Microemulsion Rheological Analysis of Alkaline, Surfactant, and Polymer in Oil-Water Interface

: Injection of alkaline (A), polymer (P), and surfactant (S) chemicals in enhanced oil recovery (cEOR) processes increases output by changing the properties of the injected ﬂuid. In this work, micellar ﬂuid interactions were studied via microemulsion rheological analysis. Crude oil and stimulated brine with ASP or SP was used for bottle testing. The results revealed that no microemulsion was produced when ASP (Alkaline, Surfactant, and Polymer) or SP (Surfactant and Polymer) was left out during the bottle testing phase. The addition of ASP and SP led to the formation of microemulsions—up to 29% for 50% water cut (WC) ASP, and 36% for 40% WC SP. This shows that the addition of ASP and SP can be applied to ﬂooding applications. The results of the rheological analysis show that the microemulsions behaved as a shear-thinning micellar ﬂuid by decreasing viscosity with increase in shear rate. As per the power-law equation, the ASP micellar ﬂuid viscoelastic behavior shows better shear-thinning compared to SP, suggesting more e ﬃ ciency in ﬂuid mobility and sweep e ﬃ ciency. Most of the microemulsions exhibited viscoelastic ﬂuid behavior (G’ = G”) at angular frequency of 10 to 60 rad s − 1 , and stable elastic ﬂuid behavior (G’ > G”) below 10 rad s − 1 angular frequency. The viscosity of microemulsion ﬂuids decreases as temperature increases; this indicates that the crude oil (i.e., alkanes) was solubilized in core micelles, leading to radial growth in the cylindrical part of the wormlike micelles, and resulting in a drop in end-cap energy and micelle length. No signiﬁcant di ﬀ erence was found in the analysis of viscoelasticity evaluation and viscosity analysis for both ASP and SP microemulsions. The microemulsion tendency test and rheology test show that the addition of ASP and SP in the oil-water interface yields excellent viscoelastic properties. SP, more shear thinning (effect) micellar microemulsion as the shear rate rises. The viscoelasticity evaluation shows that microemulsions at a low shear The viscoelastic properties of microemulsions at frequencies between 10 to 60 rad s − 1 . The viscosity of the microemulsions produced also decreases when the temperature increases, as the mobility of the microemulsions increases with The formation of microemulsions via the bottle test and the results from the rheological analysis prove that the addition of alkaline, surfactant, and polymer leads to better sweep efficiency and mobility control of microemulsions in chemical EOR flooding.


Introduction
Chemical EOR (cEOR) is a method to increase oil recovery by improving the properties of reservoir fluids to make them more favourable for oil extraction. The introduction of cost-effective surfactants and polymers, coupled with improved reservoir modelling, has re-invigorated the surfactant, polymer (SP) and alkaline, surfactant, polymer (ASP) flooding in current cEOR processes [1]. ASP and SP flooding is a technique based on gradual enhancement of oil recovery by decreasing the interfacial tension (IFT) value, increasing the capillary number, enhancing microscopic displacing efficiency, improving mobility ratio, and increasing macroscopic sweep efficiency [2].  Table 2. Crude oil composition and properties [12]. The composition of crude oil and its properties were obtained from our previously published paper, with permission from Borhan

Emulsion Tendency Test
The emulsion tendency test was performed according to procedure given in [13]. The test was initiated to select the optimum ratio of ASP: brine that gives the highest volume of microemulsions for its respective water cut (WC). Table 3 shows the percentage of crude oil in different water cuts used in bottle testing. The crude oil was placed in a water bath at 60 • C for 5 min to stimulate its temperature with actual offshore temperature conditions. The mixtures of brine, ASP, or SP, and crude oil were filled into 25 mL test tubes at different water cuts and percentages of crude oil. The test tubes were secured using aluminium foil and shaken for 100 times manually. The amount of water and oil separated was recorded at 0, 5, 10, 15, 20, and 30 min. The separation was recorded directly from the test tube.

Emulsion Rheology Test
Rheological measurement of microemulsion is a valuable tool in understanding the behaviour of microemulsion formation during the storage and injection process [14]. The microemulsion produced during the bottle testing was analysed by using a rheometer equipped with an interfacial rheology system (IRS) to study the behaviour of microemulsion as a bulk viscosity, viscoelastic, and under shear. The IRS consists of a bicone (radius of 34.12 mm and cone angle of 5 • ) and a glass of cup (inner radius of 40 mm and height of 20 mm) surrounded by an insulation jacket and covered by a metallic cap. Microemulsion samples of 3 mL were used to fill the measuring cup for a test. The frequency sweep was performed from 0.00628 to 62.8 angular frequencies at 0.1 to 100% shear rate. For viscosity analysis, the samples were analysed at various temperatures from 30 to 80 • C, fixed at 60 • C, and 1 to 70 s −1 shear rates. The micellar fluid samples were analysed using an instrument of the Anton Paar model of rheometer MCR 302, equipped with Rheocompass software. The sample preparation included removing the sample from the container, shaking or stirring it, filling it into the measuring system, positioning the measuring system as well as the subsequent waiting time, or a pre-defined shear before starting the actual measurement. The time taken to stabilize the microemulsions in the instrument was 15 min. Each microemulsion formulation of 3 mL taken from 21 samples was used to fill until the level marked on the measuring cups. The sample was always poured well above the upper rim of the inner cylinder.

Results and Discussion
The results obtained were segmented into two parts: Bottle testing for ASP and SP at five different concentrations and rheology analysis for the emulsion phase produced from the bottle testing experiment.

Emulsion Tendency Test
Emulsion tendency testing was performed to investigate the microemulsion tendency of ASP and SP in crude oil and to prepare the microemulsion for the rheology test. Figure 1 reveals the results of emulsion tendency bottle testing for a 100% WC solution at five different crude oil percent ratios after 30 min. All five crude oil percent ratios had no emulsion or microemulsion, and separation of oil and brine was clearly seen, indicating the lack of interaction. Figures 2 and 3 show the microemulsion produced after the addition of ASP and SP in the crude oil and brine mixture. A mixture containing ASP in 50% water cut solution had the highest volume of microemulsion from 0 to 29%, respectively, while the highest volume of microemulsion for a mixture containing SP was recorded in 40% water cut solution from 0 to 36%, respectively. The results indicate that the ASP composition had a lower

Emulsion Rheology Test
An emulsion rheology test was performed to investigate the rheology properties of microemulsion produced from the bottle testing test. Figure 4a,b represent the changes in viscosity as a function of shear rate for ASP and SP microemulsions. The concentration of ASP varies from 20 to 80% WC, while the concentration of SP ranges from 20 to 60% WC. The temperature and shear rate were at 60 °C and 1 to 70 s −1 , respectively. In Figure 4a, the log/log plot shows the effect of ASP concentrations on the functional relationship of viscosity and shear rate at 60% oil concentration. The viscosity of all microemulsions decreases as the shear rate increases, showing the typical behaviour of the pseudoplastic fluid. Increasing the ASP concentration resulted in more adsorption of the thickening agent through the oil-water interface, thus producing an apparent higher microemulsion viscosity. The marked rheology of ASP microemulsion comes from the HPAM polymer's large macromolecular weight. The entanglement of macromolecule chains increases at a high concentration of ASP, which then causes an increase in viscosity of the microemulsion [10]. The shear-thinning behaviour phenomenon is related to the orientation of the macromolecule along the streamline of the flow [15]. At low shear rate, the entanglement of macromolecules causes the formation of aggregates, resulting in high viscosity fluid. As the shear rate was applied to the fluid, it destroyed the aggregates, and the dispersing molecules were arranged along the flow direction, declining the flow of the fluid resistance and subsequently decreasing the apparent viscosity [16].

Emulsion Rheology Test
An emulsion rheology test was performed to investigate the rheology properties of microemulsion produced from the bottle testing test. Figure 4a,b represent the changes in viscosity as a function of shear rate for ASP and SP microemulsions. The concentration of ASP varies from 20 to 80% WC, while the concentration of SP ranges from 20 to 60% WC. The temperature and shear rate were at 60 • C and 1 to 70 s −1 , respectively. In Figure 4a, the log/log plot shows the effect of ASP concentrations on the functional relationship of viscosity and shear rate at 60% oil concentration. The viscosity of all microemulsions decreases as the shear rate increases, showing the typical behaviour of the pseudoplastic fluid. Increasing the ASP concentration resulted in more adsorption of the thickening agent through the oil-water interface, thus producing an apparent higher microemulsion viscosity. The marked rheology of ASP microemulsion comes from the HPAM polymer's large macromolecular weight. The entanglement of macromolecule chains increases at a high concentration of ASP, which then causes an increase in viscosity of the microemulsion [10]. The shear-thinning behaviour phenomenon is related to the orientation of the macromolecule along the streamline of the flow [15]. At low shear rate, the entanglement of macromolecules causes the formation of aggregates, resulting in high viscosity fluid. As the shear rate was applied to the fluid, it destroyed the aggregates, and the dispersing molecules were arranged along the flow direction, declining the flow of the fluid resistance and subsequently decreasing the apparent viscosity [16].   Figure 4b indicates the viscous behaviour of SP as a function of shear rate. The viscosity of SP emulsions was in the range of 0 to 0.8 mPa.s, which is slightly lower than the viscosity of the ASP emulsion with a range of 0.2 to 1.4 mPa.s. The presence of alkaline in the system leads to an increase in the microemulsion viscosity. The high viscosity produced by the addition of alkali is due to a rise in pH value. As the pH value increases, the degree of hydrolysis increases and thrusts more negative charges on the polymer chain; thus, the polymer molecule expands and produces a higher viscosity [17]. The Power Law Equation (1) illustrates the relationship between viscosity and shear rate [18]:   Figure 4b indicates the viscous behaviour of SP as a function of shear rate. The viscosity of SP emulsions was in the range of 0 to 0.8 mPa.s, which is slightly lower than the viscosity of the ASP emulsion with a range of 0.2 to 1.4 mPa.s. The presence of alkaline in the system leads to an increase in the microemulsion viscosity. The high viscosity produced by the addition of alkali is due to a rise in pH value. As the pH value increases, the degree of hydrolysis increases and thrusts more negative charges on the polymer chain; thus, the polymer molecule expands and produces a higher viscosity [17]. The Power Law Equation (1) illustrates the relationship between viscosity and shear rate [18]:

Shear Properties
where, K, γ, and n are the viscosity, consistency index, shear rate, and power-law index, respectively. The power-law index provides information about the effects of shear on the system. The microemulsion shows shear-thinning behaviour when the value of n is below one, and the lower the value of n, more shear-thinning behaviour of the emulsion is produced. Table 4 indicates the rheological parameters obtained after fitting the data obtained in Figure 4a,b to the Power Law Equation (1). The plot from the figure and the data from the table indicate that consistency index K increases as the concentration of ASP and SP increases, and the values obtained for n by model fitting are less than 1, which appears on the shear-thinning system. The shear properties of the microemulsion can also be characterized by the Herschel-Bulkeley Equation (2) [19]: log 10 (τ − τ o ) = log 10 K + n log 10 γ where τ is the shear stress (Pa), γ is shear rate, n is flow index, τ 0 is the yield stress, and K is the consistency index. Figure 5a,b indicate the plot of shear stress as a function of shear rate for 20, 40, 50, 60, 80% WC ASP and 20, 40, 50, 60% WC SP microemulsions, respectively. Based on Figure 6, it was observed that the microemulsions of ASP and SP in different water cuts (WC) show dual flow behaviour at different shear rates. The microemulsion demonstrated Newtonian fluid behaviour (at constant viscosity) at a low shear rate (below 5 s −1 ), and the transition of Newtonian to non-Newtonian fluid behaviour occurs at 5 s −1 shear rate. Beyond the 5 s −1 shear rate, the microemulsions are found to exhibit non-Newtonian behaviour (the apparent viscosity decreases as stress increases). The shear rate value for the transition from Newtonian to non-Newtonian behaviour is called critical shear rate. The non-Newtonian behaviour microemulsion can be characterized by the value of K, n, and τ 0 from the Herschel-Bulkeley Equation (2), where the microemulsion is classified as shear-thinning fluid with yield stress when τ 0 = 0, 0 < n < 1, and K > 0 [19]. Table 5 indicates the Herchel-Bulkeley parameters obtained on fitting the data shown in Figure A1a,b (Appendix A). The parameters obtained show that  [20]. The microemulsions start to behave as a non-Newtonian fluid when the applied stress is higher than the yield stress. Separation tends to occur for a meagre yield stress value [19].   Figure A1a,b (Appendix A). system, it causes the microemulsions droplets to move further away from each other. If the magnitude of shear stress applied is smaller compared to attractive forces, it creates an elastic physical response of microemulsions and the shear forces are stored as an extension bond between the dispersed droplets. The network that arises from these weak interaction forces hinderes the flow ability of the microemulsions. Thus, the application of shear forces overcomes the flow resistance of microemulsions.

Viscoelasticity Evaluation
The viscoelastic (viscous and elastic) properties of microemulsion were studied by measuring the loss modulus, G' and storage modulus, G", where G' represents the elastic properties and G" represent the viscous properties. If the value of G' is higher than that of G", the material is regarded  Figure 6. Viscosity as a function of shear rate of 50% WC ASP in 40, 50, 60, and 80% oil concentration at 60 • C temperature. Table 5. Herchel-Bulkeley parameters obtained on fitting the data shown in Figure A1a,b (Appendix A).  Figure 6 provides a graphical illustration of how the percentage of oil affects the functional relationship between viscosity and shear rate of the 50% WC ASP. In the microemulsion tendency test, only 50% of WC ASP produced microemulsion at five different crude oil ratios, showing shear-thinning behaviour. The trends indicate that the higher the concentration of crude oil in the system, the higher the viscosity of the microemulsion produced. The shear-thinning system in microemulsions is due to the presence of weak attractive forces between the microemulsion droplets that cause the formation of a weak elastic gel-like network [19]. When shear stress is applied to the system, it causes the microemulsions droplets to move further away from each other. If the magnitude of shear stress applied is smaller compared to attractive forces, it creates an elastic physical response of microemulsions and the shear forces are stored as an extension bond between the dispersed droplets. The network that arises from these weak interaction forces hinderes the flow ability of the microemulsions. Thus, the application of shear forces overcomes the flow resistance of microemulsions.

Viscoelasticity Evaluation
The viscoelastic (viscous and elastic) properties of microemulsion were studied by measuring the loss modulus, G' and storage modulus, G", where G' represents the elastic properties and G" represent the viscous properties. If the value of G' is higher than that of G", the material is regarded as elastic; if the value of G" is higher than that of G', the material is regarded as viscous [21]. Microemulsion will be highly stable if G' value is higher than that of G" since, in EOR, the microemulsion displays gel-like behavior [19]. However, in flooding applications, the viscoelastic injected micellar fluid solution is efficient in displacing different types of residual fluid by improving sweep efficiency. Figure 7a,b shows the storage modulus, G', and loss modulus, G", as a function of angular frequency for 20, 50, and 80% WC ASP and 20, 50, and 60% WC SP microemulsions at 60 • C (PETRONAS North Sabah field temperature). All of the microemulsions produced for ASP and SP show that the G' and G" values depend on frequency. The microemulsions show elastic-like fluid behaviour at low frequency (below 10 rad s −1 ), where the value of G' > G". At the intersection point (angular frequency equal to 10 rad s −1 ) of G' and G", the elastic modulus, G', is equal to viscous modulus G," and it represents a relaxation frequency. Thus, the smaller the relaxation frequency, the higher the particle suspension capability. The crossing point of G' and G" is called specified frequency (SF). At a frequency lower than SF, the elastic-like emulsions represent a stronger network of fluid samples [20]. As the angular frequency increases, the viscoelastic behaviour of all microemulsions is observed at frequencies between 10 to 60 rad s −1 due to the network created between the dispersed microemulsion droplets [19]. Table 6 shows the rheological parameters of G' and G'/G" for ASP and SP at 20, 40, 50, 60, and 80% WC, and 20, 40, 50, and 60% WC, respectively. The ratio of G' and G" was measured to investigate the properties of the microemulsion. A ratio value lower than 3 indicates micellar fluid properties, whereas gel-like properties are found in ratio value greater than 3 [20]. For ASP, 40 and 80% WC microemulsions show the true nature of micellar fluid with G'/G" lower than 3, while the other ASP microemulsions show gel-like fluid with a high value of G'/G". On the other hand, the gel-like fluid showed G/G" ratio that is greater than 3, except for 60% WC microemulsions. For maximum gel strength, G' measurement at 20% WC microemulsion showed the highest value at 29.58 Pa for ASP and 50% WC microemulsions for SP at a maximum gel strength value of 26.41 Pa. An increase in G' value was due to the inter-particle interactions that form bonds of ASP and SP in the oil-water interface [20].

Effect of Temperature on Viscosity
The effect of temperature on microemulsion viscosity was studied to investigate microemulsion stability at different temperatures. From Figure 8a,b, it can be observed that both ASP and SP microemulsions show temperature thinning properties, where viscosity decreases when the temperature increases. The heat applied on the microemulsion increased the Brownian motion and caused the aggregation of polymer to start moving apart, thus decreasing microemulsion viscosity [20]. The viscosity of micellar fluids decreases as the temperature increases due to the crude oil (i.e., alkanes) being solubilized in the core micelles; this leads to radial growth in the cylindrical part of the wormlike micelles, resulting in a drop of end-cap energy and micelle length [22]. Based on the variation of water cut (WC) for ASP, 20% WC ASP showed the highest viscosity at 80 • C (8.2 mPa.s); 40% WC emulsion was seen for SP (7.8 mPa.s). These microemulsions indicate the highest thermal stability. The increase in thermal stability is due to the adsorption of polymer in chains onto the oil-water interface. Meanwhile, the increase in viscosity is attributed to the formation of a three-dimensional network as a result of "pseudocrosslinking", which hinder microemulsion mobility [19]. On the other hand, microemulsion with low thermal stability (80% WC for ASP and 50% WC for SP) is caused by the flocculation of microemulsion droplets that decrease the viscosity as the temperature increases [18].  Table 6 shows the rheological parameters of G' and G'/G" for ASP and SP at 20, 40, 50, 60, and 80% WC, and 20, 40, 50, and 60% WC, respectively. The ratio of G' and G" was measured to investigate the properties of the microemulsion. A ratio value lower than 3 indicates micellar fluid properties, whereas gel-like properties are found in ratio value greater than 3 [20]. For ASP, 40 and 80% WC microemulsions show the true nature of micellar fluid with G'/G" lower than 3, while the other ASP microemulsions show gel-like fluid with a high value of G'/G". On the other hand, the gel-

Conclusions
The microemulsion tendency of five water cuts of ASP and SP in five different ratios of crude oil was successfully tested via bottle testing. The result shows that the addition of ASP and SP to the mixture of crude oil and brine solution leads to the formation of a microemulsion. For ASP, 50% WC yields the best micro emulsifier with average microemulsion from 0 to 29%, respectively, whereas, it

Conclusions
The microemulsion tendency of five water cuts of ASP and SP in five different ratios of crude oil was successfully tested via bottle testing. The result shows that the addition of ASP and SP to the mixture of crude oil and brine solution leads to the formation of a microemulsion. For ASP, 50% WC yields the best micro emulsifier with average microemulsion from 0 to 29%, respectively, whereas, it was 40% WC for SP from 0 to 36%, respectively. In the rheology test involving fluid properties, shear, viscoelasticity, and viscosity analysis were studied to investigate the micellar fluid behaviour of microemulsions in the EOR application. All the microemulsions produced showed shear-thinning fluid behaviour, and viscosity decreased as the shear rate increased. Meanwhile, the higher the concentration of ASP and SP, the more shear thinning (effect) is exhibited in the micellar fluid; this increases the flow of the microemulsion as the shear rate rises. The viscoelasticity evaluation shows that elastic microemulsions were produced at a low shear rate. The viscoelastic properties of microemulsions are shown at frequencies between 10 to 60 rad s −1 . The viscosity of the microemulsions produced also decreases when the temperature increases, as the mobility of the microemulsions increases with temperature. The formation of microemulsions via the bottle test and the results from the rheological analysis prove that the addition of alkaline, surfactant, and polymer leads to better sweep efficiency and mobility control of microemulsions in chemical EOR flooding. Thus, the rheology analysis of ASP and SP microemulsions was successfully executed and further analysis on the performance of the ASP and SP formulation will help prove the efficiency of these chemicals in flooding applications.