Research Article | Open Access
Shixin Dai, Yufei Gong, Feng Wang, Pan Hu, "Synthesis and Interface Activity of a Series of Carboxylic Quaternary Ammonium Surfactants in Hydraulic Fracturing", Geofluids, vol. 2019, Article ID 4258643, 11 pages, 2019. https://doi.org/10.1155/2019/4258643
Synthesis and Interface Activity of a Series of Carboxylic Quaternary Ammonium Surfactants in Hydraulic Fracturing
Hydraulic fracturing is an important technology for the development of unconventional resources, while the foam fracturing plays an essential role for the oil recovery in hydraulic fracturing. To further explore the anion effect of quaternary ammonium cationic surfactants on their relative performances, four fatty acid surfactants were prepared (cetyltrimethylammonium acetate (CTAAC), cetyltrimethylammonium butyrate (CTABU), cetyltrimethylammonium hexanoate (CTAHE), and cetyltrimethylammonium caprylate (CTACA)). The effect of anions on surface tension and foaming properties were discussed, and the emulsion stability was also investigated. The experimental results were presented that the CTAAC possesses the highest surface activities compared with other members in the prepared surfactants. The critical micelle concentration (CMC) and surface tension at the CMC () increase as increasing methylene segments in the anions, the maximum surface excess concentration (), and minimum area per molecule () present an opposite trend with the increase of methylene segments. The CTAAC exhibits the best performances on foamability and foam stability than other synthesized surfactants at 70°C; the initial foam height () and the foam height ratio () at 0 min and 3 min are 34.9 cm and 52.9%, respectively; this is due to the lowest surface tension and shortest methylene segments. In addition, the emulsion stability was shown to follow the order of CTAAC>CTABU>CTAHE>CTACA.
Foam fluid is a gas-liquid dispersion system. Because of its excellent performance, it has been widely applied in the oil and gas field development, including enhanced oil recovery, matrix acidification, gas well breakthrough control, and plugging removal [1–3]. The liquid production profile control shows the great potential of foam as an intelligent fluid. In fact, due to the heterogeneity of formation, low permeability formation can not be effectively developed in oilfield production. Therefore, injecting foam into the formation improves reservoir heterogeneity. Foam liquid enters the high permeability layer first and traps gas, resulting in a temporary plugging effect in the high permeability layer, and the whole process needs no artificial control [4, 5]. That is to say, the bubbles will automatically adjust the injection curve, or intelligently. In addition, the foam liquid can also adjust the distribution of acid in the acidification process and obtain the same effect of less acid.
The foam system reduces the tendency of free energy by reducing the total interfacial area of the foam coalescence . Previous studies have shown that the key to the success of foam in oilfield development lies in its stability and ability to capture gas molecules. In essence, foam is thermodynamically unstable, and stability generally refers to life expectancy. Stable foam, which has a longer life, is a mixture of air, water, and surfactant (soap) [7, 8]. It is prepared by introducing a pressurized fluid into the base solution. The base solution is prepared by adding a known concentration of surfactant in water or any other fluid. Unlike incompressible fluids, absolute pressure affects the loss of foam rheology and pressure foam flow and the quality of a given foam. At a given shear rate, the pressure increases the shear stress and the apparent viscosity of the foam . The addition of polymers to the foam significantly increased the apparent viscosity of the foam. When air, nitrogen, or carbon dioxide are injected into the fluid system, the emulsion retains gas in the surfactant film to form a continuous phase . The addition of polymer changed the viscosity mode of the system and ensured the stability of the interfacial film. The effect of polymers on viscosity depends on the ionic groups of surfactants and the polymers in the system. Polymers such as guar gum and xanthan gum are used as chemicals to stabilize and change viscosity. In order to improve the stability of foam without causing too much formation damage, viscoelastic surfactants and nanoparticles can be selected. By adding electrolyte, mixing the second surfactant with an opposite charge or adjusting the pH value, the shape of micelle will be changed from spherical to wormlike, and the liquid viscosity will be significantly increased due to the formation of slender wormlike micelles. Another way to improve foam stability is to use nanoparticles. Many studies have shown that foams can be stabilized by nanoparticles, mixtures of nanoparticles and surfactants, and mixtures of nanoparticles and polymers. For polymers, the interaction between nanoparticles and polymers will give the film strength and roughness, thereby enhancing foam stability in terms of slow drainage rate and uniform bubble size distribution. Nanoparticles can be irreversibly adsorbed at the gas-liquid interface, reducing the contact area between bubbles and liquid film, thus reducing gas diffusion. In addition, the acceptance of rice particles by single or double bridges will further slow down the discharge of liquids. Finally, the mechanical stability of the plate can be enhanced by the agglomeration of nanoparticles at the gas-liquid interface [6, 8].
With the main oil fields gradually entering to the high water exploitation moment, oil production profile control has become one of the main measures to increase oil well production . The foam liquid is a surfactant which has a certain viscosity and can flow continuously. When it is used to displace oil in the formation, it can play a good role in carrying oil. In addition, foam liquid can form a foam in the formation, which plays a good role in plugging the high permeable layer of reservoirs, thereby reducing the permeability of high permeable layers. At the same time, foam fluid can also improve the mobility ratio and further expand the range of injected fluid. From the review above, most attention of foam focused on the stability control; the effects of anions on the properties of quaternary ammonium cationic surfactants have received little attention. In this research, four fatty acid surfactants are prepared and the surface activity parameters, foaming properties, and emulsion stability are analyzed; the microstructure of the foam is investigated by microscopy, and the effect of temperature, pH, and the inorganic salt on the stability of foam are analyzed. This study is of great significance for a comprehensive understanding of the stability of foam in porous media and for improving foam flooding theory.
Cetyltrimethylammonium chlorine (CTAC) of more than 99.9% purity was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Sodium hydroxide, sodium acetate, butyrate, hexanoate, and caprylic acid (purity 98%) were supplied from Xian Reagent Co., Ltd. Methanol obtained from Chengdu Aikeda Chemical Reagent Co. Ltd. was of analytical grade with a purity of 99.8%.
2.2. Synthesis of New Surfactants
CTAC and sodium hydroxide in 1 : 1 ratio in 100 mL methanol solvent were added into 500 mL round bottom flask. The end of reaction was determined by a certain amount of white powder. Then, the supernatant was reacted with acetate, butyrate, hexanoate, and caprylic acid, respectively (molar ratio of 1 : 1); the reaction mixture was vigorously stirred for 3 h at a reflux temperature of 85°C. After the reaction was finished, it was allowed to cool to room temperature and the solvent was removed under reduced pressure by rotary evaporation. Thereafter, it was extracted by acetone through filtered vacuum suction, then dried in vacuum oven. Finally, the target products were obtained. In addition, the esterification system includes a Dean-Stark side arm in order to collect the produced water to prevent the backward reaction. The overall scheme for the synthesis of the cationic surfactants is illustrated in Scheme 1.
2.3. Surface Tension Measurement
The platinum ring method for measuring the static surface tension of synthesized cationic surfactants is common in laboratory . The surface tension of distilled water was measured to calibrate instruments before the surface tension measurements, which obtained the standard surface tension value ( mN/m) at 25°C. The prepared different surfactant solutions, with a concentration range of 80-5000 mg/L, were poured into a clean 30 mL teflon cup with 28 mm mean diameter ; the ring was initially washed with pure water and acetone between the measurements of values . All measurements were repeated at least thrice until the values were reproducible [2, 4]. The platinum ring method using SITE 100 surface tension instrument was used to determine the surface tension. The surface tension instrument was calibrated by distilled water before each measurement. The purpose was to obtain a standard surface tension value (72 mN/m). Different concentrations of surfactant solutions were prepared at room temperature. Each experiment needed to be repeated three times to obtain reproducible experiment results.
2.4. Foaming Properties
There are many methods for generating foam, such as the sparge tube technique or gas flow and whipping . The standard method for evaluating the foam properties is the Ross-Miles method. First, 300 mL surfactant aqueous solutions, with a concentration range of 80-5000 mg/L, were prepared at room temperature. Among those solutions, 50 mL aqueous solution was poured into the bottom of a thin tube with 90 cm height and 2.9 mm inner diameter. Then, 200 mL of the solution in a pipette placed into the thin tube from the top of the apparatus was impacted in 50 mL of the same solution to produce foam. After the solution in the pipette had run out, the foam height was immediately recorded at 0 min and 3 min. Foamability was determined by the initial foam height. Foam stability was determined by the ratio of the foam height at 0 min and 3 min . All measurements were repeated at least thrice until the values were reproducible . 100 mL of 3000 mg/L surfactant solutions was prepared and blended at 7000 rpm for 180 s by means of the Waring Blender. Foam was firstly produced, and then the foam microstructure was measured with an optical microscope, whose light source was the polarized light .
In order to determine the emulsion stability capacity of the prepared cationic surfactants. 3000 mg/L surfactant solutions (25 mL) and crude oil (25 mL) (Table 1) were firstly added in cone-shaped graduated tubes (100 mL capacity), which were placed in a thermostatic water bath at 60°C for 30 min. Then, the heated surfactant and crude oil were added in a measuring cylinder (50 mL capacity). The measuring cylinder was quickly shaken up and down for 10 times every 1 min and allowed to keep standing . Finally, the volume of phase separation was recorded at 0 min, 2 min, 4 min, 6 min, 8 min, 12 min, 15 min, and 20 min. The smaller volume of phase separation is, the better emulsion stability of the surfactant have exhibited.
3. Results and Discussion
3.1. RT-IR Analysis
The target product (CTAAC) is analyzed and characterized by the RT-IR method, the result can be indicated that the relatively wide peak at 3500-3400 cm−1 assigned to the stretching vibration absorption peak of the hydroxyl group is chosen as characteristic changes. The FT-IR bands at 2923 and 2653 cm-1 are the stretch vibration absorption spectra of the methyl and methylene, respectively. The narrow peak at 1460 cm−1 is attributed to the bending vibration of alkane C-H. A peak near 1252 cm−1 may be corresponded to the stretching vibration absorption peak of C-N bond. A peak at 726 cm−1 is narrow, which is assigned to absorption of (CH2)S rocking vibrations Figure 1. RT-IR spectral analyses indicate that the characteristic absorption peaks in this figure correspond to the functional groups in the target product, which illustrates that the synthesized surfactant is the target product.
3.2. Surface Tension Properties
To determine the surface activity of the prepared cationic surfactant solutions, the surface tension measurement is used in the study. The surface tensions of samples were measured from a series of aqueous solutions with a platinum ring using a Biolin Sigma 700 Processor Tension meter at . The surface tension of double distilled water was measured to calibrate instruments, which was generally mN/m. Every sample solution was prepared with the double distilled water which was stabilized for 5 min in the instrument before measurements and was repeated three times to reduce the error. The platinum ring method using the SITE 100 surface tension instrument was used to determine the surface tension. The surface tension instrument was calibrated by distilled water before each measurement. The relation between surface tension and log C is clearly shown in Figure 2, where the surface tension values firstly decrease continuously with the increase of solution concentrations; the tendency is attributed to the gradual adsorption of surfactant molecules at the air/water interface. Then, the surface tension values by increasing concentration were kept almost constant, which illustrates complete surface saturation by surfactant molecules . The clear turning point in the -log C curve indicates the formation of micelle. The CMC of the four surfactants can be obtained from the turning points of the curve.
The CMC values are presented in Table 2. For the four surfactants, the CMC values show an obvious difference; this is due to the different structures of the surfactant molecules which are the same with those of the hydrophilic and hydrophobic groups except for anions. Therefore, the anions should be the principal factor to determine the differences between the aggregation properties . Through the comparison of CTAAC, CTABU, CTAHE, and CTACA, it can be concluded that as the number of methylene segments increases (, 4, 6, 8), the CMC values gradually increase. This difference may explain that the methylene segments in the anion directly affect the distribution of surfactant molecules at the interface. For CTAAC molecule, it has the shortest methylene segments, the shortest distance between the hydrophobic groups has exhibited, and surfactant molecules are arranged more compactly with each other, which leads to adsorption of more surfactant molecules at the interface. These contributions all act to help these surfactant molecules to aggregate lower concentration, with a decreased length of the methylene segments in anions.
To better demonstrate the capacity of the surfactants to reduce the surface tension of solutions, , namely, the effectiveness of surface tension reduction is introduced here and the data are listed in Table 1. By comparing the values of four surfactants, the conclusion is that the surface activities increase with decreasing methylene segments in the anion with maximum activity corresponding to CTAAC .
value is used to determine the adsorption of surfactant molecules at the air/water solution interface and calculated according to the Gibbs adsorption isotherm equation using Equation (1) . These data are obtained from Table 1, which indicate that the larger value represents the greater preference of surfactant molecules to be absorbed at the interface; this is ascribed to the difference of molecule structure.
The minimum area, , is a traditional measure of packing densities and calculated according to the Gibbs adsorption isotherm equilibrium using Equation (2) . The values of the four surfactants in this article are determined at 298 K and summarized in Table 1. From Table 3, it can be observed that the methylene segment in the anion increases, and the values also increase. As a result, for the four surfactants, the shorter methylene segments present higher packing densities at the interface . In view of the surfactant structure, this is likely due to the longer methylene segments in the anion causing more likely to curl. Hence, the values are easily getting larger. in which is the saturated adsorption amount in mol·cm-2, is the gas constant, is the absolution temperature, is the slope of the linear part of the γ-log C curve, is Avogadro’s number, and is the area occupied by the surfactant molecule at the interface in nm2.
3.3. Foaming Properties
Foam plays an important role in the field of petrochemical industry. Therefore, it is necessary to research foaming properties when exposed to various circumstances. The foamability and foam stability are effective methods to determine foaming properties. Among them, the former is used to measure to the foam height at 0 min and the latter is used to measure the foam height ratio at 0 min and 3 min.
It can be seen from Table 4 that, in the use of the same surfactant, solution concentration is a critical factor affecting foam generation ability; the general observation is that the foam height at 0 min for four surfactants is strongly promoted by increasing concentration until the surfactant concentrations reach 3000 mg/L. Above this concentration, there is no significant increase in the foam height. Therefore, it can be determined that concentration of 3000 mg/L is considered to possess corresponding high foamability compared to other concentrations. So, 3000 mg/L is fixed for the optimum surfactant concentration for further investigation. In the use of the same concentration, the CTAAC obtains the most prominent performance on foamability and foam stability by having the greatest foam height compared with other members in the prepared surfactants.
The reasons can be attributed to two aspects. On the one hand, the phenomenon is directly traceable to surface tension, since the CTAAC has the lowest surface tension among those for a series of prepared surfactants; it is consistent with the idea that the low surface tension could promote foaming according to Rosen’s study . On the other hand, surfactant molecule structure plays an exceedingly significant role on the difference of foamability. The four surfactants possess quite different anions with the same charges even though they are the same with those of the hydrophilic groups and hydrophobic groups. Hence, the foam stability of the four surfactants is ultimately determined by the anions. Analyzing the data represented in Table 3 indicates that the gradual decrease of the values by increasing the methylene segments in the anion is attributed to the arrangement of surfactant molecules at the gas/water interface. The graph of schematic representation of the four surfactants is presented in Figure 3, where the molecules are arranged more closely at the interface, and the strength of the liquid film increases, finally resulting in the increase of the foam stability. These illustrate that the foam stability of CTAAC is the strongest compared with other surfactants.
3.4. Effect of Temperature on the Foaming Abilities
Excellent foam produced from a surfactant system should have some characteristics conducing to maximize their correlative performances under actual condition. Hence, in the study, it is highly necessary to investigate the effect of different temperatures on foamability and foam stability of surfactant in aqueous solution.
Based on the results obtained, the presence of high temperature may have great influence on foamability and foam stability when the surfactant solution concentrations were kept constant; this is observed from Figure 4, which shows that the values of the prepared surfactants began to increase above 30°C and continued to do so with increasing temperature; however, the and values for the four surfactants slightly drop below 50°C, and those values reach the lowest at 70°C. It is also clear that the foamability benefits from a rise in temperature, but higher temperature has an adverse effect on the foam stability in turn. That is, the values are negatively correlated with foaming temperature. The values of all members in the prepared surfactants are maximum at 30°C; this is ascribed to their lower gas diffusion rate. As the temperature continually rises, the motion of surfactant molecule in bubbles is considered to increase rapidly and gas expansion also tends to increase, which results in more rapid gas diffusion through liquid phase causing bubble disproportion ; this phenomenon is consistent with previous foam stability studies at high temperature [18–20].
Although the foamability and foam stability of the four surfactants show the same trend at different temperatures, the subtle differences are also present among those surfactants. In Figure 4, it is showed that the and values of CTAAC have the maximum and values at an average of 34.9 cm and 52.9% at 70°C, far larger than those of other surfactants (CTABU: 31.8 cm and 48.2%; CTAHE: 29.8 cm and 32.5%; and CTACA: 19.7 cm and 20.5%). Therefore, the CTAAC obtains the best performance on foamability and foam stability, regardless of high temperature; its highlight performance may be ascribed to the fact that its molecules are likely to form closely packed arrangement at the gas/water interface, which is expected to be favorable in producing a more elastic and strong liquid film in foam .
3.5. Microstructure of Foam
Form the above experimental results, it is established that the CTAAC is selected as a prominent foaming agent in terms of its foamability and foam stability but does not show specific research of a foam microstructure. In order to further analyze this problem, the microscope with 40-fold magnification is used to record foam images. Foam was first generated by using Waring Blender, and then the microstructure of gas bubbles was measured by using a Leica microscope (LM) (DMLB2, Leica Ltd., Germany) and the liquid films of foam were investigated by using environmental scanning electron microscopy (ESEM) (Quanta 450, FEI Ltd., America). Foam was first generated by using a Waring Blender, and the foam microstructure was measured with an optical microscope, whose light source was the polarized light.
It is observed from Figure 5 which shows that the foam microstructure of foam due to low temperature (around 30-40°C) hardly present obvious change when the time is less 8 min; that is, bubbles show good shapes with regular circles or ellipses, and the size distribution of bubbles is relatively uniform . This also indicates that foam is comparatively stable in this condition. At 50°C, the shapes of bubbles are regular circles in the initial stage of bubbles produced, since they did not experience a significant process of liquid drainage. As time passes, the exceedingly obvious change of a foam microstructure is taking place; that is, the shapes of bubbles are regular polygons, and in-between bubbles occur to direct contact due to remarkable coalescence. At high temperature (around 60-70°C), it is observed that the small bubbles are merged into the big ones and a spontaneous and continuous reduction of foam density is presented in this graph before 2 min; the phenomena result in bubble disappearance. Therefore, high temperature becomes responsible for the foam stability; this is owing to high pressure difference and gas diffusion among bubbles, ultimately leading to obvious bubbles coarsening and lamella breakage under this condition .
3.6. Effect of NaCl on the Foamability
To further understand the arrangement rules of different surfactant molecules at the interface, the foam properties under the fixed surfactant solution concentration of 3000 mg/L are studied.
Table 5 is complicated by the presence of additional NaCl; the initial foam volume and half-life time first increase and then decrease with the increase of NaCl concentrations. It is illustrated in our study that there are NaCl concentrations of 8000 mg/L that allows initial foam volume and half-life time obtain optimal values. The inspection of Figure 6 shows the presence of three processes describing the arrangement of surfactant molecules at the interface. Figure 6(a) shows that surfactant molecules are loosely arranged and more evenly spaced from each other when no NaCl is added. Figure 6(b) shows that, when NaCl concentrations are less 8000 mg/L, negatively charged ions are attracted by the positively charged or polar head groups and then distributed around head groups. This makes the electrostatic repulsion between the ion head groups decrease, which leads to more surfactant molecules being absorbed in the interface. Hence, it is beneficial to decrease surface tension. From this view of Rosen’s study, the foamability is enhanced by the low surface tension. In Figure 6(c), more negatively charged ions are distributed near head groups once NaCl concentrations exceed 8000 mg/L, leading to desorption of surfactant molecules at the interface. As a result, it is not beneficial to generate foam . Compared with foam properties of the four surfactants in the presence of additional NaCl, the abilities of foam to generate and stabilize foam are illustrated: CTAAC>CTABU>CTAHE>CTACA.
3.7. Effect of pH on the Foamability
In this experiment, the pH regulator of pHS-25pH produced by Shanghai Elite REE Company was used to adjust the pH value of foam base solution with NaOH solution of 1 mol/L HCl and 5 mol/L, and the adjusted solution was foamed with a Waring Blender high speed agitator [23–25]. The foam stability and foaming height and half-life of foam liquid at different pH values were measured. The effect of the pH value on foam stability was investigated (Figure 7). From the experiment, it can be found that different pH has different effects on different surfactants. The foam volume and half-life of CTAAC and CTAHE increased with the increase of the pH value, but the foam volume and half-life of CTABU and CTAHE changed little with the change of the pH value and basically remained stable. In addition, the foaming volume of CTAAC and CTAHE was much higher than that of CTABU and CTACA, and the half-life of CTAAC and CTAHE was also greater than that of CTABU and CTACA. Compared with foam properties of the four surfactants under different pH, the abilities of foam to generate and stabilize foam are illustrated: CTAAC>CTAHE>CTABU>CTACA.
3.8. Emulsion Stability
The emulsion is a thermodynamically unstable multiphase dispersion system, in which one liquid is dispersed in the form of tiny droplets in the other that is incompatible with it . This characteristic is mainly determined by the complex composition of emulsion, with three components: an aqueous phase, an oil phase, and an emulsifying agent; the surfactant is usually considered to act as emulsifying agents, and it can effectively reduce the interfacial tension and form interface film at the oil/water interface; therefore, it can accelerate as well as stabilize water-in-oil emulsions. It is observed from Figure 8 which simply shows the processes of emulsion formation and separation . The first step of separating the emulsion is started owning to relatively weak repulsive forces, and if the adhesion energy is sufficiently large, adhesion is sure to be facilitated, which is the so-called flocculation phenomenon . Once the interface film breaks, a significant coalescence process of droplets is immediately taking place.
From Figure 6, the relationship between the dewatering rates of the emulsion prepared by the four surfactant solutions with the same concentration and time can be shown. Owning to the unstable system, the emulsion system, the dewatering rate spontaneously and continuously increases as time goes on. Yet, for CTAAC and CTABU, the slow increase of dewatering rate is observed before 15 min. It is indicated that the low dewatering rate can determine the better performance of emulsion stability. From Figure 9, it is determined that the CTAAC shows the best performance on emulsion stability compared with other members in the prepared surfactants. The reason is due to the fact that the closely packed surfactant molecule arrangement at the oil/water interface makes the interfacial film become stronger; thus, it can successfully protect droplets in the dispersed phase, ultimately leading to the significant reduction of droplet coalescence. This explanation is in good agreement with the relevant regularity of experimental results represented in the graphs.
In order to research further the relationship between the anions of quaternary ammonium cationic surfactants and their relative performances, a series of quaternary ammonium cationic surfactants with novel anions are synthesized and its basic surface tension, foam properties, and emulsification power were amply investigated in this study. (1)The estimated results of surface tension indicated that the CMC, , and values increase with the increase of methylene segments in the anions; however, values present an opposite trend. The test of foaming properties shows that the CTAAC exhibits the best performances on foamability and foam stability compared with other members in the prepared surfactants, regardless of experimental temperatures. The emulsion test presents that the emulsion stability followed the order of CTAAC>CTABU>CTAHE>CTACA(2)Compared with foam properties of the four surfactants under different pH, the abilities of foam to generate and stabilize foam are illustrated: CTAAC>CTAHE>CTABU>CTACA. From the foam properties under NaCl, the abilities of foam to generate and stabilize foam are illustrated: CTAAC>CTABU>CTAHE>CTACA
The data for the paper were used in the manuscript.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB0605503 and 2018YFC0807801), the National Natural Science Foundation of China (Grant No. 51804112), and the Natural Science Foundation of Hunan Province (Grant No. 2018JJ3169).
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