A review of petroleum emulsions and recent progress on water-in-crude oil emulsions stabilized by natural surfactants and solids

A review of petroleum emulsions and recent progress on water-in-crude oil emulsions stabilized by natural surfactants and solids

Accepted Manuscript A review of petroleum emulsions and recent progress on water-in-crude oil emulsions stabilized by natural surfactants and solids A...

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Accepted Manuscript A review of petroleum emulsions and recent progress on water-in-crude oil emulsions stabilized by natural surfactants and solids Abubakar Abubakar Uma, Ismail Bin Mohd Saaid, Aliyu Adebayo Sulaimon, Rashidah Bint Mohd Pilus PII:

S0920-4105(18)30198-0

DOI:

10.1016/j.petrol.2018.03.014

Reference:

PETROL 4755

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 15 April 2017 Revised Date:

15 September 2017

Accepted Date: 2 March 2018

Please cite this article as: Uma, A.A., Saaid, I.B.M., Sulaimon, A.A., Pilus, R.B.M., A review of petroleum emulsions and recent progress on water-in-crude oil emulsions stabilized by natural surfactants and solids, Journal of Petroleum Science and Engineering (2018), doi: 10.1016/j.petrol.2018.03.014. 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.

ACCEPTED MANUSCRIPT

Available online at www.sciencedirect.com

Journal of Petroleum Sciences and Engineering

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Abubakar Abubakar Umara, Ismail Bin Mohd Saaid*b, Aliyu Adebayo Sulaimonc, Rashidah Bint Mohd Pilusd

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Abstract

During production of conventional as well as heavy oils, emulsions occur at well bore, in pipelines, and at surface facilities. As the production time of the oil wells increases, there will be an increased coproduction of oil and water in the form of emulsions. These emulsions are undesirable because they result in high pressure drop due to their high viscosity. They also cause serious corrosion problems due to presence of chlorides dissolved in water. These emulsions must be treated to meet production and transportation requirements, and to maximize the overall profitability of the crude oil production. Apart from the undesirable emulsions, the petroleum industry is witnessing a surge in the application of emulsions for beneficial processes. Both cases would be discussed in this review. This subject has experienced an extensive research over the years, with highly complicated theories regarding the phenomena involved in its formation (emulsification) and breaking (demulsification). Crude oils, irrespective of their origin, contain certain components or characteristics which tend to make them emulsifiable. These crude oil components are referred to as emulsifiers, and they vary so widely with the nature of the crude oil. The natural interfacially active components responsible for emulsion stability undoubtedly come from the resin and asphaltenes of the crude oil. However, the presence of other solids like crystalline waxes, clays, corrosion products and mineral scales may lead to the formation of very stable water-in-oil (w/o) emulsions. The nature of these particles controls the type as well as the stability of emulsions produced. Apart from the natural emulsifiers, chemical enhanced oil recovery (EOR) techniques have been reported to produce stable w/o and o/w emulsions. It is believed that the alkali, surfactant and polymers used in these techniques are responsible for these stable emulsions. When they form, these emulsions increase pumping costs, heighten the chances of pipeline and equipment erosion, corrosion rate, scaling and lower the produced oil API gravity. These emulsions have to be treated to remove the dispersed water and accompanying inorganic salts in order to meet market specifications, transportation requirement and to reduce corrosion and catalyst poisoning in downstream processing. Despite the huge and concerted efforts by researchers from the academia and the petroleum industry, there are few fundamental and applied investigations into the roles of native solids in combination to natural emulsifiers on the stabilization of petroleum emulsions. This paper presents a comprehensive overview of the progress made in the field of petroleum emulsions, principally the roles of particles in combination to asphaltene and resin in stabilizing w/o emulsions. The study also charts a way for emulsions studies that could lead to an effective demulsification via thorough characterizations of the solids believed to the enhancers of emulsion stability in order to tailor demulsifiers based on the characteristics of such emulsifiers.

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Department of Petroleum Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak Malaysia

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A Review of Petroleum Emulsions and Recent Progress on Waterin-Crude Oil Emulsions Stabilized by Natural Surfactants and Solids

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Keywords: Water-In-Oil Emulsions (W/O); Oil-in-water (O/W); Pickering, Emulsifiers; Demulsifiers; solid particles; Surfactants.

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1. Introduction The process of stabilization of oil and water droplets by solid particles has been acknowledged for over a

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century. Currently, these particle-stabilized emulsions are largely known as Pickering emulsions. They occur in

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personal care products, the food industry and have long been used in oil recovery and mineral processes

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(Bernardini 2015). These are industries where such emulsions are desirable for achieving certain characteristics of

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the products. In the oil and gas industry however, the formation of emulsion during oil production is a pricy

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problem, both in terms of chemicals used and the production lost (Kokal 2005). These emulsions have to be treated

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to remove the dispersed water and accompanying inorganic salts in order to meet market specifications,

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transportation requirement and to reduce corrosion and catalyst poisoning in downstream processing (Kokal and

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Wingrove 2000). Surfactants with small molecular weight or amphiphilic polymers have long been employed in

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certain industries in emulsion stabilization, either by reducing interfacial tension or forming a viscoelastic

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interfacial film. Although these surfactants have been well understood and are widely in use, they are not the only

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potential sources of stabilization of emulsions. Colloidal particles with suitable wettability partially in both the

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dispersed and continuous phases can function as Pickering-type stabilizer by forming a physical barrier at droplet

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interface. This phenomenon is discussed in detail in section 6.2 of this review paper.

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Several other researches have shown that asphaltenes are the prime stabilizers of water-in-oil emulsions and that

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resins are needed to solvate the asphaltenes (Fingas, Fieldhouse et al. 1999, Fingas and Fieldhouse 2003, Fingas

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and Fieldhouse 2004, Fingas 2014). Certain studies however found out that a synergy in stabilization occurs when

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asphaltenes and fine solids jointly stabilize an emulsion based on a certain fractional area ratio of 2:1 of asphaltenes

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to solids (Sztukowski and Yarranton 2005). In a similar work, (Bobra, Fingas et al. 1992) established that waxes

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cannot act as emulsifiers by themselves, but can stabilize emulsions in combination with asphaltenes or resins.

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Thus, according to their findings, a concentration of 0.01g/ml of asphaltenes did not produce a stable w/o emulsion,

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but when 0.05g/ml of wax added, stable emulsions were formed. However, an asphaltene concentration of 0.03g/ml

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without the presence of wax produced a stable emulsion. These findings were repeated by the authors of this paper,

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and the results found corroborated this result.

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2. The Emulsion Problem The problem of separating water from produced crude oil is as old as the oil industry itself. At the beginning of

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the oil industry, the water separation problem was handled by settling the free water from oil in open tanks or pits

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and the sludge (an intermediate phase between clean water and clean oil) was disposed of, ordinarily by burning

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(Meyer 1964). It was not more than a century ago that attention was drawn to the fact that “sludge” is an emulsion

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of crude oil and water, and that substantial amount of merchantable oil can be recovered from the emulsion (API

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1961). By crude oil emulsions, we are referring to water-in-oil (W/O) emulsions because most emulsions are this

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type (Kokal and Wingrove 2000). Although oil-in-water (O/W) emulsions also form and are encountered in the

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industry, they are generally resolved in the same way W/O emulsions are resolved, except electrostatic treaters

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cannot be used on O/W emulsions (Kokal 2005). At the time when crude oil and water are leaving the wellbore of

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an oil well, tight w/o emulsions can form due to the turbulence in the choke valve at the wellhead (Janssen, Noïk et

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al. 2001). The formation of emulsion during crude oil production is a very costly operational problem. It occurs

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when hydrocarbon and formation water in the reservoir and in production pipes are extremely mixed under

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shear/turbulence, and in the presence of surface active agents (Fingas, Fieldhouse et al. 1999, Opawale and

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Osisanya 2013) (Ngai and Bon 2014).

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The continuous phases of these emulsions depend on the water to oil ratio, the natural emulsifier systems contained

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in the oil, and the origin of the emulsion. The emulsifiers are complex chemically, and they come in different

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shapes and sizes. As new oil fields are developed and as production conditions change in older fields, there is a

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constant need for new, effective demulsification methods. The emulsion must be separated before the crude oil can

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be accepted for transportation, to meet the residual salt and water content quality criteria for a delivered crude oil.

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The water content must be less than 1% (Fink 2015).

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2.1. Definitions of Emulsions Emulsions are thermodynamically unstable systems, since they will separate to reduce the interfacial area

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between the oil phase and the water phase, as a function of time (Sjoblom 2001). Emulsions are metastable systems

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typically formed in the presence of surfactant molecules, amphiphilic polymers or solid particles. The relative

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balance of the hydrophilic and lipophilic properties of these emulsifiers is known to be the most important

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parameter dictating the emulsion type: oil-in-water (O/W) emulsions are preferentially obtained with molecules

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which are rather hydrophilic whereas water-in-oil (W/O) emulsions are produced in the presence of hydrophobic

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molecules (Leal-Calderon and Schmitt 2008).

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(Manning and Thompson 1991) defined emulsion as a quasi-stable suspension of fine drops of one liquid in

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another liquid. (Roberts 1926) defined emulsion as a system containing two liquid phases, one of which is

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dispersed as globules in the other. Other researchers defined emulsion as a mixture of two mutually immiscible

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liquids, one of which is dispersed as very small droplets in the other, and is stabilized by an emulsifying agent

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(Aziz, Darwish et al. 2002) (Singh, Thomason et al. 2004) (Kokal 2008).

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Another definition of oil field emulsions was proposed by (Roberts 1926). According to his work, he maintained

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that oilfield emulsions vary from extremely unstable types, which should more accurately be called suspensions, to

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extremely stable ones. Based on that, he classified emulsions into three classes, according to their behavior in the

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hand centrifuge. These are: (a) Emulsions that show only clear oil and; and are better referred to as suspensions,

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thus if allowed to stand will generally separate into their different phases without any form of treatment. However,

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certain unstable emulsions occur which are capable of resolution in the centrifuge, especially when diluted with

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gasoline, but which will not settle to oil and water without any form of treatment. (b) Emulsions that show the

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emulsion phase, with or without water, and a clear oil phase. These are real emulsions and must be treated to

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recover the emulsified oil (c) those that may or may not show emulsion and water phases, but which also show

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cloudy oil after centrifuging.

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Something common to all the definitions provided in this review and many others not stated here is the fact these

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emulsions are thermodynamically unstable and separate into two phases if allowed to sit for a long time (Singh,

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Thomason et al. 2004). These emulsions, which fall under macro-emulsions (having dispersed phase diameters

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greater than 0.1µm) are said to be thermodynamically unstable systems because the contact between the oil and

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water molecules is unfavourable, and so they will always break down over time. There has been more

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comprehensive studies and a lot has been known about the formation and stabilization of oil-in-water emulsions

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than of the water-in-oil emulsions type (Oliveira and Goncalves 2005). To understand the W/O emulsions, more

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information is needed on the materials responsible for their formation and stabilization, and especially how solid

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particles form or enhance their stabilizations.

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2.2. Classifications of Emulsions

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Decades of far-reaching research work on water-in-oil emulsions (often called “chocolate mousse”) that form

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after oil spill, (Fingas and Fieldhouse 2009) found that four classes of emulsions form when crude oil mixes with

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water. Among the leading studies on classification of crude oil emulsion according to their stability are the works

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of (Fingas, Fieldhouse et al. 1999, Fingas and Fieldhouse 2003, Fingas 2014, Fingas and Fieldhouse 2014) who

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proposed new empirical data and corresponding physical knowledge of emulsion formation. Based on their studies,

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the density, viscosity, saturates, asphaltene, resins and fine solids were used to propose an emulsion type

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classification index which gives either an unstable, entrained, meso-stable or stable water-in-oil class of emulsion.

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From the four classes, only stable and meso-stable states can be considered as emulsions. It is assumed that the

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stability derives from the tough visco-elastic interface, triggered by asphaltenes and resins. Mesostable emulsions

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are the emulsions between stable and unstable emulsions. It is thought that meso-stable emulsions lack sufficient

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quantities of asphaltenes to render them completely stable. The meso-stable emulsions may degrade to form layers

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of stable emulsions. Given the oil and water phases, the type of emulsion formed depends on several factors. As a

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rule of thumb, when the volume fraction of one phase is very small compared with the other, the phase that has the

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smaller fraction is the dispersed phase and the other is the continuous phase. When the volume-phase ratio is close

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to 1 (a 50:50 ratio), then other factors determine the type of emulsion formed (McClements 2008) (Kokal and

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Wingrove 2000). Although emulsions are defined as stable dispersion of one liquid in another, not every mixture

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or dispersion of water in oil is an emulsion. For a dispersion to qualify as an emulsion, it has to be a stable

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dispersion (Bansbach 1965).

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Figure 1 (a) Water-in-Oil Emulsion

(b) Oil-in-Water Emulsion (Nalco)

Based on this, (Bansbach 1965) classified emulsions as Tight and Loose. Tight emulsions are those emulsion

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characterized by very small sizes of the dispersed phase, while a relatively larger dispersed phase droplet

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characterizes loose emulsions. According to (Bobra 1992) (Meyer 1964), the type of emulsifying agent determines

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the type of emulsion that would form, either w/o or o/w. If the emulsifying agent is more favorably wetted by the

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oil phase, the contact angle between the oil-water-solid boundaries, Ɵ, is greater than 90o and a w/o emulsion

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forms. However, if the water phase more favorably wets the particle, then Ɵ is <90o and an o/w emulsion will

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form. If the contact angle is much greater or much lesser than 90o, the emulsion will be unstable. According to

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(Wang and Alvarado 2011), Stable emulsions form when the contact angle is near 90o As a rule of thumb, the

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continuous phase of the emulsion is normally the one in which the particles are preferentially dispersed.

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(Fingas, Fieldhouse et al. 1999, Fingas and Fieldhouse 2003, Fingas 2014) classified emulsions into stable,

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mesostable, entrained and unstable. Each of these emulsion types has unique characteristics and is believed to be

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non-convertible to other types once formed.

(Kokal 2008, Fink 2015) in separate studies upheld that w/o emulsions form as a result of asphaltene and resin

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surfactant behaviors in oil of moderate viscosities (50-2000mPa.s) and that oil field emulsions are sometimes

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classified based on their degree of kinetic stability as Loose emulsions; those that will separate within a few

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minutes, medium emulsions; that will separate in approximately ten minutes; and tight emulsions; that will

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separate within hours, days, or even weeks, and even then, not completely.

Table 1 Some desirable and undesirable emulsions in the petroleum Industry (Schramm 2000).

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Occurrence

Usual Type

Well-head Emulsions Fuel Oil Emulsions (Marine)

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Undesirable Emulsions

Oil Flotation Process (Froth) emulsions

Oil Sand Flotation Process (Diluted froth) Oil Spill Mousse Emulsions

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Tanker Bilge Emulsions

W/O W/O

W/O and O/W O/W/O W/O O/W

Desirable Emulsions

Heavy oil pipeline emulsions

Oil Flotation Process froth emulsions Fuel-oil emulsion (70% heavy oil)

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Emulsion Drilling Fluid (Oil-Base Mud) Asphalt Emulsions

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Enhanced Oil Recovery in-situ Emulsions

O/W O/W O/W W/O O/W O/W

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Earlier, (Surfluh 1937) has classified emulsions as temporary and permanent. While a temporary emulsion will

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break down into oil and water by settling methods, a permanent emulsion would remain stable until it is treated

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effectively. Any method of preventing the formation of emulsions in oil and water mixtures must either reduce the

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degree of agitation or must employ the use of chemicals to produce physicochemical changes which will aid in

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emulsion prevention (Becker 1997). (Singh, Thomason et al. 2004) (Becker 1997) classified emulsions based on the size of the dispersed phase.

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When the dispersed droplets are larger than 0.1 µm, the emulsion is referred to as macro-emulsion. Emulsions of

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this kind are normally thermodynamically unstable (i.e., the two phases will separate over time because of a

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tendency for the emulsion to reduce its interfacial energy by coalescence and separation). Emulsions are called

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Micro-emulsions when the dispersed phase droplet sizes are less than 0.1µm. Micro emulsions are transparent and

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can occur both as water-in-oil, or oil-in-water.

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Technically speaking, micro-emulsions differ from macro emulsions in several ways (McClements 2008)

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(McClements 2015), while there exist a direct oil-water contact at the interface of a macro emulsions, such direct

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contact is not present in micro emulsions. Also, macro emulsions are cloudy colloidal systems, while micro

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emulsions are optically transparent (isotropic).

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2.3. Crude Oil Emulsions formed during Enhanced Oil Recovery (EOR) Processes

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Enhanced Oil Recovery (EOR) technique has always been a subject of interest in the oil and gas industry. Prior to

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the fall in oil price which started June 2014, the high oil prices and energy demand all over the world has

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necessitated the needs for Enhanced Oil Recovery (EOR) methods. Of recent, EOR techniques are getting more

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attention by many countries since energy crises are getting worse and frightened. One of the reasons for this is due

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to the shortage of current oil resources and difficulties in finding new oil fields all over the world. EOR has been

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classified into five (5) categories, with general intent of reducing the mobility ratio between injected and in-situ

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fluids, eliminating or reducing interfacial tension or doing both simultaneously (Sheng 2014, Standnes and

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Skjevrak 2014, Talebian, Masoudi et al. 2014). Application of EOR technology gives an additional chance to get

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out more oil from the reservoir, possibly about another 20 - 40%. These classes are Mobility-control, chemical,

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miscible, thermal and other processes such as microbial. Despite the recorded successes, EOR processes are always

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accompanied with varying problems. The formation of strong emulsions and excessive formation of silicate scales

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(Umar and Saaid 2013) especially with the application of high concentration of alkali.

200 (Li, Lin et al. 2005) studied the effect of alkaline–surfactant–polymer (ASP) flooding using sodium

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hydroxide as the alkali constituent to enhance oil recovery of an onshore oilfield in Daqing, China. Although it has

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increased the oil recovery, it has also created a new problem for the industry. Although the crude oil is paraffinic

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(contains very little asphaltene), the alkali added formed stable w/o emulsion. The study reveals that the sodium

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hydroxide solution reacts with fatty acids in the aliphatic fraction of the crude oil and/or with the fatty acids formed

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from the slow oxidation of long chain hydrocarbons, and form soap like interfacially active components. These

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accumulate at the crude oil–water interface and contribute to the stability of the oil/water emulsion.

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(Li, Xu et al. 2007) investigated the effects of HPAM on crude oil/water Interfacial properties and the

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stability of petroleum emulsions formed by Gudong crude oil. The investigation was conducted via measurement

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of interfacial shear viscosity, interfacial tension, Zeta potential, and emulsion stability. They found out that HPAM

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has the ability to adsorb at the interface between the oleic phase and water without decreasing the interfacial

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tension. Increasing the HPAM concentration however, leads to increase in the interfacial shear viscosity, Zeta

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potential, and stability of the emulsion.

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(Abidin, Puspasari et al. 2012) in a comprehensive review of polymers used in EOR processes believe that

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there is an immense optimism that the use of polymer may play a significant role in resolving the current energy

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crisis since its applications in some EOR fields has shown some successes to recover more than 20% additional oil

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from OOIP. However, HPAM one of the most common polymers used in the EOR so far, was found to enhance the

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stability of o/w emulsions and makes the water treatment difficult (Li, Xu et al. 2007).

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(Ahmadi and Shadizadeh 2012) investigated the implication of adsorption equilibrium when different

types of nanosilica and Zyziphus Spina Christi, a novel surfactant, were combined in aqueous solutions for EOR

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and reservoir stimulation purposes. The study employed a conductivity technique to evaluate the adsorption of the

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surfactant and nanosilica in the aqueous phase. Batch experiments were used to understand the effect of adsorbent

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dose on sorption efficiency as well. The results from this study can help in appropriate selection of surfactants in

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the design of EOR schemes and reservoir stimulation plans in carbonate reservoirs.

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chemical nature of the emulsions produced by Da Qing crude oil (paraffinic crude oil- that contains very little

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asphaltene and very low acid number). In this chemical flood where sodium hydroxide was used as the alkaline

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component in the recovery of crude oil, production was enhanced but the recovered oil was accompanied by a

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severely stabilized water-in-crude-oil emulsion. Certain studies however pointed to the fact that neither the

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surfactant nor polymer are responsible for the stabilization of the w/o emulsion.

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In a series of articles, Ahmadi and co-workers (Ahmadi and Shadizadeh 2012, Ahmadi and Shadizadeh

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2013, Ahmadi and Shadizadeh 2013, Ahmadi and Shadizadeh 2015, Ahmadi and Shadizadeh 2016) have

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conducted several studies ranging from estimation of adsorption behaviour of surfactants with nanosilica,

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adsorption of solid surfaces like carbonate reservoirs, adsorption of new plant derived surfactants on quartz among

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others. Results from these studies can help in making the right selection of surfactants in the design of chemical

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EOR schemes and reservoir stimulation plans in carbonate reservoirs. Also, the studies presented economically

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viable and environmentally friendly options for use in EOR techniques, particularly chemical flooding. Also, the

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studies are very helpful in understanding the mechanism of surfactant loss into sandstone reservoirs.

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2.4. Some Industry Applications of Emulsions- Desirable Emulsions

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Despite the numerous posed to the oil and gas industry by the formation of emulsions during crude oil

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production, emulsions and other materials like foams, have been used as mobility control or diverting agents in

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different EOR processes and many other useful applications in the oil and gas industry, food industry, construction

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industry, among others (Islam and Ali 1989, Israelachvili 1994).

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(Mendoza, Thomas et al. 1991) studied the effect of injection rate on emulsion flooding for a Canadian and a

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Venezuelan crude oil. The study was conducted using a porous media consisting of crushed Berea sandstone

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packed in 5x30cm (diameter x length). The study employed a Lloydminster crude oil 16.4° API and 12o API

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Morichal crude oils. The brines used for the runs and for the preparations of the emulsions had sodium chloride

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concentrations of 3.3 and 7.5% by weight, respectively. The emulsions were prepared by adding 0.04% and

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0.0004% by weight sodium hydroxide with pH = 12 and 10 respectively, to mixtures of crude oil and water. The

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study revealed that water driven emulsion flooding may offer a viable alternative to thermal recovery of

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moderately viscous oils.

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(Abdul and Ali 2003) carried out a study to examine the effective techniques that can better water-flood bottom

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water reservoirs using polymer and emulsion as mobility control and/or blocking agents. In the provinces of

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Alberta and Saskatchewan certain light and moderately heavy oil reservoirs have a high-water saturation zone in

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connection with the oil zone. Using a conventional water-flood to produce from such reservoirs gives poor

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performance. This is attributed to insufficient and incomplete sweep of the reservoir by the injected water, which

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tend to move to the producing wells via those portions of the reservoir that have higher permeability. This leads to

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low recovery. In this study, polymer was used to control the movement of water in the oil zone while the emulsion

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was used to block the injected water from routing into the bottom water zone. The study found out that when

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producing from reservoirs with water leg, the use of 10% quality oil-in-water emulsion as a blocking agent and

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polymer solution as mobility control agent is the most successful strategy.

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(Mandal, Samanta et al. 2010) investigated the efficiency of o/w emulsions as a displacement fluid during EOR

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process. In the study, they used synthetic emulsions prepared by gear oil, and experiments were conducted using

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sand pack flooding tests to observe the efficiency of the emulsion as displacing fluid. They found a substantial

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additional recovery (more than 20% of original oil in place) over conventional water flooding.

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(Ashrafizadeh, Motaee et al. 2012) in a study of emulsification of heavy crude oil by surfactants reported

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several findings on the application of emulsions in the oil industry. He reported the work of (Kessick and Denis

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1982) on pipeline transportation of heavy crude oil. According to them, conventional pipelining is not suitable for

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transporting heavy crudes from the reservoir to the refinery because of the high viscosities involved. This requires

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alternative transportation techniques. (Saniere, Hénaut et al. 2004) in their study outlined several alternative

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transportation methods that have been proposed. Among the techniques proposed, transporting such viscous crudes

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as concentrated o/w emulsions is believed to be one of the most favorable ones (Poynter and Simon 1970, Marsden

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and Raghavan 1973, Sifferman 1981).

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Water-in-diesel emulsions (WiDE) has been studied and applied as fuel for regular diesel engines for the

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reductions in the emissions of nitrogen oxides and particulate matters, which are both hazardous to our health, and

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reduction in fuel consumption due to better burning efficiency (Lif and Holmberg 2006). This leads to

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improvement in combustion efficiency when water is emulsified with diesel as a result of the micro-explosions,

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which assist atomization of the fuel. Several studies (Lin and Wang 2003, Abu-Zaid 2004, Lif and Holmberg 2006,

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Ghannam and Selim 2009, Alahmer, Yamin et al. 2010, Alahmer 2013, Fahd, Wenming et al. 2013, Ithnin, Noge et

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al. 2014) have been conducted on the viability of diesel emulsion as an alternative fuel. Most of the studies pointed

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to the fact that thermal efficiency is increased by using WiDE fuel compared to clean diesel fuel. Most of the

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studies also agree that WiDE result in improvements in brake power, torque and specific fuel consumption

286

measurements when the total amount of diesel fuel in the emulsion is compared with that of the neat diesel fuel.

287

3. The Emulsification Process

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Crude oil emulsions form when oil and brine come into contact with each other, with the influence of sufficient

289

mixing, and in the presence of an emulsifying agent or emulsifier. The amount of mixing and the presence of

290

emulsifier are critical for the formation of an emulsion (Kokal and Wingrove 2000, Herrera 2012). Several sources

291

of mixing are available during the process of crude oil production, a factor frequently referred to as the amount of

292

shear. These include; Flow through reservoir rock, bottom-hole perforations/pump, flow through tubing; flow lines,

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and production headers, valves, fittings, and chokes, surface equipment, gas bubbles released because of phase

294

change etcetera (Fingas 1995, Kokal and Wingrove 2000, Langevin, Poteau et al. 2004) as indicated with letters A

295

to F on Fig. 2.

296

(Fingas, Fieldhouse et al. 1998, Langevin, Poteau et al. 2004, Fingas and Fieldhouse 2009) studied different

297

emulsions and opined that the amount of mixing depends on several near-unavoidable factors. High speed agitation

298

and shear causes vigorous mixing of oil and water and leads to smaller dispersed droplet sizes that are more stable.

299

This is, however as a result of the increased energy transferred for the break-up process which eventually lead to

300

small droplets and more stable emulsions. The sources responsible for this agitation may be present between the

301

time at which the oil enters the well and the time when the produced phases are separated at the surface (Jackson,

302

Harrington et al. 2012). Undoubtedly, certain methods of production contribute to the formation of emulsions.

304 305

Figure 2 A schematic diagram of crude oil flow from the reservoir to the storage tanks.

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Although, there seems to be no universal theory that has been postulated for all emulsions, several theories

307

have been suggested to explain variations in emulsions formation processes (Lowe 1955).

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308

According to (Clayton 1923, Becher 1988, Schramm 1992, Bhardwaj and Hartland 1994, Binks 2002, Fingas

309

2014, Umar, Saaid et al. 2017), many factors play different roles in the stabilization of emulsions, but the

310

significance of such roles vary even as they combine in a single emulsion. (Clayton 1923) presented various

311

theories for emulsions. We would mention few of such theories here in section 3.1.

All crude oils have four main constituents belonging to four broad classes of compounds. These are classified

313

as alkanes (also called saturates or aliphatics), aromatics, resins, and asphaltenes, SARA components. The

314

lower-molecular-weight compounds in crude oils are generally alkanes and aromatics, while Asphaltenes,

315

resins, and waxes (which are high-molecular-weight alkanes) account for the higher-molecular-weight

316

compounds. In a complex mixture like petroleum, all these compounds interact in such a way that all

317

components are maintained in the liquid oil phase. In other words, the lighter components of the oil act as

318

solvents for the higher molecular-weight compounds. As long as this solvency interaction is maintained in the

319

oil and thermodynamic conditions remain constant, the oil will remain stable. Should this equilibrium state be

320

changed, a point will be reached where the solvency strength of the oil is insufficient to maintain the heavy

321

components in solution, and as a result, they will precipitate out as solid particles. This is a frequent and

322

problematic occurrence during petroleum production, transportation, and storage (Griffith and Siegmund 1985,

323

Kawanaka, Leontaritis et al. 1989, Bobra 1991).

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3.1. The phase-volume theory.

325

This theory holds that, if small spheres of the same diameter are packed as closely as possible into a given space,

326

they will occupy 74·048 per cent of the available volume, irrespective of the size of the spheres This fact was

327

employed by Ostwald as the basis for a theory of emulsion, generally referred today as the "phase-volume theory."

328

According to Ostwald, 2 two types of emulsions are only possible over a certain range of concentration and that an

329

emulsion of one liquid in another was only possible when the volume concentration of the dispersed liquid was less

330

than 74 per cent, the double series being possible only over the range of 25·96 percent to 74·04 percent by volume.

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331 332 333

1.1. The Hydration Theory of Emulsions

334

liquid which would form the continuous phase is all used in the formation of a hydrated compound of the

335

emulsifying agent employed. Thus, substances such as acacia, soap, gelatin, casein, dextrin, and albumens are

336

considered to act as emulsifiers in virtue of their ability to form colloidal hydrated compounds. The emulsifying

337

efficiencies of these substances vary, since their "hydratability" varies qualitatively and quantitatively.

338

It is postulated by this theory that oil cannot be emulsified in a hydrated colloid until a certain minimum amount of

339

water is present; correspondingly, too much water presence (exceeding the amount used in hydrating the colloid),

340

makes the formation of stable emulsion impossible. It is reasonably accurate to emphasise the importance of

341

hydrophilic colloids in forming oil-in-water emulsions, but it is only reasonable to extend this and debate that oil-

342

soluble colloids (hydrophobic colloids) promote the formation of the water-in-oil type of emulsions.

343

3.2. Oriented wedge theory.

344

This theory has been developed from the work of Langmuir and Harkins (Clayton 1923). It postulates the manner

345

in which emulsions are stabilized. The theory is established upon the perception that the molecules of the

346

emulsifier orientate themselves in the interface between the dispersed and continuous phases, forming a wedge,

347

whose curvature determines the size of the dispersed phase.

348

3.3. The Adsorbed Film and Interfacial Tension Theory

349

At present, the Interfacial tension theory is probably the most universally accepted theory of emulsions formations.

350

Several works done by (Quincke 1889); in which he created emulsions from different oils in solutions of NaOH or

351

gum Arabic. He found out that the interfacial tensions between the oils and these solutions were lower than those

352

between the oils and pure water. Previous works (Langmuir 1917, Clayton 1923) had shown that oils containing

353

free fatty acids result to better emulsions in dilute solutions of borax or sodium carbonate than those created by

354

purer oils. (Quincke 1889), commenting on such works, recommended that the simplicity of emulsification differ

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According to this theory proposed by Fischer (Finkle, Draper et al. 1923), emulsions can only be created if the

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with the acidity and viscosity of the oil, the concentration of the alkaline solution, and the solubility in water of the

356

resulting soap. (Clayton 1923) holds that with this theory "emphasis is laid upon the fact that emulsification is

357

influenced by (1) the mass of the emulsifying agent present, (2) the ease with which this agent is adsorbed at the

358

interfacial separating surface, and (3) the nature of the ions adsorbed by the resultant film."

359

4. Mechanism of Emulsification

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Emulsification is a process of agitating two or more immiscible liquids, which result in heterogeneous systems,

361

consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets, whose

362

diameters, generally exceeds 0.1 µm (Baloch and Hameed 2005). The emulsification process comprises of a certain

363

number of diverse chemical and physical processes and mechanisms, with many theories out forth to justify how

364

different emulsions are stabilized by the emulsifying agents. The emulsification history can begin right inside the

365

reservoir where the crude oil and water comingle and squeezed through constricted pores. When the crude oil is

366

produced from the well-head to the manifold (as shown in Fig. 2), there is usually a considerable pressure decrease

367

with a pressure gradient over chokes and valves where the mixing of oil and water can be intense (Sjöblom, Aske

368

et al. 2003).

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As earlier discussed in the definitions of emulsions, they are thermodynamically unstable material systems

370

formed by at least two immiscible liquid phases, with one dispersed in the other(s). When such emulsions separate

371

into their different phases, there is reduction in the free energy of the system as a result of the large decrease in

372

interfacial area. However, the presence of a third component (referred to as a surfactant) in the erstwhile unstable

373

system makes the spontaneous formation of thermodynamically stable dispersions (Shahidzadeh, Bonn et al. 1999,

374

López-Montilla, Herrera-Morales et al. 2002). Two forms of emulsification processes are encountered and have

375

been reported in the literature; (a) Spontaneous emulsification and (b) self-emulsification.

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4.1. Spontaneous Emulsification

377

On the one hand, also called “True” Spontaneous emulsification, and it ensues when two immiscible liquids are

378

brought together, and they emulsify without the application of any form of external energy. The emulsification

379

may last for few minutes, or several days depending on the nature of liquids involved (Shahidzadeh, Bonn et al.

380

1999).

381

4.2. Self-Emulsification

382

On the other hand, emulsification in the industry is habitually accomplished with the aid of appropriate surface-

383

active agents, and is commonly called 'self-emulsification', although the emulsification process is assisted by

384

providing mechanical energy of some form, such as slight shaking, mixing (5) or sonication. In the case of self-

385

emulsifying systems, the free energy required to form the emulsion is either very low and positive or actually

386

negative (i.e., the formation is thermodynamically spontaneous) (Craig, Barker et al. 1995, Shahidzadeh, Bonn et

387

al. 1999).

388

5. Conditions necessary for Emulsion formation

389

All crude oils, whatever their origin contains certain characteristics which would likely make them emulsifiable

390

(Bansbach 1965). For emulsions to form, three conditions must be satisfied (Smith and Arnold 1992). These

391

conditions are (a) the two liquids forming the emulsion must be immiscible, (b) there must be sufficient agitation to

392

disperse one liquid as droplets in the other, and (c) the presence of an emulsifying agent (Becher 1988, Bobra,

393

Fingas et al. 1992, Smith and Arnold 1992, Fingas 2014).

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According to Hany et al., (Aziz, Darwish et al. 2002), for an emulsion to form, the system must have the

395

presence of water (brine), crude oil and sufficient agitation. (Becker 1997) documented that the formation of

396

emulsions requires; differences in solubility between the continuous phase and the dispersed phase, the existence of

397

intermediate agents having partial solubility in both phases and the presence of an energy source or sources,

398

sufficient enough to mix the phases.

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5.1. The Emulsifiers

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Also, of paramount importance are the types of emulsifying agents or simply called emulsifiers. Emulsifiers are

401

associated with the produced crude oil. Since the nature and compositions of crude oils vary so widely, there exists

402

also a great variety of crude oil emulsifiers (Meyer 1964). To formulate concentrated stable emulsions, either oil-

403

in-water or water-in-oil type, a third substance is required apart from the two liquids. This substance is called an

404

emulsifying agent, or simply an emulsifier. The nature of the emulsifying agent determines what type of emulsion

405

forms (Clayton 1923). Thus, the lastingness (known as stability) of the emulsion is dependent upon the

406

rigorousness of the agitation and upon the emulsifying agents (Surfluh 1937).

407

(Roberts 1926) hold that the emulsifying agent responsible for the formation of petroleum emulsions is not

408

categorically known. However, in a sizable number of cases, it is believed to be colloidal asphalt, which includes

409

all asphalts and similar substances which occur in colloidal dispersion in crude oil. Since the nature and

410

compositions of crude oils vary so widely, there exists also a great variety of crude oil emulsifying agents (Meyer

411

1964) . These emulsifiers include asphaltic materials, “resinous substances, soluble organic acids, particles in the

412

ocean, particles found in crude oils including waxes and asphaltenes, particles found in sea water including

413

suspended sediments, dissolved surfactants which accumulate at the water/oil interface including metallic salts,

414

organic acids, organic bases and organometallics, and other tiny particles of solids, including products of corrosion

415

of the equipment involved or particles of the producing formation, in case of wells completed in unconsolidated

416

sands and sandy shales, are also the emulsifying agents contributing toward stability of the emulsions (Lee 1999).

417 418

The absence of these emulsifying agents in a crude oil can lead to the formation of a dispersion that will separate

419

quickly due to rapid coalescence of the dispersed droplets. However, the presence of these emulsifying agents in

420

the crude oil would lead to the formation of a very stable emulsion (Bobra 1992, Smith and Arnold 1992, Kokal

421

and Wingrove 2000, Gafonova and Yarranton 2001, Janssen, Noïk et al. 2001, Sjoblom 2001, Binks 2002, Kokal

422

2002, Sjöblom, Aske et al. 2003, Fingas and Fieldhouse 2004, Sjoblom 2005, Müller and Weiss 2007). These

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natural emulsifiers form a mechanical film at the oil/water interface. The structural mechanical properties of the

424

natural crude oil emulsifiers in the interfacial layer surrounding the dispersed droplets are believed are very

425

important. This is the layer that provides resistance to coalescence in the final stage of emulsion breaking (Jones,

426

Neustadter et al. 1978). It is worthy of mention here that, understanding the chemical and physical properties of

427

these particles that reside at the interface is no doubt, key to understanding the emulsion breaking techniques. This

428

area is receiving more attention of recent. The authors of this paper intend to expound more on the effects of native

429

organic and inorganic solids on the stability of petroleum emulsions and how they can be included in emulsions

430

stability prediction models.

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431

(Gallup and Star 2004) in a study of Acidic crude oils identified that apart from their tendencies to cause scale

433

formation in production tubing or in surface installations, acidic crude oils also have high tendencies of forming

434

stable emulsions. The scale is often a mixture of calcium soaps associated with other minerals. Nigeria, on the

435

Afia field, Indonesia, on the Attaka field, Great Britain, on the Blake field, Norway, on the Heidrun field, Angola,

436

on the Kuito field, China, on the EDC field, Cameroon, on the Kita and Asoma fields. The acidity of crude by itself

437

is not a sufficient criterion. Some weakly acidic oils in Cameroon or in Indonesia may form stable emulsions while

438

other highly acidic crudes can be treated with no problem. It is the actual structure of the naphthenic acids that may

439

explain these differences in behaviors, hence the importance of characterizing the naphthenic acids of a crude oil.

440 441 442

5.1.1. Amphiphiles

443

(‘‘liking both’’), designating that they have some affinity for two fundamentally immiscible phases. The word

444

amphiphile was created by Paul Winsor 50 years ago (Paul and Moulik 1997). It emanates from two Greek roots.

445

The prefix ‘amphi’ means "double", "from both sides", "around", as in amphibian. Then the root philos which

446

expresses affinity, as in "philanthropist" (the friend of man), "hydrophilic" (compatible with water), or

447

"philosopher" (the friend of wisdom or science) (Salager 2002). Crude oil contains particles such as silica, clay,

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(Myers 1990) defined those substances that have chemical groups leading to surface activity as being amphiphilic

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iron oxides; that are naturally hydrophilic, but can become oil-wet (hydrophobic) due to extended contact with the

449

crude oil in the absence of water. A decrease in the size of oil-wet particles results in an increase in W/O emulsion

450

stability. Emulsions with particles and asphaltenes combined can be much more stable than those stabilized by

451

asphaltenes alone, provided that enough asphaltenes are present: all the adsorption sites on the particle surface need

452

to be saturated by asphaltenes (Langevin, Poteau et al. 2004).

453

An amphiphilic substance exhibits a double affinity, which can be defined from the physico-chemical point of view

454

as a polar-apolar division. On the one hand, an amphiphiles has a polar group made up of heteroatoms such as

455

Oxygen (O), Sulphur (S), Phosphorus (P), or Nitrogen (N), incorporated in functional groups such as ether,

456

alcohol, ester, thiol, acid, sulfate, sulfonate, phosphate, amine, amide etc. Equally, it has an principally apolar

457

group which is a hydrocarbon chain of the alkyl or alkylbenzene type, sometimes with halogen atoms and even a

458

few non-ionized oxygen atoms (Salager 2002).

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Asphaltenes and resins are among the natural amphiphiles found in crude oils. In an attempt to determine the

461

contribution of indigenous amphiphiles (the light, intermediate and the heavy ones) to emulsion stability, Dicharry

462

et al.,(Dicharry, Arla et al. 2006) evaluated and compared emulsion formed by different parts of substances. They

463

found out that the emulsions formed with the light and intermediate fractions separated immediately when the

464

agitation stopped. However, the most stable emulsions form with the fraction of crude that distilled at temperatures

465

greater than 520°C. This suggests that the amphiphiles with the highest molecular weight, i.e., resins and

466

asphaltenes, play a major role in the protection of water droplet against coalescence, thus making the emulsion

467

more stable. According to (Acevedo, Escobar et al. 1999, Yan, Elliott et al. 1999, Gu, Xu et al. 2002), the key role

468

of the heaviest amphiphilic materials in the crude oil is to stabilize the interface, while the lightest ones tend to

469

lower the emulsion stability. Due to its dual affinity, an amphiphilic molecule does not feel "at home" in any

470

solvent, whether it is polar or non-polar. This is because, there always exist one of the groups which "does not like"

471

the solvent environment. This is the reason amphiphilic molecules have a very strong tendency to migrate to

472

interfaces or surfaces and to adjust so that the polar group lies in water and the non-polar group is placed out of it,

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473

and ultimately in oil. Amphiphiles have other properties other than tension lowering and this is why they are often

474

categorized based on their main use such as: soap, foaming agent, detergent, emulsifier, wetting agent, dispersant,

475

bactericide, corrosion inhibitor, antistatic agent, etc. In some cases they are known from the name of the structure

476

they are able to build, i.e. membrane, micro-emulsion, liquid crystal, liposome, vesicle or gel (Salager 2002).

477 5.1.2. Asphaltenes, Resins and Waxes as Emulsifiers

481

responsible for the formation of petroleum emulsions is not definitely known but, can be attributed to the presence

482

of colloidal asphalt, that including all asphalts and similar substances which occur in colloidal dispersion in crude

483

oil. Determining the type of an emulsion is very simple. If it is miscible with water, it is an emulsion of oil in

484

water; if miscible with oil, it is water dispersed in oil. Probably more than 95 percent of all oil field emulsions are

485

of the water in oil type (Wang, Zhang et al. 2004).

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(Roberts 1926) in a study on treating field emulsions in Mid-continent Field that the emulsifying agent

(McLean and Kilpatrick 1997) in an attempt to further investigate the effects of crude solvency and specific

487

resin–asphaltene interactions on emulsion stability, created model emulsions, using model oil formed by dissolving

488

varying amounts of resins and/or asphaltenes in a mixture of heptane and toluene. The resins and asphaltenes used

489

in this study were isolated from four different types—Arab Berri, Arab Heavy, Alaska North Slope, and San

490

Joaquin Valley. They found out that the prime factors governing the stability of these model emulsions were the

491

aromaticity of the crude medium, the concentration of asphaltenes, and the availability of solvating resins in the oil

492

(i.e., the resin/asphaltene or R/A ratio). The model emulsions were the most stable when the crude medium was

493

30–40% toluene and in many cases at small R/A ratios (i.e., R/A ≤1). This immensely supports the theory that

494

asphaltenes are the most effective in stabilizing emulsions when they are near the point of incipient precipitation.

495

The point of incipient precipitation, according to (Andersen and Speight 1992) is the point at which separation of

496

asphaltenes from a crude oil becomes apparent.

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(Sjöblom, Mingyuan et al. 1990b) in a study conducted to compare the destabilization of True petroleum

498

Emulsions from the Norwegian Continental Shelf and Model Systems. It was reported that True water-in-crude oil

499

emulsions are stabilized by a rigid interfacial film in which the surface-active material is accumulated, and that the

500

distinct components of this film seem, at least for the crude oils from the Norwegian continental shelf, to be

501

asphaltenes, waxes and other non-specified polar components. In addition, small wax particles are incorporated in

502

the film. On the other hand, inorganic particles such as clays have not been detected under laboratory conditions.

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Earlier before the works of Anderson and Speight, (Bridie, Wanders et al. 1980) investigated the roles of wax

505

and asphaltenes separately and in combination. In the study, a Kuwait 200+ fraction was first deasphaltenized

506

(thirty-fold dilutions with n-pentane) and then dewaxed. The dewaxing involves a six-fold dilution in a methyl

507

ethyl ketone/dichloromethane mixture 1/1 vol.). The asphaltenes fraction was recovered and kept under nitrogen to

508

prevent oxidation, and an emulsion of synthetic sea water (70% vol.) in the Kuwait 200 + fraction proper (30%

509

vol.) was prepared. The investigation revealed that, the de-asphaltenized, dewaxed oil did not form a stable

510

emulsion and had released 93% of its water content after standing for 15 min while the oil plus wax and

511

asphaltenes mixed to the original concentrations gave a stable emulsion.

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512

(Kim, Boudh-Hir et al. 1990) investigated the role of asphaltene both in wettability reversal and as a surfactant.

514

The change in wettability is governed mainly by interfacial properties, with interfacial tension probably being the

515

most important property. When a rock surface comes in contact with crude oil, the surface of the rock can possibly

516

be modified due to asphaltene adsorption. This could alter the wettability of the rock. Whereas the polar segments

517

of an asphaltene molecule are oriented towards the surface, the non-polar portions are away from it, causing the

518

surface to be oil-wettable. It is a well-known fact that certain solids that possess dual wettability (i.e. are wetted by

519

both oil and water can play the role of emulsifiers (Sjöblom, Söderlund et al. 1990a, Bobra 1991, Becker 1997, Lee

520

1999, Vignati, Piazza et al. 2003, Sztukowski and Yarranton 2005, Al-Sahhaf, Fahim et al. 2009). Thus, when

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asphaltene adsorbs on such solids (when they come in contact with the crude oil), there is a high possibility of

522

modification of its wettability from dual wettability to single wettability.

523

(Salager 1990) in an investigation of the most effective mechanism for destabilization of W/O or O/W emulsion

524

reported that the removal of the surfactant from the water-oil interface by trapping it in a micro-emulsion is the

525

most effective destabilization mechanism. According to the study, these emulsion stabilizers have a polar part with

526

affinity to water and a nonpolar part with affinity to oil. These substances cannot fulfil this dual affinity, except

527

when they are located at the water/oil interface, with the polar part immersed in water and the nonpolar part in oil.

528

When they are adsorbed at the interface, they result to a decrease in the free energy of the system. Such include

529

naphthenic acids, resins, asphaltenes, etc.

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A physicochemical study by (Bobra 1991) on the emulsification of water-in-oil emulsions reiterated the fact

532

that, the indigenous emulsifying agents are concentrated in the higher boiling fractions (boiling point > 370° C),

533

and predominantly in the residuum. It is largely accepted that asphaltenes, resins, and waxes play key roles in

534

emulsification, but specific mechanisms have not been clearly explained. These compounds are believed to be the

535

main constituents of the interfacial films that surround the water droplets contained in the emulsion. These films

536

have been shown to have high mechanical strength and therefore act as effective physical barriers, which prevent

537

droplet coalescence and in turn gives rise to the stable petroleum emulsions.

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(Sztukowski and Yarranton 2005) studied the stability of w/o emulsion with clays. In the study, heptane,

540

toluene, and asphaltenes were used as model fluids and native solids extracted from oilfield operations. They

541

established that a combination of asphaltene and fine solids (submicron scale) coverage produced the most stable

542

emulsions. They claim that fine solids compete with asphaltenes to adsorb at the interface and that a coverage of

543

the interface by asphaltenes between 60 to 80% and the remaining area covered by fine solids leads to the highest

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544

stability. Coarser solids (above 1 µm) can indeed stabilize emulsions, but only if they are in sufficiently high

545

concentration.

546

Figure 3 (a) Example of a hypothetical structure of asphaltene, among the many suggested, showing their aromatic character.

562

(b) Asphaltene structure deduced from microscopic and macroscopic analysis, showing their micro- and macro-molecular

563

bonding (Kawanaka, Leontaritis et al. 1989).

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(Bobra 1992) studied the emulsification of W/O emulsions, and the effects of wax, resin and asphaltenes. From

582

the studies, it was revealed that waxes cannot act as emulsifying agents by themselves but can act in combination

583

with resins or asphaltenes to produce stable oil-in-water emulsions. Thus, a concentration of 0.01 g/ml of

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Figure 4 Formation of water-in-oil emulsions (Lee 1999).

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asphaltenes did not produce a stable water-in-oil emulsion, but when waxes were added (0.05 g/ml), stable

585

emulsions were formed. An asphaltene concentration of 0.03 g/ml, with no wax added, produced a stable emulsion.

586 587 588

(Bobra 1991) established that resins alone could act as effective emulsifiers, but the most stable emulsions were

589

produced when both asphaltenes and resins were present. It was suggested that waxes could interact with

590

asphaltenes and resins to stabilize emulsions. Stable emulsions are characterized by properties which prevent the

591

coalescence of their small water droplets (1 to 10 µm), while in unstable emulsions the larger water droplets

592

quickly coalesce as in Fig. 4.

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Figure 5 (a) Schematic Diagram of dispersed water droplets in oil stabilized by asphaltenes, waxes and surfactants. An unstabilized region is shown where there is formation of an incomplete barrier. Modification by (Umar, Saaid et al. 2016)., from (Daling, Moldestad et al. 2003) (b) Modified from (Lee 1999).

(Daling, Moldestad et al. 2003) in a review of the major findings from laboratory studies and field trials conducted

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594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617

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618

on the emulsification of oils spilled at sea, reported that among other factors, the precipitation of stabilizing agents

619

(asphaltenes, photo-oxidized compounds (resins) and in some crude oils precipitated waxes) leads to the formation

620

of stable W/O emulsions. The precipitated asphaltenes make an elastic ‘‘skin’’ between the water droplets and the

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oil (see Fig. 5), leading to an increase in the stability of the emulsion- since the water droplets cannot coalesce and

622

drain so easily from the emulsion and the equilibrium will tend to favour emulsion formation. (Lee 1999) in a an

623

attempt to study the agents that promote and stabilize W/O emulsions reported that particles and surfactants found

624

in crude oil can act as emulsifying agents and thus can promote and stabilize water-in-oil emulsions. If particles

625

and surfactant concentrations are adequately high, then the coalescence of the water drops are prevented, leading to

626

stable emulsions (Fig. 5b).

627 628 629

Particles found in crude oils include waxes and asphaltenes, while particles found in sea water include suspended

630

sediments and/or particulates. Dissolved surfactants which accumulate at the water/oil interface include metallic

631

salts, organic acids, organic bases and organometallics. In oceans and during oil spills, particles in the ocean can

632

enter an oil slick and act as emulsifying agents.

633

(Bobra, Fingas et al. 1992) has advocated that asphaltenes resins and waxes must be in the form of finely divided

634

submicrometer particles before they can act as emulsifying agents.

635

According to (Filby and Van Berkel 1987), since the classification of asphaltenes and resins is an operational term,

636

i.e. they are defined based on their solubility in different solvents, there exist no clear difference between the

637

compounds found in the two fractions. Resin particles are mostly smaller than asphaltene particles and most of the

638

metal porphyrins are in the asphaltene fraction. Asphaltenes tend to have higher-molecular-weight compounds and

639

to be more polar than resins.

640

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621

(Lee 1995) disputed the fact that a direct correlation exists between the presence/concentration of asphaltene in

642

certain crude and the stability of the emulsion formed by that crude. In a study involving a Kuwait crude oil with

643

1.4% asphaltenes and TiaJuana crude oil with 3.1% asphaltenes, it was found that the Kuwait crude forms a more

644

stable emulsion than the TiaJuana crude, despite the fact that the Tiajuna crude contains a higher concentration of

645

asphaltene.

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(Jenkins, Grigson et al. 1991) carried out an empirical laboratory procedure to obtain information about the

647

different tendencies of six North Sea crude oils to form stable and highly viscous water-in-oil emulsions. They

648

found out that one crude (identified as crude A) had 1.6% asphaltenes and formed a very stable emulsion while a

649

second crude (crude B) had 1.03% asphaltenes and did not form stable emulsions. Thus, certain components of the

650

asphaltenes may play a critical role in emulsion formation. The amounts of these emulsifying agents in asphaltenes

651

are likely to vary for asphaltenes from different crude oils. However, several studies have established the

652

significance of asphaltenes, resins and waxes in promoting and stabilizing w/o emulsions.

653 654

(Brandvik 1991) in a study found that the stability of a water-in-oil emulsion was positively correlated with the

655

resin, wax and asphaltene content of the original crude oil. Also, in separate studies, (Sjöblom, Söderlund et al.

656

1990a, Ebeltoft, Børve et al. 1992, Sjöblom, Urdahl et al. 1992, Urdahl, Brekke et al. 1992, Umar, Saaid et al.

657

2017) observed that the exclusion of asphaltenes from crude oils by silica columns produced oils that did not form

658

water-in-oil emulsions.

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659

(Bridie, Wanders et al. 1980) found that after the removal of asphaltenes from Kuwait crude oil, very unstable

661

emulsions were produced. When wax crystals and asphaltenes were added back to the treated oil, a stable water-in-

662

oil emulsion was formed. Waxes cannot act as emulsifying agents by themselves but can act in combination with

663

resins or asphaltenes to produce stable oil-in-water emulsions. Thus, a concentration of 0.01 g/ml of asphaltenes

664

did not produce a stable water-in-oil emulsion, but when waxes were added (0.05 g/ml), stable emulsions were

665

formed (Bobra 1992, Bobra, Fingas et al. 1992).

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660

(Oliveira and Goncalves 2005) in a study on the rheology of emulsions held that the presence of W/O emulsions

668

may have a strong impact on the crude oil production, especially in offshore conditions. In such kind of systems,

669

the temperature of the crude oil varies widely along the flow from the reservoir to the platform storage tanks. For

670

example, in Campos basin, where most of the Brazilian crude oil is produced, typically the temperature of the oil

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671

gradually decreases from 80 ºC at the bottom of the well-bore, located 3,000 m below the seabed, to about 60 ºC at

672

the top of the well-bore, located 1,000 m below the sea level, where the sea water temperature may vary from 4 ºC

673

to 10 ºC. In order to reach the storage tank, the crude oil has to flow for several hundred meters through a pipeline

674

in a cold environment. The contact with the cold seawater imposes a major decrease in the crude oil temperature;

675

hence crude oil arrival temperatures below 30 ºC have been frequently reported in some fields.

(McMahon 1992) studied the interfacial action of crude oil emulsions at the interface. He found out that in

678

certain waxy crude oils, the size of the wax crystal has a fundamental role on the stability of W/O emulsions. Data

679

obtained of the interfacial viscosity and other physical properties of the mixture show that the crystals form a

680

barrier at the W/O interface, which retards the coalescence of colliding water droplets. To associate with the

681

interface, wax, which is normally hydrophobic, has to acquire some affinity for the water phase, possibly by

682

adsorption of polar asphaltenes and resins from the crude oil. Studies with Octacosane (n-C28H58), a model crude

683

oil wax, show that limited wax/asphaltene/resin interactions do exist. However, the adsorbed layer does not confer

684

hydrophilicity to the surface of either Octacosane or real crude oil wax. Therefore, the effect of wax on emulsion

685

stability does not appear to be through action at the interface. Instead, wax may act in the bulk oil phase by

686

inhibiting film thinning between approaching droplets or by scavenging demulsifier. It is the asphaltene and resin

687

that were found to affect stability via interfacial action. They can adsorb in either dissolved or in solid form and

688

thereby inhibiting water separation. When wax crystals occur in the continuous phase, they usually act as emulsion

689

stabilizers. However, if they are present in the dispersed phase, such crystals may stick out through the interface,

690

leading to partial or complete droplet coalescence (Becker 1997, Rousseau 2000).

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676 677

(Mouraille, Skodvin et al. 1998) reported how naturally occurring surfactants in crude oils (mainly asphaltenes

693

and resins) are important for the stabilization of water-in-crude oil emulsions. According to a research reported, the

694

stability of emulsions at room temperature was mainly due to those surface-active fractions. It is crucial to gain a

695

better understanding of the mechanisms via which the stabilization processes of water-in-crude oil emulsions

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occur, so as to solve the emulsion problem more efficiently. In general, about 2/3rd of the world's crude oil is

697

produced in form of emulsions that must be treated (demulsified) before it can be marketed. These oilfield

698

emulsions are stabilized primarily by film-forming asphaltenes and resins containing organic acids and bases

699

(Strassner 1968). They are mostly stable and form spontaneously due to the presence of natural surfactants existing

700

in the crude oil phase. It is known that the viscosity of a w/o emulsion is strongly augmented by increasing its

701

water volume fraction and by decreasing the temperature. When w/o emulsions form, oil viscosity changes from a

702

few hundred mPa.s to about 100,000mPa.s (Selvarajan, Sivakumar et al. 2001, Ge, Yang et al. 2010, Wang and

703

Alvarado 2011, Fingas 2014, Fingas and Fieldhouse 2014).

704

6. Emulsion Stabilization Mechanisms

705

There are four principal mechanisms for the stabilization of emulsions (with cases where a combination of

706

mechanisms occurs). Certain emulsions may be weakly stabilized by the presence of adsorbed ions and non-

707

surface-active salts (a). The presence of colloidal sols partially wetted by both phases of the emulsion may form a

708

mechanical barrier to drop contact and coalescence (b). Many emulsions are stabilized by adsorbed polymer

709

molecules (c). Along with polymers, adsorbed surfactant molecules represent the most common stabilization

710

mechanism (d). These are as depicted in Fig. 6 below.

711 712 713

6.1.1. Surfactants

714

other hydrophobic (water disliking). The hydrophilic part (the head) of the surfactant molecule may be positive,

715

negative, neutral, or Zwitter ionic, and the hydrophobic part (tail) consists of one or more hydrocarbon chains,

716

usually with 6–22 carbon atoms (Migahed and Al-Sabagh 2009).

717 718

According to (Salager 2002), Surfactants are substances which exhibits some superficial or interfacial activity. It is

719

a short form for Surface-Active-Agents. Other languages such as Spanish, French or German do not have the word

720

"surfactant", but however describe these substances based on their characteristics to lower the surface or interfacial

TE D

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696

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Surfactants are amphiphilic molecules comprising of two dissimilar parts: one hydrophilic (water liking) and the

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721

tension. For instance, the French would use tensioactif, German would use tenside, while Spanish would call it

722

tensioactivo.

This would suggest that surface activity is firmly the same to tension lowering, which is not absolutely general,

725

although it is true in many cases. It is worth mentioning that it is not all amphiphiles that display such activity. It is

726

only the amphiphiles with more or less equilibrated hydrophilic and lipophilic tendencies that are likely to migrate

727

to the surface or interface. It does not happen if the amphiphilic molecule is too hydrophilic or too hydrophobic, in

728

which case it stays in one of the phases.

RI PT

723 724

(Li, Guo et al. 2006) carried out a study on the formation of emulsions during chemical flooding (ASP) in Da

756

Qing and Sheng Li oil fields in China. In the study, partially hydrolyzed polyacrylamide (HPAM) was used as the

757

polymer, petroleum sulfonate (ORS-41 or TRS) as the surfactant, and sodium hydroxide (NaOH) or sodium

TE D

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729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755

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Figure 6 Main emulsions stabilization mechanisms (Myers 1990).

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carbonate (Na2CO3) as the alkaline component. Although, the technique yielded an increase in oil recovery of 15 to

759

20% compared with water-flooding, formation of stable w/o and o/w emulsions accompanied it. They found out

760

that the formation and properties of the crude emulsions formed in this technique are exceptionally complicated.

761

These emulsions do not only depend on the properties of the crude oil, the injection water and formation water, the

762

alkali, HPAM, and surfactant used, but also on the properties of the reservoir formed alkali–oil surfactant, the solid

763

particles from the reservoir among others.

RI PT

758

764 6.1.1.1. Surfactant Classifications

766

Surfactants can be classified commercially according to the uses they are put to in different industries. In the Oil

767

and Gas industry for instance, there is an increased used of surfactants in the formation of drilling fluids. In oil-

768

based drilling fluids, surfactants are used as emulsifiers and wetting agents. In water-based muds however, there is

769

a continually-growing diverse applications that include oil-in-water emulsification in base fluid formulations;

770

shale-swelling inhibitors to prevent wellbore instabilities, detergency to prevent cuttings sticking to drill bit,

771

defoaming additives to eliminate unwanted foam in water-based fluids; surfactant-polymer complexes for

772

enhanced properties in fluids for low-pressure reservoirs, etc. (Quintero 2002). Other applications of surfactants in

773

the oil and gas industry include: Gas/liquid systems; producing oil well and well-head foams, oil flotation process

774

froth, Liquid/liquid systems; emulsion drilling fluids (as explained above), Enhanced oil recovery in situ emulsions

775

(EOR), Well-head emulsions, Heavy oil pipeline emulsions, Fuel oil emulsions, etc (Schramm, Stasiuk et al. 2003,

776

Migahed and Al-Sabagh 2009). Classifications of surfactants based on their usage however, is not as important as

777

classifying them based on their dissociation in water (Salager 2002). Thus, based on their dissociation in water,

778

surfactants can be classified as:

EP

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765

779 780 781 782

These types of Surfactants are dissociated in water in an amphiphilic anion, and a cation, that is commonly an

783

alkaline metal (Na+, K+) or a quaternary ammonium. They are the most universally used surfactants across many

AC C

• Anionic Surfactants

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784

industries. They comprise alkylbenzene sulfonates (detergents), (fatty acid) soaps, lauryl sulfate (foaming agent),

785

di-alkyl sulfosuccinate (wetting agent), lignosulfonates (dispersants) etc. 50% of the world surfactants produced

786

are Anionic surfactants (Salager 2002). This moiety carries a negative charge, as in soap: C17H35COO- Na+, sodium

787

stearate (Becher 1988, Schramm, Stasiuk et al. 2003, Migahed and Al-Sabagh 2009).

788 789 790 791

These classes of surfactants are dissociated in water into an amphiphilic cation and an anion, most habitually of

792

the halogen type. These surfactants are in general more expensive than anionics, because of the high pressure

793

hydrogenation reaction to be carried out during their synthesis. As a consequence, they are only used in cases

794

where there is no cheaper substitute (Salager 2002, Migahed and Al-Sabagh 2009). The charge is positive, as in

795

quaternary ammonium salts: (C18H37)2N+(CH3)2Cl-, dimethyl dioctadecyl ammonium chloride (Schramm, Stasiuk

796

et al. 2003, Migahed and Al-Sabagh 2009).

M AN U

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• Cationic Surfactants

797 798 799 800

There are situations where a single surfactant molecule exhibit both anionic and cationic dissociations. This

801

surfactant is called an amphoteric or a zwitterionic surfactant. Syhthetic products like betaines or sulfobetaines and

802

natural substances likes aminoacids and phospholipids exhibit this behavior (Salager 2002).

803

For these surfactants, solubilization is provided by the presence of positive and negative charge in the molecule, as

804

in C12H35N+(CH3)2CH2COO-, Ɓ-N-alkyl amino propionic acid (Schramm, Stasiuk et al. 2003, Migahed and Al-

805

Sabagh 2009).

TE D

• Amphoteric or Zwitterionic Surfactants

806 807 808 809

Nonionic Surfactants do not ionize in aqueous solution, because their hydrophilic group is of a non- dissociating

810

type. These comprise of groups like alcohol, phenol, ether, ester, or amide. They come as a close second with about

811

45% of the overall industrial production (Salager 2002). According to (Migahed and Al-Sabagh 2009), the

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• Non-Ionic Surfactants

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812

solubility is provided by solvation of polyoxy ethylene oxide groups in water, such as C9H19C6H4(OCH2CH2)9OH,

813

nonyl phenol ethoxylate (Schramm, Stasiuk et al. 2003).

814 815 816 817 818

6.1.1.2. Surfactants Roles in Emulsion Stabilization

819

tension, increasing surface elasticity, increasing electric double layer repulsion (ionic surfactants), and possibly

820

increasing surface viscosity (Schramm, Stasiuk et al. 2003). Also, surfactant nature can control the arrangement of

821

the phases in an emulsion, that is, which phase will form the dispersed versus continuous phase. As discussed

822

earlier (under section 3), several experimental predictive methods based on anticipated surfactant positioning at the

823

interface exist (Schramm 2000, Dingcong 2002, Pasquali, Sacco et al. 2009). These include the Bancroft’s rule, the

824

oriented wedge theory, the hydrophile–lipophile balance (HLB), and the volume balance value (Dingcong 2002).

825

Among all the methods, the HLB has been the most widely used. The HLB dimensionless scale ranges from 0 to

826

20 for non-ionic surfactants; a low HLB (<9) refers to a lipophilic surfactant (oil soluble) and a high HLB (>11) to

827

a hydrophilic (water soluble) surfactant. Most ionic surfactants have HLB values greater than 20. Water-in-oil

828

(W/O) surfactants show HLB values in the range 3–8 while oil-in-water (O/W) emulsifiers possess HLB values of

829

about 8–18 (Schramm 2000, Pasquali, Sacco et al. 2009, Zafeiri, Horridge et al. 2017). Additionally, surfactant

830

exchanges between the interface and the bulk can drastically lower interfacial visco-elasticities (Powell, Damitz et

831

al. 2017).

• Surfactant Adsorption at Liquid-Liquid Interface

TE D

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The adsorption of surfactants at liquid interfaces can influence emulsion stability by lowering interfacial

832 833 834 835

Certain surfactants adsorb at a solid surface and by so doing, reduce interfacial tension and alters the ability of

836

water or oil to wet the solid surface. When the adsorbed surfactant positioning is in such a way that its hydrophobic

837

(water disliking) tail groups point away from the surface or along the surface, that will lead to a decrease in water-

838

wetting and an increase in oil-wetting. Similarly, if the positioning is with the polar head group away from the

AC C

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• Surfactants and Wettability Alteration

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839

surface, there will be a resultant increase in water-wetting. An example of surfactant-induced wettability alteration

840

can be found in the treatment of swelling clays, such as montmorillonite, with a cationic surfactant, such as

841

dimethyl di(hydrogenated tallow) ammonium (Schramm 2000, Schramm, Stasiuk et al. 2003).

842

6.2. Roles of Solids in w/o Emulsion Stabilizations (Pickering) The subject of Pickering emulsions was first investigated by (Ramsden 1903)(Proc. Roy. Soc., 1903, 72, 156).

844

However, (Pickering 1907) was independently working on emulsification of solids, and only got to know about the

845

work of Ramsden after he has completed his work. Thus, the credit on Pickering emulsion is mostly given to

846

Pickering. An accumulation of continuous research in the past one century led to the recognition of distinctive

847

characteristics of Pickering emulsifiers compared with conventional emulsifiers, such as, irreversible interfacial

848

adsorption, exceptional stability against coalescence and Oswald ripening , the capability to stabilize emulsions

849

with enormous droplet size (up to several millimetres) or high internal phase, irregular rheological properties, etc.

850

(Dickinson 2012, Rayner, Marku et al. 2014, Xiao, Li et al. 2015).

M AN U

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843

851

According to (Pickering 1907), the mixture of any insoluble liquid in water, where the water is broken up into

853

minute globules, like when it is forced through a syringe, the globules would remain emulsified permanently if the

854

water contains emulsifying agents. On mixing lime with paraffin and water, a kind of emulsion formed, which

855

either upsurges or descends in the excess of liquid, according to the proportions used.

856

However, describing the behaviour of the many substances that results to these different emulsions types would be

857

a tedious and unprofitable task. Thus, it is sufficient to briefly classify as those which give true emulsions, those

858

which give inadequate emulsions or quasi-emulsions, and those which do not emulsify at all.

859

This study established the fact that solids which are not adequately fine-grained to emulsify will, in most cases,

860

when present in significant amounts, form quasi-emulsions. Lime for instance forms a quasi-emulsion. Other

861

substances, including many recently formed precipitates, and, probably, all crystalline solids seem to be incapable

862

of forming even quasi-emulsions. This was the earliest documented results on Pickering emulsions, and this

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863

opened doors to further researches. (Clayton 1923) reported how finely-divided solids in certain instances can

864

promote emulsification of o/w or w/o emulsions. He found out that a North African argillaceous earth promptly

865

emulsified oils in water and could be used as a replacement for soap. (Alexander 1921) reported that emulsification is influenced by the size of the particles of the emulsifier, and that

867

the size of the emulsion droplets varied directly with the size of the particles of the emulsifying agent. He also

868

identified the ideal condition that for emulsions of oil in water the finely-divided solid emulsifying agent must be

869

more readily wetted by water than by oil while that of w/o should be more wetted by oil than water.

RI PT

866

(Briggs 1921) in a study of emulsions by finely divided solids showed that certain finely-divided solids, when

871

used together, might exert opposing emulsifying effects. For instance, adding 0·8g of carbon black (CB), an

872

amount sufficient to disperse 25 c.c. of water in 15 c.c. kerosene, to 0·1g of silica (350 mesh), no emulsion was

873

possibly formed. However, kerosene can be emulsified in a water suspension of silica, but if sufficient CB is

874

present, no emulsion is formed. Similarly, 1 part of mercuric iodide in 20 parts of silica will prevent the latter from

875

emulsifying 25 c.c. of kerosene in 25 c.c. of water.

M AN U

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870

(Mukerjee and Srivastava 1956) reported the action of colloidal clay as efficient emulsifying agents. According

877

to their findings, colloidal clay is capable of promoting the emulsification of o/w or of w/o emulsions. Such results

878

however had to be subjected to further investigation because as at that moment, what was understood was that a

879

given emulsifying agent only promotes one type of emulsion with any two given liquids to be emulsified. (Al-

880

Sahhaf, Fahim et al. 2009) while studying the factors that control emulsion stability in Burgan Oilfield Kuwait

881

reported that colloidal particles partially wetted by both the water and oil phases are capable of effectively

882

stabilizing emulsions.

EP

883

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876

In separate studies, (Binks 2002, Binks, Clint et al. 2005, Binks and Rocher 2009) also established that colloidal

885

particles such as wax crystals can originate by direct solidification at the droplet interface, or previously formed

886

crystals can migrate and attach themselves to the interface. Colloidal particles like wax, silica, clay, iron oxide and

AC C

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887

polymer lattices can provide kinetic stability to the dispersed phase of many oil-continuous emulsions via either the

888

presence of a particle network and/or interfacially-adsorbed colloids. The degree to which solids increases

889

emulsion stability depends on several factors such as particle size, shape and morphology, density, concentration,

890

and surface coverage, and wettability.

891 (Gelot, Friesen et al. 1984) investigated the effect of different combinations of finely divided solids and surface-

893

active agents on the stability of a w/o emulsion. In the study, they investigated the emulsification of w/o by the

894

clays (Ca−bentonite and Ca−kaolinite), as well as carbon black. The study revealed that by adding the surfactant

895

sodium dodecyl sulfate (SDS), the stability of the emulsion can be enhanced, and the wettability of the particles

896

changed. Thus, the principles that developed from this work and preceding works was that surface modification of

897

solid particles by adsorption of surface-active materials can modify the wettability of the particles and the stability

898

of the emulsions enhanced. (Finkle, Draper et al. 1923) are seemingly the pioneer researchers to link the three-

899

phase contact angle, (see Fig. 7 and 8) with the type of emulsion (O/W) or (W/O) stabilized by the solid particles.

900

According to them, the better-wetting liquid constitutes the continuous phase. (Reinders 1913) articulated three

901

important parameters that are some strong possibilities for the interfacial tension in a solid-liquid-liquid system;

902

γ12, γ13, γ23, where 1 and 2 denote the two liquids and 3 the solid. These possibilities are:

M AN U

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892

1.

γ23>γ12+γ13, particle wetted completely by liquid 1;

904

2.

γ13>γ12+γ23, particle wetted completely by liquid 2;

905

3.

γ12>γ13+γ23, or γij>γik+γjk for all i≠j≠k, particle wetted partially by both liquids.

907

Only in case (3) will the particle tend to situate itself at the liquid-liquid interface.

EP

906

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903

(Thompson, Taylor et al. 1985) demonstrated how wax particles and associated solids have considerable

909

influence on the emulsion stability of waxy North Sea crude. They found out that isolating the indigenous particles

910

from this oil inhibited oil’s tendency to form stable emulsions. (Bobra, Fingas et al. 1992) established that for

AC C

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solids to act emulsifying agents, the particles must be very small compared to the droplet size of the emulsified

912

phase. They must accumulate at the interface and they must be “wetted” by both the oil and water phases.

913

(Monson 1969) in a study on chemical resolution of emulsions stated that depending on the specific characteristics

914

as geological age, chemical composition and other impurities, a wide range of materials can stabilize crude oil

915

emulsions. These include materials include finely-divided solids or mechanical stabilizers such as drilling mud

916

(Colloidal clay), produced sand, iron sulphide from pipe corrosion, precipitated minerals due to scale formation,

917

asphaltenes in the crude oil.

918 919

6.2.1. Pickering Stabilization Mechanism

RI PT

911

Several studies have been dedicated to the clarification of the mechanisms behind particle stabilization by

921

focusing on the influence of particle size, hydrophobicity, surface roughness and shape (Ngai and Bon 2014). The

922

phenomenon that solid particles can reside at the interface of droplets, thereby giving them some resistance against

923

coalescence or fusion, or Ostwald ripening, is known as Pickering stabilization (Bernardini 2015) (Ngai and Bon

924

2014). (Binks and Lumsdon 2000) reviewed some experimental findings concerning the stabilization of emulsions

925

by solid particles. They further described the preparation and properties of w/o emulsions stabilized by nanometer-

926

sized hydrophobic silica particles alone.

M AN U

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927

SC

920

(Binks and Lumsdon 2000) reported, based on the works of (Abend, Bonnke et al. 1998, Lagaly, Reese et al. 1999,

929

Neuhäusler, Abend et al. 1999), at least two mechanisms by which colloidal particles stabilize emulsions

930

depending on the system. In the first mechanism, the particles are expected to adsorb at the oil/water interface and

931

stay there forming a dense monolayer or multilayer film around the dispersed droplets, thus hindering coalescence.

932

In the second, increased stabilization ensues when the particle-particle interactions are such that a three-

933

dimensional network of particles forms in the continuous phase surrounding the dispersed droplets. This has been

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934

cited mainly in clay-containing systems in which the emulsion oil drops become captured and largely immobilized

935

in the array of clay platelets in water.

936 (Kilpatrick 2012) suggested that the mechanism by which w/o emulsions are stabilized by inorganic solid particles

938

seems to be principally via the enhancement of asphaltene and/or resin or crude oil acid-stabilized emulsions by an

939

adsorption process of surface-modifying components in the crude oil to the particles that render them interfacially

940

active. There seems to be a limit to the surface coverage of inorganic particles that can effectively stabilize

941

micrometer-sized droplets, with particles in the tens to hundreds of nanometer size scale being most effective.

942

Figure 9 shows a schematic of surfactant molecule and a colloidal particle at oil-water interface. Surfactants are

943

amphiphilic molecules and they have a natural tendency to move to the oil-water interface. They decrease the oil-

944

water interfacial tension; and by so doing minimizing the energy required for emulsion formation. The adsorbed

945

surfactant molecules at the interface act as electrostatic or steric barriers against droplet coalescence thereby

946

increasing the emulsion stability. Hydrophilic-Lipophilic balance (HLB) of the surfactant molecules dictate the

947

nature of the emulsion formed; either O/W or W/O (Ramsden 1903, Pickering 1907, Clayton 1923, Binks 2002).

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948

Finely divided insoluble solid particles constitute an important class of emulsifying agents (Al-Sahhaf, Fahim et al.

950

2009) and behave in various ways like surfactant molecules, particularly if adsorbed to a fluid/fluid interface. Just

951

as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile-lipophile balance HLB

952

number, so can that of a spherical particle be described in terms of its wettability via contact angle. Important

953

differences exist, however, between the two types of surface-active material, due in part to the fact that particles

954

are strongly held at interfaces (Binks 2002).

955

According to (Myers 1990), there are three conditions a solid must satisfy before it can play the role of an emulsion

956

stabilizer: (1) Particle size; In field operations, it is found that the stabilizer particles must be smaller than the

957

emulsion droplets, (2) the state of stabilizer particle dispersion; (i.e. in a state of incipient flocculation -that is, they

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should have partial colloidal stability in both liquids, otherwise their tendency to ‘‘reside’’ at the oil–water

959

interface will not be sufficiently strong for them to act as stabilizers, and (3) the comparative wettability of the

960

particles by each liquid component of the emulsion system; the solid must exhibit a significant contact angle at the

961

three-phase (oil–water–solid) contact line, usually as measured through the aqueous phase.

962

.

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958

Figure 7 (Left)Distribution of solid particles at water/oil interface (Bobra 1991). (Right) Particles positioning at oil/water interface (Binks 2002).

For maximum efficiency, the stabilizer usually should be preferentially wetted by the continuous phase (but not excessively so). (Gelot, Friesen et al. 1984, Tambe and Sharma 1993, Yan, Elliott et al. 1999, Aveyard, Binks et al.

970

2003) in separate studies establish that the extent to which solids increases emulsion stability depends on several

971

factors. Among these factors are particle size, shape of particles and morphology, particles density, concentration,

972

surface coverage, and wettability.

973

(Rousseau 2000) reviewed the effects of fat crystals in food emulsion formation and its stability. The study of

974

colloidal particles in food emulsions, particularly the role of fat crystals, is a more recent phenomenon, being first

975

examined in 1960s (Lucassen-Reynders and Tempel 1963). In quiet a number of emulsified foods (e.g. ice crystals

976

in ice cream, egg yolk particles in mayonnaise and fat particles in whipping cream) solid particles are necessary for

977

making them stable (Darling 1982). The role played by colloidal particles in the stabilization of emulsion is

978

receiving much attention, yet, the knowledge base is relatively scarce (Rousseau 2000). Some factors that are

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intimately related by inter-particle interactions and significantly affect fat crystal stabilization of emulsions ate

980

explained below.

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981 982 983 984 985

Figure 8 Adsorption and contact angles of fat crystals at the interface of an oil-in-water emulsion (Rousseau 2000).

986

of the crystals at the interface (Friberg, Larsson et al. 2003); (2) interfacial film rheology and (Lucassen-Reynders

987

1993); (3) polymorphism and morphology of the particle (particle structure) (Ogden and Rosenthal 1998); and (4)

988

location of fat crystals [(in the dispersed (O/W emulsion) or continuous phase (W/O emulsion)] (Darling 1982). All

989

of these factors are intimately related via inter-particle interactions (Wesdorp, Human et al. 1992).

990

(Mackay, McLean et al. 1973, Edwards and Wasan 1991) proposed that, the ability of an emulsion to oppose

991

coalescence will largely be determined by the properties of the interface. A highly viscous and rigid interfacial film

992

will retard the rate of film drainage and resist rupture thereby promotion.

993

(Wang and Alvarado 2011) in a research demonstrated how particle suspension contributes to emulsion stability. In

994

the study, Kaolinite and silica particle dispersions were characterized as functions of brine salinity using a

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The significant factors that determine the influence of fat crystals on emulsion stabilization are: (1) the wettability

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reference water phase, as the reservoir brine, diluted with distilled water to obtain 10 and 100 times lower Total

996

Dissolved Solid (TDS) brine. Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD) were employed

997

in examining the morphology and composition of the clays. The emulsions used in the studies were prepared by

998

mixing a crude oil with brine, with and without dispersed particles to investigate emulsion stability. The stability

999

was measured through the conventional bottle tests and optical microscopy. Results from the experiments show

1000

that both silica and kaolinite promote emulsion stability. Also, around 1 µm in size of kaolinite stabilizes emulsions

1001

more than does larger clay particles. A total reversal to this was observed with regards silica particles. Silica

1002

particles of bigger sizes (around 5 µm) produced more stable emulsions than smaller silica particles do.

1003

(Sztukowski and Yarranton 2005) inspected the role of solids in the stability of oil filed emulsions. They found that

1004

emulsion stabilized by fine solids and asphaltenes were most stable at a 2:1 fraction area ratio of asphaltene to

1005

solids. There is a strong correlation between asphaltene content and emulsion tightness.

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(Binks and Rocher 2009) in a study on the effects of temperature on water-in-oil emulsions stabilized by wax

1008

microparticles claimed that micro-wax particles alone can stabilize emulsions. According to the authors, since wax

1009

particles are hydrophobic at the oil–water interface; such emulsions are water-in-oil. They found out that the

1010

stability of these emulsions at different temperatures to both sedimentation and coalescence depends significantly

1011

on whether particles are adsorbed to dispersed drop interfaces or they are not, prior to the temperature change.

1012

However, if the drops are formed at room temperature, increasing the temperature of the emulsion consequently

1013

leads to increase in the degree of coalescence as particles melt and cannot provide an obstruction to drop blending;

1014

the temperatures over which this occurs are in the same range as that of the melting range of the particles alone.

1015 1016

(Sharma, Velmurugan et al. 2015) conducted an EOR study, where oil-in-water Pickering emulsion stabilized by

1017

Nanoparticles was used in Combination with Polymer Flood. In this work, oil-in-water Pickering emulsion systems

1018

stabilized using nanoparticles, surfactant, and polymer were formulated and their efficiency tested for enhanced oil

1019

recovery with and without a conventional polymer flood. For the flooding experiments conducted, Berea core

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samples at 13.6 MPa and temperatures of 313 and 353 K were used. The study found out that a combination of 0.5

1021

PV polymer flood with 0.5 PV Pickering emulsion was efficient and have yielded an additional 1–6% oil recovery

1022

as compared to 0.5 PV Pickering emulsion flooding alone.

SC

Figure 9 Schematic representation of (a) surfactant molecule (b) colloidal particle at oil-water interface (Katepalli 2014).

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(Yan, Gray et al. 2001) investigated the stability of w/o emulsions stabilized by fine solids with different

1040

hydrophobicities. They used model organic solvents like light mineral oil (Bayol-35), decane and toluene. The fine

1041

solids used in the study include hydrophilic and hydrophobic colloidal silica; kaolinite clay particles treated with

1042

asphaltenes, hydrophobic polystyrene latex microspheres, as well as fumed silica dry powders treated with

1043

silanization. Experimental results showed that hydrophilic colloidal silica could only produce o/w emulsion that is

1044

stable for a very short period of time. Using hydrophobic particles (either colloidal silica or polystyrene latex

1045

microspheres) that were suspended in the aqueous phase prior to emulsification produced only o/w emulsions.

1046

However, when such hydrophobic particles were suspended in organic phase prior to emulsification, stable w/o

1047

emulsions were formed, with dispersed water droplets as small as 2 mm when the solids were 12 nm in diameter.

1048

They found out that the stability of the produced emulsions studied depended on the hydrophobicity of the

1049

particles, and only particles with intermediate hydrophobicity produced very stable w/o emulsions.

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1050

Although vast literatures on factors that affect emulsion stability exist, there is a shortage of work on inorganic

1051

and organic solids stabilized water-in-oil emulsion despite their importance (Al-Sahhaf, Fahim et al. 2009). The

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authors believe that this extensive literature review would open more doors to those in the academia and the

1053

industry, especially that want to explore deeper into the issues of petroleum emulsions.

1054

7. Conclusions

1055

The formation of emulsions in the oil and gas industry is as old as the industry itself. These produced emulsions

1056

have posed serious operational and economic problems to the industry, justifying the economic and technical

1057

reasons for the resolution of these unwanted emulsions. Resolving emulsions require a detailed understanding and

1058

thorough research into not just the consequence of its formation, but the factors that prompt its formation and those

1059

that enhance its stabilization. Numerous studies have been conducted by researchers from the Industry and

1060

academia, and several factors have been experimented with respect to their roles in producing stable petroleum

1061

emulsions. Despite the vast literatures available, there is still a significant shortage of studies that deeply examine

1062

the roles of inorganic solids in stabilizing water-in-crude oil emulsions. From this review study, we found out that

1063

the concept of study adopted by most of the researchers that studied solids stabilized emulsions involve (1) Using

1064

processed solids in the study (2) preparing emulsions in the laboratory, rather than using field emulsions, and (3)

1065

inadequate knowledge on the properties of the solids – in the case of the few studies that have used field emulsions.

1066

To the best knowledge of the authors of this work, there is no study that holistically made use of solids from a field

1067

emulsion and explicitly characterized those solids, while examining their roles in stabilizing emulsions. Hence the

1068

authors of this work focus on native solids from field emulsions (both organic and inorganic), holistically

1069

understand their properties and roles in stabilizing petroleum emulsions and predict the type of emulsion that the

1070

presence of these solids may form. It is believed that understanding the compositions, behaviors and properties of

1071

these solids (that reside at the o/w interface) is fundamental in understanding the nature, severity and type of

1072

petroleum emulsions, as well as a key in developing cost-effective demulsification strategy. From the details of the

1073

papers reviewed by the authors of this paper, they believe that tackling the issue of undesirable emulsions,

1074

especially, requires a closer investigation into the problem. Due to the peculiarity of every oilfield emulsions

1075

(because of the varying factors responsible for its formation), containing the behavior of these complex mixtures is

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no easy task. Perhaps the best way to visualize such complex emulsions is to identify the physical and chemical

1077

properties of not just the organic emulsifiers, but also of the inorganic solids which we believe are very significant

1078

in making the emulsion more difficult to treat. As mentioned, emulsion treatment has always been more of an art

1079

than science, with many such chemicals selected based on trial and error. This we believe should be replaced with

1080

a more scientific approach to the problem, where the demulsifiers are tailored based on the exact properties of the

1081

emulsifying agents. Hence, it is important to understand the nature of the solids that stabilize the emulsions at a

1082

molecular, microscopic and macroscopic level. Also, to obtain an adequate prediction model that can predict the

1083

nature and severity of petroleum emulsions, the solids that are produced together with the crude oil should be

1084

included in the prediction model. The authors of this paper have conducted series of laboratory studies as part of

1085

an extensive study that would go a long way in tackling the problem of emulsion treatment in the oil and gas

1086

industry.

1087

Acknowledgements

1088

The authors wish to acknowledge Universiti Teknologi PETRONAS, Malaysia for providing us with state-of-the-

1089

art equipment for this study. Also, Deleum Chemicals Sdn Bhd Malaysia, PETRONAS Research Sdn Bhd (PRSB)

1090

Malaysia, Vision Petroleum Sdn Bhd, Malaysia for providing us with production data and field emulsions.

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The formation and stabilization of petroleum emulsions remain a serious problem in the oil and gas industry. Increase in solids concentrations lead to an increase in the emulsion stability. The irreversible adsorption of solids has a major consequence on the long-term stability Solids sizes play significant roles in improving emulsion stability. The stability of these emulsions in the oil industry has been a fundamental issue that necessitates continuous research. This paper provides details of the petroleum emulsions in the oil and gas industry.

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