respectively. Washed product
20
was dispersed in 200 mL DMF. Then 0.30 g of 2-mercaptobenzimidazole and 0.01 g
21
of DMPA were added into the dispersion. After stirring under UV for 0.5 hours,
22
product
was
washed
by
ethanol
three
times.
Then,
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product
was
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dried in a vacuum oven at 70 °C for 24 hours and grinded to obtain benzimidazole
2
modified silica nanoparticles (BMSNPs). The reaction process of surface modification
3
of SNPs was shown in Figure 1.
4 5
Figure 1. Surface modification process of SNPs.
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2.3 Preparation of nanofluids
7
First, BMSNPs were added into synthetic brine to get 0.1 wt% dispersion. Then, pH
8
of dispersion was adjusted to 10 with 1 mol/L NaOH solution. After 2 hours’
9
ultrasonic vibration, the dispersion became clear and nanofluids were obtained.
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2.4 Characterization
11
Morphology of BMSNPs was examined by JEM-2100 transmission electron
12
microscope (TEM) of JEOL Ltd.. The effective diameter and zeta potential of
13
BMSNPs dispersed in aqueous solution was measured by NanoBrook Omni laser
14
particle size analyzer of Brookhaven Instruments Corporation. To determine the
15
functional groups of silica nanoparticles before and after modification, Fourier 8
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transform infrared spectroscopy (FTIR) spectra of samples were tested by the Nicolet
2
6700 Fourier transform infrared spectroscopy of American Thermo Nicolet Company.
3
The interfacial tensions between oil and BMNPs nanofluids, water, NaCl solution
4
were measured at 80 °C and 6000 r/min by TX-500C spinning drop interfacial tension
5
meter from Bowing Industry Corporation, USA. Cores surface morphology before
6
and after the adsorption of silica nanoparticles under scanning electron microscope
7
(SEM) were tested by S-4800 Field Emission Scanning Electron Microscope from
8
Hitachi, Ltd., Japan.
9
2.5 Spontaneous imbibition tests
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Relatively homogeneous cores were cut into columns with a length of 3.0 cm and a
11
diameter of 2.5 cm. These cores were dried in a vacuum oven at 90 °C for 24 hours.
12
After cooling down to the room temperature over a period of 4 hours, the weight of
13
dry cores was examined. Then, these cores (no initial water) were vacuumed for 4
14
hours to remove the gas and saturated with oil under the pressure of 15 MPa. After
15
aging for 48 hours, the weight of cores saturated with oil was examined. At the start
16
of imbibition tests, all the cores were immersed in synthetic brine at 80 °C for 0.5
17
hours, and part of oil was expelled because of the temperature change. To eliminate
18
the effect of temperature, this part of oil was not recorded in ultimate oil recovery.
19
Then, these cores were immersed in imbibition cells filled with liquid (0.1 wt% silica
20
nanofluids, pH 10 alkaline water and 5 wt% NaCl solution), respectively. The
21
experimental set-up for imbibition tests was supplied in Supporting Information.
22
These cells were placed in a thermostatic water bath at 80 °C. The volume of oil 9
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expelled from the cores (expressed as percentage of original oil in place -% of OOIP)
2
was monitored against time.
3
3. RESULTS AND DISCUSSION
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3.1 Characterization of modified silica nanoparticles.
5
The morphology of modified silica nanoparticles was measured by TEM. As shown
6
in Figure 2, BMSNPs have an approximate spherical morphology with an average
7
diameter of 20 nm. The FTIR spectra was obtained to identify the functional groups
8
on the surface of silica nanoparticles, as shown in Figure 3. From the FTIR spectra of
9
SNPs (a) and BMSNPs (b), the stretching vibration peak of Si-O-Si reaches near 1100
10
cm-1 and bending vibration peak of Si-O-Si reaches near 470 cm-1 for both samples. In
11
the FTIR spectra of BMSNPs, the stretching vibration peak of C-H at 2960 cm-1
12
confirms the existence of vinyl on the surface of silica after the reaction between
13
VTES and silica nanoparticles. The C=C skeleton vibration peak between 1600~1450
14
cm-1 confirms the existence of benzene. The appearance of new absorption peaks for
15
vinyl and benzene confirm that benzimidazolyl has been successfully grafted on the
16
surface of silica nanoparticles.
17 10
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Figure 2. TEM image of BMSNPs.
2 3
Figure 3. FTIR spectra of SNPs (a) and BMSNPs (b).
4
3.2 Dispersion of BMSNPs
5
The dispersion condition of BMSNPs were monitored by adjusting the pH of silica
6
nanofluids. As shown in Figure 4, the dispersion of silica nanoparticles was turbid in
7
acidic medium. Whereas, 0.1 wt% silica nanofluids gradually became clear and
8
transparent when pH increased to 10. The size distribution of BMSNPs in alkaline
9
medium (pH = 10) measured by dynamic light scattering (DLS) was shown in Figure
10
5. The results showed that the average size of BMSNPs was 57 nm with size
11
distribution of 20~110 nm.
12
In acidic medium, the zeta potential of BMSNPs nanofluids (pH = 1) was -0.70 mV,
13
which means BMSNPs dispersion was instable in acidic medium. The existence of H+
14
groups may compress the diffused double layer of nanoparticles which would cause
15
worse stability of silica nanofluids. While in alkaline medium, the zeta potential of
16
BMSNPsnanofluids (pH = 10) was -63.93 mV. OH- groups can adsorb on the surface
17
of benzimidazolyl and hydroxyl groups in alkaline environments. According to 11
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DLVO theory,34,35 the adsorption of OH- groups increased the electronegativity of
2
BMSNPs, which can raise the electrostatic repulsion between adjacent BMSNPs,
3
therefore, the stability of silica nanofluids increased.
4 5
Figure 4. Dispersion conditions of BMSNPs.
6 7
Figure 5. Particle size distribution of BMSNPs.
8
3.3 Spontaneous imbibition tests
9
In order to study the performance of silica nanofluids, the spontaneous imbibition
10
tests using 0.1 wt% silica nanofluids, pH 10 alkaline water and 5 wt% NaCl solution 12
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were conducted, respectively. The selection of silica nanofluids concentration was
2
supplied in Supporting Information. The curve of oil recovery changing in time was
3
shown in Figure 6. In comparison to alkaline water and NaCl solution, the amount of
4
oil expelled from the core immersed in BMNPs nanofluids was considerably higher.
5
The photo of cores immersed in BMNPs nanofluids, alkaline water, and NaCl solution
6
was supplied in Supporting Information. In the first 60 hours, oil recovery curve of
7
silica nanofluids increased rapidly, and about 25% of oil was recovered. After about 8
8
days, approximately 35% of oil was recovered by silica nanofluids. Whereas oil
9
recovery curve of alkaline water and NaCl solution increased slowly. After about 150
10
hours, only about 10% and 4% of oil was recovered in alkaline water and NaCl
11
solution, respectively. After about 12 days, approximately 38% of oil was recovered
12
in silica nanofluids, whereas 12% and 6% of oil was expelled in alkaline water and
13
NaCl solution, respectively. By contrast, silica nanofluids reached higher
14
displacement efficiency and ultimate oil recovery than alkaline water and NaCl
15
solution.
16 17
Figure 6. Oil recovery versus time in spontaneous imbibition tests. 13
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3.4 Mechanism of BMSNPs for enhanced oil recovery
2
To investigate the underlying mechanism of BMSNPs for enhanced oil recovery in
3
imbibition tests, the effect of BMSNPs nanofluids on the wettability of glass slide was
4
studied. Wettability was estimated by measuring the contact angle of oil/water/glass
5
slide system. All glass slides were immersed in paraffin at 90 °C for 24 hours. Before
6
the measurement, glass slides were immersed in 0.1wt% BMNPs nanofluids, pH 10
7
alkaline water and 5 wt% NaCl solution for 24 hours, respectively. Figure 7 showed
8
the results of contact angle of oil/water/glass slide system. The surface of glass slide
9
was hydrophobic due to the adsorption of paraffin, the contact angle of oil was about
10
42°, as shown in Figure 7(a). In Figure 7(b), the contact angle of oil on the surface of
11
glass slide immersed by BMSNPs nanofluids increased to 122°. Whereas, in Figure
12
7(c) and Figure 7(d), the contact angle of oil on the surface of glass slides immersed
13
by NaCl solution and water were about 45° and 47°, respectively. By comparison, the
14
adsorption of BMSNPs can obviously alter the wettability of hydrophobic surface
15
from oil-wet to water-wet. Cores surface morphology before and after the adsorption
16
of 0.1 wt% BMNPs nanofluids under SEM were shown in Figure 8. Results showed
17
that BMNPs can form a uniform adsorption layer on the surface of core. Combined
18
with the results of contact angle measurements, the adsorption of BMNPs can cause
19
the wettability alternation of core surface from oil-wet to water-wet.
14
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Figure 7. The contact angle of oil droplets on the surface of hydrophobic glass slide,
3
which was treated by deionized water (a), 0.1wt% BMNPs nanofluids (b), 5wt% NaCl
4
solution (c), and pH 10 alkaline water (d)
5 6
Figure 8. SEM images of bare core surface (a) and core surface with BMNPs (b).
7
To investigate the impact of silica nanoparticles on interfacial property of oil and
8
water, the interfacial tensions between oil and different fluids were tested. As shown 15
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in Table 1, the interfacial tension of oil/BMNPs nanofluids was roughly the same as
2
that of oil/ water and oil/NaCl solution. Therefore, BMNPs did not have an obvious
3
influence on oil/water interfacial tension.
4
Table 1. Interfacial tension of oil/fluids Fluids
Interfacial tension (mN/m)
0.1wt% BMNPs nanofluids
23.8
5wt% NaCl solution
24.5
pH 10 alkaline water
23.8
5
The interfacial tension of oil and nanofluids was roughly the same as that of oil and
6
synthetic brine. However, due to the absorption of silica nanoparticles on the surface
7
of glass slide, the contact angle of oil/glass slide/nanofluids changed from 42° to
8
122°. According to the adhesion work equation (1), adhesion work between crude oil
9
and rock surface decreased due to the wettability alternation of rock surface, which
10
means crude oil can be more easily detached from the rock surface and higher oil
11
recovery can be reached.
12
W = σ(1 + cos ߠ)
(1)
13
Where W is the adhesion work between oil and rock surface, mJ/m2; σ is the
14
interfacial tension between crude oil and nanofluids, mN/m; ߠ is the contact angle of
15
oil/rock surfaces/nanofluids, degree.
16
The change of oil droplet shape on the hydrophobic surface was monitored through
17
dynamic contact angle measurement of oil/glass/BMNPs nanofluids. The oil droplet
18
was released and captured under the surface of glass slide immersed in synthetic
19
brine. Three-phase contact angle of oil/glass slide/synthetic brine was measured. 16
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Then, the prepared BMNPs nanofluids was poured into synthetic brine and the contact
2
angle of crude oil/glass slide/silica nanofluids was measured versus time. Figure 9
3
represented the change of contact angle versus time. Initially, contact angle of oil on
4
hydrophobic surface of glass slide was about 42°. After the injection of BMNPs
5
nanofluids, the contact angle gradually increased with time. In the initial stage,
6
contact angle changed dramatically from 42° to 96°. After about 12 hours, contact
7
angle was approximately 128°. Whereas, the contact angle in NaCl solution and water
8
did not change obviously.
9 10
Figure 9. Dynamic contact angle in nanofluids, alkaline water, and NaCl solution.
11
The shape change of oil droplet in dynamic contact angle measurements is
12
consistent with the structural disjoining pressure mechanism presented by Darsh
13
Wasan et al.36-39 As shown in Figure 10, silica nanoparticles tend to form a wedge
14
film in the three-phase contact region of oil droplet, SiO2 nanofluids and rock pore
15
surface. A force, which is called structural disjoining pressure, pointing to vertex of
16
the wedge film is exerted as a result of Brownian motion and electrostatic repulsion
17
between silica nanoparticles. Structural disjoining pressure has exponent relation to 17
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the thickness of wedge film that the force increases sharply with the decrease of the
2
thickness of wedge film. The vertex of wedge film keeps moving forward when
3
structural disjoining pressure is stronger than the adsorption force between oil droplet
4
and rock pore surface. Ultimately, oil droplet is detached from the surface of rock
5
pore and is expelled by capillary force. Thus, crude oil is displaced and oil recovery is
6
enhanced.
7 8
9
Figure 10. Structural disjoining pressure mechanism.
4. CONCLUSION
10
In this study, a new self-dispersing silica nanoparticles was prepared by surface
11
modification and its performance was evaluated by imbibition tests. Silica
12
nanoparticles were modified by VTES and 2-mercaptobenzimidazole. By comparing
13
the performance of 0.1wt% BMNPs nanofluids, pH 10 alkaline water and 5wt% NaCl
14
solution in spontaneous imbibition tests, ultimate oil recovery can be enhanced by
15
BMNPs nanofluids. Approximately 38% of oil was recovered from sandstone core by
16
0.1wt% BMNPs nanofluids after about 10 days. The underlying mechanism of
17
BMSNPs for enhanced oil recovery was investigated. BMNPs did not have an
18
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obvious impact on oil/water interfacial tension. BMNPs can change the wettability of
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core surface from oil-wet to water-wet. Moreover, BMNPs can detach the oil droplet
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from a hydrophobic surface, which is consistent with the structural disjoining pressure
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mechanism. We hope this work can be helpful to the applications of nanofluids,
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especially in tight and shale reservoirs.
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Supporting Information.
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Detailed parameters of cores, experimental set-up for imbibition tests, selection of
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silica nanofluids concentration and photo of cores in imbibition tests were supplied as
9
Supporting Information.
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AUTHOR INFORMATION
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Corresponding Author
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*E-mail:
[email protected],
[email protected]; Tel: +86-532-86981183.
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ACKNOWLEDGMENT
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The work was supported by the National Key Basic Research Program (No.
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2015CB250904), the National Science Fund (U1663206), the National Science Fund
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for Distinguished Young Scholars (51425406), the Chang Jiang Scholars Program
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(T2014152), the Fundamental Research Funds for the Central Universities
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(15CX08003A).
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