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Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process
Accepted Manuscript Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process
S.A. Shafiee Najafi, P. Kamranfar, M. Madani, M. Shadadeh, M. Jamialahmadi PII: DOI: Reference:
S0167-7322(17)30294-5 doi: 10.1016/j.molliq.2017.02.092 MOLLIQ 7005
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
21 January 2017 21 February 2017 22 February 2017
Please cite this article as: S.A. Shafiee Najafi, P. Kamranfar, M. Madani, M. Shadadeh, M. Jamialahmadi , Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.02.092
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ACCEPTED MANUSCRIPT Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process S. A. Shafiee Najafia, P. Kamranfara1 , M. Madania, M. Shadadehb, M. Jamialahmadia Petroleum University of Technology, Ahwaz Faculty of Petroleum, Ahwaz, Iran
b
Memorial University of Newfoundland, Faculty of Engineering and Applied Science, Canada
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a
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Abstract
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Of the most important methods of enhancing oil extraction from petroleum reservoirs includes injecting chemical materials of which surfactants have been found to play a significant role. In surfactant concentrations higher than critical micelle concentration (CMC), formation of micelles is started which will lead to change in the properties of microemulsion. In previous studies, the effect of transformation of micelle from spherical to cylindrical shape on the viscosity of DeAC (dodecyl ammonium chloride) microemulsion has been discussed. In this paper, application of CTAB (Cetyl trimethylammonium bromide, a cationic surfactant) in above CMC ranges has been studied based on the data from laboratory tests and proposed models in order to predict microemulsion viscosity. Moreover, the size of cylindrical micelles has been calculated using some mathematical equations for the first time. The results showed that the shape transformation from spherical to cylindrical happens to CTAB surfactant leading to vigorous increase in viscosity. Additionally, size calculation of micelles in different conditions and their comparison to pore throats dimension of porous media suggest that for certain conditions, they are adequately smaller than the pore throats. It is therefore suggested that CTAB surfactant may be utilized as a sole reagent in lowering IFT and mobility ratio instead of implementation of CTABpolymer, making the possible chemical Enhanced Oil Recovery (EOR) process less costly.
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Keywords: Micelle, Surfactants, Viscosity, Enhanced Oil Recovery, CMC, CTAB
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Corresponding author E-mail address:[email protected] (Peyman Kamranfar).
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ACCEPTED MANUSCRIPT 1. Introduction It is believed that petroleum is one of the most nonrenewable energy resources in the world and as a result of progressively rapid growth in global population, a demand has been in effect in last decades to produce more of these reserves by many oil and service companies[1, 2]. Oil
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extraction involves three stages of primary, secondary and tertiary recovery. Due to existence of
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some governing constraints such as rock heterogeneity, unfavorable mobility ratio, and capillary
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pressure in the first two stages in reservoirs, primary oil recovery which utilizes natural energy of
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reservoir can usually produce oil as much as 15% of initial oil in place and in secondary oil recovery in which water and/or gas is injected into aquifer and gas cap zone, respectively, this
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value is about 30% of initial oil in-place, consequently leaving at least 55% oil in place trapped in reservoir porous medium [1, 3-5]. This has caused oil companies to incorporate many capable
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recovery techniques known as Enhanced oil recovery (EOR) including (a) thermal recovery (b)
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gas and (c) chemical injections [6].
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EOR (also referred to as tertiary) methods are exploited through the objective of extraction of isolated oil in reservoirs which cannot be produced in the first stages of recoveries [7]. Amongst
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EOR methods, chemical flooding including (a) alkaline flooding, (b) polymer flooding and (c)
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surfactant flooding has been reportedly considered a promising technique used both in laboratory and field scales [8-14]. In surfactant flooding, a slug of microemulsion (surfactant solution) is introduced into the reservoir through injection wells. The mechanism through which this process takes effect to decrease residual oil saturation in reservoir has been reported to be IFT reduction. Since the injected microemulsion cannot sweep a significant amount of trapped oil owing to unfavorable mobility ratio (low microemulsion viscosity compared with oil), polymer-augmented water is 2
ACCEPTED MANUSCRIPT also injected to push the surfactant slug and residual oil saturation to the production well. This will eventually cause a better sweep of trapped oil and consequently higher efficiency. Despite this, there are some difficulties corresponding to polymer flooding which are known as high expenses of the process itself and limitation to some reservoir properties [15-20].
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To express oil trapping phenomena quantitatively in pore scale, a quantity called capillary number can be presented which indicates the effects of both viscous and capillary forces
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simultaneously. This dimensionless property is defined as follows, representing viscous forces
u
(1)
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Nc
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per capillary forces ratio:
in which Nc is the capillary number, u is the microemulsion darcian velocity in m/s, μ is the
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viscosity of surfactant solution in Pa.s and σ is the interfacial tension in N/m [21]. Studies have
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shown that the relationship between capillary number and microscopic efficiency is direct which
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means that the lower the IFT, the higher the sweep efficiency [22].
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Surfactant is an organic substance that will be adsorbed on a solid surface provided that it is available in low concentration in contact with the solid surface. They are composed of two
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characteristic molecular structures referred to as hydrophilic (water-loving) portion and hydrophobic (water-hating) portion which together make a surfactant molecule amphiphilic. Therefore, they are soluble both in organic solvents and water. These diverse characteristics of surfactants have made them applicable in various industries including chemistry, biology, pharmacy, adhesives, agrochemical formulations, emulsions and EOR [23-31]. According to the nature of the polar head group in surfactants, they may be divided into four different types namely cationics, anionics, zwitterionic, and nonionics [1, 3]. Because of 3
ACCEPTED MANUSCRIPT exposing good properties such as relative stability, economical production and low adsorptive manner toward reservoir rocks, anionic surfactants have been found to be most extensively utilized in industrial scale[3]. Nevertheless, it has also been reported that cationic surfactants are the most effective reagents in carbonate reservoirs for oil recovery, owing to the fact that
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carbonate rocks and cationic surfactants both have the same charge[32]. In this study, in order to
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conduct additional studies, CTAB (Cetyl trimethylammonium bromide), a cationic surfactant is
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chosen along with the experimental measurement of dynamic viscosities of different solutions.
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Microemulsion solutions are in the form of monomers (single surfactant molecules) at low concentrations; however, as the concentration increases, large number of monomers are
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aggregated to form micelles in which hydrophilic groups are in aqueous solution while lipophilic
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parts associate in the inner side of the cluster. The threshold concentration at which micelles start to develop is believed to be critical micelle concentration (CMC) which is one the major
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properties of surfactants. It is known that above CMC, an increase in surfactant molecules in
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microemulsion has no effect on monomer concentration while causes micelle concentration to
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raise which means that micelles and monomers are in dynamic equilibrium [1, 23, 33-35]. Studies have shown that there could be an abrupt change in physio-chemical properties of
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surfactants below and above CMC such as detergency, density, conductivity, surface tension, osmotic pressure, equivalent conductivity and interfacial tension [36]. Micelles geometrical structure is in the form of spheres, cylinders, ellipsoids and disks[37]. It has been observed that many factors can govern micelle size and geometrical shape of surfactants including nature of salt additive, temperature, surfactant species, concentration, structural groups and ionic strength. Studies together with experiments suggest that the dominant shape of micelles in microemulsion
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ACCEPTED MANUSCRIPT solutions is in the form of spheres above the CMC at low concentrations; yet an increase in concentration may lead to transition from spherical micelles into cylindrical ones [38-40]. Investigations indicate that shape transformation from spherical to cylindrical for some surfactants including DeAC (dodecyl ammonium chloride) occurs at some threshold
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concentration, notably cth. Because of rigidity of large cylindrical micelles, it is believed that microemulsion solution at concentrations above Cth may be more viscous than below Cth
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concentrations. Besides, increasing concentration may lead to larger size of cylindrical micelles.
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Therefore, it can be concluded that an increase in microemulsion concentration above threshold concentration makes solution viscosity to rise[41, 42].
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According to our recent work, surfactants could enhance both IFT and mobility ratio in particular
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conditions, in which micelle transformation from spherical to cylindrical shape was shown to
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occur in some specific conditions, leading to severe increase in DeAC microemulsion viscosity. Also, an analytical model was proposed to predict the viscosity of the microemulsion while the
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data in the literature [37].
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micelles were in the cylindrical shape. This model was evaluated based on the existing viscosity
In the present paper, microemulsion viscosity characterized with cylindrical shaped surfactants
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are determined; in this regard, a recently mathematically presented model is utilized to calculate the K value (a parameter related for preference of cylindrical to spherical micelle) utilizing experimentally measured dynamic viscosity data of CTAB at every surfactant concentration. For this purpose, the experimental results are initially used to determine the aforementioned threshold CTAB concentration (Cth). Then, an optimized K value is obtained for desired salt concentrations at surfactant concentrations higher than Cth. The experimental CTAB viscosity data are then compared with the predicted results to validate the employed model. Eventually, 5
ACCEPTED MANUSCRIPT size of the cylindrical micelles is calculated and compared with the pore throats typical dimension to introduce inexistence of pore throat blockage imposed by viscous CTAB microemulsion at such circumstances. The overall aim of the current paper is thus to introduce the applicability of individually-implemented CTAB surfactant rather than a system of polymer-
Materials
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2.1
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2. Materials and Experimental Procedures
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surfactant regarding EOR objectives.
To make the different micellar solutions which were needed in this work, Sodium Chloride
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NaCl (Merck, ≥99%, Germany), Cetyltrimethylammonium Bromide CTAB (Merck, ≥98%, Germany), and double distilled water were utilized. Surfactant used in this study is
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Cetyltrimethylammonium Bromide abbreviated as CTAB (Chemical formula: C 19H42NBr) which
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is a cationic detergent, soluble in H2O, and readily soluble in alcohol.
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In order to perform viscosity tests with Cannon-Fenske viscometer and calibrate this set up afterward, double distilled water was employed. Periodically, traces of organic deposits were
Experimental procedures
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2.2
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removed with a cleaning solution, specifically through chromic acid (Merck, ≥98%, Germany).
2.2.1 Fluid Preparation The experiments included a series of tests at different salinities. Therefore, the brine required for each series of tests is first prepared. Then, to prepare the required micellar solution samples, different weights or volume of surfactant were mixed with prepared brine in separate beakers. The prepared solutions were then mixed very carefully to avoid foaming. Tables 1 to 4 provide general information about the prepared samples. 6
ACCEPTED MANUSCRIPT Table 1 Values of CTAB needed parameters. 0.365 18 95 0.60 2.55
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MW, (Kg/Mol) Vs/Vw, (dimensionless) gs, (dimensionless) V0, (nanometer) l0, (nanometer)
Number of samples Volume of each sample, (cm3)
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Series of experiments (Salt concentration)
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Table 2 General information of experimental tests.
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Volume of brine prepared for each series of experiments, (cm3)
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Table 3 Different salinities of CTAB solution samples.
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2 4 6 8
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B C
Salt concentration, (gr/dL)
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Sample name
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F
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Table 4 CTAB concentration of different samples. Sample No.
Surfactant concentration, (gr/dL)
1
0.10
2
0.20
3
0.30
4
0.40
5
0.50
6
0.60
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0.80 7
6 11 20 250
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1.00
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1.50
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2.00
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2.50
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2.2.2 Density and Viscosity Measurement
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In this part, kinematic viscosity values were experimentally determined with a Cannon-Fenske
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viscometer.
The Cannon-Fenske Routine viscometer which is cheap and robust can give the best results for
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transparent solutions[43]. Because dynamic viscosity data were needed and this viscometer can
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only measure kinematic viscosity, it was necessary to measure density of each solution as well. Hence, a density meter (Model: DMA 45, Anton Paar Co.) was used to determine the density of
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each micellar solution at atmospheric pressure and ambient temperature. Dynamic viscosities
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(2)
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cm3
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(cp) (cSt ) ( g
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were then calculated using below simple relation;
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3. Methodology
Based on the performed investigation, while a reservoir is flooded with surfactant, the properties and viscosity of a microemulsion of a unique type of surfactant may be predicted provided that the key parameter K, is calculated in terms of salinity of micellar solution. The following equation is used to determine microemulsion relative viscosity[37]:
(3) 8
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(relative micellar solution viscosity) or other parameters are defined as: (4)
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(5)
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(6)
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(7)
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This simplified equation is favorable for surfactant flooding conditions including dilute solution
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and constant surfactant structure[37].
In case of surfactant flooding, the only unknown variable is
, which is a K-dependent
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parameter. The relation between these parameters is briefly shown below:
(9)
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and
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(8)
In this relation, Xcmc and gs are constant and X is dependent on the surfactant viscosity. Thus, the only unknown parameter is K which is calculated here based on the presented method in our recent investigation [37]. Then, prediction model results are compared with other laboratory
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ACCEPTED MANUSCRIPT results to determine the validity of this model, after which the size of the cylindrical micelle is measured and compared with the size of pore throats typical dimension. It is clear that the micellar solutions do not flow through porous media in case the micelle size becomes greater than average pore throat size. Hence, the micelle size should be calculated to
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prevent phase trapping in the porous media. The size of cylindrical micelles can be calculated by
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the following method:
The cylindrical micelle diameter, d, is about two times of surfactant molecule length (lo). The
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parameter d is therefore calculated as below:
(10)
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The hydrophobic length of a surfactant molecule is calculated from Equation (14) against carbon
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atoms number observed in the structure of surfactant. Thus, the diameter of a cylindrical micelle
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is calculated by knowing the carbon atoms number existed in the surfactant structure.
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(11)
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Length of cylindrical micelles, L can be calculated using following relation;
Wherein, parameter L/d in Equation (11) is known as the axial ratio of a cylindrically typical surfactant micelle and is given as [37]:
(12) The volume corresponding the largest possible spherical micelle equals: 10
ACCEPTED MANUSCRIPT (13)
In which
is the aggregation number related to biggest spherical micelle. This dimensionless
parameter could be either determined experimentally or according to provided surfactant and
denote the surfactant tail volume and length, respectively,
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characteristics in Table 1.
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both in nm, defined by the two following equations[3]:
(14) (15)
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Where N is the carbon atoms number existed in the surfactant tail.
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Blending Equations (12) and (13) and after mathematical simplification, the equation (11) can be
(16)
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written in a more appropriate form as:
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The parameter δ is defined as the hydrophilic head effective length in a typical surfactant molecule in nanometer, and gw is a dimensionless parameter defined as the cylindrical micelles
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aggregation number at a certain surfactant concentration, as mathematically represented in the
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following equation:
(17)
Where K is a thermodynamic coefficient that shows the preferential in formation of cylindrical rather than spherical micelles. Based on the definition of parameter K, it is clear that as this parameter increases, spherical to cylindrical shape transition phenomena becomes more probable. A realistically true assumption is that the K value does not change for a range of concentrations in which dominant micelle shape is cylindrical [37, 42]. 11
ACCEPTED MANUSCRIPT It should be pointed out that following equation is used to determine dimensionless viscosity values [37]: (18) For the matter of simplicity in calculation, nominator and denominator of Equation (18) are
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to get:
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further multiplied by
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(19)
The aforementioned calculations are applied at every surfactant concentration to compare the
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micelle size with average pore throat size. It is known that micelle size is proportional to the
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surfactant concentration. Hence, when the micelle size is small enough at a surfactant concentration, then it is smaller than average pore size at all concentrations below that surfactant
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concentration. The achieved results are discussed in full in the next section.
4. Results and Discussion
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The results illustrate that the microemulsion viscosity can be increased to a desired value by
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adjusting the salt and surfactant concentration appropriately without extra addition of polymer. The microemulsion with high viscosity would then be favorable to be applied in the mobility control processes in the EOR, instead of utilizing both polymer and surfactant simultaneously. The micellar solution viscosity is predicted from the presented model and compared with experimental data at disparate NaCl and CTAB concentrations. Experimental data includes viscosity data of CTAB microemulsion. Initially, the micellar solutions CMC at several salinities were determined since the model requires K parameter for each K-value as tabulated in Table 5 12
ACCEPTED MANUSCRIPT and shown in Figs. 2 and 3. As it is obvious from the experimental data and the model prediction (See Table 6), the dynamic viscosity micellar solutions is influenced by NaCl and CTAB concentrations. According to obtained results of the current investigation, Figs 1-5 are plotted. Fig. 1, in which a
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graph of experimentally dynamic viscosity data versus CTAB concentration above CMC is provided for different NaCl concentrations, is employed to determine the threshold concentration
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(shape transition point), Cth. As illustrated in Figs. 4 and 5, the experimental data and proposed
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prediction model are graphed against CTAB concentrations higher than Cth (i.e., the applicable range of CTAB concentration to be used via presented model), for different salt concentrations
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illustrating that results of the introduced model are in appropriate agreement with experimentally
Impact of CTAB concentration
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4.1
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measured viscosity data.
The impact of CTAB concentration on the experimentally obtained micellar solution viscosity is
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demonstrated in Fig. 1 over a broad range of NaCl concentration, in which for surfactant
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concentrations below 0.50 gr/100ml, CTAB aqueous solution viscosity does not change for all NaCl concentration values. In contrast, as the surfactant concentration is increased above this
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threshold concentration of 0.50 gr/100ml, the microemulsion viscosity starts to considerably increase. Fig. 1 also points out that there exists an ascending dimensionless micellar solution viscosity trend against CTAB surfactant, dependent on salt (NaCl) concentration at CTAB concentrations above this threshold value. Hence, it can be concluded that the surfactant concentration of 0.50 gr/100ml is regarded as CTAB threshold concentration; above which the shape transition from spherical to cylindrical
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ACCEPTED MANUSCRIPT causes a substantial increase in the CTAB microemulsion viscosity from this concentration onward is believed to be due to transition phenomena from spherical to cylindrical micelles. The magnitude of growth in such viscosity values above this threshold concentration is affected by NaCl concentration, which is reviewed in the following.
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Impact of NaCl concentration
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The results of Fig. 1 suggest that when the NaCl concentration is more than 8 gr/dL, there exists a significantly direct relation between microemulsion viscosity and CTAB concentration, while
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for NaCl concentrations equal to 8 gr/dL or less, no significant increase in the microemulsion viscosity is observed imposed by increasing CTAB concentration. On the contrary,
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microemulsion viscosity trend does not depend on NaCl concentration in low CTAB
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concentrations.
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Impact of NaCl concentration on the obtained K parameter is shown in Figs. 2 and 3. The effect of salinity on the dimensionless viscosity of CTAB solution is also illustrated in Figs. 4 and 5.
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The results show that as the salt concentration increases, the K value and consequently, the value
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of viscosity are increased. It is also concluded that increase in the estimated K value is negligible, provided that salt concentration is below 8 gr/dL as illustrated in Fig. 2 and Table 6,
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which is due to the transition phenomena from spherical to cylindrical micelles. Fig. 3 is also plotted in the semi-log coordinate which illustrates that a linear trend line can be employed to determine the K value at different salinities in the range of experimental data. The predictions of the model and experimental results are compared at different concentrations of CTAB and NaCl in Figs. 4 and 5, respectively. A very reasonable agreement between experimental results and predicted values exists in these two figures for CTAB concentrations
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ACCEPTED MANUSCRIPT above threshold value; therefore, results of the model are trustworthy in the range of experimentally obtained concentrations. This is in good agreement with what has already been proved that cylindrical micelles have very rigid structures and, consequently, higher viscosities
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in comparison to the same solution with spherical micelles[42].
Optimum K value, (dimensionless) 3.151 ×108 3.734×108 7.781×108 1.809×109 4.562×109 1.598×1010
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CMC, (gr/dL) 0.036 0.035 0.033 0.030 0.026 0.022
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Salt Concentration, (gr/dL) 2 4 6 8 10 12
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Table 5 The CMC of CTAB micellar solutions for different salt concentrations at 25 °C.
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Surfactant concentration (gr/dL) 0.10 0.20 0.30 0.40 0.50 0.60 0.80 1.00
Relative Viscosity of CTAB micellar solution at 25 °C Salinity Salinity Salinity Salinity Salinity Salinity (2 gr/dL) (4 gr/dL) (6 gr/dL) (8 gr/dL) (10 gr/dL) (12 gr/dL) 1.12 1.12 1.13 1.15 1.18 1.35 1.13 1.13 1.14 1.15 1.19 1.37 1.13 1.13 1.14 1.16 1.20 1.40 1.14 1.14 1.15 1.16 1.21 1.42 1.14 1.14 1.15 1.17 1.22 1.45 1.14 1.14 1.16 1.17 1.27 1.55 1.16 1.16 1.18 1.20 1.36 1.76 1.18 1.18 1.19 1.24 1.53 2.03 15
ACCEPTED MANUSCRIPT 1.50 2.00 2.50
1.22 1.24 1.27
1.22 1.27 1.32
1.28 1.42 1.68
1.44 1.83 2.59
1.96 3.16 5.28
3.23 9.76 18.97
Table 6 Experimentally obtained data corresponding to CTAB micellar solution relative viscosity against surfactant concentration for disparate NaCl Concentrations at 25 oC 2.20 CTAB
Experimental
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Shape transition
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1.60 1.40
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Dimensionless viscosity, μd
1.80
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12% 10% 8% 6% 4% 2%
2.00
1.00 0.00
0.20
0.40
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1.20
0.60
0.80
1.00
1.20
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Surfactant concentration, Cs, (gr/dLl)
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Figure 1 CTAB micellar solution relative viscosity against surfactant concentration for various salt concentrations (in gr/dL), considering experimental data for surfactant concentrations higher than determined CMC at 25 °C. 20.0
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CTAB Microemulsion
15.0
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K valu, dimensionless, ×109
Estimated K value
10.0
5.0
0.0 0
2
4
6
8
Salinity (NaCl Concentration), (gr/dL)
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10
12
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1.0E+11
Estimated K value
K value, dimensionless
Expon. (Estimated K value)
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1.0E+10
y = 9E+07e0.3998x R² = 0.9587
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CR
1.0E+09
1.0E+08 0
2
4
6
8
10
12
14
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Salinity (NaCl Concentration), (gr/dL)
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Figure 3 Semi-log plot of the estimated K value of CTAB microemulsion versus salt (NaCl) concentration (gr/dL) at 25 °C
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Cs=2.50 model
Cs=2.00 exp
Cs=2.00 model
Cs=1.50 exp
0.7
Cs=1.00 exp
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0.8
0.6 0.5
CTAB
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Cs=2.50 exp
Cs=1.50 model Cs=1.00 model
Cs=0.80 exp
Cs=0.80 model
Cs=0.60 exp
Cs=0.60 model
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Dimensionless viscosity, μd
0.9
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
Salinity (NaCl Concentration), (gr/dL)
Figure 4 CTAB micellar solution dimensionless viscosity based on experimental and model-predicted values against NaCl concentration (gr/dL) for several CTAB concentrations (gr/100ml) above the Cth at 25 °C.
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1.00 0.90 0.80
10% exp
10% model
8% exp
8% model
6% exp
6% model
4% exp
4% model
2% exp
2% model
CTAB Microemulsion
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12% model
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0.60
CR
0.50 0.40 0.30
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Dimensionless viscosity, μd
0.70
12% exp
0.20
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
2.20
2.40
2.60
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0.00 0.40
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0.10
Surfactant concentration, Cs, [gr/dL]
Comparative analysis between micelle size and pore size
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4.3
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Figure 5 CTAB micellar solution dimensionless viscosity based on experimental and model-predicted values against CTAB concentration (gr/dL) for several NaCl concentrations (gr/dL) above the Cth at 25 °C.
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The micelle size is a function of both salt and surfactant concentrations. The micellar solution flows through porous media only when the micelle size is smaller enough than the pore throat. The minimum size of pores in the non-tight reservoirs is usually greater than 1-2 microns[44]. Hence, it is necessary to compare the micelle size with the pore size at different salt and surfactant concentrations. For this purpose, the results are given in Fig. 6 for CTAB microemulsions from which it can be deduced that micelles are smaller enough than the smallest pores in the porous media at all salt and surfactant concentrations carried out in this work;
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ACCEPTED MANUSCRIPT therefore, pore plugging due to this microemulsion flooding in porous medium may not be
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CR
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considered.
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Figure 6 Comparative analysis between size of CTAB micelles and pore throats of the porous media at different salt and surfactant concentrations.
5. Conclusions
The present paper provided a method for prediction of CTAB cylindrical microemulsion viscosity against various surfactant and salt (NaCl) concentrations, based on an already presented mathematical model in the literature. The model was found to be remarkably effective due to emerging great agreement when compared to experimental viscosity data. It was also demonstrated that higher than a threshold CTAB concentration, microemulsion viscosity 19
ACCEPTED MANUSCRIPT increases as CTAB concentration increases, which was ascribed to spherical to cylindrical shape transition phenomenon. Moreover, using analytical equations, cylindrical micelle size was measured as a function of surfactant concentration and water salinity, followed by a comparison with pore throat size in different conditions. Results illustrated that under the presented
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conditions, micelle sizes are considerably smaller than the pore throat size, therefore, making
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them easily pass through pores network without being plugged. Therefore, it is finally deduced
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that CTAB surfactant may be used as a chemical additive to activate two major oil recovery mechanisms of IFT and mobility ratio reduction at certain salt and surfactant concentrations in a
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typical surfactant flooding process instead of employing both surfactant and polymer, which
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therefore suggests a more economically chemical EOR process.
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Nomenclature
Nc u
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CTAB concentration above which spherical to cylindrical micellar shape transition happens, gr/dL
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Micelle concentration, gr/dL
Aggregation number existing in the biggest spherical micelle, dimensionless
Surfactant tail length, nm Axial ratio of cylindrical micelle, dimensionless
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Aggregation number corresponding to cylindrical micelles, dimensionless
Surfactant molecular weight, Kg/Mol Number of carbon atoms existing in the surfactant molecule, dimensionless Capillary number, dimensionless
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N
Surfactant concentration, gr/dL
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C C th Cm gs gw lo l /d MW s
X cmc
Surfactant mole fraction at critical micelle concentration, dimensionless
cmc
Microemulsion viscosity used in equation 1, Pa.s Micellar solution viscosity at critical micelle concentration, , Pa.s Microemulsion relative viscosity, dimensionless
r d K
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V s /V w
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vo
Dimensionless micellar solution viscosity Thermodynamic coefficient of the employed model, dimensionless Effective length existing in the hydrophilic portion of surfactant molecule, nm IFT utilized in equation 1, N/m
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c
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X
Microemulsion darcy velocity used in equation 1, m/s Shape factor characterized with subscript “c” in order to indicate its dependency on concentration, dimensionless Surfactant tail volume, nm Surfactant per water molecular volume ratio, dimensionless Surfactant mole fraction, dimensionless
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15.
16. 17. 18.
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6.
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5.
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4.
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3.
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ACCEPTED MANUSCRIPT Research Highlights
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A mathematical model was evaluated by using new experimental data. Micelle shape is investigated as surfactant (i.e. CTAB) concentration is increased. Shape transition from spherical to cylindrical strongly increases the viscosity. For CTAB, the Shape transition occurs at threshold concentration of 0.50 gr/100ml. The micelle size of CTAB microemulsion was determined at different conditions. Some surfactants can play the role of both polymer and surfactant simultaneously.
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S.A. Shafiee Najafi, P. Kamranfar, M. Madani, M. Shadadeh, M. Jamialahmadi PII: DOI: Reference:
S0167-7322(17)30294-5 doi: 10.1016/j.molliq.2017.02.092 MOLLIQ 7005
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
21 January 2017 21 February 2017 22 February 2017
Please cite this article as: S.A. Shafiee Najafi, P. Kamranfar, M. Madani, M. Shadadeh, M. Jamialahmadi , Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.02.092
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ACCEPTED MANUSCRIPT Experimental and theoretical investigation of CTAB microemulsion viscosity in the chemical enhanced oil recovery process S. A. Shafiee Najafia, P. Kamranfara1 , M. Madania, M. Shadadehb, M. Jamialahmadia Petroleum University of Technology, Ahwaz Faculty of Petroleum, Ahwaz, Iran
b
Memorial University of Newfoundland, Faculty of Engineering and Applied Science, Canada
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Abstract
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Of the most important methods of enhancing oil extraction from petroleum reservoirs includes injecting chemical materials of which surfactants have been found to play a significant role. In surfactant concentrations higher than critical micelle concentration (CMC), formation of micelles is started which will lead to change in the properties of microemulsion. In previous studies, the effect of transformation of micelle from spherical to cylindrical shape on the viscosity of DeAC (dodecyl ammonium chloride) microemulsion has been discussed. In this paper, application of CTAB (Cetyl trimethylammonium bromide, a cationic surfactant) in above CMC ranges has been studied based on the data from laboratory tests and proposed models in order to predict microemulsion viscosity. Moreover, the size of cylindrical micelles has been calculated using some mathematical equations for the first time. The results showed that the shape transformation from spherical to cylindrical happens to CTAB surfactant leading to vigorous increase in viscosity. Additionally, size calculation of micelles in different conditions and their comparison to pore throats dimension of porous media suggest that for certain conditions, they are adequately smaller than the pore throats. It is therefore suggested that CTAB surfactant may be utilized as a sole reagent in lowering IFT and mobility ratio instead of implementation of CTABpolymer, making the possible chemical Enhanced Oil Recovery (EOR) process less costly.
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Keywords: Micelle, Surfactants, Viscosity, Enhanced Oil Recovery, CMC, CTAB
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Corresponding author E-mail address:[email protected] (Peyman Kamranfar).
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ACCEPTED MANUSCRIPT 1. Introduction It is believed that petroleum is one of the most nonrenewable energy resources in the world and as a result of progressively rapid growth in global population, a demand has been in effect in last decades to produce more of these reserves by many oil and service companies[1, 2]. Oil
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extraction involves three stages of primary, secondary and tertiary recovery. Due to existence of
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some governing constraints such as rock heterogeneity, unfavorable mobility ratio, and capillary
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pressure in the first two stages in reservoirs, primary oil recovery which utilizes natural energy of
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reservoir can usually produce oil as much as 15% of initial oil in place and in secondary oil recovery in which water and/or gas is injected into aquifer and gas cap zone, respectively, this
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value is about 30% of initial oil in-place, consequently leaving at least 55% oil in place trapped in reservoir porous medium [1, 3-5]. This has caused oil companies to incorporate many capable
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recovery techniques known as Enhanced oil recovery (EOR) including (a) thermal recovery (b)
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gas and (c) chemical injections [6].
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EOR (also referred to as tertiary) methods are exploited through the objective of extraction of isolated oil in reservoirs which cannot be produced in the first stages of recoveries [7]. Amongst
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EOR methods, chemical flooding including (a) alkaline flooding, (b) polymer flooding and (c)
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surfactant flooding has been reportedly considered a promising technique used both in laboratory and field scales [8-14]. In surfactant flooding, a slug of microemulsion (surfactant solution) is introduced into the reservoir through injection wells. The mechanism through which this process takes effect to decrease residual oil saturation in reservoir has been reported to be IFT reduction. Since the injected microemulsion cannot sweep a significant amount of trapped oil owing to unfavorable mobility ratio (low microemulsion viscosity compared with oil), polymer-augmented water is 2
ACCEPTED MANUSCRIPT also injected to push the surfactant slug and residual oil saturation to the production well. This will eventually cause a better sweep of trapped oil and consequently higher efficiency. Despite this, there are some difficulties corresponding to polymer flooding which are known as high expenses of the process itself and limitation to some reservoir properties [15-20].
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To express oil trapping phenomena quantitatively in pore scale, a quantity called capillary number can be presented which indicates the effects of both viscous and capillary forces
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simultaneously. This dimensionless property is defined as follows, representing viscous forces
u
(1)
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Nc
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per capillary forces ratio:
in which Nc is the capillary number, u is the microemulsion darcian velocity in m/s, μ is the
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viscosity of surfactant solution in Pa.s and σ is the interfacial tension in N/m [21]. Studies have
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shown that the relationship between capillary number and microscopic efficiency is direct which
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means that the lower the IFT, the higher the sweep efficiency [22].
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Surfactant is an organic substance that will be adsorbed on a solid surface provided that it is available in low concentration in contact with the solid surface. They are composed of two
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characteristic molecular structures referred to as hydrophilic (water-loving) portion and hydrophobic (water-hating) portion which together make a surfactant molecule amphiphilic. Therefore, they are soluble both in organic solvents and water. These diverse characteristics of surfactants have made them applicable in various industries including chemistry, biology, pharmacy, adhesives, agrochemical formulations, emulsions and EOR [23-31]. According to the nature of the polar head group in surfactants, they may be divided into four different types namely cationics, anionics, zwitterionic, and nonionics [1, 3]. Because of 3
ACCEPTED MANUSCRIPT exposing good properties such as relative stability, economical production and low adsorptive manner toward reservoir rocks, anionic surfactants have been found to be most extensively utilized in industrial scale[3]. Nevertheless, it has also been reported that cationic surfactants are the most effective reagents in carbonate reservoirs for oil recovery, owing to the fact that
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carbonate rocks and cationic surfactants both have the same charge[32]. In this study, in order to
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conduct additional studies, CTAB (Cetyl trimethylammonium bromide), a cationic surfactant is
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chosen along with the experimental measurement of dynamic viscosities of different solutions.
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Microemulsion solutions are in the form of monomers (single surfactant molecules) at low concentrations; however, as the concentration increases, large number of monomers are
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aggregated to form micelles in which hydrophilic groups are in aqueous solution while lipophilic
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parts associate in the inner side of the cluster. The threshold concentration at which micelles start to develop is believed to be critical micelle concentration (CMC) which is one the major
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properties of surfactants. It is known that above CMC, an increase in surfactant molecules in
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microemulsion has no effect on monomer concentration while causes micelle concentration to
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raise which means that micelles and monomers are in dynamic equilibrium [1, 23, 33-35]. Studies have shown that there could be an abrupt change in physio-chemical properties of
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surfactants below and above CMC such as detergency, density, conductivity, surface tension, osmotic pressure, equivalent conductivity and interfacial tension [36]. Micelles geometrical structure is in the form of spheres, cylinders, ellipsoids and disks[37]. It has been observed that many factors can govern micelle size and geometrical shape of surfactants including nature of salt additive, temperature, surfactant species, concentration, structural groups and ionic strength. Studies together with experiments suggest that the dominant shape of micelles in microemulsion
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ACCEPTED MANUSCRIPT solutions is in the form of spheres above the CMC at low concentrations; yet an increase in concentration may lead to transition from spherical micelles into cylindrical ones [38-40]. Investigations indicate that shape transformation from spherical to cylindrical for some surfactants including DeAC (dodecyl ammonium chloride) occurs at some threshold
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concentration, notably cth. Because of rigidity of large cylindrical micelles, it is believed that microemulsion solution at concentrations above Cth may be more viscous than below Cth
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concentrations. Besides, increasing concentration may lead to larger size of cylindrical micelles.
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Therefore, it can be concluded that an increase in microemulsion concentration above threshold concentration makes solution viscosity to rise[41, 42].
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According to our recent work, surfactants could enhance both IFT and mobility ratio in particular
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conditions, in which micelle transformation from spherical to cylindrical shape was shown to
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occur in some specific conditions, leading to severe increase in DeAC microemulsion viscosity. Also, an analytical model was proposed to predict the viscosity of the microemulsion while the
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data in the literature [37].
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micelles were in the cylindrical shape. This model was evaluated based on the existing viscosity
In the present paper, microemulsion viscosity characterized with cylindrical shaped surfactants
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are determined; in this regard, a recently mathematically presented model is utilized to calculate the K value (a parameter related for preference of cylindrical to spherical micelle) utilizing experimentally measured dynamic viscosity data of CTAB at every surfactant concentration. For this purpose, the experimental results are initially used to determine the aforementioned threshold CTAB concentration (Cth). Then, an optimized K value is obtained for desired salt concentrations at surfactant concentrations higher than Cth. The experimental CTAB viscosity data are then compared with the predicted results to validate the employed model. Eventually, 5
ACCEPTED MANUSCRIPT size of the cylindrical micelles is calculated and compared with the pore throats typical dimension to introduce inexistence of pore throat blockage imposed by viscous CTAB microemulsion at such circumstances. The overall aim of the current paper is thus to introduce the applicability of individually-implemented CTAB surfactant rather than a system of polymer-
Materials
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2.1
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2. Materials and Experimental Procedures
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surfactant regarding EOR objectives.
To make the different micellar solutions which were needed in this work, Sodium Chloride
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NaCl (Merck, ≥99%, Germany), Cetyltrimethylammonium Bromide CTAB (Merck, ≥98%, Germany), and double distilled water were utilized. Surfactant used in this study is
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Cetyltrimethylammonium Bromide abbreviated as CTAB (Chemical formula: C 19H42NBr) which
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is a cationic detergent, soluble in H2O, and readily soluble in alcohol.
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In order to perform viscosity tests with Cannon-Fenske viscometer and calibrate this set up afterward, double distilled water was employed. Periodically, traces of organic deposits were
Experimental procedures
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2.2
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removed with a cleaning solution, specifically through chromic acid (Merck, ≥98%, Germany).
2.2.1 Fluid Preparation The experiments included a series of tests at different salinities. Therefore, the brine required for each series of tests is first prepared. Then, to prepare the required micellar solution samples, different weights or volume of surfactant were mixed with prepared brine in separate beakers. The prepared solutions were then mixed very carefully to avoid foaming. Tables 1 to 4 provide general information about the prepared samples. 6
ACCEPTED MANUSCRIPT Table 1 Values of CTAB needed parameters. 0.365 18 95 0.60 2.55
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MW, (Kg/Mol) Vs/Vw, (dimensionless) gs, (dimensionless) V0, (nanometer) l0, (nanometer)
Number of samples Volume of each sample, (cm3)
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Series of experiments (Salt concentration)
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Table 2 General information of experimental tests.
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Volume of brine prepared for each series of experiments, (cm3)
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Table 3 Different salinities of CTAB solution samples.
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2 4 6 8
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Salt concentration, (gr/dL)
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Sample name
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Table 4 CTAB concentration of different samples. Sample No.
Surfactant concentration, (gr/dL)
1
0.10
2
0.20
3
0.30
4
0.40
5
0.50
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0.60
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0.80 7
6 11 20 250
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1.00
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1.50
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2.00
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2.50
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2.2.2 Density and Viscosity Measurement
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In this part, kinematic viscosity values were experimentally determined with a Cannon-Fenske
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viscometer.
The Cannon-Fenske Routine viscometer which is cheap and robust can give the best results for
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transparent solutions[43]. Because dynamic viscosity data were needed and this viscometer can
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only measure kinematic viscosity, it was necessary to measure density of each solution as well. Hence, a density meter (Model: DMA 45, Anton Paar Co.) was used to determine the density of
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each micellar solution at atmospheric pressure and ambient temperature. Dynamic viscosities
)
(2)
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cm3
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(cp) (cSt ) ( g
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were then calculated using below simple relation;
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3. Methodology
Based on the performed investigation, while a reservoir is flooded with surfactant, the properties and viscosity of a microemulsion of a unique type of surfactant may be predicted provided that the key parameter K, is calculated in terms of salinity of micellar solution. The following equation is used to determine microemulsion relative viscosity[37]:
(3) 8
ACCEPTED MANUSCRIPT Where
(relative micellar solution viscosity) or other parameters are defined as: (4)
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(5)
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(6)
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(7)
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This simplified equation is favorable for surfactant flooding conditions including dilute solution
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and constant surfactant structure[37].
In case of surfactant flooding, the only unknown variable is
, which is a K-dependent
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parameter. The relation between these parameters is briefly shown below:
(9)
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and
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(8)
In this relation, Xcmc and gs are constant and X is dependent on the surfactant viscosity. Thus, the only unknown parameter is K which is calculated here based on the presented method in our recent investigation [37]. Then, prediction model results are compared with other laboratory
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ACCEPTED MANUSCRIPT results to determine the validity of this model, after which the size of the cylindrical micelle is measured and compared with the size of pore throats typical dimension. It is clear that the micellar solutions do not flow through porous media in case the micelle size becomes greater than average pore throat size. Hence, the micelle size should be calculated to
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prevent phase trapping in the porous media. The size of cylindrical micelles can be calculated by
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the following method:
The cylindrical micelle diameter, d, is about two times of surfactant molecule length (lo). The
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parameter d is therefore calculated as below:
(10)
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The hydrophobic length of a surfactant molecule is calculated from Equation (14) against carbon
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atoms number observed in the structure of surfactant. Thus, the diameter of a cylindrical micelle
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is calculated by knowing the carbon atoms number existed in the surfactant structure.
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(11)
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Length of cylindrical micelles, L can be calculated using following relation;
Wherein, parameter L/d in Equation (11) is known as the axial ratio of a cylindrically typical surfactant micelle and is given as [37]:
(12) The volume corresponding the largest possible spherical micelle equals: 10
ACCEPTED MANUSCRIPT (13)
In which
is the aggregation number related to biggest spherical micelle. This dimensionless
parameter could be either determined experimentally or according to provided surfactant and
denote the surfactant tail volume and length, respectively,
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characteristics in Table 1.
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both in nm, defined by the two following equations[3]:
(14) (15)
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Where N is the carbon atoms number existed in the surfactant tail.
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Blending Equations (12) and (13) and after mathematical simplification, the equation (11) can be
(16)
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written in a more appropriate form as:
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The parameter δ is defined as the hydrophilic head effective length in a typical surfactant molecule in nanometer, and gw is a dimensionless parameter defined as the cylindrical micelles
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aggregation number at a certain surfactant concentration, as mathematically represented in the
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following equation:
(17)
Where K is a thermodynamic coefficient that shows the preferential in formation of cylindrical rather than spherical micelles. Based on the definition of parameter K, it is clear that as this parameter increases, spherical to cylindrical shape transition phenomena becomes more probable. A realistically true assumption is that the K value does not change for a range of concentrations in which dominant micelle shape is cylindrical [37, 42]. 11
ACCEPTED MANUSCRIPT It should be pointed out that following equation is used to determine dimensionless viscosity values [37]: (18) For the matter of simplicity in calculation, nominator and denominator of Equation (18) are
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to get:
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further multiplied by
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(19)
The aforementioned calculations are applied at every surfactant concentration to compare the
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micelle size with average pore throat size. It is known that micelle size is proportional to the
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surfactant concentration. Hence, when the micelle size is small enough at a surfactant concentration, then it is smaller than average pore size at all concentrations below that surfactant
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concentration. The achieved results are discussed in full in the next section.
4. Results and Discussion
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The results illustrate that the microemulsion viscosity can be increased to a desired value by
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adjusting the salt and surfactant concentration appropriately without extra addition of polymer. The microemulsion with high viscosity would then be favorable to be applied in the mobility control processes in the EOR, instead of utilizing both polymer and surfactant simultaneously. The micellar solution viscosity is predicted from the presented model and compared with experimental data at disparate NaCl and CTAB concentrations. Experimental data includes viscosity data of CTAB microemulsion. Initially, the micellar solutions CMC at several salinities were determined since the model requires K parameter for each K-value as tabulated in Table 5 12
ACCEPTED MANUSCRIPT and shown in Figs. 2 and 3. As it is obvious from the experimental data and the model prediction (See Table 6), the dynamic viscosity micellar solutions is influenced by NaCl and CTAB concentrations. According to obtained results of the current investigation, Figs 1-5 are plotted. Fig. 1, in which a
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graph of experimentally dynamic viscosity data versus CTAB concentration above CMC is provided for different NaCl concentrations, is employed to determine the threshold concentration
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(shape transition point), Cth. As illustrated in Figs. 4 and 5, the experimental data and proposed
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prediction model are graphed against CTAB concentrations higher than Cth (i.e., the applicable range of CTAB concentration to be used via presented model), for different salt concentrations
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illustrating that results of the introduced model are in appropriate agreement with experimentally
Impact of CTAB concentration
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measured viscosity data.
The impact of CTAB concentration on the experimentally obtained micellar solution viscosity is
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demonstrated in Fig. 1 over a broad range of NaCl concentration, in which for surfactant
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concentrations below 0.50 gr/100ml, CTAB aqueous solution viscosity does not change for all NaCl concentration values. In contrast, as the surfactant concentration is increased above this
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threshold concentration of 0.50 gr/100ml, the microemulsion viscosity starts to considerably increase. Fig. 1 also points out that there exists an ascending dimensionless micellar solution viscosity trend against CTAB surfactant, dependent on salt (NaCl) concentration at CTAB concentrations above this threshold value. Hence, it can be concluded that the surfactant concentration of 0.50 gr/100ml is regarded as CTAB threshold concentration; above which the shape transition from spherical to cylindrical
13
ACCEPTED MANUSCRIPT causes a substantial increase in the CTAB microemulsion viscosity from this concentration onward is believed to be due to transition phenomena from spherical to cylindrical micelles. The magnitude of growth in such viscosity values above this threshold concentration is affected by NaCl concentration, which is reviewed in the following.
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Impact of NaCl concentration
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The results of Fig. 1 suggest that when the NaCl concentration is more than 8 gr/dL, there exists a significantly direct relation between microemulsion viscosity and CTAB concentration, while
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for NaCl concentrations equal to 8 gr/dL or less, no significant increase in the microemulsion viscosity is observed imposed by increasing CTAB concentration. On the contrary,
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microemulsion viscosity trend does not depend on NaCl concentration in low CTAB
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concentrations.
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Impact of NaCl concentration on the obtained K parameter is shown in Figs. 2 and 3. The effect of salinity on the dimensionless viscosity of CTAB solution is also illustrated in Figs. 4 and 5.
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The results show that as the salt concentration increases, the K value and consequently, the value
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of viscosity are increased. It is also concluded that increase in the estimated K value is negligible, provided that salt concentration is below 8 gr/dL as illustrated in Fig. 2 and Table 6,
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which is due to the transition phenomena from spherical to cylindrical micelles. Fig. 3 is also plotted in the semi-log coordinate which illustrates that a linear trend line can be employed to determine the K value at different salinities in the range of experimental data. The predictions of the model and experimental results are compared at different concentrations of CTAB and NaCl in Figs. 4 and 5, respectively. A very reasonable agreement between experimental results and predicted values exists in these two figures for CTAB concentrations
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ACCEPTED MANUSCRIPT above threshold value; therefore, results of the model are trustworthy in the range of experimentally obtained concentrations. This is in good agreement with what has already been proved that cylindrical micelles have very rigid structures and, consequently, higher viscosities
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in comparison to the same solution with spherical micelles[42].
Optimum K value, (dimensionless) 3.151 ×108 3.734×108 7.781×108 1.809×109 4.562×109 1.598×1010
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CMC, (gr/dL) 0.036 0.035 0.033 0.030 0.026 0.022
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Salt Concentration, (gr/dL) 2 4 6 8 10 12
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Table 5 The CMC of CTAB micellar solutions for different salt concentrations at 25 °C.
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Surfactant concentration (gr/dL) 0.10 0.20 0.30 0.40 0.50 0.60 0.80 1.00
Relative Viscosity of CTAB micellar solution at 25 °C Salinity Salinity Salinity Salinity Salinity Salinity (2 gr/dL) (4 gr/dL) (6 gr/dL) (8 gr/dL) (10 gr/dL) (12 gr/dL) 1.12 1.12 1.13 1.15 1.18 1.35 1.13 1.13 1.14 1.15 1.19 1.37 1.13 1.13 1.14 1.16 1.20 1.40 1.14 1.14 1.15 1.16 1.21 1.42 1.14 1.14 1.15 1.17 1.22 1.45 1.14 1.14 1.16 1.17 1.27 1.55 1.16 1.16 1.18 1.20 1.36 1.76 1.18 1.18 1.19 1.24 1.53 2.03 15
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1.22 1.24 1.27
1.22 1.27 1.32
1.28 1.42 1.68
1.44 1.83 2.59
1.96 3.16 5.28
3.23 9.76 18.97
Table 6 Experimentally obtained data corresponding to CTAB micellar solution relative viscosity against surfactant concentration for disparate NaCl Concentrations at 25 oC 2.20 CTAB
Experimental
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Shape transition
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1.60 1.40
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Dimensionless viscosity, μd
1.80
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12% 10% 8% 6% 4% 2%
2.00
1.00 0.00
0.20
0.40
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1.20
0.60
0.80
1.00
1.20
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Surfactant concentration, Cs, (gr/dLl)
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Figure 1 CTAB micellar solution relative viscosity against surfactant concentration for various salt concentrations (in gr/dL), considering experimental data for surfactant concentrations higher than determined CMC at 25 °C. 20.0
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CTAB Microemulsion
15.0
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K valu, dimensionless, ×109
Estimated K value
10.0
5.0
0.0 0
2
4
6
8
Salinity (NaCl Concentration), (gr/dL)
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10
12
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1.0E+11
Estimated K value
K value, dimensionless
Expon. (Estimated K value)
IP
T
1.0E+10
y = 9E+07e0.3998x R² = 0.9587
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CR
1.0E+09
1.0E+08 0
2
4
6
8
10
12
14
AN
Salinity (NaCl Concentration), (gr/dL)
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Figure 3 Semi-log plot of the estimated K value of CTAB microemulsion versus salt (NaCl) concentration (gr/dL) at 25 °C
1
Cs=2.50 model
Cs=2.00 exp
Cs=2.00 model
Cs=1.50 exp
0.7
Cs=1.00 exp
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0.8
0.6 0.5
CTAB
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Cs=2.50 exp
Cs=1.50 model Cs=1.00 model
Cs=0.80 exp
Cs=0.80 model
Cs=0.60 exp
Cs=0.60 model
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Dimensionless viscosity, μd
0.9
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
Salinity (NaCl Concentration), (gr/dL)
Figure 4 CTAB micellar solution dimensionless viscosity based on experimental and model-predicted values against NaCl concentration (gr/dL) for several CTAB concentrations (gr/100ml) above the Cth at 25 °C.
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1.00 0.90 0.80
10% exp
10% model
8% exp
8% model
6% exp
6% model
4% exp
4% model
2% exp
2% model
CTAB Microemulsion
T
12% model
IP
0.60
CR
0.50 0.40 0.30
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Dimensionless viscosity, μd
0.70
12% exp
0.20
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
2.20
2.40
2.60
M
0.00 0.40
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0.10
Surfactant concentration, Cs, [gr/dL]
Comparative analysis between micelle size and pore size
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4.3
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Figure 5 CTAB micellar solution dimensionless viscosity based on experimental and model-predicted values against CTAB concentration (gr/dL) for several NaCl concentrations (gr/dL) above the Cth at 25 °C.
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The micelle size is a function of both salt and surfactant concentrations. The micellar solution flows through porous media only when the micelle size is smaller enough than the pore throat. The minimum size of pores in the non-tight reservoirs is usually greater than 1-2 microns[44]. Hence, it is necessary to compare the micelle size with the pore size at different salt and surfactant concentrations. For this purpose, the results are given in Fig. 6 for CTAB microemulsions from which it can be deduced that micelles are smaller enough than the smallest pores in the porous media at all salt and surfactant concentrations carried out in this work;
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CR
IP
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considered.
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Figure 6 Comparative analysis between size of CTAB micelles and pore throats of the porous media at different salt and surfactant concentrations.
5. Conclusions
The present paper provided a method for prediction of CTAB cylindrical microemulsion viscosity against various surfactant and salt (NaCl) concentrations, based on an already presented mathematical model in the literature. The model was found to be remarkably effective due to emerging great agreement when compared to experimental viscosity data. It was also demonstrated that higher than a threshold CTAB concentration, microemulsion viscosity 19
ACCEPTED MANUSCRIPT increases as CTAB concentration increases, which was ascribed to spherical to cylindrical shape transition phenomenon. Moreover, using analytical equations, cylindrical micelle size was measured as a function of surfactant concentration and water salinity, followed by a comparison with pore throat size in different conditions. Results illustrated that under the presented
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conditions, micelle sizes are considerably smaller than the pore throat size, therefore, making
IP
them easily pass through pores network without being plugged. Therefore, it is finally deduced
CR
that CTAB surfactant may be used as a chemical additive to activate two major oil recovery mechanisms of IFT and mobility ratio reduction at certain salt and surfactant concentrations in a
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typical surfactant flooding process instead of employing both surfactant and polymer, which
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therefore suggests a more economically chemical EOR process.
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Nomenclature
Nc u
T
CTAB concentration above which spherical to cylindrical micellar shape transition happens, gr/dL
IP
Micelle concentration, gr/dL
Aggregation number existing in the biggest spherical micelle, dimensionless
Surfactant tail length, nm Axial ratio of cylindrical micelle, dimensionless
CR
Aggregation number corresponding to cylindrical micelles, dimensionless
Surfactant molecular weight, Kg/Mol Number of carbon atoms existing in the surfactant molecule, dimensionless Capillary number, dimensionless
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N
Surfactant concentration, gr/dL
AN
C C th Cm gs gw lo l /d MW s
X cmc
Surfactant mole fraction at critical micelle concentration, dimensionless
cmc
Microemulsion viscosity used in equation 1, Pa.s Micellar solution viscosity at critical micelle concentration, , Pa.s Microemulsion relative viscosity, dimensionless
r d K
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V s /V w
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vo
Dimensionless micellar solution viscosity Thermodynamic coefficient of the employed model, dimensionless Effective length existing in the hydrophilic portion of surfactant molecule, nm IFT utilized in equation 1, N/m
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c
M
X
Microemulsion darcy velocity used in equation 1, m/s Shape factor characterized with subscript “c” in order to indicate its dependency on concentration, dimensionless Surfactant tail volume, nm Surfactant per water molecular volume ratio, dimensionless Surfactant mole fraction, dimensionless
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ACCEPTED MANUSCRIPT Research Highlights
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A mathematical model was evaluated by using new experimental data. Micelle shape is investigated as surfactant (i.e. CTAB) concentration is increased. Shape transition from spherical to cylindrical strongly increases the viscosity. For CTAB, the Shape transition occurs at threshold concentration of 0.50 gr/100ml. The micelle size of CTAB microemulsion was determined at different conditions. Some surfactants can play the role of both polymer and surfactant simultaneously.
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