A Novel Nanofluid Based on Fluorescent Carbon Nanoparticles for

Oct 16, 2017 - A novel nanofluid based on fluorescent carbon nanoparticles for enhanced oil recovery (EOR) was developed. Fluorescent carbon nanoparti...
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A novel nanofluid based on fluorescent carbon nanoparticles for enhanced oil recovery Yuyang Li, Caili Dai, Hongda Zhou, Xinke Wang, Wenjiao Lv, Yining Wu, and Mingwei Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03617 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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A novel nanofluid based on fluorescent carbon

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nanoparticles for enhanced oil recovery

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Yuyang Li, Caili Dai,* Hongda Zhou, Xinke Wang, Wenjiao Lv, Yining Wu and Mingwei Zhao*

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School of Petroleum Engineering, State Key Laboratory of Heavy Oil, China University of

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Petroleum (East China), Qingdao, Shandong 266580, China

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Email: [email protected] (Caili Dai), [email protected] (Mingwei Zhao)

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Tel: (86) 532-86981183, Fax: (86) 532-86981161

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ABSTRACT: A novel nanofluid based on fluorescent carbon nanoparticles for enhanced oil

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recovery (EOR) was developed. Fluorescent carbon nanoparticles prepared by a simple and rapid

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method were used as a chemical agent for EOR and fluorescence imaging. Transmission electron

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microscope (TEM) and Fourier transform infrared spectrometer (FTIR) were employed to

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observe the shape, size and surface components of the fluorescent carbon nanoparticles. The

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fluorescent carbon nanoparticles could be instantly dispersed in water without any auxiliary

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equipment. The nanofluid showed excellent anti-temperature, anti-salinity, oil displacement and

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wettability alteration properties. The nanofluid (0.1 wt%) could reduce the oil-water interfacial

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tension to 13.4 mN/m. The oil recovery of a core immersed in nanofluid was significantly

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improved. The core intersection was observed by a fluorescence microscope. The fluorescence

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image demonstrated that the fluorescent carbon nanoparticles had seeped into the core. The

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fluorescent carbon nanoparticle-based nanofluid provides a promising and efficient chemical

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agent for EOR.

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KEYWORDS: Fluorescent carbon nanoparticles; Nanofluid; Wettability alteration; Structural

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disjoining pressure; Fluorescence image; Enhanced oil recovery

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1. Introduction

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The world is facing serious challenges to meet our energy needs because the available

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conventional energy is becoming increasingly difficult to extract. As tight reservoirs and ultra-

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low permeable reservoirs are constantly being discovered, the crude oil reserves in tight

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reservoirs and ultra-low permeable reservoirs are taking up an ever-greater proportion in the

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global crude oil reserves.1-5 However, tight reservoirs and ultra-low permeable reservoirs, have

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low porosity and low permeability. The pore-throat size of an ultra-low permeable reservoir is

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mainly distributed on the sub-micron and micrometre scale. Primary and secondary oil

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recoveries in ultra-low permeable reservoirs are low. It is thus a great challenge to enhance the

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oil recovery in an ultra-low permeable reservoir.

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Due to the nanoscale and large surface/volume ratio, nanoparticles have a high surface energy,

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which results in their adsorption on a solid surface or at a water/oil interface. The surface

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wettability or interfacial tension is affected because the surface or interface energy of the system

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is changed by nanoparticles.6-9 In the field of oilfield development, using nanomaterials as a new

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cost-effective material for high-temperature, high-salinity, and low permeable reservoirs has

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attracted increasing attention due to their promising applications in enhanced oil recovery

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(EOR).10-15 Some nanoparticles, such as silicon dioxide, titanium oxide and aluminium oxide,

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have been reported to have potential applications in EOR.16-19

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However, nanoparticles aggregate easily and are unstable in harsh environments. To disperse

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nanoparticles in water, surfactants have generally been used as a dispersing agent.20 There is

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competitive adsorption between the surfactant and nanoparticles at a liquid-liquid or solid-liquid

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interface.21 The potential mechanisms of using nanofluids (suspensions of nanometre-sized

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particles) for EOR cannot clearly confirm whether surfactants or nanoparticles are working in the

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enhanced oil recovery process. In addition, an important issue is whether the nanofluid can seep

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into the ultra-low permeable core. Hence, it is necessary to prepare a stable self-dispersing

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nanoparticle for EOR and to develop an easy-to-use method that can prove that nanofluids can

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seep into the pores of ultra-low permeable core.

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Wasan et al. investigated oil displacement from a solid surface using a nanofluid. Based on

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their experiment results and theoretical calculations, they proposed the novel concept of a

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nanoparticle structural wedge film in the confines of a solid-oil-aqueous phase contact region.

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The structural disjoining pressure in the wedge film is enhanced by the nanoparticles’ structural

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wedge film. As nanoparticles come closer to the tip of the wedge film, the structural disjoining

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pressure increases. With the increase in the structural disjoining pressure, the oil/aqueous

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interface moves onward, and the nanofluid spreads over the solid surface, causing the oil to

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detach from the solid surface.7, 22-26

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In this work, we present a novel nanofluid based on fluorescent carbon nanoparticles for EOR

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of ultra-low permeable core. The fluorescent carbon nanoparticles can be instantly dispersed in

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water without any auxiliary equipment. Based on its features of excellent anti-temperature and

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anti-salinity properties, a stable nanofluid for EOR of an ultra-low permeable core was prepared.

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This nanofluid exhibited an excellent capability to make oil displace from a solid surface and

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changed the surface wettability from oil-wet to water-wet. A larger oil recovery was achieved by

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using the nanofluid as a highly efficient chemical agent for EOR. To prove that the fluorescent

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carbon nanoparticles can seep into an ultra-low permeable core, fluorescence imaging was used

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to observe the core intersection. The proposed nanofluid can be used as an effective chemical

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agent for EOR in ultra-low permeable reservoirs. This strategy shows potential application value

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for tracing and locating nanoparticles in an ultra-low permeable core.

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2. Experimental section

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2.1. Materials and apparatus

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Glacial acetic acid (CH3COOH), diphosphorus pentoxide (P2O5) and sodium hydroxide (NaOH)

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were purchased from Xilong Chemical Co., Ltd. China. Crude oil was obtained from Xinjiang

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Oilfield. The oil phase used in this study was a mixture of dehydrated crude oil and kerosene

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with a volume ratio of 1:19. The density and dynamic viscosity of oil were 0.801 g/cm3 and 1

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mPa·s at 25 °C, respectively. NaCl solution (3 wt%) was used as reservoir brine with a density of

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1.021 g/cm3 and a dynamic viscosity of 0.91 mPa·s at 25 °C. Ultra-low permeable sandstone

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cores (length 2.5 cm and diameter 2.5 cm) with a gas permeability of 0.6 mD and a porosity of

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14% were purchased from Haian Oil Scientific Research Apparatus Company.

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Transmission electron microscope (TEM) images were obtained using a FEI-Tecnai-G20

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microscope (USA). Infrared spectroscopy was conducted using a Fourier transform infrared

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spectrometer (FTIR, NEXUS, Thermo Nicolet, USA). Zeta potential and dynamic light

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scattering (DLS) measurements were carried out using a laser particle size analyser (Zetasizer

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Nano ZSP, Malvern, England). Interfacial tension was measured by a spinning drop interfacial

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tensiometer (TX500C, Kono, USA). The contact angle measurement was carried out using a

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contact angle measuring system (Tracker, Teclis, France). The transmittance of the nanofluid

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was measured by a UV-vis spectrophotometer (UV-5, Mettler Toledo, USA). Microscopic

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fluorescence imaging was observed by a self-constructed fluorescence microscope which was

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composed of a microscope (XSP-35TV, CIWA, China) and a spot UV lamp (UV-L03, Tank,

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China). The pore size distribution of the ultra-low permeable core was measured by a mercury

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intrusion porosimeter (AutoPore IV 9500, Micromeritics, USA).

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2.2. Preparation of fluorescent carbon nanoparticles

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Fluorescent carbon nanoparticles were synthesized in large quantity according to Wang’s

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reported method.27 Fluorescent carbon nanoparticles fabrication process is shown in Figure 1.

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Approximately 25 g of P2O5 was placed into a 500 mL beaker. Glacial acetic acid (10 mL) and

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water (0.4 mL) were added to P2O5. A large amount of heat was released, and the temperature

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increased rapidly during the reaction. Glacial acetic acid was carbonized under high temperature

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and carbon nanoparticles were formed. After cooling down to room temperature, 300 mL of

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water was added to the reaction mixture, and the dark brown solid was separated from the

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mixture by centrifugation. The dark brown solid was washed with water three times. After

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purification, the obtained dark brown solid was dried at 110 °C for 48 hours. Finally, 5.2 g of

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fluorescent carbon nanoparticles was obtained.

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Figure 1 Schematic illustration of the fluorescent carbon nanoparticles fabrication process.

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2.3. Preparation of nanofluid

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Approximately 1.0 g of fluorescent carbon nanoparticles was added to 1.0 L of brine. When the

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pH of the mixture was adjusted to 8 by 1 mol/L NaOH solution, a dark brown nanofluid was

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instantly obtained, appearing similar to instant coffee, without any auxiliary equipment. The

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preparation process of fluorescent carbon nanoparticles aqueous solution was shown in Figure

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1S.

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2.4. Spontaneous imbibition tests

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Cores were dried at 120 °C for 48 hours. After cooling down to room temperature, the weight of

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the dry cores was measured by an electronic analytical balance. Then, these cores were

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vacuumed to remove the gas in the cores. After 5 hours, these cores were saturated with oil under

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a pressure of 15 MPa for 48 hours. Cores that had been saturated with oil were removed from the

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oil. To avoid the effect of oil expansion at a high temperature, the cores were immersed in oil at

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60 °C for 24 hours. Then, the cores were removed from the oil, and their weight was measured

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quickly. In different imbibition devices, these cores were immersed in brine or different

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concentrations of nanofluids. These imbibition devices were placed in a constant temperature

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bath at 60 °C. The volumes of oil discharged from these cores with time were recorded.

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3. Results and discussion

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3.1. Characterization of fluorescent carbon nanoparticles

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The structure and morphology of fluorescent carbon nanoparticles were observed through TEM

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(Figure 2). The fluorescent carbon nanoparticles were generally of spherical shape, and their size

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was approximately 10 nm, which would be beneficial to flowing in a low permeable porous

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medium.

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Figure 2 TEM image of fluorescent carbon nanoparticles.

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Surface chemistry is key to the properties of fluorescent carbon nanoparticles. The fluorescent

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carbon nanoparticles were analysed by FT-IR (Figure 3). Peaks at 2921 cm-1 and 2970 cm-1 were

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ascribed to the group of -C-H. A peak at 1616 cm-1 was presented due to the conjugated C=C

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stretching vibration. An absorption peak at 3426 cm-1 was ascribed to the group of -OH. An

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absorption peak at 1660 cm-1 was due to the C=O group conjugated with condensed aromatic

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carbons. The peaks at 3426 cm-1 and 1660 cm-1 indicated that the fluorescent carbon

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nanoparticles had a large content of carboxylic groups, which gave excellent water dispersibility

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properties to the fluorescent carbon nanoparticles.

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Figure 3 FT-IR of fluorescent carbon nanoparticles.

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3.2. Dispersion of fluorescent carbon nanoparticles

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The fluorescent carbon nanoparticles aqueous solution (0.005 wt%) was light yellow and

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transparent in daylight (Figure 4a), but it changed to bright green under UV excitation (365 nm)

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(Figure 4b). DLS was employed to characterize the size distribution of fluorescent carbon

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nanoparticles in water. As shown in Figure 4, the distribution of particle size was narrow,

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between 10 and 30 nm. The Zeta potential of fluorescent carbon nanoparticles in water was -35

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mV, which improved the electrostatic repulsion among fluorescent carbon nanoparticles. The

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results indicated that the obtained fluorescent carbon nanoparticles were well dispersed in water.

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Figure 4 Fluorescent carbon nanoparticles in water (0.005 wt%) (left) under daylight (a) and UV light (365 nm,

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b). Size distribution of fluorescent carbon nanoparticles in water (right).

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3.3. Performance of nanofluid

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Low oil-water interfacial tension is conducive to enhance oil recovery. The oil-water interfacial

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tensions of nanofluids with different concentrations were measured (Table 1). The oil-water

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interfacial tension decreased with an increasing concentration of nanofluid. The nanofluid (0.1

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wt%) could reduce the oil-water interfacial tension to 13.4 mN/m.

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Table 1 Oil-water interfacial tension of different concentrations of nanofluid at 60 °C. Concentration of nanofluid (w/w)

0.1 wt%

0.05 wt%

0.01 wt%

0.005 wt%

0.001 wt%

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Oil-water interfacial tension (mN/m)

13.4

16.4

21.2

22.6

22.9

26.2

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Figure 5 shows the dynamic contact angle of oil on the oil-wet surface in different

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concentrations of nanofluid (0.1 wt%, 0.05 wt%, 0.01 wt%, 0.005 wt%, 0.001 wt%) and brine.

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To obtain an oil-wet surface, glass slides as a model of sandstone surface were immersed in

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paraffin at 80 °C for 72 hours. The oil droplet was captured on the oil-wet surface of the glass

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slide immersed in brine. The dynamic contact angles of oil on the oil-wet surface in different

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liquid phases were measured by contact angle measurement. Different concentrations of

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nanofluid were obtained by adding nanofluid at a high concentration into the brine. In the initial

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stage, the contact angle of oil on the surface in the nanofluid increased rapidly, and oil was

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displaced from the surface gradually. With a decrease in the nanofluid concentration, the change

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rate of the contact angle decreased. After the rapid change, the contact angle increased gently. In

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the end, the contact angle almost did not change, and the change of the contact angle increased

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with an increasing concentration of nanofluid. The contact angle of oil on the oil-wet surface

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changed from 36° to 120° at the highest concentration of nanofluid and changed merely from 36°

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to 38° at the lowest concentration of nanofluid. Compare with the nanofluid, the contact angle of

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oil on the oil-wet surface in brine barely changed. According to the results, the fluorescent

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carbon nanoparticles exhibited an excellent capability that made oil displace from an oil-wet

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surface.

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Figure 5 Dynamic contact angles of oil on the oil-wet surface in different concentrations of nanofluid (a-0.1

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wt%, b-0.05 wt%, c-0.01 wt%, d-0.005 wt%, e-0.001 wt%) and brine (f).

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Wettability alteration is an extremely important influence factor for EOR.28-30 The capillary

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driving force for the spontaneous imbibition process is strong in a water-wet core. Wettability

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alteration can enhance the spontaneous imbibition of water into the core. The ability of

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wettability alteration was estimated by measuring the contact angle of oil/water/solid. Oil-wet

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glass slides were immersed in different liquid phases at 60 °C for 24 hours. The surface of the

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glass slide was oil-wet due to the adsorption of paraffin on the surface (Figure 6a). After the

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surface of the glass slide was treated with the 0.1 wt% nanofluid, the wettability of the glass slide

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was altered to a water-wet state (Figure 6b). The good wettability alteration of fluorescent carbon

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nanoparticles gave the essential foundation for the future application in spontaneous imbibition.

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Figure 6 Oil droplets on oil-wet glass slides treated by brine (a) and nanofluid (b).

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Due to the high temperature of reservoir, temperature resistance is an important indicator for

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an oil recovery chemical agent. As temperature increases, the movement of nanoparticles

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becomes increasingly severe, which can cause the agglomeration of nanoparticles. A high

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temperature may produce a negative effect on the stability of nanofluid. To evaluate the stability

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of the nanofluid, it was stored under different temperatures for 1 day, and the transmittance of

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the nanofluid at different temperatures was measured by a UV-vis spectrophotometer. As shown

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in Figure 7, the transmittance of the nanofluid exhibited basically no change with an increasing

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temperature. To further explore the stability of the nanofluid at high temperature, it was stored at

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90 °C. After 30 days, the transmittance of nanofluid basically had no change. According to the

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results, the nanofluid had good temperature resistance.

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Figure 7 Effect of temperature on the stability of nanofluid. Error bar=RSD (n=5)

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Salt tolerance is an important indicator for an oil recovery chemical agent. The surface charge

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density of nanoparticles can be affected by cations and anions. As the surface charge density

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changes, the interactions among nanoparticles may be changed, which can cause the

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agglomeration of nanoparticles. To evaluate the salt tolerance of the nanofluid, different amounts

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of NaCl were added to the nanofluid, and its transmittance with different concentrations of NaCl 12 ACS Paragon Plus Environment

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was measured by a UV-vis spectrophotometer. As shown in Figure 8, the transmittance of the

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nanofluid exhibited basically no change with an increase in the NaCl concentration. The results

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showed that salinity did not have much impact on the stability of the nanofluid.

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Figure 8 Effect of salinity on the stability of nanofluid. Error bar=RSD (n=5)

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3.4. Spontaneous imbibition tests

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In different imbibition devices, cores were immersed in different liquid phases. These imbibition

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devices were placed in a constant temperature bath at 60 °C for verifying the EOR ability of the

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nanofluids. The imbibition device was shown in Figure 2S. The volumes of oil discharged from

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these cores with time were recorded. The results of spontaneous imbibition tests using different

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concentrations of nanofluids and brine are shown in Figure 9. For the first 80 hours, the oil

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recovery of the core immersed in nanofluid increased with an increasing nanofluid concentration.

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After 80 hours, the oil recovery of 0.1 wt% nanofluid was below that of 0.05 wt% nanofluid, and

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the oil recovery of 0.05 wt% nanofluid (39.1%) was slightly higher than that of 0.1 wt%

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nanofluid (38.3%) in the end. The reason for this may be that some pores in the ultra-low

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permeable core were partially blocked by a high concentration of fluorescent carbon

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nanoparticles. Starting from a 0.05 wt% nanofluid, the oil recovery of nanofluid decreased with a

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decrease in the nanofluid concentration. The oil recovery of the core immersed in nanofluid was

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significantly higher than that of core immersed in brine. After the spontaneous imbibition tests,

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38.3%, 39.1%, 37.3%, 34.4%, 24.4% and 16.8% of oil were extracted in cores immersed in

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different concentrations of nanofluid, 0.1 wt%, 0.05 wt%, 0.01 wt%, 0.005 wt% and 0.001 wt%,

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and brine, respectively. The results from our work showed that the obtained nanofluid had a

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great potential for EOR.

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Figure 9 Oil recovery with time in spontaneous imbibition tests.

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3.5. Fluorescence imaging in porous media

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The pore size distribution of an ultra-low permeable core is shown in Figure 3S. The pore sizes

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were mainly distributed in the range of 0.4 µm to 3 µm. The fluorescent carbon nanoparticles

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with a size of 10 nm should be able to seep into the ultra-low permeable core. To prove that

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fluorescent carbon nanoparticles can seep into the ultra-low permeable core, cores were cut, and

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the core intersection was observed by a self-constructed fluorescence microscope. As shown in

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the bright field and fluorescence images of the core intersection (Figure 10), the pore wall 14 ACS Paragon Plus Environment

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showed light green or yellow photoluminescence when fluorescent carbon nanoparticles were

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excited at 365 nm. The fluorescence image demonstrated that fluorescent carbon nanoparticles

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had seeped into the ultra-low permeable core in the process of spontaneous imbibition.

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Figure 10 Bright field image (left) and fluorescence image (right) of core intersection (excited at 365 nm,

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scale bar = 10 µm)

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3.6. Mechanism of fluorescent carbon nanoparticles for EOR

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Comparing with surfactant, the nanofluid did not reduce the oil-water interfacial tension to 10-2

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mN/m or lower. The effect of this nanofluid on the oil-water interfacial tension is not a major

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factor for EOR. The result of the spontaneous imbibition tests was consistent with the oil

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displacement from a solid surface and wettability alteration. According to the formula of

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capillarity (Pc=2σcosθ/r), capillary force increases with a decrease in pore diameter. In the

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process of spontaneous imbibition, nanofluid seeped into the ultra-low permeable core through

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small pores, and oil was expelled from the core through large pores. According to the above

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experiment of oil displacement from oil-wet solid surface by nanofluid, the results were in

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accordance with the structural disjoining pressure mechanism reported by Darsh Wasan.7, 22-26

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The fluorescent carbon nanoparticles in the oil/nanofluid/solid three-phase contact region tended

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to form a wedge film. Due to Brownian motion and electrostatic repulsion among nanoparticles,

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a structural disjoining pressure, which was conducive for displacing oil from the solid surface,

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formed in the wedge film. Oil was displaced from the pore channel surface, and the wettability of

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the pore channel surface changed from oil-wet to water-wet. The spontaneous imbibition process

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is the most efficient in a water-wet core where the capillary driving force is strong. The recovery

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efficiency can be improved by the nanofluid, which alters the wettability of the reservoir rock to

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a water-wet state. The spontaneous imbibition mechanism of nanofluid includes three main

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aspects: capillary force, structural disjoining pressure, and wettability alteration. The

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spontaneous imbibition mechanism of nanofluid is shown in Figure 11.

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Figure 11 Spontaneous imbibition mechanism of nanofluid.

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4. Conclusions

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In this work, we have developed a novel nanofluid based on fluorescent carbon nanoparticles

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for the EOR of an ultra-low permeable core. The highlights of the developed nanofluid are as

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follows: (1) fluorescent carbon nanoparticles can be instantly dispersed in water without any

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auxiliary equipment; (2) the nanofluid showed excellent anti-temperature, anti-salinity, oil

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displacement and wettability alteration properties; and (3) carbon nanoparticles with a good 16 ACS Paragon Plus Environment

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fluorescence property were used to prove the existence of fluorescent carbon nanoparticles in the

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pores of an ultra-low permeable core using fluorescence imaging technology. In an operating

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site, the nanofluid based on fluorescent carbon nanoparticles could be obtained easily because

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fluorescent carbon nanoparticles can be instantly dispersed in water without any auxiliary

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equipment. The proposed nanofluid showed excellent performance for EOR of ultra-low

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permeable core. The fluorescence image demonstrated that the fluorescent carbon nanoparticles

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had seeped into the ultra-low permeable core in the process of spontaneous imbibition. The

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spontaneous imbibition mechanism of nanofluid includes three main aspects: capillary force,

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structural disjoining pressure, and wettability alteration. Thus, the fluorescent carbon

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nanoparticle-based nanofluid provides a promising and efficient chemical agent for EOR of an

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ultra-low permeable reservoir.

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Supporting Information

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Photographs in the preparation process of fluorescent carbon nanoparticles aqueous solution,

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photograph of imbibition device, and pore size distribution of the core.

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript. The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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

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2015CB250904), the National Science Fund (U1663206, 51425406), the Chang Jiang Scholars

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Program (T2014152), Climb Taishan Scholar Program in Shandong Province (tspd20161004),

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and the Fundamental Research Funds for the Central Universities (15CX06028A).

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