water emulsion in gravity separation

Accepted Manuscript Influence of alkali-surfactant-polymer flooding on the coalescence and sedimentation of oil/water emulsion in gravity separation Javed A. Khan, Hussain H. Al-Kayiem, Waqas Aleem, Ahmed B. Saad PII:

S0920-4105(18)30923-9

DOI:

https://doi.org/10.1016/j.petrol.2018.10.055

Reference:

PETROL 5418

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 24 May 2018 Revised Date:

15 October 2018

Accepted Date: 16 October 2018

Please cite this article as: Khan, J.A., Al-Kayiem, H.H., Aleem, W., Saad, A.B., Influence of alkalisurfactant-polymer flooding on the coalescence and sedimentation of oil/water emulsion in gravity separation, Journal of Petroleum Science and Engineering (2018), doi: https://doi.org/10.1016/ j.petrol.2018.10.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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Experimental measurement demonstrated the need for new prediction model when the oil/water emulsion is flooded with ASP additives for EOR. The graphs show a comparison with experimental results which illustrate the accuracy of the developed model to estimate the coalescence and sedimentation.

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Influence of Alkali-Surfactant-Polymer Flooding on the Coalescence

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and Sedimentation of Oil/Water Emulsion in Gravity Separation

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Javed A. Khan1, Hussain H. Al-Kayiem1*, Waqas Aleem2 and Ahmed B. Saad1

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Centre of Research in Enhanced Oil Recovery (COREOR), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Malaysia.

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Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Malaysia.

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*Corresponding email address: [email protected] Abstract:

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The emulsification and stabilization of the residual chemical within the recovered oil pose

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difficulties in the primary separation process. This study emphasis on the effectiveness of ASP

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injection on coalescence and sedimentation of oil and aqueous phases. In addition, the present

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work is focused at modifying an existing oil/water separation prediction model applicable in the

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presence of ASP fluids, as the existing model is applicable only to oil/water emulsion.

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Experimental and empirical techniques have been adopted as a methodology for investigating

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this work. The modification of the existing model for the prediction of the separation as well as

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the interaction effect of the parameters were explored. The influence of ASP on separation time

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in oil-water emulsion were explored in terms of sedimentation and coalescence profiles.

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Empirical expressions were generated from the batch experiments to find the coalescence and

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sedimentation constants for various alkali/surfactant/polymer concentrations. The outcomes of

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the study show that the presence of various ASP concentrations in the oil-water emulsions

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resulted in a variation in the required separation time. The results also show that alkali in the

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500-1500 ppm range has the most significant negative impact on sedimentation. The effect of

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alkali in reducing sedimentation increases very significantly with surfactant interaction (200 –

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600 ppm). While the polymer in the range of 400 to 800 ppm and alkali significantly reduced

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coalescence, as well as the interaction of alkali-surfactant and surfactant-polymer, also

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contributed significantly to reducing coalescence.

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Keywords: Alkali-Surfactant-Polymer Flooding; Batch Separation; Coalescence; Dispersion;

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Oil/Water Emulsion; Sedimentation

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1.

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The crude oil produced from the reservoir usually contained a complex mixture of formation

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water and hydrocarbons. These mixtures of multiple components flow from the reservoir to the

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primary separator through the casing perforations, the production tubes and the chokes where

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high shear is introduced into the produced mixture, as shown in Fig. 1. During the flow from

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these areas, multiple types of fluids are mixed under high shear; and in particular, the water

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phase begins to disperse in the oil phase and form a stable emulsion (Kokal, 2005). There are

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various factors affect separation, such as mixing intensity, physical properties of components, oil

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and water phases ratio (Nadiv et al., 1995; Aleem et al., 2015; Jeelani and Hartland, 1998).

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Another stabilizing factor is the chemical EOR which is mainly performed to improve the

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production of oil, chemicals such as ASP are injected into the reservoir.

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Fig. 1. Formation of emulsion during production of crude oil (Kokal, 2008)

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INTRODUCTION

A design study of the separator was carried to separate the alkali/surfactant/polymer

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produced emulsions with coalescence plates (Zhang et al., 2007). The requirement of oil

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production industries is to separate oil-water proficiently with minimal entrainment to handle a

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high volume of crude oil. The basic approach is to examine batch tests because of their

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simplicity. It can also decrease the cost if the data acquired from small batch tests can be utilized

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for scale up reasons. To decrease scaling costs, the researchers proposed linking the batch

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investigation to the dynamic operation of the gravity separator (Madhu et al., 2007).

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Experimental studies associated with the separation of fluids in the light of the dispersion

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technique are reported in the literature (Mungma et al., 2013 and Noïk et al., 2013). The

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separation of crude oil emulsions with improved gravity settling is considered a basic facilitating

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the evaluation of the interface. The evolution of the coalescence is recorded by the quantity of

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settled water-phase and by the increase of the size of the drops in the upper layer of the emulsion

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(Krebs et al., 2012). Recently, a survey showed that the presence of multiple emulsions increases

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sedimentation but does not speed up the phase separation process as a whole (Hohl et al., 2017).

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After the flooding of the chemicals, there is a breakthrough in the primary separator that causes

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the stabilization of the water in oil emulsion (Khan et al., 2015).

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Emulsion formation with chemical flooding is a recognized problem on the facility side

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because it stabilizes the emulsion (Dalmazzone et al., 2012). Alkalis play a significant role in

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ASP floods (Olajire, 2014). Alkali is injected to create micro-emulsion to upsurge stability due to

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electrostatic and steric effects. Alkali also starts reaction with the acidic constituents present in

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the crude oil and form soap which results in the reduction of interfacial tension. Then, this also

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reduces the surfactant absorption in the formation; alkali has been considered as an emulsifying

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agent in ASP floods that lead to a stable emulsion (Li et al., 2005; Wang et al., 2012). Although,

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for oils with low acid number, the interaction of surfactant with strong alkali, such as NaOH,

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reduced the interfacial tension (Guo et al., 2017; Guo et al., 2017). The presence of strong alkali

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is more problematic on separation as compared to weak alkali (Guo et al., 2017).

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With the presence of bound molecules (surfactant) at the oil and water interface hampers

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the process of coalescence of the droplets. Thus, the coalescence profile depends on the size

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distribution of the droplets and the amount of surfactant adsorption on the oil and water interface.

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The bound molecules can affect all the mechanisms of the sedimentation process and, therefore,

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provide major insight into the destabilization of the emulsion (Abeynaike et al., 2012). Thus,

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surfactant with a high concentration in the main slug is performed as it has a significant role in

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the displacement of oil from the reservoir (Huang et al., 2017). A study shows that with the

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upsurge in the dispersed phase of the oil in water emulsion with the surfactant results in the

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increase in the energy destruction rate significantly as a result of an increase in the emulsion

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viscosity (Pal, 2014). The reason why the surfactant stabilizes the emulsion: at a higher

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concentration of surfactant, the solubilization of water and oil and in type III form to increase oil

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recovery (Hirasaki et al., 2010). Although the higher concentration of surfactant injection is necessary. Whereas, it must

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be noticed that at a higher concentration of surfactant results to stopover the effectiveness from

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the normal. Past studies have found that if the concentration of surfactant is significantly high,

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this leads to an increase in the pressure gradient and this will be in the opposite of the direction

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(Apaydin and Kovscek, 2001). Shupe, 1978 studied that anionic surfactants stabilize the

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emulsion for a numerous of crude oils at reservoirs conditions, though, not so stable and

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effective at high alkali concentration. Surfactants impact may vary depending on the condition

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and alkali which can cause the production of surfactant in-situ are cause of more concern

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(Argillier et al., 2014; Delamaide, 2015). Polymer has been used in large scale as compared to

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surfactant and is normally necessary even in surfactant based process thus its effect on

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production process has been studied (Zhu et al., 2012; Zheng et al., 2011). Due to polymer the

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viscosity of the emulsion increases, which considerably affects the separation performance of

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induced gals floatation units and hydro cyclones (Zheng et al., 2011; Argillier et al., 2014). A

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debate is in progress to decide the conclusion of the polymer on the stability of the emulsion

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(Koh, 2015; Seright, 2017).

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The current problem of the stabilization of emulsions concerns surface facilities, mainly

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because of crude separation problems, such as design of the gravity separator and the choice of

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demulsifier (Kokal and Wingrove, 2000). Further research is required on the mechanism of

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emulsion, which can express the information for the improvement of demulsifiers (Zhang et al.,

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2017). A study found the effect of the molecular weight of polymers and reservoir permeability

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assessed the performance of each molecular weight for specific reservoir permeability (Huang et

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al., 2017). An evaluation of the EOR was also performed to correlate the properties of the

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reservoir with the gradual recovery of oil with chemicals floods and another study in which the

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correlation of EOR chemicals on stability of produced emulsion at low water cut has been

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studied (Wong et al., 2015; Al-Kayiem and Khan, 2017). The present study investigates the

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impact of ASP and interactions of each additive on the sedimentation and coalescence of high

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water cut emulsion produced in the separator.

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Literature review demonstrating that coalescence and sedimentation separation profiles of

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liquid-liquid phases are performed without taking into account the impact of ASP (Jeelani and

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Hartland, 1998; Hartland and Jeelani, 1988). Coalescence of droplets is the rupturing/thinning a

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films of fluid among a couple of drops and the consequent combination of the drops. In this case

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of coalescence, total separation of the crude emulsion occurs in two separate phases (Tadros,

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2009). The phenomena of coalescence govern by many steps (Sztukowski and Yarranton, 2005;

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Frising et al., 2006). Initially the drops approach to the nearby molecular dimensions and

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followed by dimpling and distortion to create a plane interface among the droplets which

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outcomes during the continuous phase drainage to from the plane region, and forming a thin

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film. Then, the bridging among the droplets is almost quickly start and irreversible combination

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to form single drop. The phenomena of coalescence are dependent on the interaction rate of the

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droplets as well as the surface properties of the droplets (Binks and Horozov, 2006).

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Sedimentation and creaming happen autonomously from the interactions of droplets and the

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drops

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Sedimentation/creaming generally occurs due to external forces, for example, gravitational and

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centrifugal forces because of the differences in the densities between each phases. On account of

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water in oil emulsion, droplets of water settle to the bottom then, in the case of an oil in water

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emulsion, lighter drops of oil begin to cremate at the top (Tadros, 2009). The creaming /

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sedimentation formation can be suppressed by decreasing the density dissimilarity of each phase,

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by decreasing the size of the droplet or by continuously thickening the phase (Farn, 2008). An

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attempt has been made to modify these existing model to accurately measure the separation time.

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The modification was made to the sedimentation and coalescence profiles by taking different

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mixing time and mixing intensity (Aleem and Mellon, 2016). In addition, studies on the

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separation of oil in water emulsions are less numerous than those on the separation of water in

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oil emulsion (Feng et al., 2008). Recently, a separation prediction model has been explored to

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express the separation mechanism of oil in water emulsions taking into account the coalescence

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and creaming processes. However, the model was devolved based on batch separation of

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kerosene and distilled water (Aleem and Mellon, 2018).

in

the

emulsion

remains

same

(Binks

and

Horozov,

2006).

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distribution

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ASP additives play a significant role in the crude emulsion stabilization process, but the

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impact of ASP additives on separation prediction profiles have not been emphasized. The

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objective of this research is to present the modification of the coalescence and sedimentation 5

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profiles in the existing model (Jeelani and Hartland, 1998; Hartland and Jeelani, 1988). In

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addition, the interaction of each additive on sedimentation and coalescence in terms of separation

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performance have been analyzed and presented. As well as to identify the influence of the ASP

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on the separation of oil/water in the primary separator. The modification is based on

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experimental measurements that were made to study the impact of ASP injection on an oil-in-

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water emulsion. Experimental research was focused on the 60% water cut emulsion flooded by

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various ASP compositions. The measurements were made using a laser light transmission and

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backscattering from the emulsion. The coalescence and sedimentation heights have been

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measured at various time intervals up to 24 hours. In addition, experimental results were

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compared with an existing and modified models. By comparing experimental and model profiles,

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coalescence and sedimentation constants have been obtained, which correspond to the model

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data. Then, a new correlation was developed by combining all the data, which makes it possible

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to evaluate the constants for coalescence and sedimentation profile measurement.

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2.

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Turbiscan test was conducted to inspect the stability of the oil/water/ASP emulsion, and then the

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measured coalescence and sedimentation separation profiles are compared with the existing

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separation prediction model. A statistical method is carried out to find the coalescence and

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sedimentation constant for various ASP concentrations. The steps of the study are as follows.

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2.1

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The measurement of the stability of emulsion is a main test that is initiated with crude emulsion.

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It describes the capacity with which oil and water phases separates in the produced emulsion.

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The most used method by far is the bottle test. This test is performed using the laser light in the

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Turbiscan to determine stability. The scattered light in this equipment can accurately calculates

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the amount of disperse phase deposition. In the literature, studies on the stabilization of emulsion

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was carried out with transmission and backscattering of laser light through emulsion samples

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(Khan et al., 2015; Jing et al., 2016).

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2.1.1

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The material required for the emulsion are crude oil, reservoir brine, alkali, surfactant and

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polymer. ASP is being used in some Malaysian fields for Enhanced Oil Recovery purpose. In the

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METHODOLOGY

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Experimental Procedure

Materials

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combination of ASP, a weak alkali, Sodium-Carbonate (Na2CO3) is being used as it has

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significant effect in the recovery of crude production and it has less effect on corrosion in the

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production lines as compared to strong alkali. The surfactant used in this combination of ASP is

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Anionic Alpha Olefin Sulfonate (AOS). The polymer used in the combination of ASP is anionic

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hydrolyzed polyacrylamide. The amount of ASP was calculated from the chemical breakthrough

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in the primary separator. ASP used in the experiment included alkaline, Na2CO3, 5%–15%, An

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ionic surfactant, AOS, 20%–40% and an anionic hydrolyzed polyacrylamide polymer, GLP 100,

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60%–70%.

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2.1.2

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The emulsion sample was produced by mixing brine (60%) with an alkali, a surfactant and a

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polymer and tremendously mixed with oil (40%) at high shear with a disperser rotating at 12,000

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rpm for 2 minutes. It has been calculated that the shear provided to produce an emulsion in the

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laboratory is identical to the shear energy experienced by the produced emulsion in the reservoir.

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2.1.3

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The Turbiscan was used to notice the stability of emulsion. It consists of a scanning device

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composed of a source of laser. This equipment can accurately compute the separated oil and

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water phases when it is difficult to observe the clarity of water and oil phase. The phase

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separation rate is achieved by calculating the amount of light transmittance in the emulsion. The

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measuring procedure is shown schematically in Fig. 2. It shows the Turbiscan measurement

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principle of for coalescence and sedimentation in an emulsion sample.

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Measurement Method

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Emulsion Preparation

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Fig. 2. Measuring principle of Turbiscan

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2.2

Separation Profile Prediction Model

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A model has been developed in the past for predicting the separation profiles of oil-water in

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gravity separator (Jeelani and Hartland, 1998). The authors divided the sedimentation and

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coalescence interfaces into two sections each (Jeelani and Hartland, 1998). One before the

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inflection point 0

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developed by Jeelani and Hartland is only valid for pure oil and water system (Aleem and

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Mellon, 2018; Frising et al., 2006). To use the model developed by the Jeelani and Hartland for

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the ASP containing system, the model is modified and correction factors are introduced to

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incorporate the effect of ASP on the separation profile. The separation experiments performed in

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this study are used to test the accuracy of the modified model developed and the model of Jeelani

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and Hartland. The equations of the modified model for the calculation of the separation profiles

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in terms of coalescence and sedimentation are mentioned in the following section.

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2.2.1

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The height of coalescence with time before the inflection point can be predicted using the

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following equation.

. The model

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Coalescence Height/Profile

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ℎ =

−

+

Δℎ

1−

0

1

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The height of coalescence at the final time

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the point of inflection in the profile is calculated by the following equation: 1−!

+ !" #ℎ

&



%$Sedimentation Height/Profile

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ℎ =

when complete sedimentation occurs

' 2

2.2.2

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The sedimentation height/profile need to be calculated in two different intervals: 0 < t < ti

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and

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the following equation:

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ℎ) =

−

−

2

*

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. The height of sedimentation for the time interval 0 < t < ti is calculated using

) 0



3

Where V0 and Vi are the velocity of sedimentation of the droplets, initially and the velocity of

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sedimentation of the droplets at ti respectively. The change in height of the sedimentation profile

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after the inflection point, t > ti is calculated by equation (4) as modified in this study. ℎ) =

1−!

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− &1 − !, 'Δℎ

) &



%$' 4

Constant Estimation Procedure

2.2.3

217

In this procedure, a response surface method is applied to evaluate three different chemical

218

additives that influence coalescence and sedimentation phenomena in the emulsion. In order to

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perform an analysis of the experimental data, regression is carried out with response surface

220

method. This methodology was also adopted by Al-Kayiem and Khan, 2017. The second order

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polynomial used in this fitting is governed by following equation, 5;

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2

2

34

34

2 4

2

. = / + 0 / 1 + 0 / 1 + 0 0 / 5 1 15 5 *

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Where, Y represents the response output, Xi and Xj denotes independent variable. The regression

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coefficient for the intercept term, linear tem, quadratic and the interaction term are βo, βi, βii,

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and βij, respectively.

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2.2.4

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By introducing all the terms, the coalescence constant, (Kc) and the sedimentation constant (Ks)

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of an emulsion produced with ASP fluids can be calculated using Equations (6 and 7). The R2

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values of 0.89 and 0.86 shows a good fitting. By substituting the coefficients of main and

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grouped parameters in Equation (5) we get the following equations:

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K C = 0 .96 + 0 .00001 A + 0 .0001 S − 0 .0001 P

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Prediction expressions for coalescence and sedimentation constants

+ ( A − 1000 ) * (( A − 1000 ) * 0 .0000001 ) − ( A − 1000 ) * (( S − 400 ) * 0 .0000003 ) − ( S − 400 ) * (( S − 400 ) * 0 .0000003 ) + ( A − 1000 ) * (( P − 600 ) * 0 .0000003 ) +

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(6)

( S − 400 ) * (( P − 600 ) * ( 0 .00000057 ) + ( P − 600 ) * (( P − 600 ) * ( 0 .0000008 ) K S = 0 .89 + 0 .00077 A + 0 .0014 S − 0 .00022 P

− ( A − 1000 ) * (( A − 1000 ) * 0 .0000005 ) + ( A − 1000 ) * (( S − 400 ) * 0 .0000055 ) + ( S − 400 ) * (( S − 400 ) * 0 .00000066 ) + ( A − 1000 ) * (( P − 600 ) * 0 .0000008 ) −

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( S − 400 ) * (( P − 600 ) * ( 0 .0000045 ) − ( P − 600 ) * (( P − 600 ) * ( 0 .00000025 )

With the above prediction expressions, the alkali-polymer (AP) term showed statistically

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significant responses to increase the coalescence constant. The interaction term of the surfactant-

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polymer (SP), the surfactant (S) alone and the square term of the alkali has an intermediate

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significance for increasing the coalescence constant. The interaction of the alkali-surfactant (AS)

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and the presence of the polymer (P) alone showed significant responses to reduce the coalescence

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constant. The term alkali-surfactant (AS) has shown highly significant responses to increase the

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sedimentation constant. The presence of alkaline (A) and surfactant (S) terms alone also has a

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significant response to increase the sedimentation constant.

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3.

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The experiments have been carried out by taking into account the influence of various

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compositions of ASP on the separation profiles. The heights of the coalescence and the

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RESULTS AND DISCUSSION

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sedimentation of the oil-in-water emulsion (60% water cut) were calculated as a function of time

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(0-24 hours) at 60o C.

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3.1

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Fig. 3 (a-b) demonstrates the kinetics of separation obtained by laser scanning at various

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concentrations of alkali and at the fixed concentrations of surfactant-polymer. The measurements

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are made at a particular period to notice sedimentation and coalescence. The results of the

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sample scan show the reduction of light transmission over the sample height with the increase of

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the alkali concentration, as shown in Fig. 3 (b). The height of sedimentation is 22 mm at low

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alkali concentration, while the sedimentation height is reduced to 12.5 mm at 1500 ppm of alkali

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at 24 hours, as shown in Fig. 3 (a-b).

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Determination of sedimentation and coalescence heights

At low alkali concentration, around 34% of the light transmitted from the water phase, while

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about 31% of the light transmission is observed at a higher concentration of alkali. The separated

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water therefore contained more oil droplets. Light transmission from the samples shows that 83%

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water is separated at a low alkali concentration, while 38% water separation occurs at a high

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alkali concentration at 24 hours. The increase in water separation at low alkali clarifies that the

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unresolved emulsion in the mid-section of sample is unstable. The lack of stability in the

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emulsion at low concentration of weak alkali was also confirmed with back scattering

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measurements that indicates a significant decrease in the backscattering i.e. 42% to 27% at 0

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second and 5 minute, respectively. Although, at high alkali concentration, the backscattering

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percentage is marginally reduced, from 33% to 31% at 0 second and 4 minutes, respectively.

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Therefore, the height of the residual emulsion is low at low alkali concentration with respect to a

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high alkali concentration which are respectively 20 mm and 37 mm at 24 hours.

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(b)

Fig. 3. Measurement of ASP produced emulsion separation with light transmission and back-

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scattering (a) A = 500 ppm, S = 600 ppm, P = 800 ppm (b) A = 1500 ppm, S = 600 ppm, P = 800

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ppm.

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Fig. 4 shows the coalescence and sedimentation profiles of actual data obtained from laser

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scanning measurements and results from the model. The results of the present study show a

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significant stabilization of the high water cut emulsion. The stability of the emulsions and residual

275

emulsions increases with high water cuts (Wong et al., 2018). The sedimentation profiles show a

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significant difference, whereas the coalescence is slightly change in the presented case of the

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emulsion produced by ASP. The fit in the model is done with the constants and new separation

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profiles are obtained, which also show the same amount of residual emulsion as found in the

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experimental measurements [see Fig. 4 (b)].

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Experimental Data 280

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Jeelani & Hartland Model

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Proposed Model

Fig. 4. Comparison of the experimental measured coalescence and sedimentation profiles with

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the existing model and the modified model. ASP concentrations: A =1500 ppm, S = 600 ppm, P

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= 800 ppm

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Fig. 5 shows the comparison of the sedimentation heights measured from the emulsion in

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the presence of ASP with the past model and modified model. The ASP concentrations injected

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into this emulsion sample are the lowest concentrations of alkali, surfactant and polymer.

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However, there is a significant difference between experimentally measured sedimentation

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heights and previous model. The comparison of the sedimentation height with the modified

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model and the experimental measured showed a good agreement.

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Experimental Data Jeelani & Hotland Model Proposed Model

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60000

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100000

120000

Time, sec

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Fig. 5: Comparison of the experimental measured coalescence and sedimentation profiles with

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the existing model and the modified model. ASP concentrations: A = 500 ppm, S = 200 ppm, P =

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400 ppm

In this section, the concentration of ASP is increased to identify the difference in

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coalescence and sedimentation height between the experimental measurements and the past

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model. Fig. 6 shows the comparison of the sedimentation heights measured from the emulsion in

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the presence of ASP with the past model and the modified model at an increased concentration of

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ASP. The ASP concentrations injected into this emulsion sample are high concentrations of

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alkali, surfactant and polymer. Surprisingly, there is a significant difference between

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sedimentation and coalescence heights measured experimentally and in the past model.

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Sedimentation and coalescence constants are introduced into the model to predict actual

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separation. Comparison of sedimentation and coalescence heights with a modified model and

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experimental measurements showed good agreement.

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Experimental Data Jeelani & Hartland Model

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Time, sec

304 305

Fig. 6: Comparison of the experimental measured coalescence and sedimentation profile with the

306

existing model and the modified model. ASP concentrations: A = 1000 ppm, S = 600 ppm, P =

307

600 ppm

As there are inconsistent differences in sedimentation and coalescence heights measured

309

at different concentrations of ASP additives. Therefore, various ranges of ASP depending on the

310

design of the experiment were used to prepare the emulsion and to obtain sequential variation

311

and sensitivity of the emulsion at a specific additive. Table 1 shows the input parameters to find

312

the coalescence and sedimentation constants for the modification of the analytical model. A

313

statistical analysis was performed to determine the significance of each factor contributing to the

314

variation of the separation profiles. Correlations were also obtained from the batch experiments

315

to find the coalescence and sedimentation constants for various alkali/surfactant/polymer

316

concentrations.

317

3.2

318

The experiments were performed by taking into account the effect of various concentrations of

319

ASP on the separation profiles. The coalescence and sedimentation heights of the oil-in-water

320

emulsion (60 % water cut) were measured at 60oC. A statistical analysis was performed to

321

determine the significance of each factor contributing to the variation of separation profiles.

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Determination of the input parameters of the separation profiles

15

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Correlations were also obtained from the batch experiments to find the coalescence and

323

sedimentation constants for various alkali/surfactant/polymer compositions. The effect of various

324

ASP concentrations on the coalescence and sedimentation constants is presented in Table 1. A

325

statistical analysis was performed to determine the significance of each factor contributing to the

326

variation of separation profiles. Correlations were also obtained from the batch experiments to

327

find the coalescence and sedimentation constants for various ASP compositions.

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Table 1. Definitions and levels of ASP and experimental responses of constants

328

1000

Coalescence constant, Kc 1.04 0.86 1.06 1.02 1.03 1.02 0.97 1.01 0.96 0.96 0.87 0.96 0.99 0.93 0.97

Sedimentation constant, Ks 1.80 1.70 1.28 1.15 1.15 2.06 3.53 3.02 2.00 2.03 1.91 2.43 2.44 1.82 1.97

EP

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Polymer ppm 400 800 400 800 600 600 600 400 800 600 400 800 400 800 600

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Surfactant ppm 200 200 600 600 400 200 600 400 400 400 200 200 600 600 400

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Alkali ppm

Experimental Responses

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Input Parameters

3.3

Fitting the model and validation

330

The value of coefficient of determination (0.89) indicates that the proposed model is satisfactory

331

for finding the influence of the ASP on the coalescence and sedimentation of the emulsion as

332

shown in Fig. 7. Therefore, the fitting with second-order polynomial model has proved effective

333

in describing experimental responses. The model shows an agreement of the actual and predicted

334

data with high significance (P-value 0.026). Small P-values defines the significance of the

335

parameter or correlation.

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16

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(a)

338 339 340

(b) Fig. 7. Actual and predicted plot (fit of the data) (a) RMSE = 0.03, R2 = 0.89 and P-Value = 0.026

EP

342

(b) RMSE = 0.3667, R2 = 0.86 and P-Value = 0.047

The validity of the model was assessed on the base of adjustments summary as shown in

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336 337

343

Table 2. The values of determination coefficients are 0.89 and 0.86, representing a similarity

344

between model prediction and experimental data. The probabilities of the generated correlation

345

are significant (Prob.>F = 0.026 and 0.047). Thus, the proposed design of experimental method

346

has given sufficiently accurate correlation. To determine the difference between the prediction

347

and the actual data, the standard error was estimated.

348 349 17

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Table 2. Prediction model validation parameters (Adjustment summary) Source

coalescence constant

Sedimentation constant

R Square

0.89

0.86

R Square Adj.

0.73

0.66

Prob. > F

0.026

0.047

Root Mean Square Error

0.03

Mean of Response

0.97

Observations (or Sum Wgts.)

15

0.36 2

15

SC

351

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350

3.4

Interaction effect of ASP on coalescence and sedimentation constants

353

The combined effect of various ASP compositions on coalescence and sedimentation constants is

354

shown in Figs. 8 and 9. The predicted profiles represent the variation of the constants as a

355

function of the alkali, surfactant and polymer. The surfactant has been found to contribute to

356

increase coalescence and sedimentation constants. Fig. 8 shows that with the presence of alkali,

357

the concentration in the range of 1000-1500 ppm also increases the coalescence constant. The

358

average value of coalescence constant is 0.94 in the presence of 500-1500 ppm of alkali, 200-600

359

ppm of surfactant and 400-800 ppm of polymer.

360

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361

Fig. 8. Influence of various ASP concentrations on the coalescence constant in the separation

362

prediction model

363

The upsurge in the alkali and surfactant concentrations had a significant influence on the

364

sedimentation constant. The influence of the polymer is negative on the coalescence constant,

365

while it has a negligible effect on the sedimentation constant, as shown in Fig. 9. The average 18

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value of the sedimentation constant is 2.1 in the presence of 500-1500 ppm of alkali, 200-600

367

ppm of surfactant and 400-800 ppm of polymer. A minimum constant value is found at 500 ppm

368

alkali and 200 ppm surfactant.

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366

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369 370

Fig. 9. Effect of various ASP concentrations on the sedimentation constant in the separation

371

prediction model 3.5

373

Table 3 summarizes the analysis of the data regarding the effect of ASP on sedimentation of

374

water at 24 hours. It shows the estimates of the effect of process parameters on the separation of

375

water. The table displays estimates of linear terms and collective factors. P value shows the

376

variation from zero to 1 and the significant range will be 0 ≥ 0.05. The effectiveness of the alkali

377

is clearly the most significant factor (P = 0.0023) to reduce the separation of the water phase.

378

The interaction of the alkali-surfactant is also significant with P value 0.017, which decreases

379

sedimentation with a high negative effect, as predicted by the t-ratio is -2.99. The interaction of

380

the polymer-surfactant shows the significance with a P-value 0.077), having a negative influence

381

on sedimentation (t-Ratio -2.02). The interaction of the polymer-alkali as well as the polymer

382

alone has an insignificant impact on the separation.

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Effectiveness of ASP on water phase separation/sedimentation

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Table 3. Sorted parameter estimates for ASP effectiveness on sedimentation at 24 hours Term A (A-1000)*(S-400) (S-400)*(P-600) (A-1000)*(P-600) P S

Std. Error 0.005082 2.841e-5 0.000071 2.841e-5 0.012705 0.012705

t-Ratio -4.41 -2.99 -2.02 1.14 1.06 -0.91

Effect direction

Prob>|t 0.0023 0.0173 0.0776 0.2857 0.3190 0.3918 19

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Fig. 10 represents an interaction of the surfactant-alkali, the polymer-alkali and the

385

polymer-surfactant on the sedimentation at 24 hours. The large time period is taken to obtain the

386

final influence of ASP at complete sedimentation. Alkali and surfactant interaction have a

387

negative influence on water separation. The interaction effect of the polymer with the alkali is

388

insignificant on the separation. The interaction of the surfactant-alkali shows a stabilization of

389

the emulsion which leads to a less separated amount of water phase. The presence of weak alkali,

390

Na2CO3 in the emulsion triggered the carbonate ions and resulted in obtaining protons from

391

water droplets. It increases the amount of hydroxide ions therefore the pH of the solutions

392

increases. It has been explored in the past that in the presence of weak alkali, separation is less

393

problematic than that of strong alkali (Guo et al., 2017).

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384

394

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Fig. 10. ASP interaction profiles for sedimentation/water separation

395 396

3.6

Effectiveness of ASP on the oil phase separation/coalescence

397

Table 4 provides a summary of the data analysis for the ASP effect on coalescence at 24 hours.

398

The interaction of the alkali-surfactant and the interaction of surfactant-polymer have

399

significantly affected to reduce the coalescence, the significance being P = 0.023 and P = 0.033,

400

respectively. However, the effect of the polymer is also significant in stabilizing the oil droplets,

401

which resulted in a decrease in coalescence of the oil phase with a high significance (P = 0.028).

402

The significance factor shows that coalescence is suppressed with increasing polymer 20

ACCEPTED MANUSCRIPT

403

concentration. A recent study has also shown that the addition of polymer to an alkaline and

404

surfactant solution considerably improves the rheology of the ASP slug (Pal et al., 2018).

405

Surprisingly, the surfactant alone has negligibly affected the coalescence of the oil phase.

Term (A-1000)*(S-400) P (S-400)*(P-600) A S (A-1000)*(P-600)

Std. Error 3.231e-5 0.014451 8.078e-5 0.00578 0.014451 3.231e-5

t-Ratio -3.03 -2.86 -2.74 -2.30 1.83 1.82

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Table 4. Sorted parameter estimates for ASP effectiveness on coalescence at 24 hours Effect direction

SC

406

Prob>|t| 0.0230* 0.0288* 0.0336* 0.0613 0.1164 0.1194

Fig. 11 displays the effect of a surfactant-alkali, a polymer-surfactant and a polymer-

408

alkali on the coalescence/oil phase separation at 24 hours. The interaction of the additives show

409

that the alkali-surfactant has reduced the separation of the oil phase, the effect is highly

410

significant at high concentration of these additives. The interaction of the surfactant-polymer

411

also reduced the separation of the oil. The interaction effect of the polymer-alkali has

412

significantly increase the separation of the oil. The coalescence of oil is suppressing and resulted

413

in the 40% separation of the oil with the interaction of high concentration of the additives i.e. A:

414

1500 ppm-S: 600 ppm and S; 600 ppm-P: 800 ppm. Although coalescence resulted in a 72%

415

separation of the oil at A: 500 ppm-S: 600 ppm and S: 600 ppm-P: 400 ppm. The presence of

416

NaOH results in a very stable emulsion as compared to the Na2CO3 (Gregersen et al., 2013).

417

While, the interaction of strong alkali with the surfactant indicates that absorption of the

418

surfactant is improved over weak alkali at similar concentrations (Krumrine and Falcone, 1987).

419

The absorption of the surfactant is greater because of the presence of monovalent ion in NaOH,

420

while absorption declines due to the presence of divalent in weak alkali, Na2CO3. Although the

421

high concentrations of weak alkali have similar stabilizing effect like strong alkalis in the Daqing

422

field (Guo, et al., 2017).

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423

Fig. 11. ASP interaction profiles for coalescence/oil separation

424 425

CONCLUSION

427

The actual data from the experiments were compared to the existing model used in the industry

428

for the prediction of separation and the design of gravity separators. The final separation time

429

calculated by the existing model is not perfect because it has been developed without taking into

430

account the effect of the ASP. An improvement of the existing model was carried out to ensure

431

its accuracy in predicting the actual separation time in the presence of ASP. The modified model

432

was developed more accurately to predict the coalescence and sedimentation profiles. The

433

interaction of the alkali-polymer and the surfactant-polymer has a significant effect in increasing

434

the difference between the coalescence heights of the model and actual data. Conversely, it was

435

realized that the difference in coalescence heights of model and the actual data decreased with

436

increasing concentration of the polymer alone and the interaction of alkali-surfactant

437

concentrations. The difference in sedimentation height between the model and actual data

438

increases with increasing concentrations of alkali and surfactant alone, as well as the interaction

439

of the alkali-surfactant. The results of the study also indicate that alkali is the most significant

440

parameter for reducing separation/sedimentation with all surfactant and polymer compositions.

441

The presence of weak alkali stabilizes the emulsion at higher concentration. The effectiveness of

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22

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the surfactant is also increased to reduce the separation significantly at high concentration of

443

alkali. In conclusion, the presence of ASP in oil-water emulsions has increased the time for

444

complete separation. It is recommended to optimize ASP formulation injected into the reservoir,

445

particularly the alkali injection, in order to overcome the reduction of coalescence and

446

sedimentation for high water cut emulsions.

447

ACKNOWLEDGMENT

448

The authors would like to thank the Enhanced Oil Recovery Center - Universiti Teknologi

449

PETRONAS for the fund and supply of required material i.e. crude oil, reservoir brine and EOR

450

chemicals to carry out the research.

451

NOMENCLATURE

452

EOR = Enhanced Oil Recovery

453

ASP = Alkali/Surfactant/Polymer

454

Ws = Separated water, %

455

Wo = Separated oil, %

456

KC = Coalescence constant

457

KS = Sedimentation constant

458

hc = Coalescence interface height (m)

459

hci = Coalescence interface height at the point of inflection (m)

460

Ho = Initial dispersion height (m)

461

hs = Sedimentation height (m)

462

hsi = Sedimentation interface height at the point of inflection (m)

463

∆hi = Height of dense layer at the inflection point (m)

464

v = Sedimentation velocity (m/s)

465

Vo = Initial sedimentation velocity (m/s)

466

Vi = Sedimentation velocity at inflection point (m/s)

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442

23

t = time (s)

468

ti = time required to complete sedimentation (s)

469

εo = Holdup fraction of initial dispersed phase

470

ψ = Interfacial coalescence rate (m/s)

471

ψi = Interfacial coalescence rate at inflection point (m/s)

472

φ = Diameter of the drop at the coalescence interface (m)

473

φo = Diameter of initial droplet (m)

SC

467

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474 475

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Highlights •

Influence of EOR chemicals on coalescence and sedimentation of the oil / water emulsion in the primary separator. An existing model for estimating the coalescence and sedimentation heights of the emulsion has

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•

been modified with better accuracy. •

The interaction and significance of the alkaline-surfactant, the surfactant-polymer and the alkali-polymer on the coalescence and sedimentation of the oil-in-water emulsion are

The sedimentation height decreases significantly with increasing concentrations of alkali and

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surfactant alone, as well as the interaction of alkali-surfactant.

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elaborated.