Experimental Study on the Stabilization Mechanisms of CO2 Foams by

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Experimental Study on the Stabilization Mechanisms of CO2 Foams by Hydrophilic Silica Nanoparticles Pan Wang, Qing You, Li Han, Weibing Deng, Yifei Liu, Jichao Fang, Mingwei Gao, and Caili Dai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04125 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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Energy & Fuels

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Experimental Study on the Stabilization Mechanisms of CO2 Foams by Hydrophilic

2

Silica Nanoparticles

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Pan Wang1,2, Qing You1,2*, Li Han3, Weibing Deng3, Yifei Liu4, Jichao Fang4,

5

4

4*

Mingwei Gao , Caili Dai

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1. School of Energy Resources, China University of Geosciences, Beijing, 100083, China.

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2. Key laboratory of marine reservoir evolution and hydrocarbon enrichment mechanism, Ministry

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of Education

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3. Engineering and Technology Research Institute, Xinjiang Oilfield, CNPC, Karamay, 834000,

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China

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4. China University of Petroleum, Qingdao, Shandong, 266580, China.

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Abstract

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The use of nanoparticles (NPs) for foam stabilization to enhance oil recovery has

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shown great promise in oil & gas development. It has been proven that NPs-based

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foam stabilizers achieve good foam stability. We chose four kinds of surfactants,

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including a cationic surfactant (CTAB), an anionic surfactant (SDBS), a nonionic

18

surfactant (TX-100), and a zwitterionic surfactant (OA-12) to investigate the

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influences of hydrophilic NPs on the stability of the CO2 foam. We determined the

20

effects of the surfactant concentration, NPs concentrations, temperature, and salinity

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on the stability of the CO2 foam. According to previous investigation, a better

22

synergistic effect was assumed to generate between NPs and OA-12 because of

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opposite charge and shorter molecular chain. The experimental results showed a

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synergistic effect between the hydrophilic SiO2 NPs and the zwitterionic surfactant.

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When CO2 was dissolved in an aqueous solution, the solution turned acidic and the

26

zwitterionic surfactant exhibited cationic surfactant characteristic, suggesting that

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different mechanisms occurred due to the different interaction between the NPs and

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the surfactant. An investigation of the surface characteristics demonstrated a “division

29

of labor” between the NPs and the surfactants; the NPs resulted in a high elasticity

30

modulus with the zwitterionic surfactant in aqueous solution and the surfactant caused 1

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a low surface tension. These mechanisms resulted in the stability of the

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NPs-surfactant CO2 foam. The results of this research provide insights into the

3

selection of stabilized CO2 foam and broaden its potential application in enhanced oil

4

recovery.

5 6

Keywords: hydrophilic NPs, zwitterionic surfactant, stability mechanism, synergistic

7

effect, EOR

8

1. Introductions

9

As the demand for energy increases along with the difficulty of oil recovery,

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more efficient and environmentally friendly methods of oil recovery are required.

11

Dating back to the 1950s, foam has been used to improve the oil displacement for

12

enhanced oil recovery (EOR); this method is especially effective in a

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non-homogeneous formation1. CO2 flooding has been widely used as an efficient

14

EOR method since 19722, but due to its low viscosity and low density, CO2 flooding

15

can

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high-permeability formations, therefore, has low sweep efficiency3. Foam flooding

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own strong profile control ability and today’s new generation of CO2 foams which

18

contain surfactant are often utilized to control the mobility during CO2 flooding4,5;

19

however, the foam generated by the surfactant is unstable and does not last very long6.

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The solid particles played an important role in forming a three-dimensional structure

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to slow down or stop the breaking of the bubbles7. Hunter et al.8 observed that the

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foam stability was proportional to the concentration of the solid and inversely

23

proportional to the particle size. Therefore, in recent decades, the rise of

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nanotechnology has led to increased interest in adding nanoparticle (NPs) to foam as a

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foam stabilizer9,10. NPs are adsorbed onto the gas-liquid interface irreversibly10, 11, and

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this is an important mechanism because compared to surfactants, NPs require

27

considerably more energy to desorb from the gas-liquid interface12. Under certain

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conditions, surfactants can form synergistic relationships with NPs13 and, as a result,

29

the foam becomes more stable14. Research has shown that hydrophilic NPs have an

easily

result

in

gas

channeling

and

gravitational

2


differentiation

in

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excellent foam-stabilizing capability15,16. SiOH-- groups form on the surface of the

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NPs; the particles have hydrophilic properties and are negatively charged and

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interactions occur between the NPs and the surfactant17, between the NPs, and

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between the surfactants in the solution. Revera et al.18 researched the dispersion of

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cationic surfactants and determined that when hexadecyl trimethyl ammonium

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bromide (CTAB) molecules adsorbed onto the hydrophilic NPs particles, the particles

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were changed into hydrophobic particles. Binks et al.19 confirmed that an optimal

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concentration ratio exists to obtain the most stable foam. Emrani et al.20 developed a

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three-dimensional network structure model for NPs absorbed on CO2 foam. For

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CTAB/SiO2 dispersions stabilize CO2 foams, NPs adsorb at the interface can slow the

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Ostwald ripening process, can improve the interfacial rheological properties and

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enhance the strength of the bubble film21. It has been shown that the synergistic effect

13

differs for different concentrations and different kinds of NPs and surfactants.

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To apply in EOR process, researcher paid a lot of efforts to investigate the

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mechanisms NPs could present during EOR process; Yuan et al.22,23 presented the first

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analytical model to evaluate the mechanism of nanofluid to mitigate fines migration

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for enhanced oil recovery; Furthermore. Yuan et al.24 also applied both experimental

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works and modelling efforts to evaluate the transport and adsorption phenomenon of

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NPs in porous media, and how to apply nanofluids to improve both well injectivity

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and oil recovery for low-salinity water flooding; All the above research gave us a

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determination that NPs would play an important role in EOR process. In regard to

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foam flooding, NPs could not make difference by itself, the choice of foaming agent

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concern to the EOR result. Mohd et al.25 chose Alpha-Olefin Sulfonate(AOS) as

24

foaming agent and explain the result it performed in EOR process; Guo et al.26 chose

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by-product fly ash and recyclable iron oxide NPs to improve carbon utilization in

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EOR process; all the above researcher gained reasonable results, and their choice of

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foaming agent and foam stabilizer was different. Thus, the following work is about

28

the choice of foaming agent and the reason to choose. 3


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The objective of this study is to evaluate the stability of CO2 foam for different

2

concentrations of hydrophilic NPs and surfactants, including CTAB as a cationic

3

surfactant, sodium dodecyl benzene sulfonate (SDBS) as an anionic surfactant,

4

polyethylene glycol tert-octyl phenyl ether (TX-100) as a nonionic surfactant, and

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dodecyl dimethyl ammonium oxide (OA-12) as a zwitterionic surfactant. The four

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kinds of surfactants used in this work are typical and representative, they are common

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and cheap, wildly used in oil field, so we chose these four kinds of surfactant. The

8

surfactants were combined with hydrophilic SiO2 NPs. Then interfacial characteristics

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were evaluated to determine the interactions between the NPs and the surfactant and

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the foam stabilization mechanisms of the NPs.

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We used a Ross-Miles foam-analyzer to produce the CO2 foam and measured the

12

foam’s stability and interfacial properties. Compared with classical foaming method,

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Ross-Miles method are closer to the foam generating method used in oil field. Our

14

goal was to determine the optimal and stable conditions and find the relationship

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between foam stability and system properties. The results of this study are expected to

16

provide petroleum engineers reference data for selecting a CO2 foam agent and

17

stabilizer.

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2. Materials and methods

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2.1 Materials

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The hydrophilic NPs dispersion (particle size is about 8 nm, concentration is 30

21

wt%) used in the experiment was obtained from Sigma-Aldrich. The pure NPs

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solution was maintained at a pH of 9.5, after being diluted by ultrapure water. The

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NPs concentration was 0.5 wt% and the particle size increased slightly to 15 nm and

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the pH was decreased to 8.

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Four kinds of surfactants were used: TX-100 (C34H62O11, nonionic surfactant,

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purity is 99%); CTAB (C16H33(CH3)3NBr, cationic surfactant, purity is 99%); SDBS

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(C18H29NaO3S, anionic surfactant, purity is 90%); OA-12 (C14H31NO, zwitterionic

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surfactant, purity is 60 %) . 4


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2.2 Experimental methods

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2.2.1 Preparation of CO2 foam

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A Ross-Miles foaming instrument was used in the experiment to generate the

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foam; Details of Ross-Miles instruments was introduced by Pásztor-Rozzo F.27. Outer

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annulus of Ross-Miles instrument is used to flow thermostatic water, and inner space

6

is used to generate foam through the sieve plate at the bottom. Gas could pass through

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the sieve, but the liquid could not. (shown in Fig. 1). The NPs, NaCl, and the four

8

kinds of surfactants were used at different concentrations. A solution of 5 ml was

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added to a foam container with a volume of 300 ml. Water was circulated and the

10

temperature in the water bath was controlled with a thermostat. The bottom of the

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foaming instrument was connected to a gas flowmeter and the entry was connected to

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the CO2 cylinder; the CO2 gas entered the foaming vessel at 20 ml/min and the CO2

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was mixed with the dispersion solution using a sieve plate. The apparatus is shown in

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Fig. 1. It is well known that the adsorption of NPs on liquid film is a slow process28,

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however, the Ross-Miles method is a quick-foaming method. Thus, the foaming

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process should be controlled in a certain rage, and give NPs enough time to adsorb on

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fluid film. When the foam volume reached 30 ml, the supply of CO2 was stopped. The

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changes in the foam height over time were recorded.

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Fig. 1. Schematic of CO2-foam generating apparatus

2.2.2 The optimum of CO2 foam system 5


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2.2.2.1 Comparison of foam stability

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The stabilization of the CO2 foam is measured by the foam half-time t1/2(min),

3

which is the time required for the foam height H(ml) to decrease to half of its original

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height H0 (30 ml). The original foam liquid volume is l0(5ml); t1/2 can be expressed as:

5

‫ݐ‬ଵ/ଶ = ‫ݐ‬൫‫ܪ‬ଵ/ଶ ൯

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‫ܪ‬ଵ/ଶ =

ுబ ି௟బ ଶ

(1) (2)

2.2.2.2 Surface tension and elasticity modulus

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The surface tension and elasticity modulus influence the bubble size and strength

9

of the fluid film respectively. An automatic interfacial rheometer (Tracker-H, TECLIS,

10

France)was used to measure these two parameters. The instrument consists of a

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dosing system, a light source, a CCD camera, a frame grabber, and a cuvette. When

12

measuring a bubble’s surface tension, a needle filled with CO2 generates a bubble in a

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cuvette containing the dispersion solution; after the bubble is stable, the CCD camera

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and frame grabber acquire an image of the bubble, which is uploaded to a computer

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The image is analyzed with software to determine the volume and area of the bubble

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to determine the dynamic surface tension. At the same time, the dosing system

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increases and decreases the surface area periodically by 10 wt%; the changes are

18

recorded and the elasticity modulus is calculated using the Young-Laplace equation.

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3. Results and discussion

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3.1 Concentration optimization of NPs

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In the experiment, four kinds of surfactant solutions with various NPs

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concentrations were evaluated. Figure 2 shows the relationship between the half-life

23

time of the foam and the NPs concentration. It is observed that the hydrophilic NPs

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have different effects on the CO2 foam stabilization for the different kinds of

25

surfactants.

6


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Fig. 2. Foam half-time at different NPs concentrations. (a) 0.02 wt% surfactant. (b) 0.1% wt%

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surfactant.

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In the Ross-Miles foaming test, the surfactants are adsorbed first on the liquid

4

film because of the slow adsorption of the NPs; therefore, the stabilization process

5

occurs later.

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The data in Fig. 2 show that the NPs do not result in the intensification of the

7

foam for the TX-100. The curve of TX-100/NPs first fall and then rise whether in (a)

8

or (b). As a kind of long molecular chain surfactant, TX-100 surfactant has a certain

9

viscosity. When the NPs are added into TX-100 surfactant solution, the aggregated

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TX-100 molecules were broken up, so the viscosity of the surfactant solution decrease

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making the foam perform poor in stability. But with the increase of NPs concentration,

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the viscosity of solution increases again. Therefore, at a low concentration of NPs, the

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foam gives a poor performance in stability. It could be concluded that the NPs with

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negative charge did not react with the nonionic surfactant based on electrical analysis,

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and the change of solution viscosity affected by NPs and the surfactants resulted in a

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decline or a rise in the foam stability.

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The NPs were able to stabilize the CO2 foam for certain SDBS concentrations.

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When the surfactant concentration is high and the NPs concentration is low, the

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hydrophilic groups repulse the dissociative negatively charged NPs because of the

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saturated adsorption on the fluid film. In addition, the NPs repulse each other, thus 7


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there exists a specific concentration at which a stable foam can be formed (Fig. 3(a)).

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When the SDBS concentration is low (Fig. 3(b)), the unsaturated adsorption of the

3

surfactant does not result in a stable foam. As the NPs concentration increases, Fig.

4

3(c) indicates that the foam becomes more stable regardless of the SDBS

5

concentration. We can conclude that an excess of NPs results in the desorption of the

6

SDBS, increased surface tension, and a decline in the foam stability (Fig. 3(d)).

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8 9 10

Fig. 3. Foam textures at concentrations of NPs and SDBS. (a) high SDBS & low NPs, (b) low SDBS & low NPs, (c) high NPs, (d) changes in surface tension

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When CO2 is dissolved in aqueous solution, it becomes acidic which lead OA-12

12

to show up a characteristic of cationic surfactant, compared with cationic surfactant

13

CTAB, The change in the concentration of the NPs had a similar influence on the

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foam stability of the CTAB and OA-12 solutions. At a low surfactant concentration,

15

an obvious positive effect on foam stability was observed; in contrast, when the

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surfactant concentrations were high, the NPs decreased the foam stability.

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Furthermore, starting from a certain concentration, the NPs and the CTAB or OA-12

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molecules flocculated together and precipitated, as shown in Fig. 4(a) and (b). Due to

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the sedimentation, the foam stability mechanism occurred at the macroscale rather

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than the microscale; however, compared with CTAB, OA-12 produced less 8


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flocculates.

2 3

Fig. 4. Sedimentation characteristics at various concentrations of CTAB and OA-12. (a)

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OA-12. (b) CTAB

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Therefore, as can be observed in Fig. 2(a), at relatively low surfactant

6

concentrations and a NPs concentration range from 0.4 wt% to 0.6 wt%, the NPs play

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an important role in contributing to the foam stability.

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3.2 Effect of surfactant concentrations

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The results indicated that a NPs concentration of 0.5 wt% led to the most stable

10

foam. Thus, for the next experiment, we used 0.5 wt% hydrophilic NPs as a base fluid

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and used different concentrations of the surfactants. This resulted in different

12

outcomes, as shown in Fig. 5.

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The nonionic surfactant, cationic surfactant, and zwitterionic surfactant exhibited

14

the same characteristics. At a certain surfactant concentration, the foam stability

15

reaches a maximal value; subsequently, the foam stability first increases and then

16

decreases as the surfactant concentration increases. In contrast, for the anionic

17

surfactant, the foam stability appears to be independent of the surfactant concentration,

18

with the exception that the NPs decrease the foam stability. This occurs because the 9


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desorption of the SDBS molecules results in a repulsion between the SDBS and the

2

NPs.

3 4

Fig. 5. Foam stability at various surfactant concentrations(0.5wt% NPs).

5

As a result, OA-12 generated the most stable CO2 foam with the NPs and the

6

foam was 2 to 5 times more stable than for the other surfactants. In the following

7

experiment, we evaluated the effects of temperature and salinity on the foam

8

formation.

9

3.3 Effect of temperature

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We used four groups of dispersion with the best foam stabilization capability:

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TX-100 and SDBS prepared at 0.05 wt% mixed with 0.5 wt% hydrophilic NPs

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respectively and CTAB and OA-12 at 0.02 wt% mixed with 0.5 wt% hydrophilic NPs

13

respectively. The effects of different temperatures on the four groups are shown in Fig.

14

6.

10


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Fig. 6. Foam stability at various surfactant concentrations and different temperatures.

3

The data indicate that the foam stability declined dramatically at high

4

temperatures. Compared with CTAB, SDBS, and TX-100, the OA-12 CO2 foam

5

maintained a better foam stability; in contrast, at temperatures of 50 ℃ or higher,

6

CTAB, SDBS, and TX-100 exhibited nearly the same foam characteristics.

7

We conclude that the combination of OA-12 and the NPs resulted in the best

8

synergistic effect; this suggests that this combination resulted in a tight fabric.

9

However, at high temperatures, the tight fabric slows down the evaporation of the

10

fluid film, similar to a lid on a pot. This indicates that a better synergistic effect

11

between the NPs and the surfactant might result in better heat resistance.

12

3.4 Effect of salinity

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Similar to the experiment described in section 3.3, various concentrations of

14

NaCl were added to the four dispersions to determine the effect of the salinity on the

15

foam half-time. The results are shown in Fig. 7.

11


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1 2

Fig. 7. Foam stability at different concentrations of surfactant and NaCl

3

As we can see in the figure, as the salinity increased, the foam stability decreased

4

sharply for all surfactants except for SDBS, which resulted in increased foam stability.

5

San et al.28 found that the amount of CO2 foam and the foam stability increased

6

with increases in the NaCl concentration from 1.0 wt% to 10 wt%. Their experiment

7

did not involve surfactants. However, we determined that the CO2 foam stability

8

decreased for some surfactants and increased for others with the increase in the NaCl

9

concentration. The SDBS CO2 foam was not very sensitive to changes in the salinity

10

but for the other three kinds of surfactants, the CO2 foam became rapidly unstable as

11

the salinity increased. The OA-12 surfactant resulted in more stable foam at high

12

salinity compared to TX-100 and CTAB.

13

These results indicate that the salinity had positive effect on the foam stability for

14

the SDBS surfactant compared with other surfactants.

15

3.5 The interfacial properties of CO2 foam

16

On a microscopic scale, the NPs concentration, surfactant concentration,

17

temperature, and salinity influence the interfacial properties and the CO2 foam

18

stability.

12


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Fig. 8. Surface tension at different concentrations of OA-12

3

The interfacial properties include the surface tension and elasticity modulus(1).

4

Generally, as the surface tension increases, the foam stability decreases. To verify this

5

assumption, we measured the surface tension and elasticity modulus for the

6

surfactants (Fig. 8 and Fig. 9).

7

Within a certain concentration range, the hydrophilic NPs and the cationic

8

surfactant exhibit a good synergistic effect, which is reflected in a decrease in the

9

surface tension (Fig. 8) and a marked increase in the elasticity modulus (Fig. 9). Thus

10

we can conclude that the surfactant and the hydrophilic NPs play different roles in the

11

foam stabilization; the surfactant decreases the surface tension and the hydrophilic

12

NPs increase the elasticity modulus.

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Fig. 9. Elasticity modulus at different concentrations of OA-12

3

Furthermore, the NPs play an active part only when the hydrophilic NPs react

4

with the cationic surfactant as the increase in the elasticity modulus shows. A pure

5

NPs solution or pure surfactant solution does not result even in a small increase in the

6

elasticity modulus. In addition, as the NPs concentration increases, the elasticity

7

modulus increases and then decreases, indicating that there is an optimal

8

concentration at which the synergistic effect between the surfactant and the

9

hydrophilic NPs is best.

1

Fig. 10. The morphology of different CO2 foams. (a) conventional aqueous foam (0.02 wt%

2

OA-12). (b) NPs-enhanced foam (0.02 wt% OA-12/0.5 wt% NPs)

3

In addition, we chose two surfactant/NPs groups to observe the CO2 foam 14


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morphology: ①0.02 wt% OA-12 solution, and ② 0.02 wt%OA-12/0.5 wt% NPs

2

solution. Figure 10(a) shows that the conventional aqueous foam (consisting of only

3

surfactant aqueous foam) is big, loose, and nonhomogeneous; most bubbles range in

4

diameter from 1 mm to 3 mm and this indicates that, as the foam is forming, small

5

bubbles easily change into large bubbles due to the Ostwald ripening effect30. In

6

contrast, Fig. 10(b) shows that the NPs-enhanced foam is tight and homogeneous. The

7

interfacial properties experiment showed that a low surface tension and high elasticity

8

modulus led to tight and small bubbles, which indicates that the NPs adsorption on the

9

fluid film results in a more stable CO2 foam.

10 11

Therefore, we propose a model that shows the mechanism of CO2 foam stabilization by hydrophilic NPs and the zwitterionic surfactant (Fig. 11).

12

13 14

Fig. 11. The mechanism of CO2 foam stabilization by hydrophilic NPs and a zwitterionic

15

surfactant. (a) inadequate surfactant concentration. (b) moderate surfactant concentration.

16

(c) excess surfactant concentration. (d) surfactant and NPs flocculation and precipitation. 15


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Figure 11 shows four states of the bubbles in the foam when the solution is acidic.

2

Figure 11(a) shows a state when the zwitterionic surfactant concentration is

3

inadequate; in this case, the surface tension and elasticity modulus have a higher value.

4

As the surfactant concentration increases (Fig. 11(b)), the surfactant reaches an

5

optimal concentration, in which the surfactant and the NPs achieve saturation

6

adsorption; the surface tension declines to the minimum value and the elasticity

7

modulus increases to the maximum value. As the surfactant concentrations increases

8

(Fig. 11(c)), some of the hydrophilic NPs are changed into hydrophobic particles by

9

the cationic surfactant. The adsorption of the NPs decreases and the solution start to

10

be feculent. Under this condition, the surface tension is invariable and the elasticity

11

modulus begins to decline rapidly due to the destruction of the three-dimensional net

12

structure formed by the NPs and the cationic surfactant. Figure 11(d) shows a state

13

when the cationic surfactant concentration is too high for the NPs; there are hardly

14

any NPs adsorbed onto the fluid film and nearly all of them are flocculated by the

15

surfactant and are precipitated. In addition, micelles also exist in the solution. Because

16

of the saturation adsorption of the surfactant, the surface tension remains low but the

17

elasticity modulus declines to the minimum value, as the dispersion of the none-NPs

18

foam shows.

19

However, even in the state shown in Fig. 11(d), a slight stabilization effect is still

20

observed as Fig. 2(c) and (d) shows. We can confirm that the flocculation or

21

sedimentation stabilizes the foam to some extent7,31, but these influences occur on the

22

macroscale instead of the microscale as the well-dispersed solution shows. Moreover,

23

once flocculation generates in the reservoir, they may bring formation damage,

24

consequently this system cannot be applied in EOR.

25

According to the Fig. 6 and Fig. 7, the foam stability sharply decreases under the

26

high salinity and high temperature. This may caused by the decrease of maximum

27

adsorption of surfactant and NPs in Fig. 11. For NPs/OA-12 system, high salinity or

28

high temperature may make maximum adsorption value decrease, the tendency of 16


Page 16 of 21

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Energy & Fuels

1

foam stability with varied concentration of NPs or surfactant is still consistent, which

2

is in line with the previous works32.

3 4

4. Conclusion

5

(1) Compared four kinds of CO2 foam that were generated by different surfactants

6

and hydrophilic NPs, the foam generated by the OA-12 surfactant was the most stable.

7

The interfacial properties indicated that a synergistic effect occurred between the

8

hydrophilic NPs and the zwitterionic surfactant under acidic conditions.

9

(2) The synergistic effect suggested that the hydrophilic NPs and the zwitterionic

10

surfactant play different roles with regard to the foam stability. The hydrophilic NPs

11

reinforce the intensity of the fluid film by forming a three-dimensional network

12

structure. The surfactant decreases the surface tension by adsorbing onto the fluid film.

13

The synergistic effect between the NPs and the surfactant has two aspects: one is the

14

decrease in the surface tension due to the surfactant and the other is the high elasticity

15

modulus due to the NPs bound by the zwitterionic surfactant.

16 17

Corresponding Author

18

*E-mail:

19

+86-532-86981183.

[email protected],

[email protected],

Tel:

+86-010-82322754,

20 21

Acknowledgment

22

This work was supported by the National Science Fund (51504222 and U1663206),

23

and the Climb Taishan Scholar Program in Shandong Province (tspd20161004). The

24

authors express their appreciation to the technical reviewers for their constructive

25

comments.

26 27

References

28

(1) Ye Zhongbin, principle of enhanced oil recovery [M], Beijing: Petroleum

29

Industry Press, 2007: 162-163 17


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