Crude-Oil Emulsions: A State-Of-The-Art Review PDF

Preview

Summary

Crude-Oil Emulsions: A State-Of-The-Art Review Sunil Kokal, SPE, Saudi Aramco Summary paper provides a review of crude-oil emulsions; however, the re- The formation of emulsions during oil production is a costly prob- view is limited to the produced oilfield emulsions at the wellhead lem, both in te...

Description

Crude-Oil Emulsions: A State-Of-The-Art Review Sunil Kokal, SPE, Saudi Aramco

Summary The formation of emulsions during oil production is a costly problem, both in terms of chemicals used and production lost. This paper discusses production and operational problems related to crude-oil emulsions and presents a review that will be useful for practicing engineers. The first part of this paper presents why emulsions form during oil production, the types of emulsions encountered, and new methods for characterizing them. Crude-oil emulsions are stabilized by rigid interfacial films that form a “skin” on water droplets and prevent the droplets from coalescing. The stability of these interfacial films, and hence, the stability of the emulsions, depends on a number of factors, including the heavy material in the crude oil (e.g., asphaltenes, resins, and waxes), solids (e.g., clays, scales, and corrosion products), temperature, droplet size and droplet-size distribution, pH, and oil and brine composition. The effects of these factors on emulsion stability are reviewed within this paper. The second part of this paper presents methods to tackle crudeoil emulsions. The focus is on the destabilization of emulsions and the demulsification process. Emulsions are destabilized by increasing temperature and residence time, removal of solids, and controlling emulsifiers. The mechanisms involved in demulsification (e.g., flocculation, aggregation, sedimentation, creaming, and coalescence) are discussed in terms of the stability of the interfacial films. The methods involved in demulsification—including thermal, mechanical, electrical, and chemical—are also presented. Experience and economics determine which methods are used, and to what degree, for emulsion treatment. Finally, a section on field applications also is included that should be useful for the practicing engineer who deals with emulsions either regularly or on a limited basis. Herein the fieldemulsion treatment program is discussed, and more importantly, methods to prevent emulsion problems are highlighted. Recommendations are made for reducing and optimizing demulsifier dosage and controlling emulsion problems. Introduction Crude oil is seldom produced alone. It is generally commingled with water, which creates a number of problems during oil production. Produced water occurs in two ways: some of the water may be produced as free water (i.e., water that will settle out fairly rapidly), and some of the water may be produced in the form of emulsions. Emulsions are difficult to treat and cause a number of operational problems, such as tripping of separation equipment in gas/oil separating plants (GOSPs), production of off-specification crude oil, and creating high pressure drops in flowlines. Emulsions have to be treated to remove the dispersed water and associated inorganic salts to meet crude specification for transportation, storage, and export and to reduce corrosion and catalyst poisoning in downstream-processing facilities. Emulsions can be encountered in almost all phases of oil production and processing: inside the reservoirs, wellbores, wellheads, and wet crude-handling facilities; transportation through pipelines and crude storage; and during petroleum processing. This

Copyright © 2005 Society of Petroleum Engineers This paper (SPE 77497) was first presented at the 2002 SPE Annual Technical Conference and Exhibition, San Antonio, 29 September–2 October, and revised for publication. Original manuscript received for review 19 April 2004. Revised manuscript received 2 November 2004. Paper peer approved 7 December 2004.

February 2005 SPE Production & Facilities

paper provides a review of crude-oil emulsions; however, the review is limited to the produced oilfield emulsions at the wellhead and at the wet crude-handling facilities. It looks at the characteristics, occurrence, formation, stability, handling, and breaking of produced oilfield emulsions. Crude-oil emulsions is a broad area and several books have been written on the subject.1–3 This paper provides an overview that is primarily targeted towards the practicing engineers with the objective of familiarizing them with the most important issues. For in-depth details and further reading on the subject matter, the reader is directed to the textbooks1–3 and website addresses4–9 provided in the references. Definitions. A crude-oil emulsion is a dispersion of water droplets in oil. Produced oilfield emulsions can be classified into three broad groups: • Water-in-oil (W/O) emulsions. • Oil-in-water (O/W) emulsions. • Multiple or complex emulsions. The W/O emulsions consist of water droplets in a continuous oil phase, and the O/W emulsions consist of oil droplets in a continuous water phase. In the oil industry, W/O emulsions are more common (most produced oilfield emulsions are of this kind), and therefore, the O/W emulsions are sometimes referred to as “reverse” emulsions. Multiple emulsions are more complex and consist of tiny droplets suspended in bigger droplets that are suspended in a continuous phase. For example, a water-in-oil-in-water (W/O/W) emulsion consists of water droplets suspended in larger oil droplets that in turn are suspended in a continuous water phase. Fig. 1 shows the various types of emulsions. Given the oil and water phases, the type of emulsion that is formed depends on a number of factors.1,2,10 As a rule of thumb, when the volume fraction of one phase is very small compared with the other, then the phase that has the smaller fraction is the dispersed phase and the other will form the continuous phase. When the phase-volume ratio is close to 1 (both phases of approximately the same magnitude), then other factors will determine the type of emulsion formed. Emulsions are stabilized by emulsifiers (i.e., surface-active agents, or surfactants) that tend to concentrate at the oil/water interface where they form interfacial films. This generally leads to a reduction of interfacial tension (IFT) and promotes dispersion and emulsification of the droplets. Naturally occurring emulsifiers in the crude oil include higher boiling-point fractions, such as asphaltenes and resins, and organic acids and bases. These compounds are believed to be the main constituents of interfacial films, which form around water droplets in an oilfield emulsion. Other surfactants that may be present are from chemicals that are injected into the formation or wellbore (e.g., drilling fluids; stimulation chemicals; and injected inhibitors for corrosion, scale, waxes, and asphaltenes control). Fine solids can also act as mechanical stabilizers. These particles, which have to be much smaller than emulsion droplets, collect at the oil/water interface and are wetted by both the oil and water. The effectiveness of these solids in stabilizing emulsions depends on a number of factors, such as particle size, particle interactions, and the wettability of the particles.11 Finely divided solids found in oil production include clay particles, sand, asphaltenes and waxes, corrosion products, mineral scales, and drilling muds. Oilfield emulsions are characterized by a number of properties including appearance, basic sediment and water, droplet size, bulk 5

Fig. 2—Droplet-size distribution of petroleum emulsions. (Typical Saudi Arabian crude oil emulsions. Distributions obtained using Nikon microscope and image-analysis software.) Fig. 1—Photomicrographs of emulsions.

and interfacial viscosities, and conductivities. Some of these properties are described below, while others are described in other publications.12 Droplet Size and Droplet-Size Distribution. Produced oilfield emulsions generally have droplet diameters exceeding 0.1 ␮m and may be larger than 50 ␮m. Droplet-size distributions of typical petroleum emulsions are shown in Fig. 2. The droplet-size distribution in an emulsion depends on a number of factors, including the IFT, shear, nature of emulsifying agents, presence of solids, and bulk properties of oil and water. Droplet-size distribution in an emulsion determines—to a certain extent—the stability of the emulsion and should be taken into consideration in the selection of optimum-treatment protocols. As a general rule of thumb, the smaller the average size of the dispersed water droplets, the longer the residence time required (which implies larger separating-plant equipment sizes). Viscosity of Emulsions. Viscosity of emulsions can be substantially higher than the viscosity of either the oil or the water. This is because emulsions show non-Newtonian behavior1 caused by droplet “crowding” or structural viscosity. At certain volume fractions of the water phase (water cut), oilfield emulsions behave as shear-thinning, or pseudoplastic, fluids; as shear rate increases, their viscosity decreases. Fig. 3 shows the viscosities of a very tight emulsion at different water cuts. The viscosity data shown in Fig. 3 (for Saudi Arabian crude emulsions) indicate that the emulsions exhibit Newtonian behavior up to a water content of 30% (this is indicated by constant values of viscosity for all shear rates or a slope of zero). At water cuts above 30%, the slopes of the curves deviate from zero, indicating non-Newtonian behavior.

Fig. 3—Viscosities of very tight emulsions at 125°F. (Safaniya crude emulsions. Data obtained using Haake Rheostress RS150 rheometer.) 6

Also, the non-Newtonian behavior is pseudoplastic, or shearthinning, behavior (i.e., viscosity decreases with increasing shear rates). Fig. 3 shows the very high viscosities achieved as the water cut increases up to 80% (compare with viscosities of oil ≈20 cp and water ≈1 cp). At approximately 80% water cut, an interesting phenomenon takes place. Up to a water cut of 80%, the emulsion is a W/O emulsion; at 80%, the emulsion “inverts” to an O/W emulsion, and the water, which was the dispersed phase, now becomes the continuous phase. In this particular case, multiple emulsions (W/O/W) were observed right up to very high water concentrations (greater than 95%). The viscosity of emulsions depends on a number of factors: • Viscosities of oil and water. • Volume fraction of water dispersed. • Droplet-size distribution. • Temperature. • Shear rate. • Amount of solids present. The relative viscosity of an emulsion is shown in Fig. 4 for several different types of emulsions. While these data are for Saudi Arabian crude emulsions, such plots can be generated easily for any crude-oil emulsion. Emulsion viscosity depends on several factors, and Fig. 4 provides only an estimate. For more precise values, experimental data must be used. Emulsion viscosity is measured by standard viscometers and rheometers, such as capillary-tube and rotational viscometers (e.g., concentric cylinder, cone and plate, and parallel plate). It is important that the temperature is constant and quoted with the viscosity data. Special procedures must be adopted for measuring the rheology of emulsions.1 Stability of Emulsions From a purely thermodynamic point of view, an emulsion is an unstable system. This is because there is a natural tendency for a

Fig. 4—Relative viscosities of emulsions. (The curves are based on typical Saudi Arabian crude emulsion viscosities measured using Haake Rheostress RS-150 rheometer.) February 2005 SPE Production & Facilities

Interfacial Films. As mentioned previously, produced oilfield emulsions are stabilized by films that form around the water droplets at the oil/water interface. These films are believed to result from the adsorption of high-molecular-weight polar molecules that are interfacially active (i.e., exhibit surfactant-like behavior). These films enhance the stability of emulsion by (a) reducing IFT and (b) increasing the interfacial viscosity. Highly viscous interfacial films retard the rate of oil-film drainage during the coagulation of the water droplets by providing a mechanical barrier to coalescence. This can lead to a reduction in the rate of emulsion breakdown. The characteristics of interfacial films are a function of the crude-oil type (e.g., asphaltic and paraffinic), composition and pH of water, temperature, the extent to which the adsorbed film is compressed, contact or aging time, and concentration of polar molecules in the crude oil.11,13–15 A good correlation exists between the occurrence of incompressible interfacial film and emulsion stability. These films are classified into two categories on the basis of their mobilities.13–14 Rigid, or Solid, Films. These are like an insoluble skin on water droplets and are characterized by very-high interfacial viscosity. There is considerable evidence that these films are formed by polar fractions of the oil and other emulsifiers and may be further stabilized by fine solids. These films play a significant role in hampering the droplet-coalescence process. They provide a structural barrier to droplet coalescence and increase emulsion stability. These films also have viscoelastic properties. Mobile, or Liquid, Films. These films, as the name implies, are mobile and characterized by low interfacial viscosities. These are formed, for example, when a demulsifier is added to an emulsion. They are inherently less stable. Coalescence of water droplets is enhanced. Stability of emulsions has been correlated with the mobility of interfacial films.14 Surfactants that modify the rigidity of the film can considerably speed up the demulsification process. This will be discussed further under the section “Demulsification of Emulsions.”

in the formation of a rigid film. An asphaltene-stabilized water droplet is shown in Fig. 5. When such a film is formed, it acts as a barrier to droplet coalescence. For two drops to coalesce together, the film has to be drained and ruptured. The presence of the asphaltenes can naturally retard the drainage of this film. Reference 18 provides an excellent discussion on the mechanism of asphaltene-stabilized emulsions. The state of asphaltenes in the crude oil also has an effect on its emulsion-stabilizing properties. While asphaltenes will stabilize emulsions when they are present in a colloidal state (not yet flocculated), there is strong evidence that their emulsion-stabilizing properties are significantly enhanced when they are precipitated from the crude oil and are present in the solid phase. Resins are complex high-molecular-weight compounds that are not soluble in ethylacetate, but are soluble in n-heptane. The role of resins in stabilizing emulsions has also been debated in literature. Some researchers believe that resins have a tendency to associate with asphaltenes and, together form a micelle. The resulting asphaltene/resin micelle plays a key role in stabilizing emulsions. It appears that the asphaltene/resin ratio in the crude oil is responsible for the type of film formed (solid or mobile) and, hence, is directly linked to the stability of the emulsion.14,16 However, there remains considerable debate on this topic. Waxes are the high-molecular-weight paraffin substances present in the crude oil that crystallize out when the oil is cooled below its cloud point. They are insoluble in acetone and dichloromethane at 30°C. The effect of waxes on emulsion stability is not clear from the literature. Waxes by themselves are soluble in oil and, in the absence of asphaltenes, do not form stable emulsions in model oils.16 However, the addition of a nominal amount (an amount insufficient by itself to produce emulsions) of asphaltenes to oils containing wax can lead to the formation of stable emulsions. Therefore, waxes can interact synergistically with asphaltenes to stabilize emulsions. The physical state of the wax in the crude oil also plays an important role in emulsion stabilization. Waxes are more apt to form a stable emulsion when they are present as fine solids in the emulsion. Crudes that have a low cloud point generally have a greater tendency to form stable and tight emulsions than crudes with high cloud points. Similarly, lower temperatures, in general, enhance the emulsion-forming tendencies of crude oils. Solids. Fine-solid particles present in the crude oil are capable of effectively stabilizing emulsions. The effectiveness of these solids in stabilizing emulsions depends on such factors as the particle size, interparticle interactions, and the wettability of the solids.11,21 Solid particles stabilize emulsions by diffusing to the oil/water interface where they form rigid structures (films) that can sterically inhibit the coalescence of emulsion droplets. Furthermore, solid particles at the interface may be charged, which may also enhance the stability of the emulsion. Particles must be much smaller than the size of the emulsion droplets to act as emulsion stabilizers. These particles typically range from smaller than 1 ␮m to several ␮m in size,1 and they are suspended colloidally in the liquids. The wettability of solid particles plays an important role in the emulsion-stabilizing process. If the solid remains entirely in the oil or water phase, it will not be an emulsion stabilizer. It must be

Factors Affecting Stability. The important factors that affect emulsion stability include the following.13–16 Heavy Fraction in Crude Oil. It is now well recognized that the naturally occurring emulsifiers (or stabilizers) are concentrated in the higher-boiling-point, polar fraction of the crude oil.13–19 These include asphaltenes, resins, and oil-soluble organic acids (e.g., naphthenic and carboxylic acids) and bases. These compounds are the main constituents of the interfacial films surrounding the water droplets that give the emulsions their stability. While it is well established that the heavy asphaltenic material stabilizes oil-continuous emulsions, there is considerable debate on the precise mechanism of stabilization. The asphaltenes reside at the oil/water interface because of their surface-active properties.18–20 The accumulation of asphaltenes at the interface results

Fig. 5—Mechanism of emulsion stabilization by asphaltenes.

liquid/liquid system to separate and reduce its interfacial area and, hence, its interfacial energy. However, most emulsions are stable over a period of time (i.e., they possess kinetic stability).1 Produced oilfield emulsions are classified on the basis of their degree of kinetic stability as follows: • Loose emulsions. Those that separate in a few minutes. The separated water is sometimes referred to as free water. • Medium emulsions. Will separate in ten minutes or more. • Tight emulsions. Will separate (sometimes only partially) in a matter of hours or even days. Emulsions are considered special liquid-in-liquid colloidal dispersions. Their kinetic stability is a consequence of a small droplet size and the presence of an interfacial film around the water droplets. Emulsion kinetic stability is attained by stabilizing agents (or emulsifiers) that could be naturally occurring in the crude oil (asphaltenes, for example) or added during production (stimulating chemicals, for example). These stabilizers suppress the mechanisms involved (i.e., sedimentation, aggregation or flocculation, coalescence, and phase inversion) in emulsion breakdown.

February 2005 SPE Production & Facilities

7

present at the interface and must be wetted by both the oil and water phases for it to act as an emulsion stabilizer. When the solids are preferentially oil-wet (more of the solid in the oil phase), then a W/O emulsion will result. Oil-wet particles will preferentially partition into the oil phase and will prevent the coalescence of water droplets by steric hindrance. Similarly, water-wet solids will stabilize a water-continuous or an O/W emulsion. Examples of oil-wet solids are asphaltenes and waxes. Examples of water-wet solids are inorganic scales (e.g., CaCO3 and CaSO4), clays, and sand. Water-wet particles can be made oil-wet with a coating of heavy-organic-polar compounds.22 When solids are wetted by both the oil and water (intermediate wettability), they agglomerate at the interface and retard droplet/ droplet coalescence. These particles will have to be relocated into either the oil or water for coalescence to take place. This process ...