Chapter 2: Emulsions
Chapter 2: Emulsions
2.1 Problem
The most important objective of any oil production facility is the separation of water and other foreign materials from the produced crude. The breaking of these “crude oil and water emulsions” constitutes one of the more challenging problems in today’s oil producing industry.
During the productive life of an oil or gas well, a stage is reached when water will be co-produced in unacceptable quantities. This water coexists with the hydrocarbons in the reservoir and gradually infiltrates into the hydrocarbon-bearing region of the formation. Eventually water becomes part of the production from the wells regardless of the method of recovery.
Figure 1 on the next page shows a simplified view of how water may be produced. In the early life of the producing field some wells that are drilled close to the oil-water contact level will begin to produce water. Other wells drilled higher in the reservoir will produce dry oil. Later, as the oil in the reservoir becomes depleted and the water expands upward, the oil-water interface level rises until the wells higher in the reservoir begin to produce water. In some cases, it is possible to exclude some or most of the water by plugging back the lower part of the wellbore with cement and perforating an interval higher up in the formation. This can at least delay water encroachment for a time.
Secondary or tertiary recovery methods are another cause of water encroachment. These recovery methods are employed to increase the amount of oil recovered from the reservoir, and they involve many different techniques. A number of these methods require the injection of water or steam into the reservoir, and of course, the water is often produced again with the oil.
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Chapter 2: Emulsions
Figure 1
Oil leaving the producing facility has to meet a low water content specification. Too high a level of produced water in the exported oil would severely reduce pumping and other transport capacity. Even a small percentage of emulsified water in crude oil increases the cost of pumping due to the larger volume and the higher viscosity of the oil. In addition, the high salinity of the water would cause corrosion and scaling in downstream operations. It is therefore necessary to remove the water and associated salts from the crude oil.
Production of immiscible oil and water through wellhead chokes and valves, along with the simultaneous action of shear and pressure reduction, often produces stable water-in-oil mixtures. The relative stability of these mixtures depends upon many factors such as water cut, the nature of salts present, the viscosity of the oil, and in particular, the indigenous surfactants present in the oil.
Some of the water does not mix with the oil to give a stable mixture. This “free water” readily separates from the oil. More often, the conditions of production are such that a stable mixture is formed. Such a mixture is called an emulsion and must be specially treated before separation can occur.
To appreciate the difficulties associated with the production and treatment of emulsions it is helpful to have some basic knowledge of emulsion theory.
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Chapter 2: Emulsions
2.2 Emulsion Theory
An emulsion is a mixture of two immiscible liquids, one of which is dispersed as droplets in the other. The liquid in an emulsion that is broken into droplets is known as the dispersed or internal phase, whereas the liquid surrounding the droplets is called the continuous or external phase. Emulsions formed in oil producing operations are predominantly water-in-oil.
2.2.1 Types of Emulsion
Emulsions are classified according to which phase is dispersed and which phase is continuous.
1.Water-in-Oil Emulsions (W/O)
Water is dispersed in oil: water is the dispersed or internal phase, and oil is the continuous or external phase.
This type is often referred to as a “regular emulsion” or an oil continuous emulsion. Water-in-oil emulsions are the type most frequently encountered when oil and water are produced. An oil-in-water emulsion may contain anywhere from a trace to 90 plus percent water.
Treating this type of emulsion is called dehydration.
2.Oil-in-Water Emulsion (O/W)
Oil is dispersed in water: oil is the dispersed or internal phase, water is the continuous or external phase.
This type can also be called a “reverse emulsion” or water continuous emulsion. These emulsions exist naturally in certain parts of the world. Oil-in-water emulsions can also be encountered in the water that has been separated from the oil during dehydration.
Treatment of this type of emulsion is sometimes referred to as de-oiling.
3.Multiphase Emulsions
It is common to find both oil-in-water and water-in-oil emulsions occurring simultaneously. This is frequently encountered in slop oil systems and storage tanks where various emulsions have mixed and been allowed to stand for a period of time. It can also result from various secondary and tertiary recovery processes.
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2.2.2 Formation of Emulsions
A stable emulsion is one that will not break down without some form of treating. Three conditions are necessary for the formation of a stable emulsion:
1.The liquids must be immiscible.
2.There must be an emulsifying agent, or emulsifier, present.
3.There must be sufficient agitation to disperse one liquid as droplets in the other.
Many emulsions are prepared for commercial use, such as insecticides and medicines. These are made up of two or more liquids that will not normally mix, plus the emulsifying agent. A common household emulsion is mayonnaise. Mayonnaise is made of vegetable oil and vinegar with eggs used as the emulsifying agent. This combination would not remain mixed if the eggs, or some other emulsifying agent, were not present. They could be mixed by violent agitation, but they would soon separate after agitation was stopped. Similarly, to form a stable emulsion of crude and water, an emulsifying agent must be present.
The stability of petroleum emulsions depends upon the presence of an emulsifying agent that is soluble, dispersible, or wettable in or by the oil or the water. If the emulsifying agent is soluble, dispersible, or wettable more easily in or by oil than water, then the oil will be the external phase and water the dispersed phase. Whereas, if the emulsifier is soluble, dispersible, or wettable more readily in or by water than oil, then the opposite type of emulsion will be formed.
The most common emulsifying agent found in petroleum emulsions include asphaltenes, solid paraffins, resinous substances, napthenic and other oil soluble organic acids, and finely divided materials that are more soluble, wettable, or dispersible in oil than water. Also found are zinc, iron, aluminum sulfates, calcium carbonate, silica, and iron sulfide. These substances are usually found at the interface between the oil and droplets of water in the form of a film around the droplet. Other emulsifying agents may be drilling, stimulation, or production chemicals. These emulsions are referred to as “chemically stabilized emulsions.” Care should be taken in the selection of chemicals to prevent formation of chemically induced emulsions. For example, corrosion inhibitors should be tested for emulsion tendency before a product is selected in order to prevent emulsification of the well during batch treatment. In addition, demulsifiers should be tested for overtreatment during Bottle Testing to prevent the application of a demulsifier which may overtreat or “burn” the oil.
The agitation necessary to form most petroleum emulsions is caused by gas bubbling through the oil and water or by the two liquids being forced through relatively small openings, such as chokes, at high velocity. It is a recognized fact that emulsions are formed rarely, if ever, in the oil reservoir, although some may be formed where the water and oil enter the well.
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Chapter 2: Emulsions Emulsification generally occurs at some stages in production. Various sources of agitation sufficient to cause emulsification may be present between the time when the oil enters the well and the time when they are separated at the surface. There is evidence that emulsions are formed in wells and in the mechanical equipment used in production, or even later in the flowlines on the surface. Undoubtedly, certain methods of production contribute to the formation of emulsions. Naturally, flowing wells produced through chokes and wells produced by gas lift or air lift usually cause the most difficult emulsion problems. Most emulsions are formed before the fluid leaves the wellhead.
2.2.3 Other Factors Affecting the Stability of Emulsions
Other factors that can affect the stability of emulsions are:
Viscosity
Specific gravity
Water percentage
Total dissolved solids
Age of emulsion
Each of these is described in this section.
2.2.3.1 Viscosity
The viscosity of a liquid may be thought of as its resistance to flow: the higher the viscosity, the greater the resistance of a liquid to flow. Conversely, the lower the viscosity, the more readily the liquid flows. Often, if a liquid of high viscosity is heated, the viscosity decreases so that the liquid flows more freely. Therefore, heating a crude oil of high viscosity lowers the viscosity and makes it flow easier.
An oil of high viscosity requires more time for the water droplets to coalesce and settle out than does an oil of low viscosity. This is because the water droplets cannot move as rapidly through a high viscosity oil as they can through a low viscosity oil. A common example of this may be seen by observing the slow rate at which air bubbles rise in syrup, which has a high viscosity, as compared to the fast rate at which they rise in water, which has a low viscosity. Air bubbles rise, whereas water droplets in oil settle, but the effect is the same.
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2.2.3.2 Specific Gravity
Specific gravity should not be confused with API gravity. The specific gravity of a liquid substance is the weight of a given amount of that liquid at a given temperature compared to the weight of an equal volume of water at the same temperature. For example, if 1 cubic inch of water at 39ºF weighs 1 unit, and 1 cubic inch of another liquid at 39ºF weighs 95 percent of that unit, then the specific gravity of the liquid is 0.95. On the other hand:
degrees API = / 1415. / - 131.5specific gravity
Thus, the water in the example of specific gravity above has an API gravity of 10 degrees, while the liquid with a specific gravity of 0.95 has an API gravity of approximately 17.5 degrees.
The difference in specific gravity between the oil and water has a bearing on the stability of the emulsion. The greater the difference, the faster the water can settle. For instance, in a water-in-oil emulsion, a heavy oil (one with a high specific gravity and low API gravity) tends to keep water droplets in suspension longer than an oil with low specific gravity and high API gravity. On the other hand, a lighter water such as freshwater does not settle out of any oil as rapidly as salt water because salt water is heavier. The fact that heavier liquids or objects do not stay suspended in a liquid for as long as do lighter liquids or objects can be illustrated by dropping a steel roller bearing and rubber pencil eraser of the same size and shape into a tall glass of water. The steel bearing, which is considerably heavier, goes directly to the bottom, but the lighter rubber eraser sinks slower.
Heating the emulsion increases the specific gravity difference between the oil and water (lowering that of oil) in addition to lowering viscosity.
2.2.3.3 Water Percentage
A factor that influences, to a certain degree, the tendency of oil and water to emulsify is the relative proportion of oil and water produced. Laboratory tests conducted to determine the influence of oil and water concentrations in emulsions show that emulsification occurs over a wide range of mixtures and that maximum emulsibility is reached at some definite ratio of water to oil.
A small percentage of water in oil often emulsifies much more thoroughly and permanently than a large amount. In fact, in many wells producing only small quantities of water, tight emulsions are formed that disappear almost completely if the percentage of water is increased beyond a certain limit. In general, the severity of an emulsion problem usually will diminish when the quantity of water produced by a well approaches or exceeds the quantity of oil produced.
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Chapter 2: Emulsions
2.2.3.4 Total Dissolved Solids
The total dissolved solids (TDS) or salinity of the water also influences settling rates: the heavier the water, the faster the settling. Salinity also influences demulsifier or surfactant partitioning, as high TDS brine may remain clear but relatively freshwater may become cloudy using high RSN demulsifiers.
Freshwater emulsions are usually more difficult to treat.
2.2.3.5 Age of Emulsion
Crude oil emulsions are systems that are not in stable equilibrium. According to the laws of thermodynamics, such systems change continually in an effort to attain equilibrium. As a result, emulsions increase in stability with age, which generally increases their resistance to dehydration. With time, emulsifying agents can migrate to the dispersed water droplets and coat these droplets completely. Solids (paraffin, clay, etc.) may then coat the emulsified water drops. Age stabilized emulsions may require a much higher chemical rate to treat or even a different chemical from the fresh emulsion.
2.3 Theories of Demulsification
There are many theories that have been advanced regarding the problem of resolving crude oil emulsions. Unfortunately, these are as diverse as the emulsions they concern, and no one theory is equally applicable in all emulsions.
2.3.1 Reverse Phase
In some cases, the breaking of emulsions has been based on the theory that the addition of a reagent, which would produce an oil-in-water emulsion, will break a water-in-oil emulsion by attempting to reverse the phases; and that in so acting, the intermediate condition of complete demulsification will be accomplished. Though this may sometimes be true, it is not always the case.
2.3.2 Rigid Film
There is one school of thought that the emulsion-breaking reagent may have the action of making the interfacial film rigid or to convert it from a plastic, somewhat distensible envelope to a glasslike one that has a relatively low coefficient of expansion. When the enclosed water is expanded by heating, the envelope is shattered and the emulsion is broken. To extend this suggestion and assume that the reagent has not only the power of making the film rigid, but actually of contracting it slightly is to supply an explanation of the efficacy of such reagents in the absence of heat.
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2.3.3 pH
Other schools of thought postulate that the emulsifier is rendered inactive by the addition of the demulsifier through neutralization, change in pH, or loss of solubility. Reverse emulsions especially may be treated by charge neutralization (most reverse emulsion breakers are cationic) or pH change. Most regular emulsions are treated with nonionics.
2.3.4 Electronic Charge
Still others believe that the emulsifying agents are polar bodies and function because of their electronic charges, and any disturbance of these charges by electron carrying molecules will result in breaking the emulsion. This is especially applicable to reverse emulsions.
2.3.5 Temperature
Another possible explanation of the great effect of small temperature increases in some cases is that such added heat is sufficient to cause a change of state in the film ( i.e., converts it from a solid to a liquid and thereby affects its stability greatly). Likewise, the effects of reagents in the absence of added heat have been asserted to be dependent on their power to cause such a change of state in the substance comprising the film, thereby dissolving it from the interface.
2.3.6 Surface Tension
The theory that petroleum emulsion breaking is caused by a reduction in surface tension is probably the most common. This phenomenon is often referred to without any suggestion as to what constituent is having its surface tension lowered. It is likewise generally predicated on a two-component system, whereas petroleum emulsions are definitely three-component systems. The reagents used may have the incidental effect of reducing the surface tension of either the water or the oil or both, but it is not identical with predicting any emulsion resolution on such reduction as a cause.
In any case, the most widely accepted general explanation is that the interfacial surface between the dispersed component and the continuous component is modified in some manner. It is generally recognized that the liquid having the greater surface tension will form the inner, or dispersed, phase. Hence, a change in the surface tension of either component could result in resolution, provided that the surface tension lowering is stopped short of the point of reversing the emulsion.
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2.4 Treating Methods
The factors involved in treating water-in-oil emulsions include:
1.Breaking the film surrounding the small water droplets and coalescing the droplets to produce larger drops.
2.Settling the water drops during or after their coalescence.
Theoretically, all emulsions separate into oil and water if allowed to settle for an unlimited time. Much of the water produced with petroleum does separate without the assistance of heat, chemicals or other devices. However, the small water droplets in water-in-oil emulsions are usually surrounded by a tough film that gives the appearance of a plastic wrap when viewed under a microscope. This film resists being broken, and until the film is broken, the water droplets do not merge together into coalescence (at least in any reasonable length of time).
The higher the viscosity of a water-in-oil emulsion, the slower is the settling rate of the water in it. Thus, if the emulsion is at a low temperature and its viscosity is high, the separation of water from the oil is slow. Also, the smaller the water droplets are in the oil, the longer it takes for