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Ind. Eng. Chem. Res. 2009, 48, 3222–3227
Transient Flow Behavior of Crude Oil-Alcoflood Polymer Emulsions Mamdouh T. Ghannam* Department of Chemical and Petroleum Engineering; College of Engineering, United Arab Emirates UniVersity, P. O. 17555, Al-Ain, United Arab Emirates
The transient flow behavior of crude oil-Alcoflood polymer emulsions is investigated in this study. A wide range of crude oil, 0-75 vol %, and polymer concentrations, 0-104 ppm, were employed to study the transient flow behavior in terms of viscosity and shear rate. Three different Alcoflood polymers were tested in this investigation, i.e., AF1235, AF1275, and AF1285. A 1 vol % Triton X100 was used as an emulsifying agent to form a stable emulsion of crude oil-Alcoflood polymer. Constant shear rates of 5, 25, and 50 s-1 were applied for 300 s to measure the emulsion viscosity of all examined samples. There was no transient response reported for polymer concentration of e1000 ppm. The polymer concentration and shear rate influenced the transient flow behavior of polymer aqueous solution. Aqueous solutions of AF1275 and AF1285 show rheopectic behavior, whereas the aqueous solution of AF1235 provides a slight thixotropic flow. The emulsion viscosity increases significantly with crude oil concentration without modifying the nature of the transient flow behavior and transient period of the polymer aqueous solutions. 1. Introduction Several technical and commercial fluids show time-dependent flow behavior. Rheological properties of time-dependent flow behavior are not easy to characterize.1 Some examples of these materials are nondrip latex paints, starch-thickened foods, semisolid foodstuffs, and macroscopic suspensions of metals. This behavior implies that the fluid viscosity gradually changes with time when a constant shear rate is suddenly applied. The time-dependent non-Newtonian fluids show either thixotropic behavior or a rheopectic one. Thixotropic behavior can be defined as the gradual decrease of fluid viscosity with time under constant shear rate or shear stress, followed by a gradual recovery with time when the shear rate or shear stress is removed. Rheopectic behavior exhibits an opposite behavior to the thixotropic one (i.e., antithixotropic). The characterization of time-dependent non-Newtonian fluid flow properties is important in establishing flow and piping systems, quality control, and storage. It is known in the literature that the colloidal suspensions show thixotropic behavior, i.e., continuous reduction in viscosity with time of shearing and a subsequent recovery under reduced shear.2-5 It was proposed that the change in viscosity results from the formation of structure within the colloidal suspension under shear.2-5 A literature survey shows that numerous research works have been done on the time effect of rheological properties for foodstuffs.6-9 The characterization of the time-dependent flow behavior of crude oil-Alcoflood polymer emulsions is important for petroleum industries, and to the best of the author’s knowledge, has not been investigated. Emulsions, in general, can be found in numerous industrial applications such as the pharmaceutical, paper, food, and oil industries. Emulsions consist of one immiscible liquid phase such as oil dispersed within a continuous aqueous phase such as water. This emulsion will lead to the formation of oil-inwater emulsion (O/W emulsion). To increase the oil production rate, during the enhanced oil recovery stage, aqueous polymer solution is injected into the oil well. This process usually forms a crude oil-aqueous polymer emulsion state. Alocflood polymer * Tel.: (9713) 7133635. Fax: (9713) 7624262. E-mail: mamdouh.
[email protected].
is heavily used in enhanced oil recovery. The Alcoflood polymers offer good stability and handling characteristics to enhance injection properties, mobility ratio, and sweep efficiency through formation permeability, which eventually improves the rate of crude oil production. Investigating and understanding the transient flow behavior of the crude oil droplet phase within the polymer continuous phase is necessary for the oil displacement mechanism by the Alcoflood polymer solution during the stage of enhanced oil recovery, storage, and pipeline transportation. Several experimental studies were done on the flow behavior of emulsions in which the continuous phase is a Newtonian fluid.10,11 For a very dilute concentration of oil phase into the aqueous phase, the viscosity increases linearly with the oil concentration. For medium oil phase concentration, viscosity increases nonlinearly while the emulsion is still Newtonian. However, at high oil phase concentration, emulsions behave in a non-Newtonian fashion with associated yield stress.12,13 Few experimental rheological investigations on oil-aqueous phase emulsions have been found in which a non-Newtonian polymeric solution is employed as a continuous phase.14-16 In the current work, a wide range of crude oil concentrations are mixed with different types of Alcoflood polymer solutions to study the transient flow behavior in terms of shear rate, polymer concentration, and oil concentration. 2. Experimental Work Alcoflood polymer aqueous solution was prepared by adding Alcoflood polymer materials to fresh water. Ample time was allowed to obtain complete polymer dissolution without external mixing to avoid any mechanical degradation for polymer networks. Then the crude oil was added gradually to the aqueous solution of polymer that contained 1 vol % Triton X100 as emulsified agent. The transient behavior was investigated for all crude oil-Alcoflood polymer emulsions over the concentration range 0-75 vol % crude oil and polymer concentrations of 0-104 ppm. This investigation utilized a controlled rate mode to apply different values of constant shear rate. 2.1. Surfactant Material. A surfactant material is usually added to the oil-aqueous emulsion as an emulsifying agent to lower the oil-aqueous solution interfacial tension, i.e., formation of the emulsion system, and to stabilize the presence of the oil
10.1021/ie801388z CCC: $40.75 2009 American Chemical Society Published on Web 02/10/2009
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 3223 Table 1. Crude Oil Specifications viscosity at 40 °C, mPa · s density, kg/m3 acid value, kg of KOH/kg asphaltene content, wt %
7.16 880.6 1.2 × 10-3 0.42
droplet phase within the aqueous continuous phase, i.e., to avoid the oil droplet coalescence mechanism.18 Triton X100 from Sigma-Aldrich Canada Ltd. was utilized as a surfactant agent. 2.2. Crude Oil. Crude oil-polymer emulsions were prepared from crude oil, Alcoflood aqueous solution, and surfactant material. The crude oil from the North Sea supplied by Shell Canada Ltd. was used in all of the transient behavior tests with specifications listed in Table 1. 2.3. Alcoflood Polymers. To investigate different transient flow behaviors of crude oil-polymer emulsions, three Alcoflood polymers were used for the emulsion investigation. These Alcoflood polymers, AF1235, AF1275, and AF1285, were supplied by Ciba Specialty Chemicals (Bradford, West Yorkshire, England). Alcoflood polymers are high molecular weight polyacrylamide copolymers supplied in a granular powder with a bulk density of 800 kg/m3. The intrinsic viscosities of AF1235, AF1275, and AF1285 are 12, 20, and 24, respectively. The three Alcoflood polymers are designed for use in enhanced oil recovery programs for polymer augmented water floods or alkali/surfactant/polymer water floods. AF1235 has good handling characteristics with excellent solubility to promote better injection into low-medium permeability reservoirs (50-500 mD). AF1275 and AF1285 are employed for high permeability reservoirs, and they offer good handling characteristics with excellent viscosity alteration power in water.17 2.4. Rheometer RS100. A rheometer RS100 from Haake was used to carry out all the transient tests of this investigation. The RS100 has several operating modes, which are the controlled rate (CR) mode, the controlled stress (CS) mode, and the oscillation (OSC) mode. The driveshaft was centered by an air bearing to deliver an almost frictionless transmission of the applied stress or shear rate to the test fluid. The resulting deformation of the tested material was analyzed with a digital encoder that processes 106 impulses. This resolution made it possible to measure small shear rates or strains. The RS100 can be easily switched between the CS and CR modes. A controlled variable lift speed was used to position the cone on the plate. The measured data was collected using a cone-plate sensor. The sensor system consists of a stainless steel cone and plate with 35 mm diameter, a 0.137 mm gap at the cone tip, and a 4° cone angle. A water bath was connected to the RS100 to control the applied temperature. All experimental runs were carried out at 22 °C. The Haake software package was utilized to operate and control the RS100, and was also used for data evaluation and analysis. To study the transient flow behavior of crude oil-polymer emulsions, the CR mode of the RS100 was used. Constant shear rates of 5, 25, and 50 s-1 were applied for 300 s to study the emulsion viscosity for all tested samples. 3. Results and Discussion Oil-aqueous phase emulsion is a complex mixture of a multicomponent and multiphase system. In a previous study of steady flow properties of crude oil-Alcoflood polymer emulsions,16 a non-Newtonian flow behavior of shear-thinning response was observed. The viscosity of O/W emulsions increased with concentration of Alcoflood polymer and crude oil, and it decreased with shear rate. The previous investigation also showed that the well-known Casson model,19 eq 1, very
Figure 1. Transient flow behavior for AF1275 polymer solution.
adequately fit the flow behavior of Alcoflood aqueous solutions and crude oil-Alcoflood polymer emulsions. τ ) (τo0.5 + (γ˙ ηc)0.5)2
(1)
where τ is the applied shear stress in Pa, γ˙ is the corresponding shear rate in s-1, τo is the apparent yield stress parameter determined by the Casson model in Pa, and ηc is the Casson apparent viscosity in Pa · s. Unlike the Bingham model, eq 2, the Casson model describes the nonlinearity of plastic behavior. τ ) τo + γ˙ η
(2)
3.1. Transient Behavior of Aqueous Polymer Solutions. The viscosity of each sample was measured in mPa · s versus time in seconds at different applied values of shear rates. In general, transient behaviors for aqueous polymer solutions were not observed for concentrations of Alcoflood polymer at or below 1000 ppm due to quite dilute solutions. However, starting from a concentration of 2000 ppm, Alcoflood polymer solutions show significant transient flow behavior. Figure 1 shows a typical example for the transient flow behavior of Alcoflood polymer aqueous solution at different shear rate values. Figure 1a displays the transient behavior for the concentration of 104 ppm AF1275 aqueous solution, whereas Figure 1b reports the same behavior for a lower concentration of 1000 ppm. As can be noted from Figure 1, the low polymer concentration of 1000 ppm reports no transient behavior for all of the tested shear rates. However, the higher polymer concentration of 104 ppm has a strong influence on the transient flow behavior in which it is shear rate dependent. Figure 1a shows that the viscosity level and transient period, tr, decrease significantly with shear rate. Therefore, it can be concluded from Figure 1 that the polymer concentration and shear rate strongly influence the transient flow behavior of Alcoflood polymer aqueous solution.
3224 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 Table 3. Predicted Coefficients of Eq 3 2000 ppm
104 ppm
5000 ppm
polymer
t0
a
t0
a
t0
a
AF1285 AF1275 AF1235
95.6 68.0 68.5
-0.458 -0.520 -0.74
180.8 107.9 78.0
-1.351 -1.143 -0.72
219.7 116.4 84.9
-0.458 -0.683 -0.454
Table 4. Modeling Parameters of Eq 4 (a) For AF1235 Aqueous Solutions 5 s-1 concn
ηo
ηf
25 s-1 λ
ηo
ηf
50 s-1 λ
ηo
ηf
λ
2000 72.9 77.1 29.3 47.9 50.4 21.2 5000 243.8 237.9 344.8 154.1 149.9 55.9 10000 1894.0 2000.5 17.4 657.7 623.8 34.5 393.6 383.2 26.0 (b) For AF1275 Aqueous Solutions 5 s-1 concn 2000 5000 104
ηo
25 s-1
ηf
λ
ηo
50 s-1
ηf
λ
ηo
ηf
λ
132.3 140.4 163.9 42.9 51.0 21.2 26.3 33.6 14.6 840.6 1149.236.9 162.7 312.5 13.9 65.7 183.3 9.8 2235.5 2620.5 32.3 444.8 883.3 16.7 371.4 487.4 26.7
(c) For AF1285 Aqueous Solutions 5 s-1 concn
Figure 2. Transient flow behavior for all Alcoflood polymer solutions. Table 2. Transient Time, tr, for Alcoflood Polymer Solutions AF1235
AF1275
AF1285
concn, ppm 5 s-1 25 s-1 50 s-1 5 s-1 25 s-1 50 s-1 5 s-1 25 s-1 50 s-1 2000 5000 104
83.0
50.0 60.0 73.0
31.5 42.0 62.5
104.0 114.3
55.0 76.0 97.0
42.0 52.2 83.3
93.7 83.3 175.0 145.3 217.8 207.4
73.0 114.0 197.1
Figure 2 shows the influence of shear rate on the transient flow behavior of 104 ppm polymer concentration for AF1235, AF1275, and AF1285 at two different levels of shear rate, i.e., 5 and 25 s-1. Two distinct flow behaviors can be noticed in Figure 2. Aqueous solutions of AF1275 and AF1285 exhibit strong rheopectic flow response for both shear rates; i.e., solution viscosity increases gradually with time at the applied shear rate. However, aqueous solution of AF1235 responds differently with applied shear rates of 5 and 25 s-1. At 5 s-1, it shows insignificant time behavior over a limited period of time. However, at 25 s-1, it shows a slight decrease of viscosity over a period of 73 s; i.e., it shows thixotropic response. Table 2 shows the transient time reported for each polymer aqueous solution as a function of polymer concentration and shear rate. Table 2 shows that the measured transient time depends on both polymer concentration and shear rate level. The transient time of Alcoflood polymer aqueous solutions decreases with shear rate and increases with polymer concentration. Linear regression is carried out for all tested samples to model the transient behavior as a function of shear rate. This model can be given by tr ) to + aγ˙
(3)
Table 3 shows the values of the predicted coefficients for all polymer solutions. The regression coefficients of these calculations were found to be higher than 0.97 for all samples. Table 3 shows that the values of t0 increased significantly with polymer
ηo
ηf
25 s-1 λ
ηo
ηf
50 s-1 λ
ηo
ηf
λ
2000 92.6 108.4 112.4 28.7 38.7 33.1 21.3 29.0 31.2 5000 658.8 996.3 129.9 151.5 253.6 51.8 100.7 140.5 81.9 10000 2076.0 2500.2 108.7 614.4 753.9 75.2 290.5 350.9 80.9
concentration over the range 2000-104 ppm and decreased with polymer type in the order AF1285, AF1275, and AF1235. To understand the transient behavior of polymer aqueous solutions and emulsions, it is useful to investigate their responses under constant shear rate over a period of time. A mathematical model of time-dependent viscosity variation can be used as given by eq 4. ηt ) (ηo - ηf)e-t⁄λ + ηf
(4)
where λ is the relaxation time in seconds, ηo is the initial viscosity at time zero in mPa · s, and ηf is the final viscosity at the end of the shearing process in mPa · s. The relaxation time λ, which is a fluid characteristic, reflects that if a fluid is strained under a fixed value, the stress will decay as exp(-t/ λ).20 For the polymer concentration range of 2000-104 ppm, the viscosity transient behaviors of all Alcoflood polymer aqueous solutions are modeled through eq 4. Table 4 shows the modeling parameters for the three Alcoflood polymer aqueous solutions. Table 4 shows a significant increase for ηo and ηf with polymer concentration and a gradual reduction with shear rate. In most cases of Alcoflood polymer aqueous solutions, the highest value of relaxation time occurs at the shear rate of 5 s-1. By increasing the shear rate to higher values of 25 and 50 s-1, the relaxation time drops significantly. The ramp for the shear rate cycles of 5, 25, and 50 s-1 was not applied in a repeated sequence for the same aqueous solution, but it was applied for a fresh sample of aqueous solution. The initial and final viscosities (i.e., ηo and ηf) of all polymer aqueous solutions displayed a power-law decline versus shear rate with a regression modeling coefficient exceeding 0.999 according to eqs 5 and 6. The results of the modeling analysis are reported in Table 5. ηo ) aγ˙ b
(5)
ηf ) cγ˙ d
(6)
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 3225 Table 6. Fitting Parameters of Crude Oil-Polymer Emulsions
Table 5. Modeling Coefficients of Eqs 5 and 6 104 ppm
2000 ppm polymer AF1235 AF1275 AF1285
a
b
c
d
a
b
c
(a) For AF1235 d
508.9 -0.61 556.4 -0.62 5559 -0.67 6384 -0.72 408.8 -0.70 384.2 -0.63 9608 -0.91 8089 -0.67 278.3 -0.69 287.1 -0.61 7407 -0.79 8886 -0.79
3.2. Transient Flow Behavior of Crude Oil-Polymer Emulsions. Figure 3 shows a typical example for the transient flow behavior of crude oil-Alcoflood polymer emulsions. This investigation covered a wide range of crude oil concentrations from 0 to 75 vol % and polymer concentrations up to 104 ppm for the three tested Alcoflood polymers. Figure 3a reports the transient viscosity behavior for crude oil-AF1235 emulsion. As can be seen, this type of emulsion showed transient flow behavior of a thixotropic response in which the emulsion viscosity increased significantly with crude oil concentration. This strong response of the presence of crude oil on the emulsion viscosity behavior is reported for AF1235 with other concentrations of 2000 and 5000 ppm. This behavior can be attributed to
Figure 3. Transient flow behavior for crude oil-polymer emulsions.
104 ppm
2000 ppm crude oil, vol %
ηo
ηf
λ
ηo
ηf
λ
10 25 50 75
81.0 90.8 114.3 144.7
74.8 92.2 139.5 141.7
3333.3 1111.1 2500 28.6
748.2 777.3 872.4 992.6
725.8 754.2 851.6 939.0
100.0 65.8 30.0 38.3
(b) For AF1275 104 ppm
2000 ppm crude oil, vol %
ηo
ηf
λ
ηo
ηf
λ
25 75
94.8 151.7
105.8 316.6
36.0 3333.3
559.5 779.9
872.2 982.7
15.7 12.1
(c) For AF1285 104 ppm
2000 ppm crude oil, vol %
ηo
ηf
λ
ηo
ηf
λ
25 75
78.5 133.7
98.7 215.4
48.3 1111.1
599.3 808.7
756.2 951.6
50.0 15.7
the influence of the crude oil presence on the three-dimensional network structure of the AF1235 polymer that formed within the aqueous solution under no shear conditions. This reveals that the presence of crude oil droplets within the polymer emulsion significantly enhances the droplet interaction in which the apparent viscosity rises with crude oil concentration. The other two types of crude oil-Alcoflood polymers of AF1275 and AF1285, Figure 3b,c show behavior similar to that in Figure 3a of crude oil-AF1235 emulsion in terms of emulsion viscosity increasing gradually with oil concentration. Figure 3b,c shows typical examples for the transient viscosity behavior for the other two types of crude oil-Alcoflood polymer emulsions. These behaviors, unlike the AF1235 emulsions, display a rheopectic response for all examined polymer and oil concentrations. It is important to conclude here that the presence of crude oil within the emulsions does not change the transient flow nature behavior and transient period of the polymer aqueous solution. The transient flow behavior of crude oil-Alcoflood polymer emulsions was modeled using eq 4. Table 6 reports all the modeling parameters in terms of polymer and crude oil concentrations for AF1235, AF1275, and AF1285, respectively. For all polymer emulsions, Table 6 showed a significant increase for ηo and ηf with polymer and crude oil concentrations. The relaxation time λ, for crude oil emulsions of AF1275 and AF1285, decreases with polymer concentration over the range 2000-104 ppm. Figures 4 and 5 show the comparisons between the transient flow behaviors of crude oil-polymer emulsions for different concentrations of crude oil and Alcoflood polymers. Figure 4 displays the transient flow behavior for the higher concentration of 104 ppm Alcoflood polymers, whereas Figure 5 shows a similar behavior for the lower concentration of 2000 ppm Alcoflood polymers. It can be concluded from Figures 4 and 5 that the crude oil-AF1275 and crude oil-AF1285 emulsions display strong transient behaviors of rheopectic response in all concentrations of crude oil and Alcoflood polymer. In the case of AF1235 emulsions, a low concentration of polymer displays insignificant transient flow behavior regardless of the oil concentration. However, the high concentration of 104 ppm AF1235 emulsion shows significant transient behavior in the presence of 25 vol % crude oil. This transient flow behavior enhances even more when crude oil increases up to 75 vol %.
3226 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009
state condition. For the thixotropic behavior, the microstructure changes can be attributed to the competition between the breakdown mechanism due to the flow stresses and the buildup mechanism due to the nonflow collisions. At rest, because of a high entanglement polymer density, the crude oil-Alcoflood polymers form a three-dimensional network structure under no shear conditions. This leads to a high value of emulsion viscosity. When the applied shear exceeds the yield stress value, the three-dimensional network structure deforms or even breaks down, leading to a lower value of emulsion viscosity. Rheopectic fashion behaves in an opposite way to the thixotropic response.4 4. Conclusions
Figure 4. Transient flow behavior of crude oil and 10 000 ppm polymer emulsions.
The current investigation is carried out to study the transient behavior of crude oil-Alcoflood polymer emulsions. The following conclusions can be made: 1. The low concentration of Alcoflood polymer aqueous solutions of e1000 ppm showed no transient behavior for all tested shear rates due to their dilute solutions. Higher polymer concentrations (i.e., g1000 ppm) provided strong transient flow behavior. 2. The polymer concentration and shear rate significantly affected the transient flow behavior of polymer aqueous solutions. The viscosity level and transient period decreased significantly with shear rate. 3. Aqueous solutions of AF1275 and AF1285 exhibited strong rheopectic behaviors, whereas aqueous solution of AF1235 showed a slight thixotropic response. 4. The initial and final viscosities, i.e., ηo and ηf, increased with polymer concentration and decreased with shear rate values. The relaxation time, λ, decreased with shear rate. 5. The emulsion viscosity increased significantly with crude oil concentration. The presence of crude oil within the emulsions did not change the nature of the transient flow behavior and transient period of the polymer aqueous solutions. The relaxation time λ for crude oil emulsions of AF1275 and AF1285 decreased with polymer concentration. 6. For all the examined concentrations of crude oil and polymer, the crude oil emulsions of AF1275 and AF1285 displayed strong transient behaviors of rheopectic response. However, the low polymer concentration crude oil-AF1235 emulsions showed insignificant transient behavior regardless of the concentration of crude oil. The high polymer concentration crude oil-AF1235 emulsions provided significant thixotropic behavior which increased with oil concentration. Literature Cited
Figure 5. Transient flow behavior of crude oil and 2000 ppm polymer emulsions.
The crude oil-Alcoflood polymer emulsions display thixotropic or rheopetic behaviors due to the microstructure changes that occurred during the flow process from one state to another
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ReceiVed for reView September 15, 2008 ReVised manuscript receiVed December 21, 2008 Accepted January 7, 2009 IE801388Z
