Polymer Flooding on Separation and

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Evaluation of Alkali/Surfactant/Polymer Flooding on Separation and Stabilization of Water/Oil Emulsion by Statistical Modelling Hussain H. Al-Kayiem, and Javed Akbar Khan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01662 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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Evaluation of Alkali/Surfactant/Polymer Flooding on Separation and Stabilization of Water/Oil Emulsion by Statistical Modelling Hussain H. Al-Kayiem and Javed A. Khan Centre of Research in Enhanced Oil Recovery, Universiti Teknologi PETRONAS, Malaysia. Tel: +605 3687008 Fax: +6053656461 Alkali/Surfactant/Polymer, Emulsion stability, Crude oil, Water in oil emulsion, Separation, and Stabilization

ABSTRACT: Alkali/Surfactant/Polymer flooding is a chemical method for enhanced oil recovery. This study emphases on the sensitivity of Alkali/Surfactant/Polymer flooding on the separation and stabilization of light oil emulsion in the primary gravity separator. A laser scattering technique by Turbiscan was used to determine the sedimentation of the water phase. A statistical modelling has been carried out to find the effectiveness of Alkali/Surfactant/Polymer on water in oil emulsion. The results show that the presence of alkali has a positive but insignificant effect on destabilization of emulsion at the concentrations ranges from 500-1500 ppm. The addition of weak alkali, Na2CO3 is less problematic on separation. It was found that surfactant alone and surfactant-surfactant cross-interaction were the most significant additives that causing reduction of water separation. However, the interaction effects between the alkali and surfactant, between the surfactant and polymer and that between polymer and polymer had significant influences on the water separation. An anionic hydrolyzed polyacrylamide polymer flooding has a positive effect on separation in the presence of low water cut and light crude oil emulsion. Polymer show positive effect on separation but the clarity (light transmission fraction) of the separated water is reduced. The clarity of the separated water is more than 83% and 1

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around 69 % at 400 ppm and 800 ppm of polymer concentrations, respectively. Increase in alkali and surfactant has insignificant effect on the clarity of separated water. Based on the statistical model, an empirical correlations were developed to predict the separation of water.

1. INTRODUCTION

The research work has been carried out in alignment with the chemical EOR programs in Malaysia oil fields. Tertiary EOR applications involving the injected combination of Alkali/Surfactant/Polymer (ASP) are expected to cause an impact on the surface facilities due to the stable emulsions that are formed in production fluids. The high stable emulsions which are often referred to as “Rag Layer” reduces the effectiveness of the bulk separation process and has potential cost implications on the EOR program. Various conventional methods to treat crude oil emulsions are existing, so far. The presence of ASP and their effects on the separation process is however not well understood. Chemical injection for EOR is influencing the separation performance of produced oil-water. The effect of alkaline flooding on the recovery of Aafaniya crude oil of Saudi Arabia was studied and found that the highest oil recovery was achieved when alkali and polymer were injected in a single slug, and moved together through reservoir [1]. In the development of enhanced oil recovery in China, the polymer flooding method had been applied successfully [2]. With the injection of chemical floods, breakthrough of the injected chemicals in the primary separator results in the formation of stabilized water-in oil emulsion [3]. Emulsion formation and stability also increase when hydrocarbons and formation water are tremendously mixed under high shear from reservoir, tubing and chokes [4]. Figure 1 is a schematic diagram that describes the sections where high shear occurs in the production system.

2


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Figure 1. Emulsion formation during crude oil production [5] The tightness and stability of emulsions are affected by various other factors which include the water-oil ratio, intensity of turbulence and physical properties [6-8]. Hirasaki et al. [9] found that asphaltene layers at the interfaces of water drop are the important factor in some cases. The effect of asphaltene undergoes by several intermolecular forces, which include electrostatic, Vander Waals/dispersion forces between aromatic rings, attraction, and repulsion, and hydrogen bonding [10]. Emulsion stability are caused by the acid and base interactions between the asphaltenes and the aqueous phase. Emulsion stability increases when the aqueous phase is acidic and emulsion stability decrease when the aqueous phase is basic in light oils [11]. Emulsification is a key mechanism in ASP injection [12]. The investigation of core flood experiments showed that the recovery of oil is increased 5% with emulsification [13]. A lab testing shows that strong alkali, NaOH formed very stable emulsion as compared to week alkali, Na2CO3 [14]. The interaction of surfactant with a strong alkali, NaOH showed that the absorption of surfactant is increased as compared to Na2CO3 at the same concentrations [15]. The absorption of surfactant is higher due to the monovalent ion in NaOH while absorption is reduced because of the divalent ion in Na2CO3. There is a similarity in the effect of week and strong alkali in the Daqing field [16]. 3


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The formation of a w/o emulsion with chemical injection is a renowned problem in the facility side, as it stabilize the emulsion [17]. In the background of ASP injection processes, research has been carried out since 2001 on the properties of emulsified and stabilized fluids produced from the reservoir [18-20]. The main difficulty that occurs is the stability of W/O and O/W emulsions that hinders the water and oil gravity separation process. Studies are found on the choice of demulsifiers (to breakdown W/O and O/W emulsions) to enhance the coalescence phenomena between the dispersed droplets [21] or on the improvement of the mechanical design of coalescence plates in the separator [20]. Stabilization of macro-emulsions is not in thermodynamic equilibrium as compared to microemulsions, contribute in the process of EOR flooding [22]. Although, the main benefit to inject an optimum concentration is that it results to minimize the emulsion stability [23, 24]. If this optimum concentration in the field can be sustained throughout the whole flooding process then the separation between oil and water is not problematic. However the injected slug concentration may be varied from optimal in essence due to the properties of reservoir which may involve several phenomena like adsorption of surfactant, change in the salinity, precipitation of salts, etc. In the particular case of ASP flooding processes, situations are even more irregular by the acid and salt production because of alkaline addition [25]. Alkali plays an important role in ASP flooding [26]. EOR flooding of alkalis is carried out to produce micro-emulsion to increase the stability as a result of steric and electrostatic effects. Alkali react with the acidic and other components in crude oil to produce in situ soap with the interaction of additional surfactant to reduce IFT. And also it reduces the absorption of surfactant in the formation; alkali was consider an emulsification agent in ASP flooding which lead to 4


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stable emulsion [27, 28]. Although, for low acid number oil Daqing oil, the interaction of surfactant with only strong alkali like NaOH was able to reduce the IFT [16, 29]. On account of surfactant EOR, an ideal injection of surfactant concentration has to be carried in the field. The formulation which can achieve a minimum interfacial tension and a maximum oil recovery might be obtained for a proper balance of the hydrophilic and hydrophobic affinities of the surfactant for the oily and aqueous phases. [30, 31]. Mechanistically, the dispersed phase droplets acquire positive or negative surface charge in acidic or basic media, respectively, due to the ionization of surfactants adsorbed at the oil-water interface induced by variation of aqueous (or oil) phase pH [17, 32, 33]. The proposed ‘‘ionization effect” is promoted by the addition of salts into the water phase, as a result of interaction/association of ions present in the brine with surface-active species at the interface [32]. In the presence of demulsifier, the optimal pH (at which separation efficiency is maximum) for the oil-brine systems stabilized by asphaltenes, which are classified as amphoteric surfactants, has mostly been reported to be neutral or nearneutral [34]. There is still a challenge to break the emulsified crude oil with the injection of ASP due to lack of emulsification and demulsification mechanism which need further research [35]. In the literature concerning polymer flooding application with a partially hydrolyzed polyacrylamide and hydrolyzed polyacrylamide, the problems of oil/water separation has been found in Daqing oilfield [36, 37]. The effect of polymer injection resulted in the reduction of separation efficiency by more than 50% because of high viscosity of the water phase caused the stability of W/O emulsions. The polymer increase the viscosity of water and decrease it mobility which enhances sweep efficiency through more favorable oil and water mobility ratio [12]. In the early days, polymer was believed not to reduce the residual oil saturation [38]. Later, many 5


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studies found that viscoelstic effect of polymer can reduce residual oil saturation [39-41]. The typical design of ASP flooding was investigated in the past and found that Polymer flooding is suitable to carry out for heavy crude oil [42]. There is still a debate to finalize the conclusion of polymer on the stabilization of emulsion [38, 42]. The present concern of emulsion stability has been on the facility side, especially due to crude quality problems, e.g., Gravity separator design and demulsifier decision [43]. There is need of research on the mechanism of complex emulsion, which can provide the base knowledge for the enhancement of demulsifiers [35]. The formation of emulsion has been demonstrated by an interaction of the various chemical constituents of ASP with the produced crude oil that enhances the stabilization of w/o emulsion. In the past, it was also found that polymer enhances flocculence phenomena but the study of alkali-surfactant-polymer interactions was required for emulsion stability conditions to optimize the demulsifier [16, 44]. The objectives of the paper are to determine the water separation and stabilization of water in oil emulsion at various concentrations of ASP for an identified oil field in Malaysia. An experimental measurements have been carried out to investigate the effectiveness of EOR flooding on 40% water cut emulsion, with fifteen different combinations of ASP. The experimental investigations focused on the water in oil emulsions imposed by different ASP flooding compositions that carry-over in the separator feed and form the emulsion. The experimental measurements have been carried out using laser transmission and backscattering technique. The separation, identified by the percentage of separated water, has been measured after 5 minutes and 24 hours. In addition, the capability of the statistical model to predict the separation behavior of complicated water-in-oil emulsions with added ASP has been explored. 6


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JMP statistical software has been used to analyze and predict the effectiveness of each individual and combined additives on the separation and stabilization of the emulsion. Then a correlation has been generated, by incorporating all the experimental results, which is able to estimate the water separation percentage.

2. EXPERIMENTAL METHODOLOGY

The present study is carried out to investigate the effect of Alkali/Surfactant/Polymer (ASP) flooding on the stabilization and separation of water in light oil emulsion. An experimental technique has been used to examine the stability of emulsion and then a statistical method is applied to find the interaction of chemicals on the separation and stabilization. Initially, emulsification was carried out according to the shear rate of crude oil production well. Secondly, the interaction effect of alkali-surfactant, surfactant-polymer and alkali-polymer on the separation and stabilization was investigated with response surface method. The steps carried out in the study are shown in scheme 1.

Scheme 1. Flow chart of the procedures The measurement of emulsion stability is an important test which is instigated at emulsion. It defines the affluence with which each phase separate in the emulsion. By far the most common method is bottle test. The present study is carried out using Turbiscan method to find the stability. It has laser scattering technique which precisely calculates the amount of dispersed

7


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phase settlement. Recently, a study on emulsion stabilization was conducted with laser transmission and backscattering technique [3, 45, 46]. 2.1 Materials Crude oil and brine water for this study were obtained from an identified field in Malaysia, where water flooding was performed. Crude oil has high wax, and density is 42 API with a live oil viscosity of 0.4 cP. The chemical used in this study was provided by PETRONAS, which is being utilized for the EOR purpose. The amount of ASP was calculated from the chemical breakthrough in the primary separator. ASP used in the experiment included alkaline, Na2CO3, 5%–15%, surfactant, AOS, 20%–40% and polymer, GLP 100, 60%–70%. Properties of crude oil and composition for the reservoir brine (salinity 18,360 ppm) are as follows in Table 1. Table 1. Crude oil properties and reservoir brine composition Crude oil properties Property

Reservoir brine compositions Value

Chemicals

g/L

Total amount (g)

Density, g/cc @ 60°C

0.79

CaCl2(H2O)2

0.8153

0.8153

Viscosity, cP @60°C

0.4

MgCl2(H2O)6

0.7517

0.7517

API

42

NaCl

9.2734

9.2734

TAN, mg KOH g-1

0.19

SrCl2(H2O)6

0.0296

0.0296

Asphaltenes Content, wt%

0.1

KCl

0.4238

0.4238

Wax Content (wt %)

25.83

NaHCO3

7.1593

7.1593

Wax Appearance Temp. (°C)

34.1

Na2SO4

0.5177

0.5177

Solid to Solid Crystalline (°C)

63.1

Na2CO3

0.0000

0.0000

Crystalline Temperature (°C)

68.1

FeCl3

0.0000

0.0000

Cumulative Mol Percent

15.21

BaCl2(H2O)2

0.0000

0.0000

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2.2 Emulsion Preparation The amount of ASP used in the experiment was based on the chemical amount that emerged in the primary separator. The amounts of alkaline, surfactant and polymer were 5%–15%, 20–40% and 60–70%, respectively, of the total ASP injections in the separator. The emulsion was produced in the lab by mixing 40 % reservoir brine solution containing alkaline, surfactant and polymer with 60% oil by shearing in heavy duty disperser at 12000 RPM for 2 min. 2.3 Measurement Method In all of the experimental tests, the volume of sample used for Turbiscan laser transmission and the scattering test was 7 mL. The Turbiscan test can calculate the separated water/oil phases when visual observation is tricky due to the less clarity. This method assists a rapid and sensitive approach for estimating the stability of the emulsion. The rate of the phase separation is determined by measuring the increase in the transmittance of light in the sample from the bottom of a test tube to the top. The change in light transmittance through the glass vial containing the emulsion is recorded by scanning the vial vertically with the optical scanning device. 2.4 Apparatus The Turbiscan equipment has been used to measure the emulsion stability. It consists of three main equipment; namely, a PC, Data Acquisition unit and Optical Scanning device. The PC, shown in Figure 2 (a), is mainly used to run a software that controls the apparatus and synchronize the operation of the optical scanning and data acquisition. Data is acquired and stored on the hard disk in the computer. The Optical Scanning Device composes of a pulsed infrared light source that uses a wavelength of 850 nm, a detector situated opposite to the light source and reads the transmittance through the glass vial containing the specimen. During a

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scanning process, the reading head moves up and down along the glass vial and scans the whole sample in the vial. The transmittance is automatically measured every 0.04 mm and store the data in the PC memory. The measuring principle is schematically shown in Figure 2 (b).

(b)

(a)

Figure 2. Apparatus to measure stability using Turbiscan. (a) PC connected with Turbiscan device (b) Optical Scanning Device which is inside Turbiscan Figure 3 shows the measurement procedure of sedimentation and coalescence in emulsion sample.

Figure 3. Experimental measurement of separation from emulsion sample


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3. STATISTICAL PROCEDURE

The main software used in the present analysis is the JMP 13 Statistical Discovery Software from statistical Analysis System (SAS). The software has been used for design of experiment (DOE), and also for statistical analysis of results. The statistical analysis of the experiment data has been carried out by nonlinear regression, by response surface method, to examine the ASP effects on the emulsion separation and stabilization. The responses of the DOE combinations have been followed as gained from the software and measured experimentally by laser transmission and backscattering technique. The next step of the investigations, which have been carried experimentally and statistically, was carried out by input the experimental results to the software. The statistical analysis are focused on the significance and effectiveness of parameter on water separation of the emulsions imposed by different Alkaline/Surfactant/Polymer concentrations. The resulted models for water separation at 5 minutes and at 24 hours have been generated, as will be shown later in the results section. The prediction results from the models have been compared with the experimental for validation. 3.1 Design of Experiment The Design of Experiment (DOE) is carried out with response surface method to identify three additive parameters that influenced the sedimentation and coalescence phenomena of the emulsion and to understand the interaction between these additive parameters upon the emulsion characteristics. The three process parameters (or factors in DOE) chosen in this study were the alkali, surfactant, and polymer. The response variables were the separation of water at equilibrium time of separation. JMP statistical software is used to create a set of experiments


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which was used for statistical analysis to find the interaction of one parameter with other as shown in Table 2. Table 2. Definitions and levels of ASP by JMP

Sample

Pattern

Alkali ppm

Surfactant ppm

Polymer ppm

ASP 1

Low A, Low S, Low P

500

200

400

ASP 2

Low A, Low S, High P

500

200

800

ASP 3

Low A, High S, Low P

500

600

400

ASP 4

Low A, High S, High P

500

600

800

ASP 5

Low A, Medium S, High P

500

400

600

ASP 6

Medium A, Low S, Medium P

1000

200

600

ASP 7

Medium A, High S, Medium P

1000

600

600

ASP 8

Medium A, Medium S, Low P

1000

400

400

ASP 9

Medium A, Medium S, High P

1000

400

800

ASP 10

Medium A, Low S, Medium P

1000

400

600

ASP 11

High A, Low S, Low P

1500

200

400

ASP 12

High A, Low S, High P

1500

200

800

ASP 13

High A, High S, Low P

1500

600

400

ASP 14

High A, High S, High P

1500

600

800

ASP 15

High A, Medium S, Medium P

1500

400

600

3.2 Response Surface Method In order to conduct a response surface method analysis, the first step is to design the experiment at identified experimental parameters. Once the experiment is conducted, the separation measurements of each combination are recorded and RSM analysis was carried to fit a


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second-order polynomial to the data [47]. Regarding real applications in the field, a response surface has been considered to optimize the alkali and surfactant concentrations in the clastic reservoir of the Angsi field in the Malay basin, as in the published works of Ghadami et al. [48]. The generalized second-order polynomial model used in the response surface analysis was as follows in eq. (1): n

n

n −1

n

Y = β 0 + ∑ β i X i + ∑ β ii X i2 + ∑ ∑ β ij X i X j i =1

i =1

(1)

i =1 j =i +1

Where, β 0 , β i , β ii and β ij are the regression coefficients for intercept, linear, quadratic and interaction terms, respectively. While, X i and X

j

are the independent variables, and Y is the

response variable. 4. RESULTS AND DISCUSSION The stability of water-in-oil emulsion (40 % water cut) was measured depending upon the separation of water phase at various ASP concentrations which were kept under 60oC. The separation time of water from emulsions were taken after 5th min and 24th hour to figure out the effectiveness of ASP at fresh emulsion and at equilibrium state of separation. 4.1 Determination of coalescence and sedimentation heights Figure 4 illustrates the emulsion kinetics curves which obtained with the laser scan of the emulsion samples by a Turbiscan test at varying polymer and at constant alkali-surfactant concentrations. The measurements are carried out at a specific interval to find the sedimentation of water phase and coalescence of oil phase. The scan of samples shows the increase of light transmission along the sample height with the increase in the polymer concentration as shown in


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Fig 4 (a-b). The sedimentation height are converted into separation percentages. Separation of water calculated from transmission values are 39 % and 66 % at low and high polymer concentrations, respectively. With the increase of polymer, there is a significant positive effect on separation but the clarity of the separated water is reduced. The clarity of the separated water is more than 83% and around 69 % at low and high polymer conc., respectively. Additionally, there is a momentous decrease in the rag layer growth. Backscattering of light at 24-hour interval show the final rag layer growth which 10.3 mm and 6.5 mm at low and high polymer, respectively. Although, polymer has shown a positive effect on separation but still there is a significant remaining amount of residual emulsion due to combined effect of ASP. As ASP flooding is carried out into the reservoir, by following an extra addition of polymer to push oil in the reservoir. Therefore main intension in the ASP process is focused upon the surfactant generation that occurs as a results of alkaline solution interaction with the acidic components present in the crude oil. The formation of these surfactant with additional surfactant results in lowering the interfacial tension and hence mobilizes the oil. But consequently, stabilizing the emulsion due to reducing the IFT with the added surfactant and produced surfactant with the interaction of added alkali and crude oil [49].


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Figure 4. Deliberate profiles of the transmission & backscattering rate for emulsion as a function of polymer at constant alkali and surfactant concentrations (a) A=1000 ppm, S=400 ppm, P=400 ppm (b) A=1000 ppm, S=400 ppm, P=600 ppm The high separation of water in the presence of high polymer indicate that the residual emulsion is unstable. The instability of emulsion in the presence of high polymer has also confirmed with back scattering measurements which shows a significant drop in the back scattering along the emulsion from 28% to 25% at 0 sec and 5 min, respectively. On the other hand, in the presence of low polymer, back scattering measurements are slightly reduced from 30% to 28% at 0 sec and 10 min, respectively. Turbiscan tests of all experimental cases were carried out as mentioned in above Table 2. Then a statistical analysis was carried out to develop the separation prediction models and significance of each factor and their interactions. 4.2 Separation Prediction Models According to the statistical analysis, water separation rate,

W s at

5th min and 24th hour from

emulsion can be modeled using equations 2 & 3, respectively. The R2 value of 0.88 for this model was obtained, it demonstrates a good fit to the observed values from the experiments. Water separation correlation after 5 minutes (fresh emulsion) Ws = 24.31 − 0.0048 A − 0.03S + 0.0145 P + ( A − 1000 ) * (( A − 1000 ) * 0.0000027 ) + ( A − 1000 ) * (( S − 400 ) * 0.000085 ) − ( S − 400 ) * (( S − 400 ) * 0.00021) − ( A − 1000 ) * (( P − 600 ) * 0.0000275 ) + ( S − 400 ) * (( P − 600 ) * 0.000056 ) + ( P − 600 ) * (( P − 600 ) * 0.0002 )

(2)


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Water separation correlation after 24 hours (equilibrium state) Ws = 46.67 + 0.0022 A − 0.036S + 0.023P + ( A − 1000) * (( A − 1000) * 0.000048) + ( A − 1000) * (( S − 400) * 0.00011625) − ( S − 400) * (( S − 400) * 0.00041) − ( A − 1000) * (( P − 600) * 0.000046) + ( S − 400) * (( P − 600) * 0.00021) + ( P − 600) * (( P − 600) * 0.00034)

(3)

With the above prediction expression, it can be seen that the factors surfactant (S), surfactants interaction (SS), alkali and surfactant interaction (AS), surfactant and polymer interaction (SP) and alkali and polymers interaction (PP) showed statistically significant responses to the separation/stabilization rate. Surfactant (S) alone has the significant effects on the stabilization rate of the emulsion. Alkali (A) and polymer (P) alone has an insignificant effect on the separation rate. However, the interaction effect between the alkali and alkali (AA), between the alkali and surfactant (AS), between the surfactant and polymer (SP) and that between polymer and polymer (PP) had an influence on the separation rate. 4.3 Validation of the models Analysis of the separation responses at each composition of ASP showed that the subsequent prediction models effectively represents the actual data with the coefficient of variation R2 being 0.81 and 0.88 after 5 min and 24 hour, respectively. This directs that the proposed models are acceptable to find the impact of the alkali/surfactant/polymer on the water separation from the emulsion. The P-values were used to determine the statistical significance of the process parameter effects [50]. The significance of model shows a good fit of the experimental and actual data with a P-value 0.09 and 0.0326 (see Figure 5), and examination of the residuals shows that the errors are normally distributed as shown in Figure 6 (a).


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(a)

(b)

Figure 5. Fitting of the model with random effects/Actual by predicted plot. (a) 5 min: RSME = 7.38; R2 = 0.81; P Value = 0.09 (b) 24 hour: RSME = 9.37; R2 = 0.88; P Value = 0.03 Residual is find with the difference between a predicted and the observed values. As the variations of different values of the input field are not the same for the residual. To interpretation for these differences, studentizing adjustment was carried out in which the residual values were divided by the standard error for the residuals as shown in Figure 6 (b). It was used for a standardized evaluation between the residuals. Studentized residuals were calculated for the exposure of data outliers. Studentized residuals were found better distributed.

(a)


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(b) Figure 6. The plot of the residuals and externally studentized residuals for the experimental data at an equilibrium state of separation; 24 hour: (a) Residual by predicted plot, (b) Externally studentized residuals with 95% simultaneous limits The model validity was evaluated on the basis of a fit summary as presented in Table 3. The design of experiments combinations performed to be satisfactory for the predictive expression of water separation at 24 hr., due to its capability to find acceptable values of determination coefficient values of 0.88, representing a similarity between the prediction of the model and the actual data. The probability of model is significant (Prob.>F = 0.032). The residual variance is measured by Root Mean Square (RMSE), also show the confidence in the prediction abilities for the separation effectiveness of parameters, giving values of 9 %. So, the suggested design of experiment methodology was found to offer a sufficiently accurate mathematical modeling with a complete coverage of experimental trials. The determination coefficient, R2 was applied to evaluate the data fitting. In this work, the determination coefficient for the water separation is 0.88. Therefore, the proposed second-order polynomial model was proven to be successful in describing the experimental responses. The standard error provides the estimation of the errors for individual estimated parameters. It is the ratio of standard deviation and square root for the individually level. It tests the hypothesis that the lack of fit error is zero. The standard error of


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Energy & Fuels

estimate is a measure of error in prediction. The larger standard error will show the less fit of the regression model with the actual data, and the poorer the prediction. Table 3. Validation parameters of prediction models (Summary of fit) R2

0.88

R2 Adj.

0.70

F Ratio

4.93

Prob. > F

0.032

Degree of Freedom

9

Root Mean Square Error

9.37

Mean of Response

54.06

Observations

15

4.4 Statistical analysis One benefit of using the design of experiment method to study emulsion stability is that it simplifies to understand the interaction effects between the additive parameters. Three factors, the alkali, surfactant, and polymer were taken into attention and symbolized as A, S, and P, respectively. The response variable was the water separation. Analysis of individual responses was carried out to estimate the significance of each factor and their interactions (see Tables 4-5). A large value of F (small value of P) in any term of the prediction model would direct the significance on the corresponding separation response. The F-ratio is the tool used to investigate the effects. It shows that the means are considerably dissimilar from each other. A high p-value means that there is not a significant lack of fit. Prob. > F is the probability, F value is the division of the mean regression sum of squares and sum of squares of mean error. Its value will range from zero to 1.


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4.5 Effectives of individual and quadratic of each factor of ASP on separation/stabilization: after 5 min: Table 4 displays the effectiveness of ASP at fresh emulsion after 5 min interval. It is found that the surfactant (S) alone was the main factor that affected the separation rate of the emulsion. Alkali (A) and polymer (P) alone has the insignificant effect on the separation rate. In the past it was also found that Na2CO3 alone has no effect on emulsion stability [51]. From the predicted equation, it was found that the effect of the surfactant concentration is insignificant (P-value 0.63) with a negative effect on separation (t Ratio -0.5). This shows that the separation percentage slightly decrease when there is variation in this parameter from low to high level. On the other hand, the alkali and polymer concentrations have positive effects on the separation percentage. This clarifies that there will be an increase in the separation percentage with the variation of polymer from low to high level. However, variation in the alkali concentration will not affect the separation percentage considerably as it has the insignificant impact (P-value 0.11). Table 4 also shows evaluations of the quadratic terms for collective factors. The interaction effect between the surfactants (S*S) and between polymers (P*P) had significant influences on the stabilization and separation rates, respectively. The effect of surfactant-surfactant interaction clarifies that increase in the surfactant concentration and keeping the alkali and polymer constant has stabilized the emulsion significantly. Whereas, polymer-polymer interaction simplifies that with the increase in polymer concentration by keeping alkali and surfactant constant resulted in the destabilization of emulsion significantly. The quadratic term of alkalis (A*A) shows the intermediate significant effect as P-value is not within the confidence level (i.e. 95%).


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Table 4. Sorted parameter estimates at a fresh state of emulsion separation/stabilization Term (A, ppm-1000)*(P, ppm-600)

Estimate Std. Error t Ratio

Effect direction

Prob.>|t|

-0.000015

2.632e-6

-5.67

0.0013

0.003

0.001177

2.55

0.0436

(A, ppm-1000)*(A, ppm-1000)

3.7359e-6

1.834e-6

2.04

0.0878

(S, ppm-400)*(S, ppm-400)

2.1724e-5

1.146e-5

1.90

0.1069

(A, ppm-1000)*(S, ppm-400)

4.925e-6

2.632e-6

1.87

0.1105

A, ppm

0.000878

0.000471

1.86

0.1115

(S, ppm-400)*(P, ppm-600)

-1.088e-5

6.58e-6

-1.65

0.1495

(P, ppm-600)*(P, ppm-600)

-1.128e-5

1.146e-5

-0.98

0.3632

S, ppm

-0.00059

0.001177

-0.50

0.6340

P, ppm

4.6 Interaction of separate factors of ASP on separation/stabilization: after 5 min Table 4 also shows the interaction effect between the alkali and surfactant (A*S), and between the surfactant and polymer (S*P) had significant influences on the separation rate with a significance of 0.011 and 0.15, respectively. The interaction effect of very high concentration of alkali with the surfactant of same type has been studied in the past and it was noticed that IFT was reduced significantly which resulted in much stabilized emulsion [51]. However, the interaction between alkali and polymer (A*P) has very high significant negative effect on separation (P value 0.001, t Ratio -5.67). It shows that the interaction of alkali and polymer has highly significant effect to stabilize the freshly produced emulsion. Figure 7 shows an interaction effect of alkali-surfactant, surfactant-polymer and the alkalipolymer on separation/stabilization at 5th min. These interaction effects of factors are shown in


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their low and high level. The interaction of alkali-surfactant is increasing the separation in the presence of low alkali, 500 ppm and low surfactant concentration, 200 ppm. Also, the interaction of alkali and polymer shows the reduction in separation. Although, the interaction of alkali with low polymer concentration is more effective to reduce the separation as compared to the high polymer concentration. The interaction of surfactant with high concentration of polymer shows the separation while there is stabilization at low polymer concentration. The interaction of polymer with alkali also show stabilization. Alkali change the viscosity of the polyacrylamide solution in two different ways. Primarily, alkali gives cations into the solution of polymer, which results in the reduction of polymer viscosity by the charge-shielding phenomena. Secondly, alkali be able to start base hydrolysis that hydrolyzes the amide groups present on the polymer chain. This phenomena can cause the increase of polymer solution viscosity. The interaction of strong alkali like sodium hydroxide and polymer has dramatic effect on base hydrolysis which results in the much higher increase in the number of negative charges on the chain of polymer. Thus, the electrostatic repulsion force increases, and the chain size elongates. The extension in the polymer-chain size improves the viscosity of polymer solution [52].

40 25 10

40 25 10

40 25 10


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Figure 7. Interaction Profiles at 5th min 4.7 Effectives of individual and quadratic of each factor of ASP on separation/stabilization: after 24 hour Table 5 displays the sequence of highest to the lowest effective terms in the model. It shows that the surfactant (S) alone was the main factor that affected the separation rate of the emulsion. Alkali (A) and polymer (P) alone has the insignificant effect on the separation rate. Even a higher concentrations of the same type of week alkali has been studied in the past and found that the effect of alkali alone in light crude was unable to stabilize emulsion [51]. From the predicted equation, it was found that the effect of the surfactant concentration is significant (P-value 0.0513) with a negative effect on separation (t Ratio -2.43). This shows that the separation percentage decrease when there is variation in this parameter from low to high level. On the other hand, the effect of alkali alone and polymer alone are found to increase the separation but the impact is insignificant with a P-values, 0.72 and 0.17, respectively. This clarifies that there will be a small increase in the separation percentage with the variation of polymer from low to high level. However, variation in the alkali concentration will not affect the separation percentage considerably as it has the insignificant impact (P-value 0.7233). The evaluations of the quadratic terms for collective factors is shown in Table 5. The interaction of alkali (A*A) shows intermediate significance (P-value 0.0812) to destabilize emulsion with a positive effect on separation (t Ratio 2.09). The interaction effect between the surfactants (S*S) and between polymers (P*P) had significant influences on the stabilization (Pvalue 0.0295, t Ratio -2.84) and separation (P-value 0.0568, t Ratio 2.35) rates, respectively. The effect of surfactant-surfactant interaction clarifies that increase in the surfactant concentration


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and keeping the alkali and polymer constant has stabilized the emulsion significantly. Whereas, polymer-polymer interaction simplifies that with the increase in polymer concentration by keeping alkali and surfactant constant resulted in the destabilization of emulsion significantly. Table 5. Sorted parameter estimates at an equilibrium state of separation/stabilization

Term

Estimate

Std. Error

t Ratio

Effects direction

Prob.>|t|

(A-1000)*(S-400)

0.00012

3.314e-5

3.51

0.0127

(S-400)*(S-400)

-0.00041

0.000144

-2.84

0.0295

(S-400)*(P-600)

0.00021

8.286e-5

2.53

0.0449

S

-0.036

0.014823

-2.43

0.0513

(P-600)*(P-600)

0.00034

0.000144

2.35

0.0568

(A-1000)*(A-1000)

0.00005

0.000023

2.09

0.0812

P

0.023

0.014823

1.55

0.1717

(A-1000)*(P-600)

-0.00005

3.314e-5

-1.40

0.2124

A

0.0022

0.005929

0.37

0.7233

4.8 Interaction of separate factors of ASP on stabilization/separation: after 24 hour Table 5 also shows the interaction of separately parameter on the stabilization/separation at equilibrium state of separation (24th hour). The interaction of alkali and surfactant (A*S) is found highly significant (P-value 0.0127), with the positive effect on separation (t Ratio 3.51). As well as, the interaction of surfactant and polymer (S*P) also has significance (P-value 0.0449), with a positive effect on separation (t Ratio 2.53). However, the interaction between alkali and polymer (A*P) has the insignificant effect on emulsion stability.


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Energy & Fuels

Surfactant injection is commonly used in EOR flooding processes to recover residual oil by reducing interfacial tension or altering rock wettability. The efficiency of process can be increased with the interaction of polymers along with surfactant. In such case normally, polymer and surfactant are mixed for injection. The capability of the injected slug to retain the mobility control throughout the ASP flooding can be determined by the interaction between alkalisurfactant, alkali-polymer, surfactant-polymer, and the interaction of all constituents [53]. The viscosity of polymer has generally negatively affect due to these interactions, and it is problematic to exclude [54]. Although, the rheology of polymer can also be affected by the salt type and concentrations present in the brine as well as the degradation of polymer due to high shear. Therefore, a comprehensive knowledge of performance of polymer interaction is of greatest value for ASP flooding and emulsion separation process. Figure 8 shows an interaction effect of the alkali-surfactant, alkali-polymer and the surfactantpolymer on separation/stabilization after 24 hr. This large time step is used to find the final interaction effect of ASP at an equilibrium state of separation/stabilization. The interaction effects are almost matching to the small time step as discussed before. The interaction of surfactant with alkali has a significant positive effect on separation at low alkali and low surfactant concentrations. The interaction of surfactant with high concentration of polymer shows the separation while there is stabilization at low polymer concentration and the separation percentage is in the range of 20-60%. The interaction of polymer with alkali shows negative effect on separation while separation percentage is in the range of 60-80%.


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80 70 60 50 40 30 20 80 70 60 50 40 30 20 80 70 60 50 40 30 20

Figure 8. Interaction Profiles at 24 hour 4.9 Prediction profiles at various parameters: Figure 9 shows the calculated profiles for the separation rate as a function of three additive parameters in the DOE. The profiles of ASP effectiveness on water separation after 5th min and 24 hour are shown in Figure 9 (A-B). The profiles at 24 hour shows the final effectiveness of ASP at equilibrium state of separation. It shows that separation rate was only slightly decreased as the alkali concentration is 1000 ppm, alkali starts destabilization the emulsion at 1500 ppm (see Figure 9 (a)). The effectiveness of alkali alone has the insignificant effect on separation with t = 0.37 and P = 0.7233. In the past, even a very high concentration of alkali tested alone to stabilize emulsion for the same type of crude oil as used in the present study and they have found insignificant effect to reduce interfacial tension [51]. They noticed that the interaction of alkali in the range of 5000-20000 ppm with surfactant with 500 ppm has stabilized the emulsion significantly. Although, in the present study, the interaction of low range of alkali (500-1500 ppm) with surfactant (200-600 ppm) as found in separator breakthrough has a positive significance (t = 3.51, P = 0.0127) effect on the separation. The interaction of alkali with


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Energy & Fuels

polymer shows a negative effect on separation with t = - 1.40 but the significance is low (P = 0.2124). A sensitivity analysis of combined effect of ASP for EOR purpose has been carried out in the past and found that combined effect of ASP has a positive effect in oil recovery [48]. The breakthrough amount of these chemical found in separator has been investigated in the present study. It is noticed that addition of surfactant had a negative effect on the emulsion separation as shown in Figure 9 (b). A small amount of surfactant addition to the emulsion increased the emulsion stability. However, the addition of surfactant resulted in an increase in the separation rate in the presence of 1500 ppm of alkali and 800 ppm of polymer. The separation trend showed that the effect of surfactant starts to stabilize the emulsion significantly at 600 ppm. The effectiveness of surfactant alone to stabilize the emulsion is significant with P = 0.0513 and t = 2.43. The interaction of surfactant with alkali has a positive significance (t = 3.51, P = 0.0127) on the separation. The interaction of surfactant with the surfactant is significantly high and it has reduced the separation with t = -2.84 and P = 0.0295. In addition, the interaction of surfactant and polymer shows a positive and significant effect on separation with t = 2.53 and P = 0.0449. The addition of surfactant had a stimulating effect on the emulsion characteristics. A small amount of surfactant addition to the emulsion increased the emulsion stability. However, the addition of surfactant resulted in an increase in the separation rate in the presence of high alkali and high polymer.


(A) Profiles at 5th min (B) Profiles at 24th hour

Water Sep. % (AVG Sep 48.3 %)

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Water Sep. % (AVG Sep 16.2 %)

Energy & Fuels

(a) Alkali, ppm

(b) Surfactant, ppm

(c) Polymer, ppm

Figure 9. Separation profile for emulsion at various ASP concentrations Figure 9 (c) shows the calculated and experimental profile for the separation rate at various alkali and surfactant concentration in the presence of 400, 600 and 800 ppm polymer. The effectiveness of polymer alone to destabilize the emulsion is insignificant as the probability is not in the significant range (P = 0.1717). The interaction of polymer and surfactant shows a positive and significant effect on separation with t = 2.53 and P= 0.0449. In addition, the interaction of polymer and polymer has a positive and significant effect on separation with t = 2.35 and P = 0.0568. The interaction of the polymer with alkali shows a negative effect on separation with t = -1.40 but the significance is low with P = 0.2124. 4.10

Justification of developed correlation

A comparison of the experimental data and model prediction is carried out to verify the accuracy of the model. Figure 10 shows the separation rate with the experimental data and the prediction model as a function of varying alkali in the presence of constant surfactant and


Page 29 of 40

polymer concentrations. Experimental data and model prediction has an agreement with a very small standard deviation (0.20, 1.28, 0.9 and 0.8). The slope of separation trend for experimental and model is m = -0.013 and m = -0118, respectively. The slope shows that there is a small decrease in the separation at the low surfactant, 200 ppm, and low polymer 400 ppm concentrations. The effectiveness of each parameter is based on various compositions of surfactant and polymer and it is found that alone alkali effect is to slightly increase separation with insignificant effectiveness (t = 0.37 and P= 0.7233). The slope of predicted values become 0.0124 with the increase in alkali concentration to 1500 ppm. The positive trend of prediction shows the agreement of experimental data with model prediction. The effectiveness of surfactant and polymer with the increase in alkali starts destabilization the emulsion in experimental data and model prediction. The comparison shows that the separation rate shows good agreement at all alkali concentration. As in the present study, Na2CO3 is utilized which is considered as a weak alkali because the carbonate ions are capable to obtain protons from water droplets. So, the amount of hydroxide ions is increased, and consequently the pH of the solutions become higher. The presence of weak alkali is less problematic on separation as compared to strong alkali [16].

100 Model Prediction Experimental Data

90

Water separation, %

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Energy & Fuels

80 70 60 50 40 30 20 10 400

600

800

1000

1200

1400

1600

Alkali, ppm


Energy & Fuels

Figure 10. Verification of model prediction @ S = 200 ppm, P = 400 ppm Figure 11 shows the separation rate with the experimental data and the prediction model as a function of varying surfactant in the presence of constant alkali and polymer concentrations. The effectiveness of alkali and polymer with the increase in surfactant has a negative effect on separation in experimental data and model prediction with very small standard deviations (0.204, 2.7, 2.811 and 3.995). There are significant negative slope of separation with m = - 0.115 and m = -0.136 for experimental and model, respectively. The stabilization rate increased drastically at 200 ppm surfactant concentration and it has an agreement with the experimental data. The comparison shows that the separation rate shows good agreement at all surfactant concentration.

100 Model Prediction

90

Experimental Data

Water separation, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

80 70 60 50 40 30 20 10 100

200

300

400

500

600

700

Surfactant, ppm

Figure 11. Verification of model prediction @ A = 500 ppm, P = 400 ppm Figure 12 shows the separation rate with the experimental data and the prediction model as a function of varying polymer in the presence of constant alkali and surfactant concentrations. The effectiveness of alkali and surfactant with the increase in the polymer has a positive effect on the separation of experimental data and model prediction. There are very small standard deviations


Page 31 of 40

with experimental data and model prediction (0.204, 3.336, 4.1 and 0.445). There are a positive slope of separation with m = 0.0075 and m = 0.0043 at experimental and model prediction, respectively. The separation rate has an agreement with the experimental data and model prediction.

100 Model Prediction

90

Water separation, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Experimental Data

80 70 60 50 40 30 20 10 300

400

500

600

700

800

900

Polymer, ppm

Figure 12. Verification of model prediction @ A = 500 ppm, S = 200 ppm As the increase in the polymer causes stabilization of water in oil emulsion at 600 ppm even in the presence of low concentration of surfactant. The similar effect of polymer to stabilize the emulsion in the presence of low surfactant concentrations is also found in the literature [3]. The effectiveness of alkali is positive on the separation in the presence of high surfactant at low and high polymer concentrations. There is a slight negative effect of alkali on separation of water at low surfactant and low polymer and high polymer concentrations. The increase in the polymer at low alkali has a positive effect on the separation of water in oil emulsion. There is a negative effect of polymer on the separation at low and high alkali/surfactant concentrations. Effect of alkali is to destabilize the emulsion in the presence of high surfactant for both concentrations of polymer respectively 400 and 800 ppm. Polymer show destabilization in the presence of low


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Page 32 of 40

alkali at 200 and 600 ppm surfactant and it stabilizes the emulsion at alkali 1500 and 600 ppm surfactant. The reason of surfactant to stabilize the emulsion as: at higher surfactant concentration, the solubilization of oil and water and form type III which is suitable for to increase the oil recovery [9]. Though, the high concentration of surfactant is required. However, it must be clear that at a high surfactant concentration stop the effectiveness from its normal. Past study showed that if the concentration of surfactant is very high, it results in the rise of the pressure gradient. The pressure gradient will be opposite of direction flow [55].

5. CONCLUSIONS

The effectiveness of ASP flooding on the stabilization and destabilization of water in light crude oil emulsion have been explored. Emulsion stability is found to be affected by the solo components and the interaction of these external emulsifiers such as alkali-surfactant, alkalipolymer and surfactant-polymer. The experimental results show that the presence of polymer in the range of 400-800 ppm are increasing the separation but the impact is insignificant at equilibrium state of separation, after 24 hour. Among all ASP components, the effect of surfactant alone is found to stabilize the emulsion with a significant impact, with a P-value of 0.051, after 24 hours. Also, it has been observed that alkali is effecting the emulsion stabilization after 5 min. The interaction of surfactant-surfactant is the most significant factor to stabilize the emulsion by reducing the separation with a high significance, with P-value 0.029. In addition, the interaction of alkali-alkali has intermediate significance, with P-value 0.0812 and with a positive effect on separation, where t Ratio = 2.09. The interactions of surfactant-polymer and polymerpolymer are found to be significant with P-value 0.044 and 0.056, respectively, with a positive


Page 33 of 40

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effect on separation. The interaction between alkali-surfactant is highly significant, with P = 0.012, to increase separation.

ACKNOWLEDGMENT The authors would like to acknowledge PETRONAS Research Sdn Bhd (PRSB), Exploration and Production Technology Department (EPTD) and Centre of Research in Enhanced Oil Recovery - Universiti Teknologi PETRONAS (COREOR - UTP) for the financial support and supply of the experiment raw material to carry out the research and to publish this work. REFERENCES

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