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The Potential of a Novel Nano-fluid in Enhancing Oil Recovery Bing Wei, Qinzhi Li, Fa-yang Jin, Hao Li, and Chongyang Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00244 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 25, 2016

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The Potential of a Novel Nano-fluid in Enhancing Oil Recovery Bing Wei, Qinzhi Li, Fayang Jin, Hao Li, Chongyang Wang State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, Sichuan, China

ABSTRACT: A surface-active and "green" flooding agent, modified nano-cellulose (NC), which is expected to be an alternative to the current flooding systems for enhancing oil recovery (EOR), was provided in this work. The physical properties of the NC samples including dispersity, rheology, phase behavior, emulsifiability, etc., as a function of mass fraction and charge density, were comprehensively studied to evaluate its EOR potential. The results indicate that this modified nano-material could be well dispersed in 1wt% NaCl brine, forming a series of homogenous nano-fluids at the concentration above 0.4wt%. Rheological analysis evidenced the viscoelastic properties and pronounced shear-thinning behavior of the nano-fluids. Due to the presence of the active groups, the dynamic interfacial tension (Oil/Nano-fluid) decreased to an order of 10-1mN/m, which accordingly promotes the microscopic recovery efficiency through emulsification effect. It was also observed that the emulsifiability of the nano-fluids was closely related to the charge density. Visual EOR experiments were conducted in a micromodel, from which two mechanisms 1) sweep volume improvement and 2) emulsification and entrainment, were established for NC nano-fluid flooding. As an eco-friendly material, this nano-fluid is supposed to be a promising flooding agent in the near future. KEYWORDS: modified nano-cellulose; nano-fluid; enhancing oil recovery; phase behavior; micro-flow; recovery mechanism. 1

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1. INTRODUCTION For most of the oilfields in the world, at least half of the reserved oil still leaves behind in the formations after the primary (natural flow or artificial lift) and secondary recovery (i.e. waterflooding) methods are exhausted.1 It has been known that the overall oil recovery is generally governed by two sub efficiencies, i.e., microscopic displacement efficiency and macroscopic sweep efficiency. The former one is a measure of oil recovery at pore-level, while the latter one refers to the area that the flooding agents are able to sweep. Therefore, the techniques which can improve either microscopic efficiency or macroscopic efficiency are acceptable to oilfields for further increasing oil production. This process is so-called tertiary or enhancing oil recovery (EOR).2, 3 In the past decades, numerous methods have been developed for enhancing oil recovery such as chemical4,

5

and thermal methods.6 As two common chemical

methods, polymer flooding and surfactant flooding have been extensively used especially in China.4,

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For example, in Daqing oilfield, almost 300,000 barrels

incremental oil can be produced per day by polymer flooding. Surfactant flooding technique is currently used in Dagang, Xinjiang, and even Daqing oilfields. The main mechanism of polymer flooding is to correct the unfavorable mobility ratio between water phase and oil phase and to create a stable propagation front, which is extremely crucial for macroscopic sweep efficiency improvement.8 While for surfactant flooding, the incremental oil recovery is produced mainly due to the mobilization of capillary force trapped oil resulting from the reduced interfacial tension.9 Therefore, in most of 2


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cases, the combination of polymer and surfactant (binary system), which is supposed to present a synergic effect, is commonly used in practical production.10, 11 Currently, partially hydrolyzed polyacrylamide (HPAM) is still the most extensively used polymer to date due to its availability in large quantity with customized properties (molecular weight, hydrolysis degree, etc.) and low manufacturing cost.12, 13

Despite the above advantages, there are two drawbacks that might affect the

acceptance of HPAM in the future. First of all, HPAM is very susceptible to harsh reservoir conditions, which severely influences its EOR efficiency particularly in elevated temperature and salinity formations.14, 15 Secondly, HPAM and derivatives (hydrophobic polyacrylamide, etc.) are acrylamide-based synthetic polymers. This kind of polymers is potentially harmful to the environment. The toxicity of acrylamide has been previously reported.16, 17 As an alternative to acrylamide-based polymers, natural or biopolymer is attracting more and more research attentions. Due to the unique properties of the polymer chain, biopolymer (xanthan gum, schizophyllan, HEC, etc.) usually experiences slight viscosity loss during flow in porous media and thus shows superior tolerance to reservoir conditions. However, it should be noticed that the synthetic polymers and biopolymers are non-surface-active. Therefore, in order to optimize the oil recovery efficiency, a small dosage of surfactant is usually incorporated with polymer (SP flooding).18 Nevertheless, when polymer and surfactant are simultaneously injected into formations, chromatographic movement, i.e. polymer ahead of surfactant, usually takes place during propagating in porous media, which, in turn, seriously weakens the synergic effect of the binary 3


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system.19-21 To eliminate the chromatographic movement, surface-active polymers, produced through introducing a small fraction of active groups onto polymer backbone, have been proposed recently. To meet the technological trend in polymer flooding, a completely novel flooding agent, nano-cellulose, was studied in this work. Cellulose is considered one of the most abundant natural polymer in wood, cotton, hemp and other plant-based materials.22-25 After cellulose is nano-sized, many distinctive chemical and physical properties are created such as high strength, large surface area and accessibility, which enhance the widespread use of this bio-material.26,

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A number of reports have

covered the physical and chemical properties of nano-cellulose as well as their applications. For example, Liu et al. studied the rheological properties of nano-cellulose colloids, and the results indicated that the viscoelasticity of NC strongly depended on the fraction and shear rate. The viscosity vs shear rate curves were divided into two regions due to the rearrangement of chains.28 Using nanofibre cellulose, Varanasi et al. prepared a biodegradable and recyclable membrane; this membrane shows great potential in ultrafiltration.29 Moreover, a composite of nano-cellulose and polyvinyl acetate (PVA) was reported by Kaboorani et al. The bonding strength of this composite with wood was significantly increased by inserting nano-cellulose into PVA.30 Nevertheless, in this paper, nano-cellulose was introduced to a new area, enhancing oil recovery (EOR). The nano-cellulose used here contains a small fraction of surface-active groups, which is expected to possess not only thickening power but also surface activity. Prior to oil recovery simulation, the 4


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physical properties of the nano-cellulose fluid that are closely related to the applicability in EOR process, were comprehensively investigated to figure out its EOR potential followed by oil recovery mechanism study using a visual micromodel. 2. EXPERIMENTAL 2.1. Materials The samples of the modified nano-cellulose (NC) were purchased from Haojia Nano-cellulose Co., LTD, Tianjin, China. The chemical structure is shown in Fig. 1. Table 1 lists the basic properties of two materials. Hydrolyzed polyacrylamide (Degree of hydrolysis, 5%; Molecular weight, 8×106g/mol) was supplied by Beijing Hengju Chemical Group Co., China. The crude oil sample was kindly provided by Sinopec Northwest Co. Figure 2 shows the viscosity-temperature curve of the oil. The density and composition of the oil are listed in Table 2. Brine (1wt%) prepared with NaCl was used throughout the experiments. Table 1. Physical properties of the nano-cellulose Length of cellulose

Charge density

(µm)

(meq/g)

NC-1

0.8-1.2

NC-2

0.8-1.1

Name

Purity (%)

Dispersant

0.72

>99

Distilled water

1.51

>99

Distilled water

Note: The charge density is proportional to the content of the active groups (-COO-) contained in nano-cellulose.

Figure 1. Chemical structure of the nano-cellulose

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100

-1

Viscosity @10s (mPa⋅s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20

0 10

20

30

40

50

60

70

80

90

100

Temperature (°C）

Figure 2. Oil viscosity as a function of temperature Table 2. Basic properties of the crude oil Density (g/cm3)@20oC

Viscosity (mPa·s)@25oC

0.882

80.4

SARA Composition Saturation hydrocarbon

Aromatic hydrocarbon

Resin

Asphaltene

(wt%)

(wt%)

(wt%)

(wt%)

47.5

23.6

9.8

14.8

2.2.Transmission Electron Microscopy The miscroscope features of the nano-cellulose samples were studied using a transmission electron microscope (TEM) (JEOL 6400 SEM, JEOL Ltd., Japan). TEM images were taken at an accelerating voltage of 80 kV. The images are shown in Fig. 3. Elongated and filamentous nano-sized cellulose chains were observed from the TEM images.

NC-1

NC-2 6


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Figure 3. TEM morphology of the nano-cellulose 2.3. Hydrodynamic size measurement The hydrodynamic size of the nano-cellulose in brine was measured using a dynamic light scattering particle size analyzer (BI-200SM, Brookhaven Instrument Co., NY, USA). The size distribution of the nano-fluids was provided in the supporting data. 2.4. Rheological analysis Viscometric measurements were carried out using an Anton Paar MCR 302 Rheometer equipped with CC27 measuring system at 25oC with shear rate ranging from from 0.1 to 1000s-1. Dynamical shear measurements were performed with the same rheometer and measuring system at 25oC in order to determine the loss (G'') and storage (G') moduli of the nano-fluids as a function of frequency ranging from 0.1 to 100Hz. It should be noted that all dynamical measurements were preceded by an oscillation strain sweep to identify the linear viscoelastic region. All of the measurements were performed in the linear response region. 2.5. Interfacial tension measurements The oil-water interfacial tension (IFT) was determined using a TX-500C spinning drop tensionmeter (Bowing, Stafford, TX) at 25oC. Normally, nearly 2h was given to allow the IFT values to level off. The instrument could automatically read the IFT with an image taking device and image acquisition software. 2.6. Emulsification test Emulsification tests were conducted using a 10mL graduated glass tubes. 7mL of nano-fluid using the concentration of 0.6wt% was firstly added to the tube, 3mL crude 7


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oil was then carefully added to the top of the aqueous phase with a syringe to avoid mechanical disturbance. Afterwards, the tubes were quickly shaken up and down 20 times, and the phase separation was recorded. 2.7. Thermal stability A temperature sweep (20-100oC) with the heating rate of 5oC/min was conducted using the Anton Paar MCR 302 Rheometer equipped with CC27 measuring system to test the temperature resistance property of the nano-fluids. The thermal stability was tested by storing the nano-fluids in sealed flasks, which were placed in an oven at a constant temperature of 60oC. The viscosity of the nano-fluid was measured along thermal aging time. 2.8. Microvisual experiment The etched glass model was used to study the displacing pattern of the nano-fluids. The etched glass model was prepared by laser etching, and channels with different sizes were distributed in the model to generate a heterogeneous porous media with permeability ratio of k2/k1=5. The size of the etched glass model in the experiment was 8cm×8cm×0.6cm. Two pores are located on the diagonal of the model to simulate injection well and production well. The basic experimental set-up is shown in Fig. 4. The procedures of the experiment are as follows: (1) saturating the porous model with brine; (2) displacing brine with oil; (3) flooding brine until the water cut reaches up to 98%; (4) injecting nano-fluids (0.3PV) into the visual model; and (5) resuming brine injection until the water cut up to 98% again.

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Figure 4. Experimental set-up of the micromodel test 3. RESULTS AND DISCUSSION 3.1. Stability of the nano-fluids The stability of the NC nano-fluids as a function of mass fraction and charge density was studied as shown in Fig. 5. Homogenous dispersions in 1wt% NaCl were obtained with the NC concentration ranging from 0.2-1.0wt%. Figure S1 presents the hydrodynamic size distribution of each sample. It was found that the hydrodynamic size of the NC in the aqueous phase is less than 10nm, suggesting the homogeneity of the nano-fluids. This property renders the nano-fluids superior injectivity in reservoirs. However, it should be noted that aggregation induced sedimentation was also observed at the concentration of 0.2wt% after about one week particularly for the sample of NC-1, which is attached by less -COO- groups compared to NC-2. This result is probably due to a fact that the repulsive forces caused by the surface negative charge in NC-1 (0.2wt%) are relatively weaker than van der Waals and hydrogen bonding forces.31 The work of enhancing NC dispersity in brine using chemical and physical methods is in progress in our group.

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Figure 5. Illustration of nano-cellulose fluids 3.2. Rheological properties The rheological properties of the nano-fluids were comprehensively studied in this section. Figure 6 shows the shear viscosity as a function of shear rate with different NC concentration. As seen, the shear dependence of the viscosity profiles is generally subjected to two distinct stages as shown in Fig. 6a and Fig. 6b. The first stage was observed at low shear rates and the viscosity decreases linearly with shear rate, suggesting a pronounced shear-thinning behavior. As the shear rate increases, a transition occurred at the second stage, i.e. shear viscosity increases with shear rate. It is believed that the networks constructed by hydrogen bonding and/or van der Waals forces were ruptured by shear stress at the first stage and thus caused the viscosity to decrease. Nevertheless, at the second stage, the nano-cellulose was rearranged to form the ordered networks again and led the viscous property to regain. Moreover, the critical shear transition shifted to higher shear rate with mass fraction, which is probably induced by the stiffness of the original networks.32 The thickening ability of the nano-cellulose as a function of mass fraction was plotted in Fig. 6(c). It can be seen that the viscosity data were well fitted to an exponential growth model with high determination coefficients (R2=0.99). NC-2 shows a higher 10


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thickening power than NC-1. The superior viscous property of NC-2 nano-fluid can be ascribed to the denser negative charges, which make the nano-cellulose chains stretched and dispersed compared to NC-1. 100000 0.2% 0.4% 0.6% 0.8% 1.0%

1000

0.2% 0.4% 0.6% 0.8% 1.0%

10000

Shear viscosity (mPa⋅s)

Shear viscosity (mPa⋅s)

10000

100

10

1000

100

10

1

1 0.1

1

10

100

0.1

1000

1

10

-1

100

1000

Shear rate (s-1)

Shear rate (s )

a

b

Shear viscosity @10s-1 (mPa⋅s)

600

NC-1 NC-2

500

400

Equation

y = Intercept + B1*x^1 + B2*x^2

Weight

No Weighting

Residual Sum of Squares

607.59784

Adj. R-Square

0.97119

2483.14284 0.97843 Value

300

Intercept NC-1

12.8983

-164.77304

60.66255

B2

360.39732

58.22897

19.87643

26.07505

-378.08464 837.11607

122.63471 117.71501

Intercept

200

NC-2

Standard Error 9.7425

B1

B1 B2

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Concentration (wt%)

c Figure 6. Shear viscosity of NC nano-fluids (a) NC-1; (b) NC-2; (c) Thickening ability at 10s-1 as a function of NC concentration 1000

1000 0.2% 0.4% 0.6% 0.8% 1.0%

100

Loss modulus (Pa)

100

Storage modulus (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

1

0.2% 0.4% 0.6% 0.8% 1.0%

0.1

10

1

0.1

0.01

0.01 0.1

1

10

100

0.1

1

Frequency (Hz)

10

Frequency (Hz)

11


100

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NC-1 1000

1000 0.2% 0.4% 0.6% 0.8% 1.0%

100

Loss modulus (Pa)

100

Storage modulus (Pa)

10

1

0.2% 0.4% 0.6% 0.8% 1.0%

0.1

10

1

0.1

0.01

0.01 0.1

1

10

100

0.1

1

Frequency (Hz)

10

100

Frequency (Hz)

NC-2 Figure 7. Viscoelastic properties as a function of frequency 100

10

NC-1 NC-2

1

G'

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

0.01

1E-3 1E-4

4.62

NC-1: y=2.93x NC-2: y=5.56x4.51 1E-3

0.01

0.1

Concentration

Figure 8. G' as a function of concentration at 1Hz The storage G' and loss G'' moduli of the nano-fluids as a function of frequency are shown in Fig. 7. It is clear that G' and G'' increase with the increasing NC concentration, indicating the formation of stiffer networks.33 Figure 7 also shows that G' is greater than G'' in the entire evaluated concentration range, which indicates the gel-like behavior of the nano-fluids. A significant increase in the storage modulus upon concentration has been previously reported.34, 35 A power-law model, G'∝ cn, is applied in most of the cases, where c denotes the concentration.36, 37 In our study, the values of n are 4.62 and 4.51 for NC-1 and NC-2, respectively, as shown in Fig. 8. 12


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3.3. Interfacial tension behavior The interfacial tension (IFT) between oil and water is a very important parameter for chemical system in EOR process and is closely related to the formation and stability of emulsions.38 The IFT behaviors between the crude oil and the nano-fluids were investigated in this part, and the results are displayed in Fig. 9. As anticipated, the IFT curves exhibit a similar trend, i.e. the IFT decreases with time and then almost hold constant at different level after 90min. The minimum IFT is about 0.7mN/m for the NC-2 nano-fluid at the concentration of 1.0wt%. It is believed that the IFT reduction is caused by the active groups loaded on NC chains. On the other hand, it was also observed that the as-prepared two nano-fluids were not able to reduce the IFT to an ultralow level (the order of 10-3mN/m). Figure 9c shows the IFT values as a function of NC concentration. It can be seen that the IFT rapidly decreases with the concentration and then almost stabilizes after the concentration of 0.8wt%. The NC-2 nano-fluid is more active in reducing the IFT than NC-1, which is highly consistent with the charge property as listed in Table 1. 25

25

0.2% 0.4% 0.6% 0.8% 1.0%

0.2% 0.4% 0.6% 0.8% 1.0%

20

IFT (mN/m)

20

IFT(mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

15

10

5

5

0 0

20

40

60

80

100

120

0

20

40

Time (min)

60

Time (min)

a

b

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32

NC-1 NC-2

28 24

IFT(mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 16 12 8 4 0 0.0

0.2

0.4

0.6

0.8

1.0

Concentration (%)

c Figure 9. IFT behavior of oil/NC nano-fluids (a) NC-1; (b) NC-2; (c) IFT as a function of NC concentration

3.4. Emulsion Stability Previous studies39,

40

proved that by using surfactants in chemical formula, the

oil/water IFT was significantly reduced and oil would be more easily dispersed into water phase forming emulsions. To study the emulsifying performance of the nano-fluids and also the stability of the formed emulsion upon storage, tube tests were carried out as shown in Fig. 10. The phase separation was recorded as a function of time. As Figure 10 shows, the oil could not be well emulsified by NC-1 and clear interface between oil and aqueous phase was observed after 5min, indicating the instability of the emulsion. On the contrary, for the NC-2 tube testing, the formed oil-in water emulsion seemed more stable than that of NC-1. The phase separation took place gently with time and reached its original state after 24h. The phase separating rate was quantified using the volume variation of the aqueous phase. The result is shown in Fig. 11. Compared the two emulsions, it can be easily seen that the oil/NC-2 emulsion separated with a much slower rate than the oil/NC-1 emulsion. 14


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This result is probably attributed to two mechanisms, 1) the relatively lower IFT between oil and NC-2 due to the presence of more surface active groups enhanced the formation of the emulsion compared to NC-1; 2) the prominent viscoelastic properties of NC-2 can stabilize the emulsion and mitigate phase separation process.41

Figure 10. Emulsification tube tests: NC-1 (left) and NC-2 (right) 1.4

Volume fraction of aqueous phase

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NC-1 NC-2

1.2

1.0

0.8

0.6

0.4

0.2 0.01

0.1

1

10

100

Time (h)

Figure 11. Volume fraction change of the aqueous phase as function of time

3.5. Thermal Stability In oilfield application the stability of the fluid viscosity is an important consideration. For enhancing oil recovery, the injected fluids have to work under elevated temperature for a long time. Therefore, the considerations of temperature resistance and thermal stability become dominant in selecting flooding systems. A temperature sweep from 20 to 100oC was conducted to study the temperature tolerance property of 15


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the NC nano-fluids (0.6wt%). The viscosity profiles against temperature are presented in Fig. 12a. The viscous properties of the nano-fluids are dependent on the exposed temperature, which would cause thermal motion of the molecules.42 Both of the nano-fluids undergo a progressive viscosity loss with temperature, and the viscous data are fitted to a linear model. The slope of the trend line can represent the rate of the thickening power loss. It was observed that the viscosity of NC-1 nano-fluid declined slightly slower than that of NC-2 nano-fluid, which corresponds to the loss rate of 0.12 and 0.3mPa·s/oC, respectively. This might be interpreted as that the repulsive forces provided by the negative charges loaded on NC-2 open more micro-pores and thus allow more water molecules to penetrate into the networks. However, under thermal effect, the rapid motion of the segments such as water and NC molecules, would cause the network unstable.43 The thermal behavior of the most commonly used EOR polymer, hydrolyzed polyacrylamide (HPAM), was also studied in this work aiming to provide a reference. As Figure S2 indicates, the viscosity loss rate of HPAM is 0.47mPa·s/oC, which is much higher than that of the NC nano-fluids. Figure 12b plots the viscosity of two nano-fluids as a function of thermal aging time. It shows that both of the NC nano-fluids experienced degradation upon thermal storage, but after a quick viscosity reduction, two viscosity curves almost leveled off after 17 days. These observations indicate that the nano-cellulose can well maintain the viscosifying power during flow in reservoirs.

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60

110

NC-1 NC-2

NC-1 NC-2

100

50

o

T=60 C

90

40

30

Equation

y = a + b*x

Weight Residual Sum of Squares

No Weighting 139.02531 249.36479

Pearson's r

-0.8992

Adj. R-Square

0.80597

Viscosity (mPa⋅s)

-1

Viscosity@10s (mPa⋅s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.97001 0.94013 Value Standard Error

NC-1

Intercept Slope

27.90434 -0.12539

0.43721 0.00709

NC-2

Intercept Slope

62.69652 -0.32641

0.58606 0.00951

20

80 70 60 50 40 30

10 10

20

30

40

50

60

70

80

90

100

0

5

10

Temperature (oC)

15

20

25

30

35

Aging time (day)

a

b

Figure 12. Viscosity change as a function of (a) temperature and (b) aging time

3.6. Microvisual tests Micromodel study of nano-cellulose flooding was conducted in a heterogeneous micromodel with permeability ratio of k2/k1=5 to investigate the displacing mechanism at pore level. The images of Fig. 13 and Fig. 14 were captured at the end of different stages in the microvisual experiments. The images of Fig. 15 are close-up (zoomed) pictures of Fig. 13c and Fig. 14c. These images give the efficiency of the nano-fluid flooding from micro-scale.

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Figure 13. Micromodel images (a) at initial oil saturation; (b) after waterflooding; (c) after nano-fluid flooding (NC-2); (d) after extended waterflooding Figure 13a illustrates the micromodel with initial oil saturation. After waterflooding as shown in Fig. 13b, a significant channel was formed along the high-permeable route leaving a great volume of oil untouched especially in the low-permeable area. It indicates the poor sweep efficiency of waterflooding in heterogeneous formations. Figure 13c shows the response of NC-2 nano-fluid injection. It is clear that the swept volume by the viscoelastic nano-fluid was increased for both of high- and low-permeable areas. It is particularly significant at the end of the extended waterflooding as shown in Fig. 13d. Most of the area in the micromodel has been reached. Figure 14 illustrates the displacing processes of NC-1 nano-fluid flooding. 18


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The comparison of the images (a-d) confirmed the improvement of the macroscopic sweep efficiency. However, it is also noticed that the swept area in Fig. 14c and Fig. 14d is not as large as that in Fig. 13c and Fig. 13d, which is caused by the relatively weak viscoelasticity of the NC-1 nano-fluid. The interaction between the nano-fluid and the oil was further investigated in the close-up images as shown in Fig. 15. Due to the surface-activity of the nano-fluids, the residual oil was emulsified and entrained in the aqueous phase, which subsequently reduced the flow resistance of the viscous oil and residual oil saturation. Because of the high IFT, the size of the oil droplet is considerably larger than that in surfactant or alkali flooding.44 From microscopic view, we can concluded that the oil entrapped in the small pores was displaced more efficiently by NC-2 than NC-1 as highlighted in Fig. 15, resulting from the higher surface-activity of NC-2 (low ITF).

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Figure 14. Micromodel images (a) at initial oil saturation; (b) after waterflooding; (c) after nano-fluid flooding (NC-1); (d) after extended waterflooding

Figure 15. Micromodel images of one location at the nano-fluid flooding stage 4. CONCLUSIONS In search of eco-friendly EOR flooding agents, a green material, modified nano-cellulose, which shows potential for EOR application, was investigated in this work. The physical properties and oil recovery mechanisms of the nano-cellulose 20


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fluids were comprehensively studied. Based on the experimental results, the following conclusions can be stated: 1. The charged nano-cellulose can well disperse in 1wt% NaCl brine forming homogenous nano-fluid upon storage with the exception of NC-1 at low concentration (0.2%) resulting from the weak repulsive forces. The dispersity improvement using chemical and physical methods is in progress in our group. 2. The nano-fluids show superior thickening ability and pronounced shear-thinning property due to the weak association forces. A gel-like behavior was observed within the NC concentration ranging from 0.2 to 1.0wt%, which can be theoretically characterized by the power-law model. 3. The oil/nano-fluid IFT can be reduced to the order of 10-1mN/m, which benefits the improvement of the microscopic oil recovery. NC-2 seems more active at the interface than NC-1; this result is in agreement with the parameter of charge density. 4. The emulsifying performance of the nano-fluids and the stability of the formed emulsion are related to the phase behavior and viscoelastic properties. Oil/NC-2 emulsion is relatively more stable than oil/NC-1 emulsion because of the charge property and viscoelasticity. 5. The nano-fluids experienced a much slower viscosity loss rate when they were subjected to elevated temperature compared to the widely used EOR polymer, HPAM. Upon thermal aging, the viscosity of the nano-fluids showed a rapid reduction with time but leveled off after 17 days. 6. The recovery mechanisms of 1) sweep volume improvement and 2) emulsification 21


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and entrainment, were established during NC nano-fluid flooding. Highly charged NC nano-fluid is generally suggested for practical use.

AUTHOR INFORMATION Corresponding author *Tel.: +86 28 83037007; E-mail: [email protected] or [email protected] (Bing Wei) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMNTS The authors wish to recognize the financial support received from PetroChina Innovation Foundation (2015D-5006-0212) and the National Key Basic Research Program of China (No. 2015CB250904). Sinopec Northwest Company is also acknowledged for furnishing the crude oil sample. The authors also would like to acknowledge Dr. Caili Dai for performing the microvisual experiments.

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Micromodel images (a) at initial oil saturation; (b) after waterflooding; (c) after nano-fluid flooding (NC-2); (d) after extended waterflooding 198x203mm (300 x 300 DPI)


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