Spontaneous Imbibition Investigation of Self-Dispersing Silica

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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

1

ACS Paragon Plus Environment

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ABSTRACT:

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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


(VTES)

and

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KEYWORDS: modified silica nanoparticles, benzimidazole, nanofluids, enhanced oil

2

recovery, structure disjoining pressure

3


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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

7

Low-permeability reservoirs are always characterized by thin pore throat structure,

8

lower porosity and permeability, which cause the difficulty of oil flowing in

9

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

2

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

4

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

6

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

9

improve oil recovery efficiency though changing rock wettability and reducing

10

interfacial tension between oil and water.21 Hendraningrat et al. investigated the effect

11

parameters of silica nanofluids, including initial rock wettability, temperature,

12

nanofluids concentration, on the ultimate oil recovery. They found that silica

13

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

15

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

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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

6

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

3

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

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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

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adjusted to 1 with 1 mol/L HCl solution. After 0.5 hours, 1.5 g VTES was added into

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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,

7


the

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

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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

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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

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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.

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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

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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

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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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1

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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1

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

2

core surface from oil-wet to water-wet. Moreover, BMNPs can detach the oil droplet

3

from a hydrophobic surface, which is consistent with the structural disjoining pressure

4

mechanism. We hope this work can be helpful to the applications of nanofluids,

5

especially in tight and shale reservoirs.

6

Supporting Information.

7

Detailed parameters of cores, experimental set-up for imbibition tests, selection of

8

silica nanofluids concentration and photo of cores in imbibition tests were supplied as

9

Supporting Information.

10

AUTHOR INFORMATION

11

Corresponding Author

12

*E-mail: [email protected], [email protected]; Tel: +86-532-86981183.

13

ACKNOWLEDGMENT

14

The work was supported by the National Key Basic Research Program (No.

15

2015CB250904), the National Science Fund (U1663206), the National Science Fund

16

for Distinguished Young Scholars (51425406), the Chang Jiang Scholars Program

17

(T2014152), the Fundamental Research Funds for the Central Universities

18

(15CX08003A).

19

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