Lionel T. Foganga,
Abdullah S. Sultan*ab and
Muhammad S. Kamalb
aDepartment of Petroleum Engineering, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Saudi Arabia. E-mail: sultanas@kfupm.edu.sa
bCenter for Integrative Petroleum Research, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Saudi Arabia
First published on 24th January 2018
Though the transition from cylindrical micelles to spherical micelles of the anionic surfactant potassium oleate in the presence of oils has been studied, these changes have not been studied for long-tail zwitterionic surfactants. The effects of n-decane, crude oil (CO), extra virgin olive oil (EVOO) and polyglycolic acid (PGA) on the zero-shear viscosity of an aqueous solution of a sulfobetaine surfactant system were investigated at 30 °C and 60 °C. The main surfactant in the system was erucamidopropyl hydroxypropyl sulfobetaine. The methods employed were rheology and cryo-TEM. The solution with 3.96 wt% surfactant system and 6.2 wt% CaCl2 was viscoelastic at both test temperatures due to the formation of entangled cylindrical micelle networks. n-Decane induced the following regimes of zero-shear viscosity change at both temperatures: (i) the high viscosity regime (HVR), (ii) the transition regime (TR), and (iii) the low viscosity regime (LVR). The HVR was characterized by high zero-shear viscosities. The TR was characterized by a sharp drop in zero-shear viscosity due to the formation of untangled micelles. The LVR was due to the formation of microemulsions. The formation of these regimes depended on the balance between micellization and oil solubilization. We reveal for the first time that the number of regimes depends on the type of oil: both CO and EVOO induced only one and two regimes at 30 °C and 60 °C, respectively. PGA did not significantly affect the solution at either temperature with increasing concentration, meaning the solution was resistant to decreasing pH even at higher temperatures.
VES have important uses in oilfield operations such as drilling, well stimulation and enhanced oil recovery.11–13 They are used in well stimulation because of their ease of preparation and their stability at high temperatures.11,14 VES are used in well stimulation to improve oil and gas well productivity because they form high viscous gels which are desirable for well stimulation.14–16 After well stimulation, the gels need to be removed from the reservoir in order to resume oil and gas production. VES gels are easily removed from oil reservoirs when compared to polymer gels14 because reservoir oils reduce their viscosities easily.11 This makes reservoir oil referred to as an external breaker of these gels.17 But the reduction of the viscosity by reservoir oil is inefficient.18,19 Moreover, the gels are detrimental to natural gas production if left in the reservoir.18 These factors led to the development of internal breakers: chemical compounds which are mixed with the VES solution to reduce the viscosity of the VES gel in the reservoir after well stimulation.20–27
Experimental studies have determined how some organic compounds, especially hydrocarbons, reduce the viscosity of surfactant solutions with cylindrical micelles. These organic compounds reduce the viscosity of the surfactant solutions by solubilizing in the core of cylindrical micelles.5,6,8,28–30 Solubilization disrupts the cylindrical micelle structure, changing longer cylindrical micelles into shorter cylindrical micelles,8 short rod micelles,29 spherical micelles or microemulsions.5,8,30
Kralchvesky and coworkers31,32 determined the solubilization kinetics of non-polar oils in mixed cylindrical micelles of a nonionic surfactant and a triblock copolymer. The micelles adsorb on the oil-aqueous solution interface before solubilization takes place.31,32 Shibaev and co-workers30 showed there are three regimes of micelle change in viscoelastic solutions of potassium oleate at 20 °C: (i) the micellar network regime (ii) the transition regime, and (iii) the microemulsion regime. The existence of micellar network regime was attributed to the reduction in length of the cylindrical micelles, disrupting the micelle network. This occurs when the oil solubilizes in the cylindrical micelles, preferably at the semispherical endcaps. The existence of the transitional regime was attributed to the simultaneous reduction in length of cylindrical micelles and the formation of microemulsions.
Long-tail surfactants have been of interest of research due to their various applications as viscosifying agents.5 Moreover, long-tail sulfobetaine surfactants have been reported to be a good candidate for especially high-temperature applications such as well stimulation.33 Despite this interest, micellar changes of long-tail sulfobetaine surfactants induced by organic compounds have not been published. Moreover, previous studies revealing the kinetics of transition from cylindrical micelles to spherical micelles has been on potassium oleate surfactant solutions at low temperatures and with only alkanes. There is no clear mechanism on the effect of different oils and temperature on such a transition. The mechanism of viscosity reduction of long-tail sulfobetaine surfactant solutions after the addition of an organic compound is especially important for any application that requires the viscosity reduction of such solutions. Furthermore, it is not known if a surfactant system with a different functional group and headgroup charge will follow the same behavior as potassium oleate when in contact with viscosity-reducing organic compounds.
Thus, the objective of this work was to investigate the mechanism of micellar changes that leads to a decrease in viscosity of a long-tail sulfobetaine surfactant solution. This was achieved by conducting rheological and cryo-TEM studies at two temperatures, and using four organic compounds at different concentrations.
η0 = G0τR | (1) |
(2) |
(3) |
(4) |
VES solutions have cylindrical micelles at high concentrations. The ends of a cylindrical micelle are semispherical in shape. According to Cates and Candau,9 the average contour length of a cylindrical micelle, L, is related to surfactant volume fraction, c, by the following equation
(5) |
(6) |
VES deviate from Maxwellian fluid behavior at high angular frequencies. At this point, breaking time greatly exceeds reptation time. This leads to the existence of a local minimum of G′′, .10 Cates and coworkers9,10 derived the following relationship between micelle mesh size, ξ, and plateau modulus
(7) |
Fig. 1 Structure of erucamidopropyl hydroxypropyl sulfobetaine.33 |
For surface tension measurements, a batch solution of surfactant was prepared and left for a day. For rheological measurements, the surfactant solutions were mixed with the organic compounds and equilibrated for one week at 30 °C and 60 °C. In case there was phase separation in the solution, the surfactant solution phase was tested.
The viscosity of the surfactant solution as a function of shear rate at both temperatures measured after the equilibration period is shown in Fig. 3. There was a transition from the Newtonian to the shear-thinning region with increasing shear rate at both temperatures. The transitions at both temperatures were evidence that the surfactant solution had cylindrical micelles that aligned themselves along the direction of flow at both temperatures.34,35 Cryo-TEM confirmed the presence of micelles in the surfactant solution equilibrated at 30 °C (Fig. 4), even after dilution with ethyl acetate (see Fig. S2 and S3†).
Fig. 4 Cryo-TEM image of 3.96 wt% surfactant solution at 30 °C. The black lines represent the micelle edges. |
There was a sharp drop in viscosity during the transition from the Newtonian to the shear-thinning region from the viscosity against shear stress plot at both temperatures (Fig. 5). This is considered a characteristic of shear-banding during the transition from the Newtonian to the shear-thinning region; the separation of a fluid into macroscopic sections with different shear rates.36 This implies micelle networks were present at both temperatures. Moreover, there was an inflection point during this transition from the shear stress versus shear rate plot at 30 °C (Fig. 6) and an apparent yield stress at 30 °C as seen from the viscosity versus shear stress graph (Fig. 5). An inflection point and apparent yield stress have been reported in another erucyl-tailed sulfobetaine surfactant, 3-(N-erucamidopropyl-N,N-dimethyl ammonium)propane sulfonate (EDAS). This was explained to be as a result of shear banding.34,37 Given the presence of shear-banding at both temperatures, the presence of the inflection point at 30 °C shows shear banding was more prominent at the lower temperature than at the higher temperature. This was most likely due to the greater closeness of the cylindrical micelles to each other at 30 °C than at 60 °C. The closeness of the cylindrical micelles might also explain why the zero-shear viscosity of the surfactant solution was higher at 30 °C than at 60 °C (Fig. 3, see estimates in Table S1†). This will be elaborated when considering the dynamic shear rheology of the surfactant solution.
The dynamic rheology of the surfactant solution at 30 °C and 60 °C is shown in Fig. 7. G′ dominated G′′ within the tested frequency range at both temperatures. Moreover, the crossover frequency was absent within the measured frequency range. Erucyl-tailed amphoteric surfactants such as erucyl dimethyl amidopropyl betaine (EDAB),5 EDAS34 and erucyldimethyl amidopropyl amine oxide (EMAO)38 have such behavior within a frequency range of 0.01–100 rad s−1 at high concentrations and low temperatures. Viscoelastic fluids typically show a region where G′′ dominates G′ at lower angular frequencies and a crossover frequency. The complete dominance of G′ over G′′ in the angular frequency range of 0.01–100 rad s−1 is attributed to gels.5 But the existence of the Newtonian region at low shear rates (Fig. 3) and G0 (Fig. 7) is evidence that the surfactant solution followed the Maxwellian fluid model at low angular frequencies at both temperatures. Chu and Feng showed that a solution with 100 mM and 250 mM of EDAS followed Maxwellian fluid behavior even with the total dominance of G′ over G′′ within an angular frequency range of 0.01–100 rad s−1.37 This implies the surfactant solution had a crossover frequency at both temperatures. The crossover frequencies and relaxation times of the surfactant solution at both temperatures are estimated from eqn (1) and (2) (Table S1†). The estimated crossover frequencies at both temperatures were below 0.1 rad s−1. Thus, the absence of the crossover frequency was because the angular frequencies used for measurement were not able to detect the crossover frequency.
According to Granek and Cates,10 another evidence for the presence of entangled wormlike micelles is the presence of on the dynamic rheology profile. The presence of shows the deviation of the fluid from Maxwellian behavior at high angular frequencies.10 was present at both temperatures. This means ξ can be estimated from eqn (6). The estimated value of ξ at 30 °C was lower than that 60 °C (see Table S1†), signifying that the micelles were highly entangled at 30 °C than at 60 °C. This explains why the viscosity of the surfactant solution at 30 °C was higher than at 60 °C, and why shear-banding was more pronounced at the lower temperature than at the higher temperature. Micelles close to each other will offer more resistance to shear than micelles far apart. Also, the more the micelles are close together, the more the micelles will experience shear banding transitions.36
Such rheological behaviors have been attributed to the fact that the tail lengths are very long, thus high hydrophobic. This disfavors the exchange of monomers from one micelle to another through water whenever a micelle breaks and reforms.5
Fig. 8 Estimated zero-shear viscosity of 3.96 wt% surfactant solution with different n-decane and octadecane concentrations. |
Fig. 9 Estimated zero-shear viscosity of 3.96 wt% surfactant solution with different crude oil concentrations. |
The estimated difference in zero-shear viscosities between the surfactant solutions with the oils and the pure surfactant solutions at the test temperatures are in Table S2.† At 30 °C, 0.9 wt% n-decane induced a difference of approximately two orders of magnitude, whereas 0.9 wt% EVOO and crude oil induced a difference of approximately one fold. The steady state rheology of the surfactant solutions with 2 wt% and 3 wt% of crude oil and EVOO were measured to check if the effect of 0.9 wt% of these oils were due to low oil concentrations. The differences induced by 3 wt% crude oil and EVOO were less than ten folds. Meanwhile, the viscosities of the surfactant solutions with 2 wt% and 3 wt% n-decane were approximately 0.001 Pa s (Fig. 8); an approximate difference of five orders of magnitude. At 60 °C, 0.9 wt% of the oils induced drastic differences, with n-decane inducing the greatest difference. This was also the same with 2 wt% and 3 wt% of the oils at 60 °C. Hence, n-decane was the most efficient breaker among the oils at both temperatures. Furthermore, temperature enhanced the breaking effect of crude oil and EVOO on the surfactant solution.
Zero-shear viscosity reduction by non-polar oils is linked to the solubilization of oil molecules in the micelle core.7,8,29,30 The solubilization of the oil molecules in the micelle core disrupts the micelle structure, leading to structural changes and reducing the zero-shear viscosity of a surfactant solution. Because crude oil and EVOO did not induce drastic viscosity differences like n-decane at 30 °C, these oils faced a barrier to solubilization in the micelle core.
The solubilization mechanism of non-polar oils by cylindrical micelles proposed by Kralchevsky and co-workers31 will be used to interpret the results, as this mechanism has been experimentally verified.32 Moreover, the formation of cylindrical micelles as soon as the surfactant system was dissolved in the aqueous solution will be taken into account. The effect of temperature explained as follows: temperature increased the average number of transient adsorptions of cylindrical micelles on the oil-aqueous solution interface. Temperature also increased the kinetic energy of the oil molecules, permitting oil molecules to migrate from the oil phase into the cylindrical micelle core. It is only in the cylindrical micelle core that oils can solubilize as they are insoluble in a salt solution. The increase in temperature provided the kinetic energy necessary for crude oil and EVOO oil molecules to overcome the barrier to solubilization. To confirm that competition between the different components in EVOO and crude oil did not affect their solubilization in the micelles, the effect of octadecane at 60 °C was tested. The viscosity of the surfactant solution with octadecane was also higher than that of the surfactant solution with n-decane (see Fig. 8 and Table S2†). This confirms oil molecular weight is a significant factor for oil solubilization in micelles.
Zero-shear viscosity dropped with increasing n-decane, crude oil, and EVOO concentration (Fig. 8, Fig. 9 and Fig. S4,† respectively). The oils induced several regimes of viscosity change, similar to Shibaev and co-workers.30 n-Decane induced three regimes at both temperatures in the following order: (i) the high viscosity regime (HVR), (ii) the transition regime (TR), and (iii) the low viscosity regime (LVR). Each regime existed within a concentration range. The other oils induced only the HVR at 30 °C, and the HVR and TR at 60 °C.
The reason for the existence of this regime depended on the nature of the oil. This is because the concentration range for which the HVR existed for n-decane was lower than that of crude oil and EVOO. It is suspected that the existence of the HVR in the case of n-decane was mainly due to the concentrations not being enough to prevent the formation of a micelle network. In the case of crude oil and EVOO, it is suspected that there were few molecules in these oils that had enough kinetic energy to penetrate the micelles at 30 °C, even at high concentrations. These oils have molecules of high molecular weight and complex structure, which might be a reason for the solubilization barrier. The HVR did not exist at high concentrations of crude oil and EVOO at 60 °C. Thus, the reason for the existence of HVR at 60 °C was possibly mainly due to the low oil concentrations.
Fig. 11 Cryo-TEM image of 3.96 wt% surfactant solution with 3 wt% n-decane at 60 °C. The white sections are the micelles. |
Thus, the formation of each regime depended on the balance between the formation of a dense micelle network and the solubilization of the oils by cylindrical micelles. Furthermore, we show for the first time that the formation of each region also depends on the type of oil which is solubilized by the micelle.
The dynamic shear rheology of the surfactant solutions with 0.1–0.9 wt% PGA at 30 °C and 60 °C was similar in some respects to that of the surfactant solutions with 0.1–0.9 wt% crude oil at 30 °C (Fig. S30 and S31†). The region where G′′ dominates G′ at lower angular frequencies and the crossover frequency were absent at both temperatures, except with the surfactant solution with 0.9 wt% PGA at 60 °C. The reason for this absence is the same for the pure surfactant solution at both temperatures.
The effect of PGA on these surfactant solutions can be understood by comparing its effect on the amphoteric carboxylic betaine surfactant, EDAB.26 Amphoteric carboxylic betaine surfactants become cationic or anionic at low and high pH respectively,39,40 unlike sulfobetaine surfactants which remain zwitterionic at all pH.40 Increasing the PGA concentration reduces the pH, changing EDAB into a cationic surfactant. This change increases the headgroup charge, thus increasing the effective headgroup area of the surfactant. The increase in the headgroup area reduces the packing parameter, changing EDAB from cylindrical to spherical micelles, which reduces the viscosity of EDAB solution. This mechanism was also proposed for the changes induced by sodium salicylate (NaSal) and sodium hydroxynaphthalene carboxylate (NaHNC) on EDAB.5 Because the surfactant was a sulfobetaine, it resisted a change in charge with reducing pH. Therefore, the micelles were insignificantly affected.
VES | Viscoelastic surfactant |
EVOO | Extra virgin olive oil |
PGA | Polyglycolic acid |
HVR | High viscosity regime |
TR | Transition regime |
LVR | Low viscosity regime |
DHR-3 | Discovery Hybrid Rheometer 3 |
EDAS | 3-(N-erucamidopropyl-N,N-dimethyl ammonium)propane sulfonate |
EDAB | Erucyl dimethyl amidopropyl betaine |
EMAO | Erucyldimethyl amidopropyl amine oxide |
Footnote |
† Electronic supplementary information (ESI) available: Steady shear viscosity plots of the surfactant solution with the oils and cryo-TEM pictures. See DOI: 10.1039/c7ra12538k |
This journal is © The Royal Society of Chemistry 2018 |