Antagonism between Crude Oil Fractions and Novel

Jan 15, 2016 - Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China. ‡. University ...
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The synergism/antagonism between crude oil fractions and Novel betaines solutions in reducing interfacial tension Jia-Hua Cao, Zhao-Hui Zhou, Zhi-Cheng Xu, Qun Zhang, Shi-Hong Li, Hai-Bin Cui, Lei Zhang, and Lu Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02566 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016

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The synergism/antagonism between crude oil fractions and Novel betaines solutions in reducing interfacial tension Jia-Hua Cao1,2, Zhao-Hui Zhou3, Zhi-Cheng Xu1, Qun Zhang3, Shi-Hong Li4, Hai-Bin Cui4, Lei Zhang1,*, Lu Zhang1,* 1

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,

China; 2

University of Chinese Academy of Sciences, Beijing 100049, P. R. China;

3

State Key Laboratory of Enhanced Oil Recovery, No.20, Xue Yuan Road, Beijing 100083, P. R.

China; 4

China Petroleum Engineering Co., ltd Beijing Company, Beijing 100086, P. R. China;

*Author to whom correspondence should be addressed. E-mail of Lu Zhang: [email protected] Lei Zhang: [email protected] Tel, 86-10-82543589

Fax, 86-10-62554670

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Abstract The Shengli crude oil fractions of saturates, aromatics, resins and asphaltenes were obtained by classic SARA method and acidic fractions were extracted by ethanol/water (v/v, 7/3) solution mixed with 1.5 mass % NaOH. The interfacial tensions (IFTs) of solutions of two different betaines with different structures against n-alkanes, kerosene and crude oil fractions dissolved in kerosene were studied in this paper. The experimental results show that, for linear betaine, a small quantity of crude oil fractions especially acidic fractions and resins can decrease IFTs of the system by forming a mixed adsorption film, which enhances the dynamic behavior and the compactness of interfacial films to some degree. Therefore it has a synergistic effect between crude oil fractions and linear betaine. However, for the branched betaine with a larger hydrophobic part, it displays absolutely opposite tendency because a small amount of active substances present in kerosene can have a synergistic effect with surfactant to achieve ultra-low IFTs, and the addition of crude fractions relatively affect the arrangement of surfactant molecules on the interface, leading to the obvious increase of IFT values. As a result, it has an obvious antagonistic effect between crude oil fractions and branched betaine.

Key Words: Betaine; Crude oil fractions; Interfacial tension; Synergistic; Antagonism

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1. Introduction Generally, it is well understood that interfacial tensions (IFTs) play a vital role in chemical flooding to enhance oil recovery (EOR) which has been the major subject of many studies after the primary and secondary production modes. Briefly, the injected chemical solutions can produce an ultralow interfacial tension (< 10−2 mN/m), resulting in the dramatical improvement of oil recovery efficiency. And the study focus for the EOR process is to search for a flooding system that will be highly efficient, low in cost, and safe for the environment [1-4]. Ultralow interfacial tension between oil and aqueous solution can be achieved by the employment of an appropriate surfactant in the EOR process. Anionic surfactants such as petroleum sulfonate and alkylbenzene sulfonate have been widely used in oilfield with the advantage of extensive source and lower cost

[5-8]

. Unfortunately, it is not widely used for anionic surfactants in high temperature and

high salinity reservoirs because of their high krafft point and poor salt tolerance, except those with polyoxyethylene group

[9-12]

, so one of the most commonly used surfactants in high temperature and

high salinity reservoirs mainly involve in zwitterionic surfactants [11-14]. Zwitterionic surfactants have both cationic and anionic centers attached to the same molecule and show many unique properties such as high foam stability, low toxicity, temperature resistance, and salt tolerance due to their special molecular structures

[12, 13]

. Betaines are an important kind of

zwitterionic surfactants and have been widely applied to EOR recently

[15-19]

. Despite large amount

of researches about IFTs between sufactant and EOR systems have been published numerously [20-26], there are few reports about the responsible mechanisms such as synergism/antagonism effect on the IFTs involving betaines solutions and crude oil fractions on the oil-water interface. Naturally, the active fractions present in crude oil are key factors in the reduction of IFTs mainly

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because of their behavior as natural surfactants

[27-39]

. Zhu et al

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[27]

detailedly discussed the

mechanisms responsible for affecting the IFT between alkylbenzene sulfonate and crude oil fractions. Zhao et al [28] also reported that acidic fractions and resins are dominant factors affecting the partition of surfactants between the oil phase and aqueous phase, which results in the great contribution to reducing IFTs. Saturates and aromatics occupy a large proportion in crude oil, but they do hardly form interfacial mixed film because of the lack of interfacial active substances [34-35]. Asphaltenes are the fraction of the crude oil soluble in toluene or benzene, but precipitating in pentane, hexane, or heptane, which can mechanically form very stable interfacial films between the oil phase and water phase [36-39]. Recently, Zhou et al

[19]

reported the interfacial interactions of betaines against hydrocarbons and

acidic model oils containing fatty acids through mixed adsorption, and their simulations illustrated that the larger hydrophilic head of betaine molecule flats at the interface, leading to a larger occupying area on the water side compared with the oil phase side. Therefore, in this paper, we mainly aim to measure IFTs between crude oil fractions and two novel betaine solutions by a spinning-drop interface tensiometer. For this purpose, the asphaltenes, saturates, aromatics, resins, and acidic fractions from Gudong crude oil and Sheng 2 block crude oil in Shengli oilfield of China were separated. Then, the IFTs between the model oils of kerosene containing crude oil fractions and the solutions with different betaines have been studied systematically. On the basis of experimental results, the possible mechanisms responsible for interfacial interaction between crude oil fractions and betaines solutions have been provided.

2 Experimental Section

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2.1 Materials Alkyl sulfobetaine (ASB) and xylyl substituted alkyl sulfobetaine (XSB) employed in this paper were obtained from Research Institute of Petroleum Exploration Development. The purity of two betaines checked by elemental analysis and 1H nuclear magnetic resonance (NMR) spectroscopy was above 95 mol %. The the critical micelle concentration (CMC) of ASB and XSB is 1.57×10-6 mol/L and 1.41×10-6 mol/L, respectively. Acidic components and asphaltenes used in this study were segregated from Gudong crude oil (density of the crude oil is 0.903 g cm−3 at 70 °C, and the acid number is 3.87 mg KOH/g) and Sheng 2 block crude oil (density of the crude oil is 0.8713 g cm−3 at 80 °C, and the acid number is 2.44 mg KOH/g), both of which came from Shengli oilfield in China. Kerosene used in the research was purified by by silica gel column chromatography (100-200 mesh) and its IFT against pure water reached a stable value of 40 mN/m. Double-distilled water (resistivity > 18.2 MΩ.cm) was used in the preparation of surfactant solutions with the approximate pH value of 7.0. H3C CH3

CH3 CH3

OH

HO

CH3(CH2)7CH(CH2)9N+CH2CHCH2SO3-

CH3(CH2)17N+CH2CHCH2SO3-

CH3

CH3

ASB

XSB

Scheme 1. Structures and abbreviations of alkyl sulfobetaine (ASB) and xylyl substituted alkyl sulfobetaine (XSB). 2.2. Separation of crude oil fractions The crude oil was separated into four components (asphaltenes, saturates, aromatics and resins)

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according to their polarizability and polarity (SARA analysis), thus we can better understand the interaction between crude components and the betaine surfactant

[27, 28, 34-39]

. Separation of

asphaltenes from crude oil was carried out by n-heptane adding 2400 ml of n-heptane to 80g crude oil at room temperature for 30 min. Then, the mixture was stirred and then left to stand for 7 days. The precipitated asphaltenes were filtered and washed with smaller portion of n-heptane until the filtrate was colorless. Later, the filtrate from the above was poured on a column containing silica-gel (100-200 mesh, activated at 110 °C for 10 hours). Besides, the saturates and aromatics were extracted by petroleum ether, toluene, and toluene/ethanol (1/9, v/v), respectively. Moreover, the acidic fractions of crude oil was extracted by ethanol/water (v/v,7/3) solution mixed with 1.5 mass % NaOH (aq). The separation of crude oil fractions is described in detail elsewhere [39].

2.3. Apparatus and Methods. The interfacial tensions were measured by Texas-500C spinning drop interfacial tensiometer (CNG USA CO.) [40-43]. The surfactant solution as outer phase was injected into the glass tube, and about 2µL oil phase as an inner phase was put into the middle of the tube. In all case, the measurements of the interfacial tension are at a rotating velocity of 5000 rpm. When the IFT is not low enough and the length of the oil drop (L, mm) is smaller than 4 times its diameter (D, mm), IFTs can be calculated according to following equation,



(ρ h − ρd )ω 2 



C

γ = 2.74156× exp(− 3)

 

L/ D<4

(1)

where ρh is the density of heavy (outer) phase (mg·L-1), ρ d is the density of light (drop) phase (mg·L-1), ω is the rotational velocity in rpm, and C is a coefficient determined by the ratio of the length to the width of the oil drop.

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For low IFT systems, where the length of the oil drop (L) in the spinning tube is greater than 4 times its diameter (D), IFTs can be calculated with a built-in software system according to the following equation,

γ = 3.42694 × 10 −7 (ρ h − ρ d )ω 2 D 3

L/D≥4

(2)

All experiments were performed at formation temperature of oilfield:Gudong (70.0 ± 0.5 °C) and Sheng 2 block (80.0 ± 0.5 °C).

3. Results and discussion 3.1 The extraction results of crude oil fractions. 3.1.1 The mass fraction of crude oil fractions. Using the classic sedimentation and column chromatography method, we obtained four crude active fractions such as asphaltenes, saturates, aromatics, and resins. The results are listed in Table 1, and the acidic components take up about 1.54 mass % and 1.82 mass % for Gudong and Sheng 2 block crude oil, respectively. Table 1. The mass fraction of crude oil fractions. Crude Oil

w (saturates) %

w (aromatics) %

w (resins) %

w (asphaltenes) %

Gudong

55.17

30.77

13.79

0.27

Sheng 2 block

49.79

20.04

27.63

2.54

3.1.2 The element analysis of crude oil fractions. We also obtained the mass fraction of different elements for five kinds of components in two crude oil samples by Elementar Vario EL (Germany) to better study the property and characteristic of each fraction, and the analysis results are shown in Table 2. As is known, the molar ratio of the elements hydrogen and carbon (n (H)/n(C)) is an important value to characterize the molecule structure of

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crude oil

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[44-45]

. According to element analysis results in Table 2, the n (H)/n(C) of saturates in two

crude oil samples is 1.77 and 1.99, respectively, which is the highest among the five fractions with the higher saturation. The outcome of 1.77 and 1.99 in the elemental analysis might be due to presence of other heteroatoms such as oxygen, or result from a certain abundance of aromatic species within the fraction; meanwhile, saturates does not contain nitrogen as a result of the weak polarity; on the contrary, the n(H)/n(C) of aromatics is lower in comparation with saturates, probably due to the exist of aromatic ring [46-47 ]; furthermore, the mass fractions of nitrogen in resins and asphaltenes are comparatively high, illustrating that the polarity of both fractions is relatively strong. Table 2 . The mass fractions of elements in each fraction of two crude oils. component

saturates

aromatics

resins

asphaltenes

Crude Oil

w (N)%

w (C)%

w (H)%

n(H)/n(C)

Gudong

-

87.01

12.85

1.77

Sheng 2 block

-

84.20

13.50

1.92

Gudong

0.83

86.53

10.16

1.41

Sheng 2 block

0.65

83.40

12.07

1.73

Gudong

1.67

78.75

10.05

1.53

Sheng 2 block

0.99

79.41

10.79

1.63

Gudong

1.15

70.26

8.46

1.44

Sheng 2 block

0.33

81.55

9.05

1.33

3.2 The IFTs between n-alkanes and betaine solutions. Surfactant has high capacity of reducing IFTs because of its amphiphilic structure, which determines the extensive applications of surfactant in the oilfield [1, 48]. The reduction of IFTs directly depends upon the replacement of solvent molecules at the interface by surfactant molecules. As the adding of surfactant in bulk aqueous, the dissimilarity between oil phase and water phase will be weakened more or less. In this case, to reach an ultralow IFTs, the interactions between the 8

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hydrophilic head of surfactant and water molecules on one side of the interface and the hydrophobic tail and oil molecules on the other side of the interface must be very similar, in this way the surfactant molecules can tightly aggregate on the interface and gradually replace interfacial solvent molecules until saturation. Upon the discussion above, the greater the concentration of a certain surfactant below CMC, the larger the number of surfactant molecules in the particular area adsorbed on the interface. However, the discrepancy of IFTs is controlled by the structures and properties of surfactant in the system [2]. Betaines are widely applied in oilfield owing to their high surface activity. To full-scale study the properity of betaine, we have measured the dynamic IFTs of ASB and XSB containing from 0.005 mass % to 0.1 mass % at 1.0 mass % NaCl, and the results are plotted in Figure 1. It can be seen from Figure 1 that both the curves of dynamic IFT display a “L” shape which is the most typical characteristic type in the reported literature

[49]

. That is, when water phase and oil phase contact,

surfactant molecules will rapidly diffuse from the bulk to oil-water interface and adsorb onto the interface. Therefore, the IFTs descend along with the increasing concentration of surfactant on the interface until it is completely covered by surfactant molecules. 10

10

ASB+1% NaCl 0.005% 0.05%

XSB+1% NaCl 0.005% 0.05%

0.01% 0.1% IFT(mN/m)

(A)

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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1

(B) 0.01% 0.1%

1

0.1

0.1

0.01 0

20

40

60

0

20

Time(min)

40

60

80

Time(min)

Figure 1. The Dynamic IFTs of (A) ASB and (B) XSB solutions with different weight fractions against decane.

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For ASB, the IFTs can only reach 10−1 mN/m order of magnitude, but the IFTs of XSB could lastly decrease to 10−2 mN/m order of magnitude at equilibrium. Obviously, it is not able to realize ultralow IFT values for bath ASB-decane and XSB-decane system, due to the hydrophilic head of betaine flatting at the interface [19], which occupys larger area compared with the hydrophobic side and thus it is not the best size distribution. However, XSB shows the higher interfacial activity than ASB against decane because of its branch structure with larger sized hydrophobic part, which relatively enlarges the area of hydrophilic side and narrows the difference of area on the interface between hydrophilic part and hydrophobic part. Therefore, the ASB moleculs can only pack loosely on the interface, resulting in the higher IFTs of ASB-decane system compared with the relatively dense packing of the adsorbed layer of XSB-decane system. 10

0.1% ASB+1% NaCl 0.1% XSB+1% NaCl 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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1

0.1

0.01 6

8

10

12

14

ACN

Figure 2. The equilibrium IFTs between ASB or XSB and n-alkanes with different carbon number. For the sake of detecting the influence of surfactant structure in reducing IFTs, the effect of n-alkane carbon number (ACN) on the dynamic IFTs between 0.1 mass % ASB or 0.1 mass % XSB has been investigated and plotted in Figure 2. As can be seen in Figure 2, the equilibrium IFTs versus ACN of 0.1 mass % ASB and 0.1 mass % XSB shift slightly from about 0.1 mN/m to 1 mN/m and 0.01 mN/m to 0.1 mN/m, respectively. In addition, the IFTs of both systems pass through a minimum

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value along with the ACN, which was defined as IFTmin. Meanwhile, the number of alkane carbons for the IFTmin is called the nmin of this surfactant solution. The nmin is very helpful for understanding the mechanisms responsible for the reduction of IFTs and can be used as an efficient way to characteristic the hydrophilic-lipophilic balance that is essential for interfacial activity [5]. The higher the nmin value, the stronger the oil-soluble ability of surfactant [1, 4] . Similar to nmin, the concept of the equivalent alkane carbon number (EACN) [1] for an oil phase will be obtained by a series of surfactant mixtures of the same structural type, and EACN is defined as the number of carbon atoms of the linear alkane that has its optimal formulation in the same conditions as the oil. For a certain oil phase such as hydrocarbon, hydrocarbon mixture, or crude oil, the EACN value can represent its influence on the partition of the surfactant in the oil phase. The higher the EACN value of the oil phase, the weaker the ability of the surfactant participating into oil phase. Therefore, when the nmin value of a surfactant solution is in line with the EACN value of an oil phase, the maximum surfactant concentration on the interface can be achieved, resulting in the achievement of IFTmin. From Figure 2 we can also find that the nmin value of ASB and XBS is 10 and 9, respectively, illustrating that the branched structure can enhance the hydrophilic ability of XBS to some extent. In addition, within the scope of the alkane chain length, the IFT of XSB is always lower than ASB, which further states that the stronger ability in reducing IFTs between XSB and n-alkanes mainly lies in its larger hydrophobic groups rather than the HLB values. Worth to mention, the EACN value of kerosene is 7 reported in the literature [27, 29-30], which is obviously not agreement with the nmin of both surfactant ASB and XSB, so the ultralow IFT value between betaines solutions and kerosene in the following discussion is probably not resulted from the HLB values.

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3.3. The IFTs between kerosene and betaine solutions. To better study the mechanism of IFTs in crude oil/ surfactant system, the crude oil fractions have been dissolved in kerosene, which is the most common means of simulation in recent years

[27-28]

.

Noticeably, the IFT values between decane and pure water at room temperature is about 50 mN/m, but in this research the IFT value of kerosene is only 40 mN/m against pure water, indicating that there still exists some active substances in kerosene after being treated. On account of above cases, we have investigated the dynamic IFTs of both ASB and XSB against kerosene and the results are shown in Figure 3. It can be seen that the IFTs of ASB are capable to reach 10−2 mN/m order of magnitude at a short time, and come back to a stable value immediately, which makes the dynamic IFTs curve present a “V” shape.

10

10

(A)

1% NaCl+ASB 0.005% 0.01% 0.05% 0.1%

1

0.1

(B)

1% NaCl+XSB 0.005% 0.01% 0.05% 0.1%

1

IFT(mN/m)

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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0.1

0.01

1E-3

1E-4

0.01 0

10

20

30

40

50

0

60

20

Time(min)

40

60

80

100

Time(min)

Figure 3. The dynamic IFTs of (A) ASB or (B) XSB solutions with different weight fractions against kerosene. The interesting results of IFTs for ASB can be explained by the arrangements of surfactant molecules at the interface in the presence of active substances, as schematically depicted in Figure 4. When oil phase and water phase contact, ASB molecules in water side and active substances in oil side will continuously diffuse towards oil-water interface. By this time, the hydrophilic group of ASB molecules tends to arrange upright at the interface, forming a tight layer of mixed adsorption 12

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film with active substances, which rapidly results in an instantaneously low IFTs; with time going on, the hydrophilic group of ASB molecules will gradually tilt from vertical to horizontal status, thereby continuously lessen active substances on the interface. This may be one reason that the dynamic IFTs of ASB system can’t keep ultralow level during a long time. Therefore, it is very reasonable that the rate of adsorption is exceeded by the rate of desorption after the rearrangement of active molecules on the interface, leading to the increasing IFTs for ASB system. Finally, the rates of adsorption and desorption are equal and the equilibrium IFT occurs. However, the IFTs of XSB even drastically decrease to the platform value after a period of time, and can eventually decrease to 10−3mN/m order of magnitude at the higher concentration, which have a great contribution toward displaying the dynamic IFTs curve of “L” shape. As shown in Figure 4, consider the large hydrophobic groups of XSB molecules, whether hydrophilic group is upright or not has little influence on the terminal number of active substances absorbed on the interface, so we can observe a continuous decrease of IFT to its equilibrium value for all XSB solutions with the increasing concentration. In a word, a small amount of active substances existed in kerosene can play a synergistic effect with betaines to reduce the IFTs to a ultralow level of the system more or less

[20-26]

. Moreover, it is

very important to point out that the synergistic effect is more obvious in XSB system in comparation with ASB. Therefore, in the following study, we will discuss the mechanism responsible for interfacial tension between crude oil components and betaines solutions with different structures. 3.4. The IFTs between model oil of crude oil fractions and betaine solutions. 3.4.1 The model oil of acidic fractions. Acidic fractions are the most important active substances present in crude oil, whose molecules

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can easily absorb onto the interface, and it also has chemical reaction with the alkaline solution and produce petroleum soap [27-31]. The IFTs between acidic fractions dissolved in kerosene and 0.1 mass % ASB or XSB have been expounded. And the dynamic IFTs between acidic fractions in Gudong crude oil and betaine solutions are plotted in Figures 4 (A) and (B). Obviously, the dynamic IFTs curve of ASB and XSB remainingly present a “V” shape and a “L” shape, respectively, which is similar with the pure kerosene system. Meanwhile, with the increasing of acidic fractions concentration, the distinct reduction and the evident increment of IFTs for ASB system and XSB system is observed, respectively, mainly because of their different molecular structures. Therefore, we will systematically study the different hydrophobic structure of betaines solutions on IFTs among the system of acidic fractions in the latter research. 10

10

(B)

(A) 1

IFT(mN/m)

IFT(mN/m)

1

0.1

0.01

Oil: Acid fraction/kerosene Aqueous: 0.1% ASB+1% NaCl 0% 0.1% 0.5% 1% 2%

1E-3

0.1

Oil: Acid fraction/kerosene Aqueous: 0.1% XSB+1% NaCl 0% 0.1% 0.5% 1% 2%

0.01

1E-3

1E-4

1E-4 0

10

20

30

40

50

60

70

0

80

20

Time(min)

40

60

80

100

Time(min)

Figure 4. The dynamic IFTs between (A) ASB or (B) XSB and acidic fractions of Gudong crude oil with different weight fractions. 1

30

(A)

Aqueous: 0.1% ASB+1% NaCl Oil: Acid fraction/kerosene Gudong Sheng 2 block

pure kerosene 0.1

20

(B)

Aqueous: 0.1% ASB+1% NaCl Oil: Acid fraction/kerosene Gudong Sheng 2 block

Time(min)

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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0.01

10

1E-3 0

0.0

0.4

0.8

1.2

1.6

2.0

0.0

0.5

Weight Fractions(%)

1.0

1.5

2.0

Weight Fractions(%)

Figure 5. The IFTmin (A) and the times (B) needed for reaching IFTmin between ASB and two 14

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acidic fractions with different weight fractions. Figures 5 (A) and (B) display the effect of acidic fractions on the minimum values of IFT and the times needed for reaching IFTmin as a function of the concentration of ASB for Gudong crude oil and Sheng 2 block crude oil. It can be seen from Figure 5 (A) that the minimum IFTs can be achieved from the order of magnitude 10−2 mN/m to almost the order of magnitude 10−3 mN/m with the gradual increasing of acidic fractions. When oil phase and water phase contact, ASB molecules will rapidly diffuse towards oil-water interface and arrange upright at the interface, and the rates of adsorption exceeds the rates of desorption, which results in an instantaneously low IFTs; but the hydrophilic group of ASB molecules will gradually tile from vertical to horizontal status, decreasing the number of ASB molecules adsorbted on the interface, so the IFT began to increases with time. Eventually, the rates of adsorption and desorption are equal and the equilibrium IFT occurs. Besides, the higher the concentration of acidic fractions, the longer the time to reach transient ultralow IFTs, especially at the concentration of 2 mass % as shown in Figure 5 (B), which may be attributed to the maximum interfacial concentration of active substances. In this case, it is also because that there exists competition between ASB and acidic fractions during mixed adsorption process, which relatively prolongs the time to reach the maximum capacity of active substances absorbted on the interface. In other words, the added acidic fractions in kerosene indeed play a good synergistic effect with ASB molecules to achieve the great reduction of IFTs to an ultralow level for two acidic fractions molecules. Oppositely, it can be seen from Figure 4 (B) that the IFTs of XSB system increase rather than decrease after adding acidic fractions in kerosene. At the same time, we can see from that the greater the concentration of acidic fractions, the higher the IFT value, meaning that there may exist an

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antagonism effect between XSB molecules and acidic fractions based on mixed adsorption. More importantly, for XSB system, the minimum value of IFT is in accordance with its equilibrium value, which illustrates that the molecular structure have an influence on dynamic IFT curve. So we assume that the adding of acidic fractions may affect the dynamic IFTs of XSB through different ways. On one hand, a small amount of active substances present in kerosene can play a valid synergistic effect with XSB molecules, which can obviously reduce IFTs to an ultralow level. The added acidic fractions will disturb the stable interfacial films through mixed adsorption, which affects the arrangement of betaine molecules on the interface and increase IFTs to a large extent; on the other hand, acidic fractions may change the polarity of oil phase, influencing the partitioning of betaines among the oil, the water, and the interface. The nmin value of XSB is 9 and the EACN value of kerosene is 7. The hydrophilic-hydrophobic balance will be more destroyed with the addition of acidic fraction because acids will decrease the EACN value of oil phase [23,29]. As a result, it tends to be an antagonism between acidic fractions and XSB. 3.4.2 The model oil of resins. 10

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0% 1%

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Figure 6. The dynamic IFTs of (A) ASB or (B) XSB against oil containing Gudong resins with different weight fractions. Resins model oil shows almost the same trend as acidic fractions in affecting IFTs, because most

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acidic fractions distribute over resins [27, 28]. The dynamic IFTs of resins for Gudong crude oil against 0.1 mass % ASB and 0.1 mass % XSB are plotted in Figure 6 (A) and (B), respectively. For ASB, the transient ultralow IFTs can reach the order of magnitude 10−3 mN/m at higher concentration. For XSB, the IFTs can exceed to 1 mN/m after adding resins in kerosene. The minimum values of IFT and the times needed for reaching IFTmin of 0.1 mass % ASB as a function of resins concentration for Gudong crude oil and Sheng 2 block crude oil have also been expounded as shown in Figure 7 (A) and (B). It appears almost the same trend as acidic fractions that the minimum IFTs of ASB system decrease along with the adding of resin fractions, and the times needed for reaching IFTmin prolong, especially for Gudong crude oil at higher concentration. Moreover, the worth mentioning aspect in Figure 7 (B) is that the the times needed for reaching IFTmin of two resins fractions are somewhat dissimilar. This phenomenon may result from the complexity of resins from different crude oil in Shengli oilfield. On the basis of the above experimental results, we can conclude that there also show a synergism or antagonism effect between resins and betaine molecules depending on their different hydrophobic part. 1

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Figure 7. The IFTmin (A) and the times (B) needed for reaching IFTmin of ASB against two oils containing resins with different weight fractions. 3.4.3 The model oil of asphaltenes. 17

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Generally, asphaltenes with low surface activity and large molecular size have a weak tendency of forming mixed adsorption films

[36, 38]

. In this paper, the dynamic IFTs between asphaltenes for

Gudong crude oil and two betaines solutions are described in Figure 8. We can see that asphaltenes with different concentration have little influence on dynamic IFTs of ASB system. Meanwhile, from Figure 9 (A), the minimum IFTs of Gudong crude oil and Sheng 2 block crude oil slightly vary around 0.01 mN/m and 0.03 mN/m, respectively, and the most noticeable difference with acidic fractons or resins is that the time to reach minimum IFT value which is almost the same at different concentration as shown in Figure 9 (B). That is to say, the interfacial concentration of ASB molecules changes little along with the variations of asphaltenes and the synergism effect between asphaltenes and ASB is relatively weaker in comparation with acidic fractons or resins. However, for XSB, it displays a similar trend as above acidic fractions or resins model oil at higher concentration by disturbing the original adsorption films, though the influence of asphaltenes is appreciably weaker at lower concentration. 10

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Figure 8. The dynamic IFTs between (A) ASB or (B) XSB and asphaltenes of Gudong crude oil with different weight fractions.

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Aqueous: 0.1% ASB+1% NaCl Oil: Asphaltene/kerosene Gudong Sheng 2 block

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Figure 9. The IFTmin (A) and the times (B) needed for reaching IFTmin between ASB and two asphaltenes fractions with different weight fractions. 3.4.4 The model oil of saturates and aromatics. To further understand the effect of crude oil characteristic on IFTs, we have investigated the IFTs between two betaines solutions and saturates or aromatics which possess relatively lower surface activity

[34, 35]

. It can be seen from Figure 10 (A) and (C) that the dynamic IFTs of saturates or

aromatics decrease as the increasing of concentration for ASB in general. Meanwhile, in Figure 10 (B) and (D), we find that the stable IFTs of XSB regularly increase with the gradual addition of saturates or aromatics, which reveals the similar curves with other fractions discussed above. These experimental results indicate that the small amount of active substances in saturates or aromatics will also show obvious synergism and antagonism for ASB and XSB respectively. 10

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Figure 10. The dynamic IFTs between betaines solutions and saturates or aromatics of Gudong crude oil with different weight fractions: (A) ASB/saturates system (B) XSB/saturates system (C) ASB/aromatics system (D) XSB/aromatics system. Similarly, the minimum values of IFT and the times needed for reaching IFTmin of ASB along with the concentration of saturates or aromatics for Gudong crude oil and Sheng 2 block crude oil are shown in Figure 11. Overall, the tendency of minimum values of IFTs for saturates and aromatics are almost the same as resins which are thoroughly studyed in the previous part. On the other hand, the variations of the times needed for reaching IFTmin are almost the same as asphaltenes. These results suggest that the synergism/antagonism effect between crude fractions and betaine molecules is not only related to the characteristic structure of betaines, but also associated with the specific properity of active fractions present in crude oil. 1

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Figure 11. The IFTmin and the times needed for reaching IFTmin between ASB and saturates or aromatics with different weight fractions: ASB/saturates system (A and B), ASB/aromatics system (C and D).

3.4.5 The comparisons of different model oils

0.1

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Aqueous: 0.1% XSB+1% NaCl Oil: Crude oil fractions of sheng 2 block/kerosene Acidic Fractions Resins Asphaltenes Saturates Aromatics

(D)

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pure kerosene 1E-4 10

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Weight Fractions(%)

Figure 12. The equilibrium IFTs between surfactant and crude oil fractions with different

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weight fractions: (A) ASB/Gudong system (B) XSB/Gudong system (C) ASB/Sheng 2 block system (D) XSB/Sheng 2 block system. The variations of equilibrium IFTs values between betaines solutions and five fractions as a function of concentration for two crude oils are plotted in Figure 12 (A), (B), (C), and (D), respectively. From Figure 12 (A) and (C), we can clearly see that the IFTs of ASB/ crude oil fractions for Gudong crude oil and Sheng 2 block crude oil are particularly similar, which are generally lower than those of ASB/kerosene system. On one hand, fractions with lower activity such as asphaltenes, saturates and aromatics have less influence on the arrangement of interfacial film, resulting in the slight reduction of IFTs after the adding of above fractions; on the other hand, the acidic fractions and resins with higher activity have preferable effect on the IFTs of ASB system, and the IFTs can even be reduced to 10−2 mN/m orders of magnitude at higher concentrations. At this moment, the acidic fractions and resins have played a good synergistic effect with ASB molecules on the interface. So we can propose that the mixed adsorption of ASB with small size of hydrophobic part and crude oil fractions is beneficial for the synergistic effect. We can find from Figure 12 (B) and (D) that a small amount of active substances in kerosene can achieve ultralow IFTs, and there exists obvious synergistic effect between active substances present in kerosene and XSB. However, it reveals an increasing trendency of IFTs when adding crude oil fractions even only a small quantity to kerosene, especially for acidic fractions and resins. By this time, it has an antagonism effect between crude oil fractions and XSB because of the complete adsorption of acids/resins and the destroying of hydrophilic-hydrophobic balance. The above-mentioned results not only show that the influences of various fractions on IFTs for different surfactant systems are different, but also prove that the importance of size compatibility of surfactant

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molecules. So the molecular structure of betaines and the activity of crude oil will be two key factors in controlling IFTs of the systems.

4. Conclusion The present work is devoted to studying the synergistic/antagonistic effect between crude oil fractions and novel betaines solutions in reducing interfacial tension. It shows that the larger hydrophilic head of betaines tends to flat at the oil-water interface, leading to the water side occupying a larger area compared with the oil side. The IFTs of branched betaine (XSB) against n-alkane is lower compared with linear betaine (ASB), mainly due to its larger hydrophobic part, which relatively enlarges the area of hydrophilic side on the interface. A small amount of active substances in kerosene can play an evident synergistic effect with two betaines of different hydrophobic part. The dynamic IFTs of ASB can be obviously reduced at a shorter time, and it will start to increase when the hydrophilic group of ASB molecules gradually tilts from vertical to horizontal. However the dynamic IFTs of XSB system show a “L” shape because of its larger hydrophobic part. By adding crude oil fractions, especially acidic fractions and resins with higher activity, the IFTs of ASB can be easily reduced to some degree through the formation of closely-packed mixed adsorption film, and it has played a good synergistic effect between crude oil fractions and ASB molecules. However, it almost displays an opposite trend for the IFTs of XSB system, because of the complete adsorption of acids/resins and the destroying of hydrophilic-hydrophobic balance, which relatively affects the original arrangement of surfactant molecules and weakens the compactness of interfacial films.

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Acknowledgemen The authors thank financial support from the National Science & Technology Major Project (2016ZX05011-003) and National Natural Science Foundation (51373192) of China.

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