Oil-Tolerant Nonionic Worm-like Micellar Solution.

The goal of this work is to study the effect of crude oil on worm-like micelles and identify any oil-tolerant systems. A new class of nonionic surfactants was synthesized that forms viscous worm-like micelles under a wide range of temperature and salinity conditions. Aqueous stability, rheology, cryogenic transmission electron microscopy imaging, and dynamic-light-scattering measurements were performed to understand properties, shape, and size of the micelles formed using these surfactants under different temperatures and salinity conditions and in the presence of hydrocarbons. These micellar solutions maintained high viscosity in the presence of small amounts (up to 8 vol %) of crude oils and pure hydrocarbons. Similar experiments were performed with conventional surfactant systems that were known to form worm-like micelles; they did not show oil tolerance. Larger alkanes and viscous crude oils affect the viscosity and transformation of cylindrical micelles less. These new surfactants are useful for oil and gas operations such as hydraulic fracturing, conformance control, and mobility control as they form viscous worm-like micelles in the presence of small amounts of crude oils.

Introduction

Surfactants have the ability to self-assemble in aqueous solutions to form microstructures, such as spherical micelles, cylindrical micelles, vesicles, and others.[1,2] Cylindrical (or worm-like or rod-like) micelles form in many aqueous surfactant systems, with or without the presence of salt,[3−10] when the surfactant packing parameter, Ns [= v/(l × a0)], is between 0.33 and 0.5, where v is the volume of the hydrophobic portion of the surfactant, l is the length of the hydrophobic chain, and a0 is the effective area of the head group.[2] Cylindrical micellar solutions are often viscous because of entanglement between micelles, while spherical micellar solutions are not viscous. Cylindrical micellar solutions are also viscoelastic; their nonlinear rheology from dilute to concentrated solutions has been reviewed by Lerouge and Berrett.[11] The dilute solutions exhibit shear thickening because of shear-induced structures. The semidilute and concentrated phases undergo shear-banding transitions. The concentrated nematic phases of micelles exhibit a tumbling instability. In this work, we study semidilute (0.1–5 wt %) solutions of surfactants.

Cationic surfactants, in particular, have been extensively studied for worm-like micelles in the presence of high concentration of salts.[12−16] Worm-like micelles have also been reported for mixtures of cationic/anionic, nonionic/ionic, and zwitterionic surfactants.[17−23] Although most studies on worm-like micelles have been reported for charged surfactant systems, a few nonionic surfactants in the presence of a co-surfactant have also been reported.[24,25] Naito et al.[26] reported the formation of viscoelastic worm-like micelles in the presence of long-chain nonionic surfactant mixture (polyoxyethylene phytosterols: mixtures of PhyEO10 and PhyEO20). Sharma et al.[27] studied a long polyoxyethylene chain of phytosterol (PhyEO30) in the presence of short-chain polyoxyethylene alkyl ether type of surfactants. Many studies have reported worm-like micelles of di- and triblock copolymers. Chaibundit et al.[28] reported micellization of diblock copolymers and transition to worm-like micelles. Schillén et al.[29] reported PEO-PPO-PEO triblock copolymers to form worm-like micelles.

Worm-like micelles can be responsive to many stimuli, such as light,[30,31] temperature,[32−34] pH,[35−38] and CO2.[39] These stimuli help to change other types of micelles to worm-like micelles. On the contrary, the presence of hydrocarbon is a stimulus that breaks viscous worm-like micelles and converts them to nonviscous spherical micelles.[40−42] A few studies have investigated the effect of hydrocarbons on worm-like micelles and found that the type of hydrocarbon plays an important role. Aromatic hydrocarbons solubilize in the palisade layer (the outer shell of the micelles consisting of ethoxy (EO) groups in nonionic surfactants) and do not break worm-like micelles unless the outer layer is completely saturated, whereas aliphatic hydrocarbons penetrate into and swell the micellar core (inner hydrophobic core) and break down worm-like micelles to spherical micelles.[43] Törnblom and Henriksson[44] reported growth of micellar size in the presence of cycloalkanes and benzene while decrease in aggregate size with n-alkanes. Shibaev et al.[41] studied transition of worm-like micelles on adding hydrocarbons and found that both shortening of worm-like micelles and transition to spherical micelles occur on adding hydrocarbon. Reverse worm-like micelles[46−49] are compatible with hydrocarbons, but they are not miscible or soluble in water. Studies reported above investigated the effect of hydrocarbon on worm-like micelles formed using cationic or anionic surfactants.[45] In this study, we report the effect of hydrocarbon on worm-like micelles of a nonionic surfactant.

Worm-like micelles of surfactants have attracted a considerable interest in various fields including oil and gas[10,11] because their rheological property can be helpful in hydraulic fracturing, drilling, and surfactant flooding processes. As surfactant molecules are small and micelles can break and reform, these surfactants may be injected into tight rocks without causing any plugging. These worm-like micelles can be used in viscosity drilling and hydraulic fracturing fluids which can transform into less-viscous fluids (with spherical micelles) during oil production. Cleaning operations after fracturing (or drilling) with viscous worm-like micellar solutions are easier than with polymeric viscous solutions. Because the temperature and salinity in oil and gas reservoirs vary in a wide range, surfactant molecules are needed that can form viscous worm-like micelles under these diverse reservoir conditions.

Compatibility of worm-like micellar solution with crude oil is an important property for applications in oil reservoirs. Most surfactants have long hydrocarbon chains, and these chains interact with crude oils changing the cylindrical micelles to spherical swollen micelles in the presence of oils. We have synthesized a new class of surfactants where the hydrocarbon chain has been replaced by a propoxy (PO) chain, and we anticipate the interaction between the oil and PO chain to be lower than that with hydrocarbon chains. The motivation of this work is to study the effect of this surfactant structural change on worm-like micellar behavior of the surfactant solutions.

In this study, a new class of surfactants was synthesized and investigated under various temperatures (20–125 °C) and salinity conditions (up to 250,000 ppm TDS). Aqueous stability, rheology, cryogenic transmission electron microscopy (CryoTEM) imaging, and dynamic-light-scattering measurements were performed to understand properties, shape, and size of the micelles formed using these surfactants under different temperatures and salinity conditions. Crude oils and other hydrocarbons were mixed with the aqueous surfactant solutions, and the effect of oil was studied. The properties of these nonionic surfactants were compared with those of a few conventional surfactants that form worm-like micelles.

Results and Discussion

Phase Behavior

The aqueous stability of 2 wt % CH3–O–70PO–100EOH is shown in Figure a as a function of salinity and temperature. The mixtures are labeled “C” for clear solutions, “H” for hazy, and “P” for those with precipitations. A solution was called “aqueous stable” if it was clear or hazy. Surfactant solutions were clear at 25 °C and at 40 °C for 9 wt % salinity and lower. These solutions were hazy at 20–25 wt % salinity. At 65 °C, the solutions containing up to 4 wt % NaCl were clear and the solutions containing 5–8 wt % NaCl were hazy. Similarly, solutions with 20–25 wt % NaCl had precipitations. As the temperature increased to 90 °C, solutions were hazy up to 2% NaCl and the higher salinity solutions had precipitations, as shown in Figure b. At 125 °C, all the surfactant solutions had precipitations.

Figure 1

(a)Aqueous phase behavior of the 2 wt % CH3O–70PO–100EOH surfactant with temperature and salinity. Experimental data points are shown in blue; the lines are interpolations from the experiments. The symbol C represents clear, H represents hazy, and P represents precipitation. (b) Appearance of 2 wt % CH3–O–70PO–100EOH surfactant solutions in the presence of 0–9 wt % NaCl (from left to right) at 90 °C. (c) Aqueous phase behavior of the 2 wt % CH3O–70PO–75EOH surfactant with temperature and salinity. Experimental data points are shown in blue; the lines are interpolations from the experiments. The symbol C represents clear, H represents hazy, and P represents precipitation. (d) Aqueous phase behavior of the 2 wt % CH3O–70PO–120EOH surfactant with temperature and salinity. Experimental data points are shown in blue; the lines are interpolations from the experiments. The symbol C represents clear and P represents precipitation.

As the salinity and temperature increase, the surfactant solutions change from clear to hazy to precipitation, as shown in Figure a. The clear solutions had a viscosity similar to water. The hazy solutions were viscous; the viscosity is discussed in the next section. As is demonstrated in the later sections, the clear solutions correspond to spherical micelles and the hazy solutions indicate cylindrical “worm-like” micelles.

Other surfactants in the homologous series (constant PO numbers with variable EO numbers and vice versa) were also studied. CH3–O–70PO-nEO compounds containing the following number of EO groups were synthesized: 60EO, 75EO, 120EO, and 150EO. In addition, the CH3O–80PO–100EOH surfactant was also synthesized. The aqueous stability results of CH3O–70PO–75EOH and CH3O–70PO–120EOH surfactant are presented in Figure c,d, respectively. The results show that the CH3–O–70PO–75EOH surfactant also forms clear, hazy, and precipitated phases. As salinity and temperature increase, the aqueous phases go from clear to hazy to precipitate. The surfactant is less hydrophilic (compared to CH3O–70PO-100E), and the range of salinity and temperature with the clear phase shrinks. For the CH3–O–70PO–120EOH surfactant, the clear solution salinity and temperature range enlarges because of increased hydrophilicity. A similar study was repeated to CH3O–70PO–60EOH and CH3O–70PO–150EOH surfactants. They also went from clear to hazy to precipitated phases as temperature and salinity increased.

Rheology of Surfactant Solutions

The viscosity of hazy CH3O–70PO–100EOH surfactant solutions was measured by a rheometer. The viscosity of the surfactant solution containing 2% CH3O–70PO–100EOH and 7 wt % NaCl at 25 and 65 °C is shown in Figure . At 65 °C, the solution was hazy; its viscosity was high and decreased with the shear rate. At 25 °C, the solution was clear; its viscosity was close to 0.001 Pa s (the water viscosity) at the room temperature and shear-independent. The shear thinning viscous behavior is possibly because of the formation of cylindrical micelles. At 65 °C, the surfactant became more hydrophobic (because of the PO groups) and the surfactant packing parameter increased close to 1/2, thus forming cylindrical micelles. To confirm the difference in sizes of the micelles at 65 °C and 25 °C, particle size measurements were performed using a Delsa Nano Particle Analyzer and CryoTEM.

Figure 2

Viscosity of the 2 wt % surfactant in the presence of 7 wt % NaCl at 25 and 65 °C.

The viscosity of the 2 wt % CH3–O–70PO–100EOH surfactant at different salinity conditions (from 4 to 9 wt % NaCl) was measured at 65 °C and reported in Figure . The 7 wt % NaCl result is already shown in Figure . The result shows that the surfactant with 4 and 9 wt % NaCl is not viscous. The surfactant in 4 wt % NaCl was clear and was not viscous (probably had spherical micelles), whereas surfactant in 9 wt % NaCl got precipitated and thus the solution was not viscous. Rest of the solutions were viscous.

Figure 3

Viscosity of 2 wt % surfactant as a function of salinity at 65 °C.

The effect of surfactant concentration (CH3O–70PO–100EOH at 25 °C) on viscosity is shown in Figure . The surfactant concentration was varied from 0.1–2 wt %. The salinity was kept constant at 20 wt % NaCl. Room temperature and high salinity (20 wt % NaCl) were chosen for this experiment because surfactant solutions form cylindrical micelles at these conditions and evaporation is not an issue during the rheological study and particle size analysis. Viscosity increases with the increase in the surfactant concentration because the number of cylindrical micelles increase. Viscosity increases sharply as the surfactant concentration goes beyond 1 wt %.

Figure 4

Viscosity of as a function of surfactant concentration in the presence of 20 wt % NaCl at 25 °C.

Size Determination

CyroTEM imaging was performed to visualize the formation of worm-like micelles. A 2% CH3–O–70PO–100EOH surfactant sample containing 25 wt % NaCl was prepared at 25 °C. This solution was found to be viscous, as discussed previously. Another sample containing 7 wt % NaCl, which was not viscous at 25 °C, was used as the control. The results from the CryoTEM are shown in Figure . The nonviscous solution has spherical structures (left picture), and the viscous solution has cylindrical structures (right picture; at the edges of the picture). These structures are micelles of the surfactant. The particle size for spherical micelles (in the nonviscous solution) is roughly 25 nm and that of the cylindrical micelles (in the viscous solution) is about 100 nm in length and 25 nm in diameter.

Figure 5

CryoTEM of the surfactant solutions (left: 7% salt, nonviscous solution, right: 25% salt, viscous solution).

The particle size of these solutions was measured by using a Delsa Nano Particle Analyzer. This analysis presumes spherical shapes and calculates a diameter distribution. For nonspherical particles, it is an “equivalent diameter distribution”. The particle size of a 2% CH3–O–70PO–100EOH surfactant solution containing 7 wt % NaCl, which formed worm-like micelles at 65 °C, was measured at both 25 and 65 °C. The particle size distributions are shown in Figures and 7, respectively. The particle size distribution was log normal, and the parameters are listed in Table . The average particle size of this sample at 25 °C was around 27.5 nm with a standard deviation of 12.4 nm, whereas the particle size at 65 °C was about 112.9 nm with a standard deviation of 99.3 nm. The particle size at 65 °C is about 3–4 times larger; the corresponding increase in viscosity indicates that the micelles have changed from spherical to elongated (worm-like) structures when the solution is heated to 65 °C.

Figure 6

Delsa “particle size distribution” of 2% CH3–O–70PO–100EOH solution at 25 °C, 7% NaCl; average particle size is 27.5 nm.

Figure 7

Delsa “particle size distribution” of CH3–O–70PO–100EOH solution at 65 °C, 7% NaCl; average particle size is 112.9 nm.

Table 1

Particles Size by the Delsa Nano Analyzer

salt (wt %)	oil (%)	temperature (°C)	particle size (nm)	polydispersity
7	no	25	27.5	0.126
7	no	65	112.9	0.288
7	5	65	24.9
359.2	0.118
25	no	25	103.8	0.182

The particle size was also studied as a function of surfactant concentration for the CH3–44O–70PO–100EOH surfactant. The particle size of the 0.1–2 wt % surfactant in the presence of 20 wt % NaCl at 25 °C is presented in Figure . The average particle size of the 0.1 wt % surfactant is 192.1 nm with a standard deviation of 111.2 nm; the average particle size of the 2 wt % surfactant is 237.9 nm with a standard deviation of 210.6 nm. Rest of the solutions had the particle sizes in between. The formation of cylindrical micelles is independent to the surfactant concentration, probably above critical micelle concentration. However, as the surfactant concentration increased, the cylindrical micelles got longer and there were more micelles.

Figure 8

Particle size distribution of CH3–O–70PO–100EOH at 25 °C, 25% NaCl for several surfactant concentrations.

Effect of Hydrocarbon on Worm-Like Micelles

Some surfactants are well known in the literature to form worm-like micelles[37,39] The cationic surfactant cetyl trimethylammonium bromide (CTAB) and anthranilic acid together have shown to form worm-like micelles[37]Figure shows the viscosity of 1.82 wt % CTAB + 0.68 wt % anthranilic acid in water at the room temperature. The solution is viscous, and shear thinning because of worm-like micelles formed. A sample of this solution was mixed with 5 wt % decane; its viscosity is also shown in Figure . This solution is Newtonian and has a viscosity close to that of water. The worm-like micelles were transformed to spherical micelles in the presence of decane. A similar test was also conducted with 5 wt % crude oil-1 with similar results. Most cylindrical or worm-like surfactant micelles are often unstable in the presence of crude oil or hydrocarbons.

Figure 9

Viscosity of 1.82 wt % CTAB + 0.68 wt % anthranilic acid in water with and without oils at 25 °C.

To test the oil compatibility of cylindrical micelles obtained with the new surfactant molecules, different amounts (4–8 vol %) of the crude oil-1 was mixed with the viscous surfactant solutions (containing 2 wt % CH3–O–70PO–100EOH and 7 wt % NaCl) at 65 °C. The solution was gently mixed and kept at 65 °C for two days. After two days, the aqueous solutions were found to be clear. The aqueous solution was carefully separated, and its viscosity was measured. These viscosities are presented in Figure . Viscosity decreased as the oil amount in the solution increased. The surfactant solution remained viscous even in the presence of 8 vol % crude oils in contrast to the CTAB system discussed in the last paragraph. This is a new finding of this work. Similar experiments were also performed by adding crude oil-2, which is a viscous oil. Crude oil-2 had a smaller effect on the micellar solution viscosity than crude oil-1, as shown in Figure .

Figure 10

Viscosity of 2 wt % MeO–70PO–100EOH in 7 wt % NaCl aqueous solution in the presence of oils: (a) with and without crude oil-1 and (b) with and without crude oil-2.

From the viscosity data, it is clear that viscosity decreases with increasing oil concentration. The viscosity of the solution in the presence of crude oil-2 is relatively higher than in crude oil-1. In general, viscous oil had a smaller effect on the micellar solution.

Similar experiments were performed in the presence of 5 vol % decane, dodecane, tetradecane, and octadecane. These results are shown in Figure . The aqueous solution of 2 wt % CH3–O–70PO–100EOH in 7% NaCl with 5 vol % hydrocarbon oil shows shear-thinning viscosity. The longer chain hydrocarbons reduce the viscosity less and have less interaction with the worm-like micelles. These results are also in agreement with crude oil results; crude oil-2 (containing heavier hydrocarbon components) had a smaller effect on the viscosity of worm-like micelles than the light crude oil-1. These solutions maintained their viscosity in the presence of crude oils and pure hydrocarbons for months.

Figure 11

Viscosity of 2 wt % MeO–70PO–100EOH in 7% NaCl with various oils at 65 °C.

The particle size distribution of the surfactant solution (of 2 wt % MeO–70PO–100EOH in 7% NaCl aqueous solution) with 5 vol % oil-1 at 65 °C is shown in Figure . It has a bimodal distribution with the peak particle sizes of about 25 and 360 nm. The particle size of the surfactant solution without the oil is shown in Figure ; the average size was about 112.9 nm with a standard deviation of 99.3 nm. Table lists the size distributions. Presumably, some of the cylindrical micelles are changed to spherical micelles after solubilizing oil.

Figure 12

Intensity distribution of particles of CH3–O–70PO–100EOH in the presence of 5 wt % oil-1 at 65 °C; average particle sizes are 24.9 and 359.2 nm.

These new class of surfactants with PO and EO groups formed worm-like micelles under specific temperature and salinity ranges. At low temperatures, high salinity (>9%) was required to form worm-like micelles and vice versa. This study is very unique because no previous study has reported a single surfactant molecule that forms worm-like micelles at such a wide range of salinity and temperature conditions. At a given temperature, the salinity range where viscous behavior was observed can be controlled by changing the number of EO and PO groups in a given molecule. In addition, these worm-like micelles were found to be compatible with crude oils to some extent. In the presence of small amounts of crude oil or pure alkanes, oil-swollen worm-like micelles were formed with these new surfactants.[13] One of these new surfactant molecules was able to keep worm-like micelles in the presence of 8 vol % crude oil or n-alkanes, unlike previous surfactant systems. This amount of crude oil (or n-alkane) is about 10 times the value reported in the literature.[42,46] The compatibility of the worm-like micelles with oil is ascribed to the unique hydrophobic structure of this molecule. Most surfactants have long hydrocarbon chains as their hydrophobes; these hydrophobes are very lipophilic and solubilize oils in inner cores of the micelles leading to break up of cylindrical micelles into spherical micelles. The surfactants studied here have PO groups as their hydrophobes; these hydrophobes solubilize less oil into the inner cores and preserve the cylindrical micelles.

Surfactant solutions with worm-like micelles are viscous. They are often not used for increasing sweep efficiency in oil reservoirs because these surfactant solutions lose their viscosity in the presence of oil. The novel surfactants studied here can develop viscous micellar solutions with limited tolerance to oil. They also maintain their viscosity at high temperatures and salinity. Polymer solutions are often used for improvement of sweep efficiency in oil reservoirs. The typical viscosity needed at the reservoir conditions is about 0.005–0.05 Pa s at about 10 s–1 shear rate. As shown in Figures –4, these micellar solutions can achieve this range of viscosity. Commonly used polymers are susceptible to chemical and thermal degradation under harsh temperatures and salinity conditions, and they do not transport well in low permeability formations.[50−52] These worm-like micellar solutions have the potential to be used for improvement in sweep efficiency during oil displacement as well as in fracturing.[53,54]

Conclusions

In this study, methanol-based surfactants having different number of PO and EO groups were synthesized and their micellar solutions were investigated under different temperature and salinity conditions. Unlike worm-like micelles of other surfactants reported in the literature, the worm-like micelles of these surfactants showed high viscosity in the presence of crude oils and pure hydrocarbons. This behavior is ascribed to having soft hydrophobic groups of PO instead of hydrocarbon chains. These surfactants can be useful for various oil and gas applications, especially for mobility control in tight formations. The following conclusions can be inferred from this work:

Single surfactants based on methanol, EO, and PO groups can form worm-like micelles.

For the same number of PO and salinity, the new surfactants form worm-like micelles at higher temperatures as the number of EO group is increased.

The surfactant concentration needs to be about 2 wt % to achieve a viscosity of 0.05 Pa s in brines.

The worm-like micelles of MeO–70PO–100EOH can tolerate crude-oil/hydrocarbon up to 8 vol %.

Heavier hydrocarbons had less effect on the viscosity of these worm-like micellar solutions than lighter hydrocarbons.

Experimental Section

Materials

The chemicals used in this study are surfactants and salts. Laboratory grade sodium chloride and CTAB were obtained from Fisher Scientific. Surfactants CH3–O–mPO–nEOH were synthesized by Harcros Chemicals (Kansas, USA). These compounds were synthesized to study the interaction of PO groups (as soft hydrophobes) with oil. These compounds have only one carbon containing the hard hydrophobe. Compounds with no hard hydrophobe were studied in the literature as Pluronic block co-polymers.[28] The starting material to synthesize these surfactants are methanol, whereas the starting material for Pluronics are ethylene or propylene glycols. Two crude oils were used. Crude oil-1 was a 38° API oil of viscosity 3.7 cP at 25 °C; its acid number was 0, and base number was 1.6 mg/gm. Crude oil-2 was a 28° API oil of viscosity 30 cP at 25 °C; its acid number was 0.1 mg/gm. Hydrocarbons such as n-decane, n-dodecane, n-tetradecane, and n-octadecane were also obtained from Fisher Scientific.

The solubility of surfactants in brine depends on salinity and temperature. A surfactant is called “aqueous stable” if it totally dissolves in an aqueous solution at a given salinity and temperature. Aqueous stability of the surfactants was studied by preparing 4 mL solutions in glass vials containing the 2 wt % surfactant and varying amounts of NaCl from 0 to 9 wt %. For CH3–O–70PO–100EOH, the aqueous stability was also studied in the range of 20–25 wt % NaCl. These vials were kept in an oven set at a given temperature for about 12 h and the precipitation, aggregation, and clarity were recorded visually. An example of these samples can be seen in Figure a.

Rheology

The rheology of aqueous surfactant solutions, prepared by adding given amounts of the surfactant and sodium chloride, was obtained with a TA Instruments ARG2 rheometer. Viscosity of these solutions was measured as a function of shear rate and temperature. These experiments were repeated after adding a given amount of crude oil or pure n-alkane to the surfactant solutions. Rheology as a function of surfactant concentration was also studied for one of the surfactants. The surfactant mixed with 20 wt % NaCl had a high viscosity even at the room temperature.

Size Determination

The size of the surfactant micelles was obtained by the dynamic light scattering (DLS) method using the Delsa Nano analyzer. The sample solutions were prepared by mixing the 2 wt % surfactant in 7 wt % NaCl solutions; the solutions were heated in an oven to 65 °C. Finally, 1 mL sample was taken in a glass cuvette for DLS measurement. Then, the sample was allowed to cool to room temperature and the particle size was measured again.

Cryogenic Transmission Electron Microscopy

The surfactant solution sample is put onto a very small grid which is plunged into a liquid ethane bath at a very low temperature to freeze the surfactant aggregate without any structural changes. In our measurement, 3 μL of the 2 wt % CH3–O–70PO–100EOH surfactant in 7 (and 25 wt % for a second sample) NaCl solutions was put onto a QUANTIFOIL grid and then plunged into ethane at −182 °C using a Leica GP. The grid was transferred to a Gatan 626 Cryo specimen holder for imaging in Tecnai Spirit BioTwin 80 kV TEM.