12 emulsions

INTEC ENGINEERING, INC. DEEPSTARMULTIPHASE DESIGN GUIDELINE

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12.0 EMULSIONS

12.1 General

Although water and crude oil are immiscible, produced fluids may be emulsified by the simultaneous action of shear and pressure drop such as at wellhead chokes and valves. The produced emulsions can be stable, i.e. the oil and water phases may not readily separate within a short time scale. Separation can take from a few minutes to a very long time (for very tight emulsions). From a multiphase flow perspective, the primary effect of emulsion formation is an increase in fluid viscosity. If the emulsion is stable, this increase in viscosity can significantly affect pressure drop.

12.2 The Design Process and Emulsions

When oil/water emulsions are present, determination of the liquid viscosity iscomplicated. The viscosity of an emulsion depends, in part, on the particle sizedistribution, which is largely determined by the flow history of the fluid and the shear imparted by chokes, pumps, etc. Dispersions that have undergone high shear (e.g. by flowing under high pressure drop through a choke) will contain water particles of a relatively small size and this "tight" emulsion will have a relatively large viscosity.

Once formed, emulsions will generally exhibit non-Newtonian shear-thinning flowbehavior so that viscosity is dependent on the shear rate in the pipeline, i.e. the prevailing flow rate.

Emulsion viscosities are dependent on:

• Temperature

• Water cut

• Flow history, including shear rate experienced when formed

• Flowing shear rate, i.e. the flow rate in the pipeline.

• Physical characteristics of the oil, the produced water, and presence/size of anysuspended particulates.

Depending on the particular multiphase simulation program being used, there are several methods to model the effect of water (in general) and emulsions on fluid viscosity:


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• The liquid-phase fluid viscosity can be estimated using a weighted average of water and water viscosity. The program should modify this liquid-phase viscosity to account for the presence of gas in the “live” oil.

• The liquid-phase fluid viscosity can be estimated using oil viscosity when water cut is below the inversion point (the point where the liquid changes from oil-continuous to water-continuous) or water viscosity when water cut is above the inversion point. The approach may be more meaningful than the “weighted average” method above.

• A special viscosity estimate may be used to account for tight emulsions when water cut is near the inversion point, such as the Woelflin correlation. Care should be taken to assure that viscosity values used in calculations are meaningful.

• When viscosity data as a function of temperature is available (it should be obtained whenever possible), viscosity can be entered as a function of temperature so as to better tune the program’s viscosity predictions. It may be necessary to run analyses over a number of temperature ranges to properly cover the fluid characteristics.

12.3 Control and Management

12.3.1 Terminology

An emulsion is a heterogeneous system consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets.

The phase present as droplets is the dispersed or internal phase. The liquid phase forming the matrix within which these droplets are suspended is called the continuous or external phase.

Two common types of emulsion are:

• oil- in-water (o/w), and• water-in-oil (w/o).

12.3.2 Emulsion Instability

The dispersed phase consists of microscopic droplets, usually within the size range 0.1 microns to 100 microns. Such dispersions are never completely stable in the absolute sense; they are thermodynamically unstable. Simply, the explanation rests with the fact that work is required to produce interfacial area, i.e. to disperse one liquid within another. Hence, coalescence of drops (resulting, therefore, in a net reduction of interfacial area) reduces the free energy of the system and is thermodynamically spontaneous.


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In discussing emulsion stability it is necessary to distinguish between the mechanisms of “flocculation” (or coagulation), “creaming”, and “breaking” as shown in Figure 12.3-1.

Figure 12.3-1: Emulsion Mechanisms

Flocculation is the association of droplets forming three-dimensional clusters without coalescence, which would form individual droplets (note that the mechanism ofcoalescence involves the drainage and subsequent rupture of films between contacting droplets).

Creaming is the term given to the rise (o/w emulsions) or fall (w/o emulsions) ofdispersed droplets under the action of gravity. Again, coalescence is not (necessarily)involved.

Creaming can only arise if the dispersed and continuous phases are unequal in density. The rate of creaming (for a single drop only) may be expressed by Stokes’ Law:

zgrV

9)(2 2 ρΔ

=

Where:V = velocity of rise (or fall)

g = gravitational constant

r = droplet radius


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Δρ = difference in density between the dispersed and continuous phases

z = viscosity of the continuous phase

Notes

i) Velocity is a function of the square of the radius.

ii) The viscosity is that of the continuous phase, not of the emulsion.

iii) Stokes’ Law is useful but often misused. In particular, no account is taken of the Brownian Motion of the droplets, which are under constant bombardment from the molecules of the external phase. A detailed comparison of the Stokes’ Law Effect versus Brownian Motion is beyond the scope of these notes. Suffice it to say that only very large droplets (typically >50 microns) truly obey Stokes’ Law, whereas the physical displacement of small droplets (typically <1 micron) is dominated byBrownian Motion.

Breaking of an emulsion occurs when droplets which are flocculated or which collide (during creaming, Brownian Motion, external turbulence) actually coalesce. To reiterate, the action of coalescence reduces the free energy of the system by reducing the interfacial area. In fact:

AG owow γ=

Where

Gow = the excess interfacial free energy associated with the oil-water interface,γow = the interfacial tension,A = the total interfacial area.

The interfacial tension (units, mN/m) is defined as the force acting parallel to the planar interface between two immiscible liquids and at right angles to a line of unit length anywhere in the interface. In effect, the tension arises because the intermolecularinteractions of molecules (of either phase) at the interface are fewer than those between molecules in the bulk (or interior) of the liquid.

To this point, the discussion has been solely in terms of two components - oil and water. It is essential that a third component (or class of materials) is present to bestow stability. This component will provide a physical (repulsive) barrier to droplet-droplet coalescence, will reduce the interfacial tension (and therefore reduce the free energy of the system), or both.


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12.3.3 Emulsion Stability

This section is divided into four parts dealing briefly with emulsion stability in terms of electrical repulsion, interfacial films, steric stabilization, and solids stabilization.

Electrical Repulsion

Electrically charged droplets may arise through:

• the presence of inorganic electrolytes, e.g. NaCl, CaCl2

• the adsorption of an ionic surfactant, e.g. C11 H23CO2Na+

• frictional contact

Notes

i) Because the permittivity of oil is low, stabilization through electrical repulsion is generally of significance only in water continuous (i.e. o/w) emulsions.

ii) Electrical stabilization is mainly ascribed to the long range forces of repulsionbetween electrical double layers.

iii) Increasing electrolyte concentration (i.e. increasing the external aqueous phasesalinity) compresses the double layer and, therefore, increases the rate of coalescence. Thus, electrical stabilization tends to be minimal or insignificant in seawater or produced water external phase emulsions.

Interfacial Films

The strength and compactness of interfacial films are primary factors favoring (o/w and w/o) emulsion stability. These are related to the type and amount of surfactant adsorbed. The formation of a highly viscous, rigid film at the oil-water interface provides amechanical barrier to the coalescence of droplets. The formation of such a film is usually also associated with a decrease in the interfacial tension. Measurements of interfacial tension versus interfacial area provide useful information about the adsorption andorientation of surfactant species; in particular, whether the film is gaseous, expanded, or condensed, as illustrated in Figure 12.3-2.


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Figure 12.3-2: Film Types

Steric Stabilization

When droplets are stabilized with long-chain surfactant molecules or polymers (notably block or graft copolymers), interpenetration of the chains as the droplets approach one another gives rise to steric repulsion. This effect can arise in both o/w emulsions, stabilized, for example, by nonionic surfactants, and w/o emulsions where thehydrocarbon tail of the stabilizer extends into the oil phase.

Solids Stabilization

Stabilization of emulsions by solid particles, such as sand or produced “fines”, depends, firstly, upon the size of the partic les in relation to the dispersed phase droplet size and, secondly, upon the relationship of the three interfacial tensions: γsw (between solid and water); γwo (between water and oil): γso (between solid and oil).

If γso > γwo + γsw, the solid remains suspended in the aqueous phase

If γsw > γwo + γso, the solid remains suspended in the oil phase

If γwo > γsw + γso, the solid will collect at the oil-water interface.

It is seldom possible, in practice, to measure γsw and γso directly. Rather, it often has to beassumed, from the preferred environment of the solid particles, just which of the above three conditions holds.


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Notes

i) the size of the solids must be smaller than that of the dispersed phase droplets typically by an order of magnitude.

ii) the bulk of the particle will be in that phase which most nearly wets it - this phase should then constitute the outer phase.

iii) coalescence is inhibited because it takes work to displace the particle from the interface.

If the conditions described above prevail, extremely stable emulsions can be formed.

12.3.4 Crude Oil Emulsions

Except when water cuts are extremely high (>70 to 80 percent), crude oil emulsions are oil continuous, i.e. of the water- in-oil (w/o) type. At present, produced crude oil- in-water(o/w) emulsions are exceptional - for our purposes their treatment can be regarded as being similar to that of oily effluent. This section will discuss only the stability and destabilization of the more commonly produced w/o type crude oil emulsions.

These produced emulsions are usually stable with respect to water separation - sometimes extremely stable, remaining unchanged even after several years storage. In the first instance, therefore, we require to consider, in the light of the preceding section, what factors (physical and chemical) give rise to pronounced stability.

Naturally Occurring (Indigenous) Surfactants

Water droplets within crude oil are often stabilized by a viscous film (or ‘skin’) ofsurface-active materials indigenous to the crude oil. This is readily observed, even with a light crude, by forming an oil droplet at the tip of a syringe immersed in water, allowing the droplet to age, then withdrawing some of the oil from the droplet via the syringe; a mechanically strong film is clearly left behind.

It is impossible to define, singularly, the chemical structure of all crude oil surfactants.Their concentration and composition vary widely from one crude oil type to another. Indeed, consideration of this fact alone reveals why crude oils of different origin (yet of similar physical characteristics, such as viscosity, API gravity) can show widely differing emulsion characteristics. It is, however, possible to categorize broadly four main classes of crude oil surfactant.

1) Low molecular weight – (< ~500) paraffinic/naphthenic hydrocarbon derivatives


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containing polar functional groups e.g. hydroxyl (phenol), ester, carbonyl, amide, sulfoxide. (These materials reduce the interfacial tension between crude oil and water).

2) Asphaltenes – these are high molecula r weight species (typically 2000 or greater).They are highly aromatic in nature and generally consist of condensed aromatic systems linked by aliphatic chains and often containing aliphatic side chains. The nucleus of the molecule usually consists of a sheet of (typically 6-16) condensed aromatic rings. Heteroatoms, e.g. metal ions, oxygen, nitrogen, and sulfur, may be present in small quantities. Asphaltene molecules have a strong tendency toassociate, the resulting structures (sometimes termed ‘micelles’) having molecular weights supposedly as great as several hundred thousand. Asphaltenes are insoluble in light n-alkane solvents (e.g. pentane, heptane).

3) Resins – these are similar to asphaltenes, but are of lower molecular weight(generally < 2000) and are soluble in light n-alkanes. They are generally more highly polar than asphaltenes, containing greater proportions of nitrogen, oxygen and sulfur; however, the dividing line between asphaltenes and resins is obscure. Theirdefinition (in literature) often being associated with the chemical means used to extract either species from the crude oil in question.

4) Waxes – in an analytical sense, wax is usually defined as that fraction of adeasphalted crude oil which is insoluble in dichloromethane at -32°C. For our purposes, the term ‘wax’ is used to describe all the high molecular weight paraffinic substances (generally > C20) which have crystallized (or precipitated) from the crude oil at a given temperature. As well as straight chain n-alkanes, the term includes ‘near-normal’ paraffins, which contain only a moderate number of short side chains (‘branching’).

Asphaltenes, and more particularly, resins, may slightly reduce the interfacial tension between oil and water (γow); the presence of waxes at the interface does not reduce γow.

The relative importance of each of the above four categories of natural crude oilsurfactants in stabilizing water- in-crude oil emulsions depends largely upon thecomposition of the crude oil. The stability of heavy (API gravity <20°) and of medium heavy (API gravity >20 to 30°) crude oil emulsions is dominated by the presence of resins and asphaltenes. All four types of surfactant may stabilize lighter crude oilemulsions (API gravity > 30°), but in particular, waxes and wax-asphaltene (-resin) co-precipitates are known to play a dominant role, especially in stabilizing


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Crude Oil Emulsion Resolution

Brief consideration will be given, first, to the physical factors, which influence the rate of sedimentation (i.e. ‘creaming’) of water- in-crude oil emulsions. Secondly, more detailed consideration will be given to those aspects paramount in assisting the actual breaking (droplet coalescence) of crude oil emulsions.

Sedimentation

This will be discussed in terms of the preceding discussion of Stokes’ Law as applied to the creaming of emulsions.

i) The rate of sedimentation will increase with increasing density difference between the crude and dispersed water phases.ρ crude oil range = 0.8 - 1.0 Kg/L @ 15°C

(i.e. 45 to 10°API)

ρ fresh water = 1.0 kg/l

ρ produced water = 1.0 - 1.20 kg /l

Note that certain heavy oils (<10°API) are denser than fresh water in certaintemperature ranges. Most crude oils vary in density from 0.8 to 0.95 kg/l and formation water densities vary from just over 1 kg/l to about 1.20 kg/l. Thus overall density differences between crude oils and their respective produced waters may vary from about 0.05 kg/l to about 0.40 kg/l.

ii) The rate of sedimentation will increase in proportion to the droplet radius squared -when Stokes’ Law is valid. The droplet size distribution of produced crude oilemulsions is polydisperse (as against monodisperse). In most cases, >90 percent by volume of the water is dispersed as droplets in the size range 5 to 25 microns, and many droplets may be smaller than 5 microns. Droplets of this size range will not sediment under gravity. Note that the droplet size distribution may be determined in part by the degree of mixing at the wellhead choke and the wellhead flowingpressures.

iii) The rate of sedimentation will decrease with increasing crude oil viscosity. Crude oil viscosity can vary widely, typically (at 15°C) from 10 mPas (light, conventionalcrude oils) to 10,000,000 mPas (heavy oils, bitumen, tar sand oils). At the appropriate temperatures and shear rates of production, most crude oils of interest will have


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viscosity in the range 5 to 50 mPas. It is important to realize that the dispersion of water as an emulsion in crude oil can substantially change the rheologic properties of the crude; the viscosity is increased and the viscosity-shear rate properties of the produced fluids may change.

The viscosity increase depends upon the temperatures of the emulsion, the shear rate applied to the emulsion, and most importantly, the droplet size and phase volume (i.e. ‘water cut’) of the emulsion.

Thus, particularly at low rates of shear (<10 sec-1), viscosity of crude oil emulsions can be several orders of magnitude greater than that of dry crude oil. Clearly, this viscosityincrease impedes sedimentation and, under worst conditions - namely, low temperatures, low shear, very high water cut (e.g. 50-65%), the resulting high viscosity can pose a serious flow impediment.

Note that Stokes’ Law, derived for the sedimentation of single spheres, takes no account of emulsion viscosity.

Breaking

The preceding section has discussed emulsion instability in terms of sedimentationwhich, while assisting overall oil-water separation, strictly only refers to the process by which produced fluids may eventually comprise an emulsion deficient (upper) andemulsion rich (lower) phase. Actual coalescence of the dispersed water droplets is a prerequisite for emulsion breaking. By some means, the barrier to droplet-dropletcoalescence, i.e. the stabilizing film or layer of natural surfactants at the water droplet-oilinterface, must be overcome. In crude oil production there are three means by which this is achieved:

• Elevated temperatures

• Electrostatic field

• Chemical demulsifier

Of these three, the use of an electrostatic field would not generally be applicable for subsea production and is, therefore, not discussed further here.

Elevated wellhead temperatures will tend to promote demulsification of emulsions that form near the wellhead (e.g., as a result of passage through a wellhead choke.) However, it will not be generally practical to add head to the flow stream for the purpose of


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emulsion breaking. Flowline insulation will provide additional time-at-temperature(relative to uninsulated systems) and, therefore, stable emulsions will be less likely in insulated lines.

If produced fluids do tend to form stable and troublesome (high viscosity) emulsions, chemical demulsifiers are the only practical tool available.

Effect of Temperature

- reduces bulk viscosity- increases Brownian Motion- increases the frequency of droplet-droplet collisions- facilitates film drainage between droplets- increases the density difference between oil and water

Hence, the overall effect of higher temperature is to increase the rate of sedimentation and the number of droplet-droplet encounters and to assist coalescence. In short, higher temperatures decrease emulsion stability.

With many light, waxy crude oils (such as are produced in the North Sea) what is observed during chemical demulsification is not a gradual change in emulsion stability versus temperature, but a discontinuity within a relatively small temperatures interval (typically 10 to 20°C).

Figure 12.3-3: Effect of Temperature on Demulsification


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This discontinuity occurs in the temperature range at which fine wax crystallites(observed by optical microscopy) dissolve. In the above example, note that emulsion stability changes little in range of 20 to 40°C, whereas considerable wax dissolution does occur. The explanation for this lie within the observation that while the overallconcentration of crystallites is essentially unaffected: as fine crystallites dissolve, they are continuously repleted in number by large wax crystals partially dissolving. Eventually, a sufficiently high temperature is reached at which an insufficient number of large particles exists to compensate for the dissolution of the small crystallites - at this point emulsion stability rapid ly decreases.

While increasing the temperature reduces emulsion stability and facilitates emulsionbreaking, it is found that, even at high temperature, a chemical demulsifier is oftenrequired.

Chemical Demulsification

The addition of a properly selected chemical demulsifier, at very low concentrations (usually 1 to 50 PPM) can cause rapid and virtually complete breakdown of otherwise stable water- in-crude oil emulsions.

Chemical demulsifiers comprise, typically, 40 to 60 percent surface active agent(polymers, usually nonionic, e.g. ethoxylated phenols) and 60 to 40 percent solvent (usually aromatic).

At present the number of formulations, patented and commercially available, isenormous; tens of thousands of different demulsifier formulations exist.

How do demulsifiers work? The active ingredient is highly surface-active and can reduce the interfacial tension (γow) between crude oil and water, from a value of~30 mNm-1 to <1 mNm-1 at the concentration of usage.

At first sight this seems a paradox, since according to the equation,

Gow = γowA

a reduction in Gow should reduce the free energy of the emulsion and, therefore, reduce the driving force behind coalescence. The explanation lies with the fact that the barrier to coalescence of water droplets in crude oil emulsions is largely physical: comprising the film of resins, asphaltenes and waxes surrounding the droplets. The primary function of


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the demulsifier is to break or displace the film of natural surfactants, which prevent coalescence.

It the stabilizing film is highly condensed, the demulsifier will require assistance to penetrate it. Increasing temperature, as well as possibly ‘dissolving’ and reducing the viscosity of the film, will also increase the rate of droplet-droplet collision; when dropletscollide, if they become non-spherical, the surface area increases and the film is expanded.

Figure 12.3-4: Stabilizing FilmNotes:

i) If ‘too much’ demulsifier is added then, in addition to having broken or displacedthe original ‘skin’ of crude oil surfactants, the resulting demulsifier-surfactant film may stabilize the water droplets. This overdosing effect can produce crude oil emulsions more stable than before demulsifier treatment and result in emulsionswhich are now extremely difficult to destabilize.

ii) The protective film of natural surfactants often takes a finite time (typically minutes, or even hours) to build up and become condensed; the emulsion ages and becomes more difficult to resolve. Clearly, in such cases, the earlier the demulsifier can be introduced to the emulsion, the more easily the emulsion will be broken.

iii) Similarly, it is vital that when demulsifier is injected it is well dispersed; most demulsifiers are not oil-soluble (but are oil-dispersible). The manufacturer’s choice of carrier solvent, and the dilution of the active ingredient within the solvent, can have a dramatic influence on demulsifier efficiency.


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iv) Other surface-active oilfield chemicals (notably corrosion inhibitors) may interfere with the demulsifier itself or the mechanism by which demulsifier displaces the natural crude oil surfactants from the oil-water interface.

12.4 Impact on Facilities and Production

As discussed previously, the primary impact of emulsions on production is a potential increase in viscosity. If water and oil do not naturally separate and/or if added chemicals are not effective, the additional fluid viscosity will have to be accommodated in the hydraulic design.

If water and oil do naturally separate in the flowlines and/or if added chemicals are effective (and if there is not a large pressure drop upstream from the separator) emulsions will not be a problem for the facility. If emulsions have not broken in the flowline and/or if there is significant mixing at the separator entrance, then methods such as:

• Chemicals,

• temperature and/or increased residence time, and

• electrostatic precipitation

will have to be used

12 emulsions

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