The structure effect on the surface and interfacial properties of zwitterionic sulfobetaine surfactants for enhanced oil recovery

Abstract

The surface and interfacial properties of five zwitterionic surfactants, including three propyl sulfobetaines CSB (where the carbon atom number of the alkyl chain is 12, 14 and 16, respectively) and two hydroxypropyl sulfobetaine surfactants CHSB (where the carbon atom number of the alkyl chain is 12 and 14, respectively), were studied at both air–water and oil–water interfaces. The surface activity of these surfactants at the air–water interface in aqueous solutions was investigated by the Wilhelmy plate method at 30 °C and ambient pressure. The values of the critical micelle concentration (CMC) and surface tension at CMC (γ_CMC) were determined from the surface tension measurements. The obtained results indicate that CMC and surface tension strongly depend on the surfactant molecular structure. An increase in the alkyl chain length results in a decrease in the CMC and γ_CMC values. The presence of a hydroxyl group causes an increase in CMC values and a decrease in γ_CMC values. The hydroxypropyl sulfobetaine surfactants have better surfacial properties. In addition, the interfacial activity at the oil–water interface among the crude oil–reservoir water–surfactant systems was investigated by use of the spinning drop method under harsh reservoir conditions of high temperature (90 °C) and high salinity (11.52 × 10⁴ ppm, including 7040 ppm Ca²⁺ and 614 ppm Mg²⁺). It is interesting that the transient minimum dynamic interfacial tension (DIT_min) could be observed in a specific concentration range. The time to reach DIT_min is different with different surfactant molecular structures and surfactant concentrations. The hydroxypropyl sulfobetaine surfactant C₁₄HSB shows excellent interfacial properties: it can reduce interfacial tension (IFT) between oil and water to an ultralow level at a very low concentration, and the ultralow IFT phenomenon only occurs in a specific concentration range from 0.03 to 0.10 wt%. In this work, hydroxypropyl sulfobetaine surfactants exhibit remarkable ability and are good candidates for chemical agents to enhance oil recovery in harsh reservoirs.

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Introduction

Zwitterionic surfactants possess both a positively charged cationic hydrophilic group and a negatively charged variable anionic hydrophilic group attached to each molecule. The cationic moiety is based on primary, secondary, or tertiary amines or a quaternary ammonium group. The variable anionic moiety includes carboxylic acids, sulfonic acids, sulfuric acid esters and phosphoric acid esters, which may not be adjacent to the cationic site. Zwitterionic surfactants have been widely used in the field of cosmetics and washing products, and underlie virtually every aspect of our daily lives.^1–4 Their colloidal behaviors in aqueous solutions have been extensively investigated within a specific range of temperature through surface tension, conductivity and light scattering measurements.^5–9 Recently, many studies have indicated that zwitterionic surfactants can be used in chemical enhanced oil recovery (EOR) because of their excellent water solubility, remarkable interfacial properties, high foam stabilities, insensitivity to temperature and salinity, and synergistic effect with ionic and anionic surfactants.^10–13 In particular, sulfobetaine surfactant, with anionic sulfonic groups in hydrophilic headgroups, is a hot topic and promising research focus in EOR for harsh reservoirs with high temperature and high salinity.^14–16

To aid the usage of surfactants for EOR, investigations of the interfacial properties at the oil–water and oil–water–rock interfaces are the most relevant.¹⁷ The interfacial phenomena of surfactant solution are more complex under reservoir conditions. The properties and compositions of crude oil and reservoir rock have significant effects.¹⁸ One of the most commonly and simply measured parameters of interfacial behavior is the interfacial tension (IFT) between crude oil and surfactant solution. For the displacement of crude oil in the pores and capillaries of petroleum reservoir rock, it is generally required to reduce the IFT to ultralow level (less than 10⁻² mN m⁻¹) through theoretical consideration and practical laboratory scale experiments.¹⁹ Ultralow IFTs can be achieved with appropriate surfactants by adsorption at the oil–water interface, which can increase the capillary numbers by several orders of magnitude and effectively displace residual crude oil from the reservoir.²⁰ Laboratory experiments and oilfield applications confirm that the sulfobetaine surfactant can reach ultralow IFTs as a single-component chemical flooding agent,^21–25 avoiding chromatographic separation due to the different adsorption capacities of each component of the multicomponent displacing fluids. Ultralow IFTs were obtained in a wide range of sulfobetaine concentration from 0.01 to 0.3 wt% for Daqing reservoirs at a temperature 98 °C, salinity of 220 000 mg L⁻¹ and divalent ions of 2300 mg L⁻¹. However, these research studies only focus on selecting optimum surfactants to certain reservoir conditions and carrying out a series of performance evaluation.

Many other experimental research studies have focused on the dynamic interfacial tension (DIT) behaviors between crude oil and single-component sulfobetaine surfactant flooding systems. Studies of the DIT give information not only on the adsorption rate, but also for the mechanism of adsorption of surfactant molecules, thus helping to reveal the factors governing the adsorption process. In order to better study the interfacial behaviors of surfactants at the oil–water interface, it is necessary to investigate their surfacial ability. A large number of research studies into sulfobetaine surfactants have concentrated on their thermodynamics of micellization.^8,26,27 The micelles are formed when the surfactant concentration in aqueous solutions reaches the critical micelle concentration (CMC). At this point, various properties of a surfactant solution are noted, including conductivity, osmotic pressure, and turbidity.²⁸ CMC can be affected by various factors, including surfactant species (hydrophobic volume, chain length and headgroup area), temperature, pressure, ionic strength, pH, etc. These factors also have some effect on IFT behaviors. The study of CMC is helpful to gain a better insight of many physical and chemical properties of surfactants.

In our laboratory, the excellent performance of sulfobetaine surfactants have been confirmed in the chemical EOR process, but lacked systematic theoretical investigation on the surfacial and interfacial properties. So in this work, five sulfobetaine surfactants with potential application in EOR with excellent abilities of temperature resistance and salt tolerance were chosen to investigate the influence of the molecular structure on the surface and interfacial properties. First, surface properties were studied at the air–water interface at 30 °C in an aqueous solution. Second, interfacial properties were studied at the oil–water interface in the crude oil–reservoir water–surfactant system under harsh reservoir conditions of high temperature and high salinity. The research results contribute to the better screening of surfactant flooding agents for harsh reservoirs.

Experimental section

Materials

The three propyl sulfobetaine surfactants, dodecyldimethylammoniopropanesulfonate (C₁₂SB), dimethylmyristylammoniopropanesulfonate (C₁₄SB), dimethylpalmitylammoniopropanesulfonate (C₁₆SB), and two hydroxypropyl sulfobetaine surfactants, dodecyldimethylammoniohydroxylpropanesulfonate (C₁₂HSB) and dimethylmyristylammoniohydroxylpropanesulfonate (C₁₄HSB), were supplied by Promise Song Industry (Shanghai, China) and used with further purification. Hydrochloric acid was dropped in the commercial product dissolved in hexane, giving hydrochloric acid salts as white solids; moreover, the solids were washed with acetone, and then recrystallized twice from methanol. Finally, white crystal powder was obtained after vacuum desiccation at 60 °C.

The molecular structures are shown in Scheme 1.

Scheme 1 Molecular structures of sulfobetaine surfactants.

Reservoir water and crude oil were supplied by the Tarim Oilfield in West China. The total dissolved solids (TDS) were 11.52 × 10⁴ ppm with a high concentration up to 7654 ppm of divalent metal ions, i.e. Ca²⁺ and Mg²⁺. The composition of the water is listed in Table 1. The crude oil was treated by dehydration and degassing. Its viscosity and density were 7.8 cP and 0.825 g mL⁻¹ at 90 °C.

Table 1 Quality analysis of the reservoir water of Tarim Oilfield

Ions	Na^+	Ca^2+	Mg^2+	Cl^−	SO4^2−	HCO3^−
Concentration (ppm)	36 660	7040	614	70 560	245.3	103

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Methods

Surface tensions. Aqueous solutions of sulfobetaine surfactants were prepared at different concentrations from 0.01 mmol L⁻¹ to 10 mmol L⁻¹ using distilled water. The surface tensions of these solutions were measured at 30 °C by Dataphysics DCAT41 Contact Angle/Surface Tension Meter (the German Dataphysics Company, Germany) using the Wilhelmy plate method. The temperature was controlled by a thermostat water bath. All the measurements were repeated three times and averaged.

To further study the effect of temperature on the surface activity, the maximum surface excess concentration Γ_max and the minimum area A_min occupied per surfactant molecule at the air–water interface were proposed and determined as follows:

where n is the number of solute species, whose concentrations at the interface change with the surfactant concentration c; the value of n is taken as 1 for a zwitterionic surfactant in aqueous solution; R is the gas constant (8.314 J mol⁻¹ K⁻¹); T is the absolute temperature; γ represents the surface tension; dγ/d(ln c) is the slope of surface tension γ vs. ln c dependence when the concentration is near CMC; and N_A is Avogadro's constant (6.022 × 10²³ mol⁻¹).

Oil–water interfacial tensions. Solutions of sulfobetaine surfactants were prepared at different concentration from 0.01 wt% to 0.3 wt% by reservoir water. They were sealed in tubes and observed visually clear after 24 h at 90 °C. The interfacial tensions between crude oil and these solutions were measured at 90 ± 0.1 °C by TX-500C spinning drop interface tensiometer (USA KINO Industry Co., LTD, USA). The rotational speed was 6000 rpm and the interfacial tension was calculated from the Vonnegut approximation as reported elsewhere.²⁹ Samples were assumed to be equilibrated when the measured IFT values were unchanged at the period of measurement (30 min at least).

Results and discussion

Surface tension

The surface tensions of five sulfobetaine surfactants as a function of concentration at 30 °C were determined, and are plotted in Fig. 1. The CMC and surface tension at CMC (γ_CMC) values were estimated from the breakpoints of these plots, and are listed in Table 2. The γ_CMC values indicate the ability of the surfactants to lower surface tensions, and accordingly the CMC indicates the efficiency. The γ_CMC values of C₁₂SB, C₁₄SB and C₁₆SB were 38.64 mN m⁻¹, 35.79 mN m⁻¹, and 33.45 mN m⁻¹, respectively. The long-chain sulfobetaine surfactants provided a higher ability to lower the surface tension of water than the short-chain ones.

Fig. 1 Variation of surface tensions as a function of concentration of sulfobetaine surfactants at 30 °C.

Table 2 Surface-active properties of sulfobetaine surfactants at 30 °C

Surfactant	CMC (mmol L^−1)	γCMC (mN m^−1)	Γmax (μmol m^−2)	Amin (Å^2)
C12SB	1.383	38.64	2.612	63.57
C14SB	0.233	35.79	2.815	58.98
C16SB	0.064	33.45	2.769	59.96
C12HSB	3.389	32.30	2.152	77.17
C14HSB	0.708	29.89	2.636	62.99

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As far as C_nSB is concerned, the relationship of surface activities and the alkyl chain length can be seen from Table 2. The CMC and γ_CMC values decrease gradually with the number of carbon atoms in the alkyl chain length increasing from 12 to 16 at 30 °C. This may be due to an increase in the hydrophobic effect³⁰ with the increase in the alkyl chain length, which promotes the micellization and the aggregation of sulfobetaine molecules. Here, the CMC values of C₁₂SB, C₁₄SB and C₁₆SB were 1.383, 0.233 and 0.0647 mmol L⁻¹, respectively. In general, the CMC value of surfactants is a sign of the surface properties, which follows the principle: the smaller the CMC value, the superior the surface activity. This means that C_nSB with a longer alkyl chain length has an excellent ability for micellization at low concentrations. The relationship between the CMC and the alkyl chain length of C_nSB is shown in Fig. 2. It is known that the variation of the CMC with the alkyl chain length can often be described by the empirical equation as follows:

Log CMC = A − Bn (3)

where A and B are constants and n is the number of carbon atoms in the alkyl chain length. The Log CMC decreases with the increasing alkyl chain length and the B value was 0.35, which was lower than those of sulfobetaine surfactants C_nSB (B = 0.44 or B = 0.48).^8,31 This indicates that the CMC of sulfobetaines in this work decreases more for each addition of two carbons in the hydrocarbon chain.

Fig. 2 Relationship between CMC and alkyl chain length of C_nSB.

The difference in molecular structure between C_nSB and C_nHSB is the spacer groups in the polar group, i.e. propyl and hydroxypropyl. From Fig. 1 and Table 2, there is an increase in the CMC values and a decrease in the γ_CMC values as a result of the introduction of the hydroxyl group. It can be concluded that the hydroxyl group in the hydrophilic headgroup has an effect on the CMC and γ_CMC. Similar conclusions had already been drawn that the introduction of the hydroxyl group to erucyl dimethyl amidopropyl sulfobetaine (EDAS) lead to an increase in CMC from 5.02 to 5.64 mmol L⁻¹ and a decrease in γ_CMC from 35.34 mN m⁻¹ to 31.41 mN m⁻¹.³² The hydroxyl group in the spacer group enhances the solubility of hydroxypropyl sulfobetaine, as the hydrophilicity increases through hydrogen bond formation between the hydroxyl group and the water molecules. As the hydrophilicity increases, a higher concentration is needed to form micelles, which leads to a slightly higher CMC.

In our work, the variations of surface activities reveals that there is a stronger tendency of solubility for C_nHSB with a flexible, hydrophilic spacer group, rather than C_nSB with a rigid, hydrophobic one. Because of the stronger intramolecular electrostatic attraction between the positive and negative charge centers caused by the flexible spacer group, the electrostatic repulsion between the headgroups of the C_nHSB molecules becomes relatively weaker.³³ Accordingly, the headgroups in C_nHSB molecules can pack together more tightly than in C_nSB ones.³⁴ This can be confirmed from the A_min values in Table 2. The larger A_min values possessed by C_nHSB are the result of the formation of hydrogen bonds between the hydroxyl groups and water molecules at the air–water interface combined with the flexible headgroups (see Fig. 3).

Fig. 3 Distribution schematic of (a) propyl sulfobetaine C_nSB and (b) hydroxypropyl sulfobetaine C_nHSB of surfactant molecules vertically staggering at the air–water interface at concentration above CMC.

In general, surfactants with the same hydrophobic tail chain and similar hydrophilic headgroup in the molecular structure have similar surface activities.¹⁹ The nature of the headgroups has a tiny effect on surface activity. In this work, there was an obvious decrease in the γ_CMC values of C_nHSB, which implies that hydroxypropyl sulfobetaine has a stronger surface activity than propyl sulfobetaine, due to the introduction of a hydroxyl group in the spacer group.

Interfacial tension

Occurrence of minimum dynamic interfacial tension. The minimum dynamic interfacial tension (DIT_min) and equilibrium interfacial tension (DIT_eq) are major parameters for evaluating the interfacial properties of surfactants. Taylor et al.³⁵ proposed that oil recovered through the surfactant-enhanced alkali flooding of linear Berea sandstone cores correlates better with the DIT_min value than with DIT_eq value. During the experimental period of DIT, there is no salting-out effect at 90 °C in surfactant solutions prepared by reservoir water with high salinity and high hardness (11.52 × 10⁴ ppm, including 7040 ppm Ca²⁺ and 614 ppm Mg²⁺). The DIT value between crude oil and reservoir water in the absence of a surfactant is always above 10 mN m⁻¹. The interaction energy across the interface must be large. This means that the nature of the material at both sides of the interface must be very different.¹⁹ Since oil and water have very different natures, the presence of surfactants will make the similar natures at both sides of the interface after the surfactants adsorb at the oil–water interface. Thus DIT will decrease sharply. Besides the chemical nature of the surfactant, the surfactant concentration has a strong effect on DIT. The effect of five sulfobetaine surfactant concentrations on DIT with time are shown in Fig. 4.

Fig. 4 Variation of the DIT between crude oil and reservoir water at 90 °C with time for sulfobetaine surfactants at different concentrations. (a) C₁₂SB, (b) C₁₄SB, (c) C₁₆SB, (d) C₁₂HSB, (e) C₁₄HSB.

As shown in Fig. 4, for a given surfactant, two interesting DIT behaviors are observed. One is that not all surfactant systems show the occurrence of DIT_min; for instance, it does not occur in very low and high concentration range where DIT values always remain higher. The other is that DIT in a low surfactant concentration range from 0.03 wt% to 0.10 wt%, decreases very rapidly over time to a transient DIT_min, followed by a gradual increase to DIT_eq. The occurrence of DIT_min in oil–water–surfactant systems has been reported in many research papers.^36,37 It may be related to the intermolecular interaction between the surface-active species present (i.e. petroleum acid) in crude oil and the added surfactants in aqueous solution. Considering surfactant diffusion-controlled adsorption, possible explanations for the occurrence of the DIT_min are that: (1) as the adsorption velocity is larger initially than the desorption velocity at the fresh oil–water interface, the rapid diffusion of added surfactant (from the aqueous phase) and surface-active species (from the oil phase) to the oil–water interface, and the interaction between each other (see Fig. 5a) results in an optimum mixed surfactant layer at the interface at an optimum concentration and ratio. This can greatly decrease IFT to DIT_min. (2) A subsequent diffusion of surface-active species at the interface into the bulk phase to form mixed micelles with the added surfactant (see Fig. 5b), which increases IFT from DIT_min to DIT_eq until an adsorption–desorption equilibrium is reached. No evidence was found that DIT_min can be reached at lower surfactant concentrations than 0.03 wt%, due to the concentration of added surfactant not being enough to form a monolayer at the oil–water interface. However, it is interesting to observe the lack of DIT_min at a higher surfactant concentration. In general, the greater the amount of surfactant solubilized in the oil–water system, the more similar the natures of the two phases approach each other, and the smaller the resulting IFT between the two phases.¹⁹ Accordingly, in the crude oil–reservoir water–surfactant systems with higher surfactant concentration, DIT should be reached for lower IFT. However, the results here show that the values of DIT are always high during the experiment process. These phenomena are probably due to the rapid diffusion of the surface-active species at the interface into the bulk phase, where high concentration of surfactant results in the instantaneous formation of an optimum mixed surfactant layer, followed by an adsorption–desorption equilibrium.

Fig. 5 Adsorption schematic of zwitterionic surfactant molecules at the oil–water interface. (a) The interaction between added surfactant and petroleum acid, which decreases IFT to DIT_min, (b) subsequent diffusion of the petroleum acid at the interface into the bulk phase to form mixed micelles with the added surfactant, which increases IFT from DIT_min to γ_eq.

Time to reach DIT_min. It is clearly observed in Fig. 4 that an amount of time is required for each oil–water–surfactant system to reach DIT_min. An increase in DIT with time is over a period generally from 10 min to 30 min. It is also found from Fig. 4 that the time to reach DIT_min varies with surfactant concentration where DIT_min exists. Many literature reports indicate that the time to reach DIT_min decreases with increasing surfactant concentrations.^36,38 Our results have similar rules by and large. The higher the surfactant concentration, the larger the concentration gradient between the bulk phase and the oil–water interface. Hence, the faster the diffusion to the oil–water interface, the smaller the time for DIT to reach the minimum values.

Göbel and Joppien³⁹ reported that in the longer time range, the plots of the dynamic interfacial tension γ of Triton X-100 versus 1/√t show a linear dependence in four cases (air, cyclohexane, n-heptane and n-hexadecane) by the drop-volume method. The relationship is as follows:

where t_ads is the adsorption time, c₀ is bulk phase concentration, D is the diffusion coefficient, R and T are the gas constant and the thermodynamic temperature, respectively. Concentration dependent adsorption (Γ_c) of the surfactant can be calculated from the relationship of the maximum adsorbed amount (Γ_∞) at the oil–water interface and the capillarity parameter b: Γ_c = Γ_∞[bc₀/(1 + bc₀)]. From the three known quantities c₀, Γ_c and the slope of the curve in a γ vs. plot, it is possible to compute the effective diffusion coefficients for the transport of surfactant to the interface. However, fewer reports have revealed a molecular thermodynamic model on the basis of IFT measured by the spinning-drop method in oil–water–surfactant systems. Explicit relationships have not been found between the adsorption properties described above in the measurement of the surface tensions of five sulfobetaine surfactants and the comprehensive effects of the testing environment (temperature, the complex composition of crude oil and reservoir water, etc.) on the surfactant adsorption–desorption equilibrium and on the molecular interaction in the oil–water–surfactant system. If mass transport is occurring at an appreciable rate compared to the time required for the oil drop to come to thermal equilibrium, the DIT calculated is path dependent. Unfortunately, because we have no method to determine the comprehensive effects of the testing environment during the experiment, we cannot resolve this issue and it is difficult to calculate thermodynamic parameters of surfactants at the oil–water interface. The variation of DIT in crude oil–reserve water–surfactant system will be studied further from the perspective of thermodynamics.

The effect of molecular structure on IFT. The values of DIT_min and DIT_eq of five sulfobetaine surfactants were extracted from Fig. 4 and are shown in Fig. 6. Because these surfactants fail to get DIT_min at high concentration range, Fig. 6a just shows the DIT_min obtained at a low concentration range. The values of DIT_min and the optimum surfactant concentration at the DIT_min (C_o-min), and the values of minimum DIT_eq (DIT_eq-min) and the optimum surfactant concentration at the minimum DIT_eq (C_o-eq) are listed in Table 3.

Fig. 6 Variation of the DIT_min and DIT_eq between crude oil and reservoir water at 90 °C as a function of surfactant concentrations. (a) DIT_min, (b) DIT_eq.

Table 3 Variation of the DIT_min and DIT_eq (in mN m⁻¹) between crude oil and reservoir water at 90 °C as a function of sulfobetaine surfactants

Surfactant	DITmin (mN m^−1)	Co-min (wt%)	DITeq-min (mN m^−1)	Co-eq (wt%)
C12SB	0.042	0.05	0.081	0.05
C14SB	0.047	0.03	0.069	0.05
C16SB	0.028	0.03	0.048	0.03
C12HSB	0.010	0.05	0.053	0.05
C14HSB	0.0009	0.03	0.003	0.03

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Comparing different molecular structure of sulfobetaine surfactants from the results of Fig. 6 and Table 3, it can be seen that the alkyl chain length has an effect on its ability to lower IFT values. Surfactant hydrophobicity increases with an increase in the alkyl chain length. For propyl sulfobetaine surfactants, increasing the alkyl chain length leads to slightly better interfacial properties. IFT values reduce at the same order of magnitude. When the alkyl chain length increases from 12 to 16, the values of DIT_min and DIT_eq reduce from 0.042 to 0.028 mN m⁻¹ and from 0.081 to 0.048 mN m⁻¹, respectively. However, for hydroxypropyl sulfobetaine surfactants, C₁₄HSB shows far better interfacial properties than C₁₂HSB. When the alkyl chain length increases from 12 to 14, the values of DIT_min and DIT_eq reduce clearly by orders of magnitude from 0.010 to 0.0009 mN m⁻¹ and from 0.053 to 0.003 mN m⁻¹, respectively. The increase in alkyl chain length and the addition of the hydroxyl group in the spacer group possibly gives C₁₄HSB a moderate hydrophilic–lipophilic ability. So C₁₄HSB has a better solubility both in the bulk phase and the oil phase. The higher the density of the surfactant at the oil–water interface layer, the greater the intermolecular interaction between the surface-active species present and the added surfactants, the lower the IFT.⁴⁰

Optimum concentration to reach DIT_min. Besides the chemical nature of the surfactant, relative changes in surfactant concentrations affected the DIT of the crude oil–reservoir water–surfactant systems. Fig. 6a shows that DIT_min increased with increasing surfactant concentration in a low concentration range. Fig. 6b shows that DIT_eq firstly decreases and then increases with increasing surfactant concentration. It is supposed that increasing the surfactant concentration results in producing micelles in both the aqueous phase and oil phase. The consequent decrease in the adsorption capacity of the surface-active species at the interface due to the solubilization of micelles, leads to the increase of DIT_min and DIT_eq with increasing surfactant concentration. From the above results, this indicates that for a given surfactant, there exists a range of optimum surfactant concentration towards certain crude oils at certain reservoir conditions. In particular, C₁₄HSB, exhibits an excellent interfacial behavior to reduce IFTs to 0.0009–0.005 mN m⁻¹ at an optimum surfactant concentration from 0.03 wt% to 0.10 wt%. A similar phenomenon was observed with many surfactants that had better interfacial properties and achieved ultralow IFT at lower concentration than at higher concentrations.^41,42 Although the values of DIT_min and DIT_eq of the other four sulfobetaine surfactants was relatively high and no ultralow IFT appears in the range of testing concentration, they still had an optimum concentration to reach DIT_min.

As is well known, the reduction of effective concentration caused by surfactant adsorption on a reservoir rock surface is a major problem in the field application of surfactant flooding.^13,43 The variation of surfactant concentration brings about changes in IFTs, which then affects the oil displacement efficiency. Accordingly, the injection concentration should be higher than the experimental results through laboratory tests. This can overcome the loss of surfactant adsorption to some extent during the flooding process and maintain the IFT between crude oil and reservoir water in a low or ultralow region. In spite of the inevitable adsorption problem, sulfobetaine surfactants are still promising chemical agents for EOR, particularly under harsh conditions with high temperature and high salinity.

Conclusions

This work shows that the molecular structure has a great effect on the surface and interfacial properties of surfactants. The surface tension of three propyl sulfobetaine surfactants and two hydroxypropyl sulfobetaine surfactants were analyzed at 30 °C, by comparing the results of different molecular structures, including the alkyl chain length and hydroxyl group. The CMC values decreased with increased alkyl chain length and increased by the addition of a hydroxyl group. The γ_CMC values decreased with increased alkyl chain length and with the addition of a hydroxyl group. The interfacial activity of these surfactants at the oil–water interface was also investigated under harsh reservoir conditions of high temperature (90 °C) and high salinity (11.52 × 10⁴ ppm, including 7040 ppm Ca²⁺ and 614 ppm Mg²⁺). DIT_min occurs in a specific concentration range and the time to reach DIT_min varies with surfactant concentration. The magnitude of DIT values varied with surfactant molecule structure and surfactant concentration. C₁₄HSB not only shows the property of lower surface tension, but also has a stronger ability to reduce IFT between crude oil and reservoir water. It can reduce DIT to an ultralow level over an optimum range of low concentration from 0.03 wt% to 0.10 wt% under harsh conditions. In evaluating the displacement agent composition used in chemical flooding, the ability of lowering IFT plays an important role. So hydroxypropyl sulfobetaine surfactants have potential wide applications in EOR.

Acknowledgements

The work was supported by National Science Fund for Distinguished Young Scholars (51425406), National Natural Science Foundation of China (51174221, 21303268), Doctoral Fund from National Ministry of Education (no. 20120133110010), China Postdoctoral Science Foundation funded project (2013T60689) and the Graduate Innovation Fund from China University of Petroleum (YCX2014015).

Notes and references

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