Properties of CO2 Foam Stabilized by Hydrophilic Nanoparticle and

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Properties of CO2 Foam Stabilized by Hydrophilic Nanoparticle and Nonionic Surfactant Songyan Li, Kang Yang, Zhaomin Li, Kaiqiang Zhang, and Na Jia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00773 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Properties of CO2 Foam Stabilized by Hydrophilic Nanoparticle and Nonionic Surfactant Songyan Li,*, a, b Kang Yangb, Zhaomin Lia, b, Kaiqiang Zhangc, Na Jiac a

Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)),

Ministry of Education, Qingdao 266580, P. R. China b

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, P. R. China

c

Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina,

Saskatchewan S4S 0A2, Canada Abstract: Nanoparticles can improve the stability of CO2 foam and increase oil recovery during CO2 flooding in reservoirs. However, there are few reports in the literature concerning the synergistic effects of hydrophilic nanoparticles and nonionic surfactants on the stabilization of CO2 foam. In this study, the mechanism of the synergistic stabilization of CO2 foam by the nonionic surfactant C12E23 and hydrophilic T40 nanoparticles (SiO2) and the flow characteristic in porous media were investigated. Furthermore, the best formula for CO2 foam has been determined. The experimental results show that the nonionic surfactant C12E23 adsorbs on the surface of T40 nanoparticles by van der Waals forces and hydrogen bonds, which increases the hydrophobicity of the nanoparticles. In the dispersion of 2.49 mM C12E23 and 1.5 wt% T40, the surfactant forms a dense monolayer on the surface of nanoparticles, the hydrophobicity of the nanoparticles reaches the maximum with a contact angle of 78°. The stability of the CO2 foam under this condition is the best with an initial foam volume of 280 ml and a half-life of 150 min. The half-life is 30-times greater than that of the C12E23 foam. The nonionic surfactant C12E23 and hydrophilic T40 nanoparticles synergistically stabilize the CO2 foam by three aspects. First, the adsorption of nanoparticles at the gas-liquid interface reduces the interfacial tension. Second, the nanoparticles increase the interfacial viscoelastic modulus, which considerably improves the mechanical strength of the foam film. Third, the nanoparticles increase the viscosity of the dispersion, which increases the liquid holdup of the CO2 foam. The C12E23/T40 foam can also increase the sweep area and displacement efficiency and thus enhance oil recovery during flooding in porous media. The results are of strong significance to improving the recovery efficiency of CO2 foam flooding.

1. INTRODUCTION Foam fluids have strong potential to be applied in the petroleum industry, food, 1 detergents, 2 cosmetics,

3

and medicine.

4, 5In

the 1950s, foam fluids were used in oilfield and gas field

development as an efficient, environmentally friendly, clean, and intelligent fluid. 6 Foam is a thermodynamically unstable system with a high surface energy, and the spontaneous decay of the surface energy of the foam gradually reaches a minimum. 7 The foam system is unstable due to its own gravity drainage, foam aggregation and diversification, and surfactant instability under hightemperature and high-salt conditions. 8-10The instability of foam is an important factor restricting 1

ACS Paragon Plus Environment

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the development of foam; therefore, how to improve the stability of foam is of great significance. After more than 50 years of research on foam fluid, the improvements to foam stability stem mainly from improving the viscosity of the foaming agent solution or adding solid particles. 10-12To suppress the aggregation and disproportionation of bubbles, many researchers add a polymer, such as polyacrylamide or polysaccharide, to increase the viscosity of the liquid phase. 12 However, these polymers possess poor temperature and salt tolerances and a poor foaming ability. 10-13 Therefore, it is necessary to seek an efficient and stable foam system. In the field of petroleum engineering, nanoparticles, as a new type of material, present great potential to stabilize foam. 10 Nanosilica can be adsorbed on the foam surface with a high adsorption energy, which improves the mechanical strength of the foam and the viscosity of the base solution.

14-15This

modification is similar to

coating the foam with “armor”. In addition, the size of nanosilica is smaller than that of the porous media. 16 Therefore, the material will not remain in the reservoir's porous media and thus block the pore channels. Combined with the above factors, nanoparticles possess a strong potential to stabilize foam. Researchers have been reported that ionic-type surfactants act synergistically with nanoparticles or solid-phase particles to stabilize foam. Binks reported that when suitable amounts of SiO2 and CTAB (cetyltrimethylammonium bromide) are added to a mixed system, the stability after the formation of bubbles is significantly stronger than that of CTAB alone. 17 However, the foaming performance of the mixed system is slightly weaker than that of CTAB by itself, which is mainly due to the decrease in the concentration of the foaming agent in the solution after the surfactant molecules are adsorbed onto the surface of the SiO2. Zhang et al. 18 formed a mixed system with lithium soapstone and CTAB, and the foam-stabilizing performance is similar to that of Binks et al. Li et al. determined the synergistic effect requires a CTAB/SiO2 concentration ratio of 0.02 to 0.07, with 0.033 representing the best concentration ratio and the foam half-life achieving 45 min 19.

Lv et al. reported that saponin molecules were adsorbed on the surface of particles, thereby

exposing hydrophobic groups to the liquid phase and that the hydrophilicity of the particles, as assessed by the contact angle, increases from 62° to 83°. 20 The stable adsorption makes the foam stable for up to 2 h. Tang et al. found that after adding SDS to SiO2 nanoparticles, the stability of the bubbles gradually increases with the SDS concentration, and the two exhibit a stronger synergistic effect on the stability of the bubbles. 21 Sun et al. attributed an improvement in micro oil displacement efficiency to the improvement in foam stability. 22 Nonionic surfactants and solid-phase particles can also synergistically stabilize foam. Binks et al. studied the synergistically stable emulsion of N20 nanoparticles and C12E7 and found that the system has the strongest synergistic effect on the stability of the emulsion and the highest system 2


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viscosity with an intermediate surfactant concentration.

23

Hunter et al. studied the synergistic

stabilization of foam by the nonionic surfactant TX-100 and nanoparticles, which was accomplished via flocculation bridging between particles at low concentrations and increasing the elasticity of the liquid film at higher concentrations.

24 25

Zhang et al. studied the synergistic

stabilization of bubbles by lithium soapstone and nanoparticles and C12E4. The adsorption mechanisms of the surfactants on the two kinds of particles are different; therefore, the bubble stabilization effects are also different. The synergistic effect of nanoparticles and C12E4 is better. 26 According to these findings, with either ionic or nonionic surfactants and at the appropriate concentration of surfactant and solid-phase particles, the surfactant adsorbs on the surface of the solid-phase particles, thereby increasing the hydrophobicity of the solid-phase particles. The change in the solid particles’ hydrophobicity greatly improves the stability of foam. At present, the way of stabilizing foam by nanoparticles has been gradually accepted by researchers. Aqueous foams stabilized by hydrophobic nanoparticles and cationic surfactants have been studied extensively; 21, 22, 27, 28 however, hydrophobic nanoparticles have some disadvantages. For example, hydrophobic nanoparticles are not easy to disperse in water, and the price is substantially higher than that of hydrophilic nanoparticles. In addition, the positively charged cationic surfactant and the negatively charged clay particles in the formation attract each other; thus, the expected effect of stabilizing foam cannot be achieved. Therefore, foams stabilized by hydrophilic particles with anionic surfactants or nonionic surfactants have great application prospects in the oilfield development. 18-20 Foams stabilized by hydrophilic particles with nonionic surfactants have great application prospects in the oilfield development. Nonionic surfactants have a strong salt resistance and a low critical micelle concentration (CMC).

29, 30, 31

However, few

reports in the literature discuss the synergistic effects of hydrophilic nanoparticles and nonionic surfactants on the stabilization of CO2 foam, and many theoretical problems remain open. In this paper, the best formula of the hydrophilic T40 nanoparticles and the nonionic surfactant C12E23 for CO2 foam has been determined based on a comprehensive experimental evaluation. The adsorption behavior of C12E23 on the surface of nanoparticles has been studied, and the mechanism of their synergistic effects on stabilizing CO2 foam has been analyzed. The results are of great significance to improving the recovery efficiency of CO2 foam flooding.

2. MATERIALS AND METHODS 2.1 Materials Lauryl alcohol polyoxyethylene ether (C12E23) with the purity of more than 99.0 % is purchased from MClean Technologies Pte., Ltd. of Singapore. C12E23 is a nonionic surfactant with an 3


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approximate relative molecular weight of 1199.55 g/mol, which is used to prepare the CO2 foam in this paper. Its appearance is a milky-white or yellowish solid, and its solution in water is clear and transparent. The pH value of a 1.0 wt% aqueous solution is 5.5-7. The molecular structure diagram of C12E23 is shown in Figure 1. The resistivity range of deionized water used in the experiment is 17-18.2 M. CO2 with a purity greater than 99.9 % is provided by Qingdao Tianyuan Gas Company of China. Four types of hydrophilic nanosilica, T40, T30, N20 and V15, are provided by Wacker Chemical Co., Ltd., Germany, and the specific surface areas of the nanoparticles are 400, 300, 200 and 150, respectively. When the specific surface area of nanoparticles is from 150 to 400, the diameter of the nanoparticles is approximately 10-30 nm.

Figure 1 Molecular structure diagram of C12E23.

2.2 Apparatus A balance (Model ML, Mettler Toledo, Switzerland, full scale of 120 g, accuracy of 0.001 g) is used to weigh the surfactant and nanoparticles. A magnetic stirrer (model TJS-3000, Changzhou Electric Appliance, stirring speed of 0-1400 r/min) is used to preliminarily disperse the nanoparticles and surfactant solution. An ultrasonic processor (YP-S17, Hangzhou Success Ultrasonic Equipment Co., Ltd., China, frequency of 20 kHz, power of 2 kW) is utilized to fully disperse the nanoparticles in the dispersion. A blender (Model GJ-3S, stirring speed of 0-15000 r/min, Qingdao Senxin Co. Ltd. of China) is used to generate CO2 foam using the Warning Blender method. 32 A drop shape tensiometer (Tracker-H, Teclis, France, a full scale of 20.0 MPa and 200 °C) is used to test the interfacial tension and interfacial viscoelastic modulus of the surfactant solution and nanoparticle dispersion. A rheometer (Model MCR 302, Anton Paar, Austria, full scale of 15.0 MPa and 300 °C) is used to measure the viscosities of the surfactant solution and nanoparticle dispersion. The microstructure of foams in the bulk phase and porous media is observed by a microscope (VHX5000, Keyence, Japan, magnification of 50-5000). 2.3 Experimental procedures

4


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2.3.1 Preparation of dispersion and foam Dispersions of hydrophilic V15, N20, T30 and T40 nanoparticles are prepared with an initial concentration of 1.0 wt%, and the volume of the solution is 100 ml. Then, the surfactant C12E23 is added into the dispersions. All of the dispersions are subjected to 40 min of ultrasonication at a frequency of 20 kHz. In one cycle, the dispersions are processed with 3 min of ultrasonication and 1 min of rest, while the temperature is maintained at 25 °C by using a water bath. To form a stable adsorption of C12E23 on the surface of the nanoparticles, the dispersions are kept stationary at room temperature for 24 h. The aqueous foams stabilized by C12E23 and C12E23/T40 are generated through the Warning Blender method. The blender is fixed at a speed of 8000 r/min for 3 min. The initial foam volume is used as a standard to measure the foaming capability, and the half-life is used as a standard to measure the foam stability. The half-life is defined as the time for the separation of 50 ml solution from the CO2 foam. 2.3.2 Contact angle of the nanoparticle The dispersion of T40 nanoparticles is mixed with different concentrations of the C12E23 solution and kept stationary for 24 h. Then, the dispersion is centrifuged for 1 h with a rotating speed of 8000 r/min. The centrifuged nanoparticles are dried and pressed under 15 MPa into a round cake by a tablet machine. The water contact angles of the original nanoparticles and surface-modified nanoparticles are measured by an interfacial tensiometer (Tracker-H, Teclis Co, France). 2.3.3 Interfacial tension and viscoelastic modulus The interfacial tension and viscoelastic modulus of the solutions and dispersions at room temperature are measured by the tensiometer (Tracker-H, Teclis Co, France). In the measurement, the oscillation period is 5 s, the oscillation frequency is 0.2 Hz, and the amplitude is 1 μm2. The dispersion is injected into the sample pool, and then, the syringe containing CO2 is adjusted to form a pear-shaped bubble. The bubble shape is inputted to a computer through a charge-coupled device (CCD) camera, and a computer software named WinDrop is used to calculate the interfacial tension based on the shape of the bubble and the Laplace equation. Finally, the obtained data are changed by the Fourier transform method, and the interfacial viscoelastic modulus is determined. The viscoelastic module is usually used to evaluate the interfacial properties of foam from the microscopic perspective. The formula of viscoelastic modulus is shown in Formula (1). 𝑑𝛾

𝐸 = 𝑑𝑙𝑛𝐴 5


(1)

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where γ is the interfacial tension, mM/m; A the surface area of the bubble, m2; and E is the viscoelastic modulus, mN/m. E is a parameter that characterizes the foam resistance to bubble volume change and is related to foam stability. 33 2.3.4 Apparent viscosity of the dispersions The viscosities of the C12E23 solution and C12E23/T40 dispersion are measured by the concentric barrel model of the rheometer. The shear rate is fixed at 170 s-1, and the temperature is kept at 20 °C. 2.3.5 Microscope structure of foams The foam generated by the blender is placed in a Petri dish, and the foam microstructure is observed by the microscope with an ultradeep view. The foam stability at room temperature and atmospheric pressure is studied by FoamScan (Teclis Co., France). The device mainly evaluates the foam stability, analyzes the liquid holdup and the distribution of bubble diameters by means of the image analysis software CSA (Cell Size Analysis) and the foam conductivity. The detailed principles behind the device can be found elsewhere. 34 2.3.6 Microscopic state of nanoparticles First, the nanoparticle solution is ultrasonically dispersed for 10 minutes, and then a certain amount of the solution is put on a copper plate with a diameter of 0.5 cm. After waiting for 15 min, the unabsorbed solution is sucked up with a filter paper. Finally, the nanoparticles are observed with TEM and the photographs are taken. 2.3.7 Flow property of CO2 foam in porous media The microscopic visualization of flooding experiment is an important method to study the flow characteristics of foam in porous media, and the experimental setup is shown in Figure 2. The microscopic models are made according to the core sample selected from the formation. The core sample is gained from coring drilling. First, the core sample is ground into thin slice, and then the laser etching method is taken to make the microscopic model according to a microscopic photograph of the rock slice. Therefore, the microscopic models used in the microscopic visualization experiment can stand for the formation. The flow of C12E23 and C12E23 /T40 foams in the microscopic models is almost the same as that in the reservoir conditions. The microscopic model is made of glass with a porous media area of 40 × 40 mm, and the depth and width of the flow channel are 60 and 80 μm, respectively. The microscopic model is placed in a holder with a 6


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pressure of less than 32 MPa and a temperature of less than 150 °C. The foam generator is 10 cm long and 1 inch in diameter. It is a miniature sandpack tube filled with 60-80 mesh sand. The foam generator can simulate the condition of underground foaming. Through the foam generator, carbon dioxide foam with good quality can be produced. First, the micromodel is vacuumed, and crude oil with a viscosity of 322 mPa.s at 25 °C is injected into the micromodel at a flow rate of 0.02 ml/min. Then, water is injected into the micromodel to simulate a water flooding process. Subsequently, a certain volume of CO2 foam stabilized by C12E23/T40 is injected into the model at the rate of 0.1 ml/min to displace the crude oil with a back pressure of 2.0 MPa, as controlled by a back-pressure regulator (BPR). The video of the foam flow in the microscopic model is recorded by a digital microscope imaging system, which can be used for the analysis of the flow characteristics. All of the experiments are conducted at under 50 °C.

Figure 2 Experimental apparatus for microscopic visualization of flooding in the porous media.

3. RESULTS AND DISCUSSION 3.1 Properties of C12E23 foam The experimental results of CO2 foam generated by a pure solution of C12E23 are shown in Figure 3. The figure depicts that the initial foam volume increases at first and then remains almost stable with increasing C12E23 concentration. When the concentration of C12E23 is greater than its CMC, which is 0.083 mM, the foam volume continues to increase. The definition of CMC means that when the interface adsorption reaches saturation at a certain gas-liquid interface, micelles are generated in the liquid phase. When a large amount of foam is formed, the total gas-liquid surface 7


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of the bubbles is much greater, and more surfactant can be adsorbed at the interface. When the concentration is greater than CMC, the foam volume continues to rise until the C12E23 concentration reaches 4.15mM. 35 With the increase in C12E23 concentration, the half-life of the C12E23 foam increases at first. When the concentration is greater than 4.15 mM, the half-life tends to be stable. This finding may be due to the unsaturated adsorption of C12E23 at the gas-liquid interface. With increasing C12E23 concentration, more molecules adsorbing at the gas-liquid interface can greatly increase the viscoelastic strength of the foam liquid film and enhance the stability of the foam. When the C12E23 concentration is greater than 4.15 mM, the adsorption of the surfactant at the interface reaches saturation. Further increases in surfactant concentration cannot improve the foam stability significantly. 600

10

Foam volume Half life 8

6 300 4

Half-life (min)

450

Foam volume (ml)

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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150 2

0

0

2

4

6

8

10

0 12

C12E23 concentration (mM)

Figure 3 Properties of C12E23 foam.

3.2 Evaluation of nanoparticles The CO2 foams are formed by the combination of the nonionic surfactant C12E23 and four kinds of nanoparticles with different hydrophilic properties. The concentration of the nanoparticles is fixed at 1.0 wt%, and the concentration of C12E23 increases gradually. The experimental results are demonstrated in Figure 4. It is noted that the trends in the foam volume and half-life of the CO2 foams formed by the four mixtures are generally the same. With increasing C12E23 concentration, the foam volume first increases and then levels off. However, the half-life of the CO2 foams first increases and then decreases, possessing a peak value. Comparing the four nanoparticles of different hydrophilic strengths, it is found that the hydrophilic property has little effect on the foam volume, but it has a strong effect on the half-life. The half-life of the C12E23/T40 foam is clearly greater than that of the other three foams, and the maximum half-life of this foam is nearly 56.2 min. 8


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Foam volume (ml)

400

C12E23+V15 C12E23+N20 C12E23+T30 C12E23+T40

350

300

250

0.0

0.5

1.0

1.5

2.0

2.5

3.0


(a) Foam volume 70

C12E23+V15 C12E23+N20 C12E23+T30 C12E23+T40

60

C12E23 Half life (min)

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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50 40 30 20 10 0

0.5

1.0

1.5

2.0

2.5

3.0


(b) Half-life Figure 4 CO2 foam properties with different nanoparticles. The concentration of nanoparticles is fixed at 1.0 wt%, and the concentration of C12E23 increases gradually.

The contact angles of V15, N20, T30 and T40 with water are measured by the experimental procedures mentioned before. The experimental results are shown in Figure 5. The hydrophilicities of the four nanoparticles are arranged as T40 > T30 > N20 > V15. Therefore, the experimental results show that the stability of the CO2 foam stabilized by the nonionic surfactant C12E23 and the nanoparticles is closely related to the hydrophilicity of the nanoparticles. Under the experimental conditions, the stronger the hydrophilicity is, the better the stabilization effect is. The contact angles of T40 in C12E23 solutions of different concentrations are also measured. It is found that the adsorption of the surfactant on the nanoparticles changes the hydrophobicity of the nanoparticles. With the increase of the concentration of C12E23, the hydrophobicity of surface-modified T40 9


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increases first, and then decreases. When the concentration of C12E23 is 2.49 mM, the hydrophobicity of T40 increases obviously, and the contact angle reaches maximum value of 78°. The specific reasons will be given in the following. The transmission electron microscopy (TEM) images of the nanoparticles are shown in Figure 6. Generally, the nanoparticles do not exist alone but in the form of aggregates. The larger the specific surface area is, the smaller the diameter of the nanoparticle is. These photographs are taken by TEM with a magnification of 10,000 times. 60

Contact angle (°)

45

30

15

0

V15

N20

T30

T40

Particle type

(a) Original contact angles of the four nanoparticles. 100

80

T40 contact angle (°)

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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60

40

20 0

1

2

3

4

5


(b) Contact angles of the surface-modified T40 nanoparticles using C12E23 Figure 5 Contact angles of the nanoparticles in water.

10


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Figure 6 TEM images of the nanoparticles. (a)-(d) are T40, T30, N20, and V15.

3.3 Properties of C12E23/T40 foam The foam volume and half-life of the C12E23/T40 foams with nanoparticle concentrations of 0.5 wt%, 1.0 wt% and 1.5 wt% are presented in Figure 7. It can be seen from the curves that the three concentrations of T40 and C12E23 exhibit a synergistic effect for stabilizing the CO2 foam; however, when the T40 concentrations are 0.5 wt% and 1.0 wt%, the synergistic effect is relatively weak. The half-life of the C12E23/T40 foam is sensitive to the concentration of T40. The basic rule is for the same concentration of C12E23, the stability of the CO2 foam increases with the concentration of T40. The half-life of the C12E23/T40 foam with 1.5 wt% T40 is substantially longer than that of the other two. Figure 7(c) depicts that the curves that can be divided into four ranges. In range I, the concentration of C12E23 is less than 0.25 mM, the foam volume is only 45 ml, and the C12E23/T40 dispersion does not completely foam. In range II, the half-life of the CO2 foam is extremely high, i.e., more than 10 h. However, the foam volume is very small, which is only approximately 180 ml. This finding may be observed because the C12E23 molecule forms a loose monolayer on the surface of the nanoparticles owing to the low concentration. Because the long chain of C12E23 molecule intertwines with each other, the surfactant adsorbs stably on the surface of nanoparticle. 36

In range III, the CO2 foam possesses the best comprehensive properties, considering both the

foam volume and half-life. This result may be attributed to the fact that the C12E23 molecules form a dense single adsorption layer on the surface of the nanoparticles. Therefore, the mechanical 11


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strength of the bubble film is the greatest, and the stability is the best. In range IV, the foam volume is higher, but the half-life of the CO2 foam is relatively poor. 30, 37, 38The reason for this behavior is that the C12E23 molecules form a double adsorption layer on the surface of the nanoparticles. If the foam volume and half-life are considered comprehensively, the properties of the C12E23/T40 foam in range III are the best. For example, when the concentration of C12E23 is 2.49 mM, the initial foam volume reaches 280 ml, and the half-life is 150 min. The half-life is 30-times greater than that of the C12E23 foam. Under this condition, C12E23 and T40 exhibit the best synergistic effect for stabilizing the CO2 foam. 600

15

Foam volume Half life

12

400 9 300 6 200 3

100

0

Half life (min)

Foam volume (ml)

500

0

1

2

3

4

5

0


(a) T40 concentration of 0.5 wt% 350

80

Foam volume Half life

60

Half life (min)

280

Foam volume (ml)

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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210

40 140

20 70

0

0

1

2

3


4

(b) T40 concentration of 1.0 wt%

12


5

0

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(c) T40 concentration of 1.5 wt% Figure 7 Properties of the C12E23/T40 foam.

3.4 Synergistic effect for stabilizing CO2 foam 3.4.1 Adsorption of C12E23 on the nanoparticle surface Nonionic surfactants cannot ionize charged ions in a water solution; therefore, the molecules cannot adsorb on the surface of nanoparticles by electrostatic forces. The approximate relative molecular weight of 1199.55 g/mol, which is significantly greater than that of the traditional ionic surfactant. Whether a nonionic surfactant molecule dissolves in a water solution and adsorbs on the surface of a nanoparticle depends on dispersion forces and hydrogen bonds. 30, 31 Due to the lack of permanent dipole moments in CO2 molecules, the dispersion van der Waals force is relatively small.

29, 39

However, it contains a large number of hydrophilic groups, i.e., the

polyoxyethylene chain and ether bond.

29, 40When

these groups contact the hydrophilic

nanoparticles, strong hydrogen bond forms. The stronger the hydrophilicity of the nanoparticles, the more stable the hydrogen bond is. C12E23 has many functional groups, such as hydroxyl group (-OH), ether bond (C-O-C) and carbon chain (-C-C). There are stable siloxane (Si-O-Si) and unstable silanol bonds (Si-OH) on the surface of nanoparticles. Therefore, hydroxyl groups, ether bonds and carbon chains all form hydrogen bonds with silanol bonds. A diagram of the adsorption of different concentrations of C12E23 on a nanoparticle is shown in Figure 8. One surfactant molecule of C12E23 has a long EO chain with degree of EO polymerization of 23 and a long carbon chain with 12 carbons. Thus, EO chain and carbon chain may adsorb on multiple sites on the surface of the nanoparticle. When the concentration of the C12E23 in solution is low, as in range II in Figure 7(c), the number of surfactant molecules adsorbed on each nanoparticle is small, and the 13


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surfactant forms a loose single adsorption layer. In addition, two surfactant molecules may be entangled because of the long EO chain of C12E23 and form a stable space bridging network structure, which benefits the stability of the CO2 foam. However, the surfactant concentration is low in the dispersion, resulting in a small foam volume. When the concentration of C12E23 in the dispersion increases to an appropriate value in range III (Figure 7(c)), the surfactant molecules can form a dense monolayer on the surface of the nanoparticle, depending on the dispersion forces and hydrogen bonds. Under this condition, the hydrophobic end of the surfactant is exposed to the liquid phase, which increases the hydrophobicity of the nanoparticle. The stability of the CO2 foam is relatively good, and the foam volume is improved greatly. With further increases in the surfactant concentration, in range IV (Figure 7(c)), the hydrophobic end of the surfactant molecule on the first layer interacts with the hydrophobic end of the molecule in the bulk phase. The hydrophilic end of the surfactant molecule on the outer layer of the nanoparticle becomes exposed to the liquid phase. The surface of the nanoparticle possesses a dense double adsorption layer, and the nanoparticle returns to hydrophilic. 19, 20, 30, 36, 38, 41Under this condition, micelles are formed in the dispersion due to the high C12E23 concentration. Therefore, the foam volume increases further, and the half-life decreases.

(a)

(b)

(c)

Figure 8 Adsorption of C12E23 on a nanoparticle with increasing C12E23 concentration. The concentration of C12E23 increases from figures (a) to (c).

Figure 9 demonstrates the change in the adsorption state of nanoparticles at the gas-liquid interface with increasing C12E23 concentration, which is related to the stability of the CO2 foam shown in Figure 7. When the concentration of C12E23 is in range A of Figure 9, the concentration of the surfactant is very low, and most of the surfactant is adsorbed on the surfaces of the T40 nanoparticles. At this time, the C12E23/T40 dispersion cannot stabilize the CO2 foam effectively, which is shown in range I in Figure 7(c). When the concentration of C12E23 is in range B, C12E23 adsorbs on the nanoparticle surface to form a loose monolayer. The nanoparticles present a weak synergistic effect with the surfactant. The stability of the foam is very good, but the foam volume is small, which is shown in range II in Figure 7(c). When the concentration of C12E23 increases further and is located in range C, C12E23 adsorbs on the surface of each nanoparticle as a dense 14


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monolayer. At this time, the nanoparticle exhibits the best hydrophobicity and the best synergistic effect for stabilizing the CO2 foam, corresponding to range III in Figure 7(c). When the concentration of C12E23 is in range D at a substantially higher concentration, C12E23 adsorbs on the surface of each nanoparticle to form a double layer. The surface-modified nanoparticles become hydrophilic again and tend to disperse in the aqueous phase. In addition, micelle molecules are formed because of the high concentration of C12E23. The stability of the CO2 foam decreases gradually, and the synergistic effect also weakens, which is shown in range IV in Figure 7(c).

Figure 9 Adsorption of nanoparticles at the interface of CO2 and the water dispersion with increasing C12E23 concentration. The concentration of C12E23 increases from figures (a) to (d).

Figure 10 depicts the experimental pictures of CO2 foam generated by C12E23/T40 dispersions after 30 min. The concentrations of C12E23, from left to right, are 0.15, 1.5, 2.49, 3.2, and 4.5. The precipitated liquid from the CO2 foam first becomes limpid from turbidity and then becomes limpid again with increasing C12E23 concentration. This finding is attributed to the hydrophobic property of the nanoparticles with different concentrations of C12E23. When the concentration of C12E23 is low, as shown in Figures 10(a) and 10(b), C12E23 adsorbs on the surface of the SiO2 nanoparticles to make them slightly hydrophobic. The nanoparticles are still generally hydrophilic and mainly disperse in the aqueous phase, so the precipitate is turbid. With increasing C12E23 concentration, the precipitated liquid becomes clear, as shown in Figure 10(c), because of the moderate concentration of C12E23 and the formation of monolayer adsorption. The nanoparticle changes from hydrophilic to hydrophobic with the greatest contact angle and mostly concentrates at the gas-liquid 15


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interface, while the precipitated liquid is limpid. With further increases in the C12E23 concentration, the hydrophobic end of the surfactant molecules in the first layer interacts with the surfactant molecules in the second layer, thus, the nanoparticle becomes hydrophilic again. The precipitated liquid from the CO2 foam becomes turbid again, as shown in Figures 10(d) and 10(e). The experimental results directly prove that C12E23 can form a single layer adsorption on the surface of SiO2 nanoparticles at low C12E23 concentrations, strengthening the synergistic effect, while a double adsorption layer forms at high C12E23 concentrations, thereby weakening the synergistic effect. 19, 20, 28, 42

(a)

(b)

(c)

(d)

(e)

Figure 10 Experimental pictures of CO2 foam generated by C12E23/T40 dispersions after 30 min.

3.4.2 Interfacial tension and viscoelastic modulus Figure 11 displays the interfacial tensions of the C12E23 solution and C12E23/T40 dispersion with CO2. As shown in Figure 11, the interfacial tension between distilled water and CO2 is 56.2 mN/m, and that between a pure nanoparticle dispersion and CO2 is 54.1 mN/m. The nanoparticles do not significantly reduce the interfacial tension. Therefore, a pure nanoparticle dispersion cannot stabilize foam. 43, 44The interfacial tension of a pure C12E23 solution under a CMC of 0.083 mM is 41.1 mN/m, and that of the C12E23/T40 dispersion is 40.2 mN/m. When the surfactant concentration is less than the CMC, the interfacial tension of a pure surfactant solution is slightly larger than that of the mixture dispersion. With increasing surfactant concentration, the interfacial tension decreases quickly. When the C12E23 concentration is higher than the CMC, the interfacial tension decreases further and then tends to gradually stabilize. The stable interfacial tensions of C12E23 and C12E23/T40 are 34.8 mN/m and 31.1 mN/m, respectively. The T40 nanoparticles interact physically with C12E23, and C12E23 strongly adsorbs on the surface of the nanoparticles. 16


45

The surface-

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modified nanoparticles reduce the interfacial tension with CO2, thereby greatly improving the stability of the CO2 foam. 60

2.49mM C12E23 2.49mM C12E23+1.5wt%T40

Surface temsion (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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50

40

30

0

1

2

3

4

5

6


Figure 11 Interfacial tensions of the C12E23 solution and C12E23/T40 dispersion with CO2.

Figure 12 displays the viscoelastic moduli of the C12E23 solution and C12E23/T40 dispersion with CO2. Interfacial viscoelasticity mainly describes the capability of a liquid membrane to resist an external disturbance and to delay the coalescence of foam bubbles. The interfacial viscoelastic modulus of the C12E23/T40 dispersion increases at first and then decreases with increasing C12E23 concentration. However, the viscoelastic modulus of the C12E23 solution is stable at approximately 5.1 mN/m against increasing C12E23 concentration. A possible reason is that the CMC of C12E23 is very small (0.083 mM), and the adsorption of surfactant molecules at the interface reaches the equilibrium state instantaneously with an oscillation period of 5 s and an oscillation frequency of 0.2 Hz.

33

The interfacial viscoelastic modulus of the C12E23/T40 dispersion peaks at a C12E23

concentration of 2.49 mM, and the values are all significantly higher than that of the pure C12E23 solution. This finding is probably observed because the surfactant C12E23 changes the hydrophobic properties of the nanoparticles. The surface-modified nanoparticles adsorb at the gas-liquid interface, increase the viscoelastic modulus of the bubble film, and improve the mechanical strength of the CO2 foam. At higher C12E23 concentrations, the surfactant molecules form a double adsorption layer on the surface of the nanoparticles, which become hydrophilic again. At this time, the nanoparticles tend to disperse into the liquid phase, and the viscoelastic modulus of the dispersion decreases. When the concentration of C12E23 is 2.49 mM, the interfacial tension reaches the minimum value, and the interfacial viscoelastic modulus reaches the peak value of 23.6 mN/m. Thus, the greater the viscoelastic modulus, the better the stability of the foam. As shown in Figure 7, the CO2 foam performance of the compound system is also the best under this condition. 17


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Viscoelastic modulus (mN/m)

30

2.49mM C12E23 2.49mM C12E23+1.5wt%T40 20

10

0

0

1

2

3

4


5

6

Figure 12 Viscoelastic moduli of C12E23 solution and C12E23/T40 dispersion with CO2.

3.4.3 Viscosity of C12E23 dispersion The viscosities of the pure C12E23 solution and C12E23/T40 dispersion are measured with C12E23 concentrations of 0-5 mM at room temperature and atmospheric pressure. The experimental results are shown in Figure 13. The viscosity of the pure C12E23 solution is basically stable at 1.5 mPa.s as the concentration of C12E23 increases. At the same concentration of C12E23, adding T40 nanoparticles can significantly increase the liquid phase viscosity. The viscosity of the C12E23/T40 dispersion reaches a peak value of 24 mPa.s with a C12E23 concentration of 2.49 mM. This finding is due to the adsorption of a dense monolayer of the C12E23 surfactant on T40 nanoparticles. The surface-modified T40 nanoparticles greatly increase the viscosity of the dispersion, increase the flow resistance, and delay the drainage of the CO2 foam. The CO2 foam generated under this condition is also the most stable. 42 30

2.49mM C12E23+1.5wt%T40 2.49mM C12E23

25

Viscosity (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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20

15

10

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0 0

1

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4

C12E23 Concentration (mM) 18


5

6

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Figure 13 Viscosities of the C12E23 solution and C12E23/T40 dispersion.

3.4.4 Liquid holdup of C12E23/T40 foam Figure 14 depicts a microstructural diagram of the CO2 foams under the three-dimensional microscope with ultradeep view. Figures 14(a) and 14(b) demonstrate the CO2 foam generated by pure C12E23 at the concentration of 2.49 mM. Figures 14(d) and 14(e) illustrate the C12E23/T40 foam with the concentrations of 2.49 mM C12E23 and 1.5 wt% T40. Figure 14(f) is a schematic diagram of the structural characterization of the C12E23/T40 foam. It can be seen from the microscope that the partial enlargement map of the C12E23/T40 foam is coarse, reflecting the adsorption of nanoparticles at the interface of gas and liquid. Compared with the adsorption of the surfactant C12E23, the adsorption of nanoparticles at the interface is more stable and irreversible because the desorption of nanoparticles from the interface is substantially harder than that of the surfactant. 46

400 μm

50 μm

Figure 14 Microstructural diagram of the CO2 foams under the three-dimensional microscope with ultradeep view.

Figure 15 shows the microscopic evolution state of CO2 foams with time, as tested by FoamScan. It is noted in the figure that the foam formed by C12E23 is clear, and the foam stability is poor. The C12E23/T40 foam is more rough and irregular, and the foam film is thicker than that of the C12E23 foam. This is because the surfactant-modified nanoparticles adsorb on the surface of the bubble to form a dense adsorption layer, which is wrapped like an armor on the surface of the foam. At the same time, this dense layer of nanoparticles also slows the disproportionation among the bubbles. The C12E23 foam nearly coalesces after 10 min; however, the C12E23/T40 foam is still stable. As 19


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time goes on, the bubbles of the C12E23/T40 foam increasingly coarsen, and the diameters of the bubbles increase due to Oswald ripening. 20, 28 However, the bubbles remain relatively stable after 30 min.

300 μm

Figure 15 Microscopic evolution states of the CO2 foams with time, as tested by FoamScan.

Figure 16 depicts the radius change with time for the different CO2 foams shown in Figure 15. At the beginning of the foam formation, the average radius of the C12E23 foam is very small (35.2 μm). The three C12E23/T40 foams almost have the same initial radius as the C12E23 foam. After 1000 s, the radius of the C12E23 foam increases to 151.6 μm due to the coarsening. However, the three C12E23/T40 foams remain very stable with average radii from 62.8 μm to 93.6 μm. 200

Average radius of bubbles (μm)

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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2.49mM C12E23 0.83mM C12E23+0.5wt%T40 1.32mM C12E23+1.0wt%T40 2.49mM C12E23+1.5wt%T40

160

120

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Time (s) 20


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Figure 16 Radius change with time for different CO2 foams.

Figure 17 shows the change in liquid holdup over time for different CO2 foams, as tested by FoamScan. Through the reference electrode at the bottom of the glass column of the FoamScan, we can obtain a linear relationship between the initial volume of the foaming agent solution and the conductivity. There are also 5 pairs of electrodes on the side of the glass column of the FoamScan, so we can get the conductivity corresponding to different foam volumes. The Cell Size Analysis (CSA) software can obtain the evolved liquid holdup with time based on the linear relationship between conductivity and liquid holdup. Almost no liquid precipitation is present at the beginning of the foam formation, and the foam holdups are more than 98 %. The clearest area of the image is selected as the analysis area. Because the size of the bubble in the analysis area is different, CSA gives an average bubble size. Thus, according to the formula (2), the liquid holdup can be calculated. The liquid precipitation of a pure surfactant is faster, and the slope of the liquid holdup curve is greater. After 30 min, the liquid holdup is decreased to 43.2 %. The addition of nanoparticles to the CO2 foam improves the foam stability greatly, resulting in a significantly higher liquid holdup. 20, 28After 30 min, the liquid holdups for the three C12E23/T40 foams are higher than 65.7 %. f 

S f －n  S m

(2)

Sf

Where f is the liquid holdup of the foam; Sf is the analytical area of the foam, 51.48 mm2; n is the number of bubbles in the area; and Sm is the average area of the foam, mm2. 120

100

Liquid holdup (%)

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 2.49mM C12E23 0.83mM C12E23+0.5wt%T40 1.32mM C12E23+1.0wt%T40 2.49mM C12E23+1.5wt%T40

20

0

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500

1000

1500

2000

2500

Time (s)

Figure 17 Change in liquid holdup over time for different CO2 foams.

3.5 Properties of C12E23/T40 foam in porous media

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3.5.1 Stability of C12E23/T40 foam in porous media The stability of the C12E23 foam and C12E23/T40 foam in porous media have been tested. The two CO2 foams are injected into the microscopic visualization model, and the entrance and exit of the model are closed. The coalescence process of the foams is tested under the static condition. The experimental pressure and temperature are 2 MPa and 50 °C, respectively. The experimental results are shown in Figure 18. Comparing the C12E23 foam with the C12E23/T40 foam, it is found that the foam diameter of the C12E23/T40 foam after 30 min and 60 min is still small. However, the C12E23 foam without nanoparticles is substantially bigger, and the foam tends to coalesce after 30 min and 60 min. The stability of the nanoparticle-strengthened foam in porous media is drastically better than that of the C12E23 foam.

200 μm (a) 0 min

(d) 0 min

(b) 30 min

(c) 60 min

(e) 30 min

(f) 60 min

Figure 18 Stability of the C12E23 foam and C12E23/T40 foam in porous media. (a)-(c) are the C12E23 foam with a concentration of 2.49 mM, and (d)-(f) are the C12E23/T40 foam with concentrations of 2.49 mM C12E23 and 1.5 wt% T40.

3.5.2 C12E23/T40 foam flooding in porous media Figure 19 illustrates the foam flooding process in the microscopic model. All of the experiments are conducted under 50 °C with a back pressure of 2.0 MPa. First, a water flooding with 1.5 times the porous volume (PV) is conducted, which is shown in Figure 19(a), and then 0.3 PV and 0.5 PV of C12E23/T40 foam flooding are conducted, as shown in Figures 19(b) and 19(c), respectively. At the end of the water flooding, a cluster of residual oil remains at the corner of the micromodel. At the end of the C12E23/T40 foam flooding, the foam has an improved sweep area and oil 22


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displacement efficiency. The foam can flow to the corner area of the micromodel and improve the oil recovery efficiency by displacing most of the residual oil. The T40 nanoparticles adsorbed on the bubble surface can provide a barrier to film rupture, and the viscoelasticity of the bubble is enhanced. Thus, the foam has the capability of improving sweep efficiency. The higher the viscoelasticity is, the more the microforce works on the oil droplet.

47-49

Therefore, the shape-

changed oil droplet is easier to move and pull out. As a result, the C12E23/T40 foam displaces more crude oil.

200 μm (a)

(b)

(c)

Figure 19 Foam flooding process in the microscopic model. (a) shows that 1.5 PV water flooding is conducted. (b) and (c) show that 0.3 PV and 0.5 PV of C12E23/T40 foam flooding are conducted.

C12E23 is a nonionic surfactant, which can decrease the interfacial tension of water and crude oil. Therefore, the mechanism of foam flooding includes the mechanism of surfactant flooding, which mainly includes reducing the interfacial tension of water and crude oil, emulsifying the crude oil and improving the efficiency of displacing oil. 47-49As shown in Figure 20, C12E23 can emulsify the residual oil on the wall of the micromodel into small oil droplets. The C12E23/T40 foam drives the emulsified droplets to flow, bringing the small oil droplets out of the micromodel to increase the oil recovery efficiency.

200 μm (a)

(b)

Figure 20 Effect of emulsifying residual oil on the wall of the micromodel by the C12E23/T40 foam. (a) shows that C12E23 emulsifies the oil into small oil droplets. (b) shows that C12E23/T40 foam displaces oil droplets. 23


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4. CONCLUSIONS The mechanism of the synergistic stabilization of CO2 foam by the nonionic surfactant C12E23 and hydrophilic T40 nanoparticles and the flow characteristics in porous media were investigated in this study. The study’s primary conclusions are outlined as follows. (1) The nonionic surfactant C12E23 has a synergistic effect with nanoparticles of T40, T30, N20 and V15 for stabilizing CO2 foam. C12E23 has the best synergistic effect with T40 nanoparticles with the smallest contact angle of 20.12°. This observation indicates that the nanoparticles with the greater hydrophilic property have a stronger the synergistic effect with the nonionic surfactant C12E23 for stabilizing the CO2 foam under the experimental conditions. (2) The nonionic surfactant C12E23 adsorbs on the surface of T40 nanoparticles by van der Waals forces and hydrogen bonds, which increases the hydrophobicity of the nanoparticles. In the dispersion of 2.49 mM C12E23 and 1.5 wt% T40, the surfactant forms a dense monolayer on the surface of the nanoparticles, the hydrophobicity of nanoparticles reaches the maximum with a contact angle of 78 o. The stability of the CO2 foam under this condition is the best with an initial foam volume of 280 ml and a half-life of 150 min. The half-life is 30-times greater than that of the C12E23 foam. (3) The synergy between the nonionic surfactant C12E23 and hydrophilic T40 nanoparticles stabilizes the CO2 foam by three aspects. First, the adsorption of nanoparticles at the gas-liquid interface reduces the interfacial tension from 34.8 mN/m to 31.1 mN/m. Second, the nanoparticles increase the interfacial viscoelastic modulus from 5.1 mN/m to 25.2 mN/m, which greatly improves the mechanical strength of the foam film, strengthens the foam resistance to external disturbances, and delays the coalescence of foam bubbles. Third, the nanoparticles increase the viscosity of the dispersion, which increases the liquid holdup of the CO2 foam. The stability of the nanoparticlereinforced foam in the bulk phase and porous media are evidently better than that of the ordinary foam. The C12E23/T40 foam can also increase the sweep area and displacement efficiency and thus enhance the oil recovery during flooding in porous media.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] for Songyan Li Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (No. 51774306), National Key Scientific and Technological Project for the Oil & Gas Field and Coalbed Methane of China (2016ZX05011004-005), and Fundamental Research Funds for the Central Universities (14CX02185A). We are grateful to the Foam Research Center at the China University of Petroleum (East China) for their assistance with the experimental research.

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Properties of CO2 Foam Stabilized by Hydrophilic Nanoparticle and

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