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Emulsions OIL AND GAS SEPARATORS
Well fluids can come in the form of an emulsion, solution gas, or can be free. In this drawing, oil is mixed with small droplets of water, which are coated with oil. A mixture like this is called an emulsion.
In an emulsion, at least one substance is finely dispersed throughout another, usually in the form of droplets.
This is another way well fluids can be mixed. The fluids in the drawing are layered. When fluids from the well are not held, as they are in an emulsion, they are free. Free fluids separate more easily than fluids in an emulsion.
Crude Oil Dehydration Removing water from crude oil is called crude oil dehydration, or emulsion treating. The water in crude oil may be free or emulsified. The water that settles out quicker is the free water.
Oil and water are immiscible. Immiscible means that the fluids will not stay mixed together. Oil and water cannot form an emulsion without an emulsifying agent like neo-sulfate, iron sulfate, or silica. These agents form a film around the droplets of water in oil, which must be broken before the two can separate. When the film around the individual droplets of water is broken, the drops coalesce into larger drops, which gradually become heavy enough to settle out of the oil. Heat, chemicals and/or electricity can rupture the film surrounding water droplets in oil.
Removing the water from crude cuts the size and operating costs of pipeline systems. And, it decreases the tankage capacity required to store the crude. Water that settles out of oil is in an emulsion.
True.
False.
Answer
Water that settles out of oil is free. Water is in an emulsion when it is held, or trapped in oil.
© 2004, Resource Development Company, LLC.
Fluid Separation OIL AND GAS SEPARATORS Three variables that control fluid separation include:
• Fluid pressure. • Fluid composition. • Fluid temperature.
Pressure Reservoir pressures are generally much higher than atmospheric pressure. As well fluids reach the surface, pressure on them is decreased, and the fluid's ability to hold gas in solution is decreased. Light fluids begin to separate naturally when the pressure is lowered. Very often, pressure is fixed according to sales-line pressure, so pressure is usually not under the lease operator's control.
Composition Gravity alone will eventually cause heavy components to settle out and light components to rise. The lease operator has little control over fluid composition.
Temperature Solution gas released as free gas is held by the surface tension of the oil. To release free gas from the oil's surface tension, the temperature of the oil is increased. Surface tension is reduced when well fluids are warmed because gases begin to separate when their temperature is increased.
In general, a decrease in temperature at constant pressure results in more liquids and less gases. More liquids are produced at the lowest possible separator temperature. However, beyond a certain point, further decreases in temperature become too expensive because the extra liquid recovery is not profitable. The optimum operating temperature is the most profitable temperature. A useful rule of thumb is that the maximum amount of profitable liquid recovery occurs between 15° and 30° F. Frequently, the only variable controlled by the lease operator is temperature.
True.
False.
Answer
The lease operator has little control over fluid composition and pressure is often fixed according to sales-line pressure.
To release gas from an oil solution, you should:
Decrease temperature.
Increase temperature.
Answer
A decrease in temperature at constant pressure will:
Produce similar amounts of liquid and gas.
Produce more gas and less liquid.
Produce more liquid and less gas.
Answer
Low-Temperature Separators Because well stream flowing temperatures average much higher than 15 - 30° F, the stream has to be cooled. A choke is a primary element in a low-temperature separating
system that uses gas expansion for cooling. Notice that the flow path through the choke is narrow compared with the line. To maintain rate through the choke, velocity must increase.
A fluid has a definable amount of energy. When its energy in the form of velocity increases, its energy in the form of static pressure decreases. The velocity of the fluid downstream of the choke is higher; therefore, fluid pressure downstream is lower than pressure upstream.
When fluid pressure decreases as it passes the choke, solution gas begins to separate from the oil. As the volume of the gas increases, its temperature decreases. So, as the gas expands downstream of the choke, its temperature decreases.
Separator temperature is important to both high- and low-pressure oil and gas wells. However, much more cooling from gas expansion is possible with a high-pressure stream.
When velocity increases, pressure decreases.
True.
False.
Answer
When gas expands, its temperature:
Increases.
Decreases.
Answer
The amount of hydrocarbons recovered from well fluids depends upon the _______ of well fluids.
Temperature.
Pressure.
Composition.
All of the above.
Answer
© 2004, Resource Development Company, LLC.
Flash, Differential, and Stage Separation OIL AND GAS SEPARATORS Flash Separation Flash separation occurs as a result of a pressure drop in tubing and lines. Because the space in lines and tubing is limited, gas remains in close contact. No means is provided for removing the gas from the liquid, so flash separation is relatively incomplete. When gases flash from reservoir fluids, condensable liquid
vapors generally do not have the time or space needed to condense. So, flash separation yields a comparatively high proportion of gaseous hydrocarbons.
Differential Separation Differential separation is the separation of gas that occurs in a separator. Differential separation is more complete than flash separation. In a separator, there is enough time and space for heavy vapors to condense. So, differential separation yields a comparatively high proportion of liquid hydrocarbons. Which type of separation yields a higher proportion of gaseous hydrocarbons?
Flash separation.
Differential separation.
Answer
Time is an important factor in differential separation.
True.
False.
Answer
Stage Separation When wellhead and separator pressures are high, the ability of fluids to hold gas in solution is high. Pressures must be lowered to get the most complete separation.
When pressure is lowered quickly, the gas is likely to flash from the liquids, causing incomplete separation with condensable vapors lost to the gas. Gradually lowering the pressure in stages results in the greatest recovery of liquid and the most complete release of gas held in solution.
Because separation continues in the stock tank, a separation system including one separator and a stock tank is a two-stage separation.
Look at this separation system.
How many stages does this system have?
Answer
As you can see from the drawing, pressure is gradually lowered from the first stage at 135 PSIA to 14.7 PSIA in the third stage.
This chart lists approximate percentages of differential separation according to stages of separation.
Number of Stages
Percent of Differential Separation
(100% = Complete Differential Separation)
2 0
3 75
4 90
5 96
6 98.5
Does a two-stage system achieve differential separation?
No.
Yes.
Answer
How many stages are required to achieve nearly total differential separation?
Answer
Increasing the number of stages is economical up to the point where enough vapors are recovered as liquids to more than pay for the cost of installation and the operation of the system. © 2004, Resource Development Company, LLC.
Vertical Two-Phase Separators OIL AND GAS SEPARATORS This is a vertical two-phase separator with a gas outlet near the top and a liquid outlet near the bottom.
Fluid entering the separator flows around baffles in the inlet element. As fluid strikes the baffles, the surface tension holding free gas in the oil is broken. Gravity causes the gas bubbles to rise up out of the liquids.
By abruptly stopping the fluids' forward movement, the baffles give the fluids a circular or swirling motion. The fluids change direction.
Inertia is resistance to change in direction. The heavier a fluid is, the greater its resistance or inertia. So, the heavy oil resists change in direction more than the light gas. This results in the oil and gas pulling in different directions.
Some of the liquid drops impinge on the sides of the separator where they are caught in drains and channeled to the bottom of the separator.
The purpose of baffles is to:
Create flash separation.
Release the gas held in the oil by surface tension.
Prevent the formation of hydrates.
Answer
Oil has a greater inertia than gas.
True.
False.
Answer
Gas rising through the separator carries some liquid in the form of spray or small droplets.
The gas-oil mixture funnels through a narrow opening in the mist extractor cone. Because the stream is restricted in the cone, it moves faster than when it entered the cone.
As the drawing shows, the cover plate stops the fast upward movemthe fluid. The fluid is forced to flow around the curved vanes in the miextractor.
ent of
st
Dense oil particles fly against the vanes and cling to the separator's sides. Oil droplets fall from the vanes onto the top of the cone and run down the separator's sides, flowing into the drain at the edge of the cone.
The light gas continues rising through
r
Droplets flow from one pocket to gh to
the separator to the secondary mist extractor. This extractor pad is madeof knitted wire mesh. The gas flows around the wires, but the oil droplets impinge on the wires. These droplets flow down the wires to pockets in the mesh pad until they form a droplet that is heavy enough to fall to a lowepocket.
another until they are heavy enou fall from the bottom of the extractor onto the cone.
Dense solids, such as sand, mud, and salt:
Settle to the bottom of the separator.
Leave through the liquid outlet.
Answer
Both water and oil leave the separator though
water and oil leave the separator though
the liquid outlet. the liquid outlet.
True.
False.
Answer
This is another vertical two-phase off the
The mist extractor in this ned
separator. Here, gas is also taken top of the separator and water and oil are removed at the liquid outlet.
separator is specially desigto remove large quantities of residual liquid mist from gases.
The stream of gas and liquids
__________ has greater inertia and therefore, resistance to change in direction of
flows through and impinges against (hits) several layers ofvanes.
_has greater
flow.
Liquid.
Gas.
Answer
Gas assumes the new direction of flow more readily than the liquid because it has less inertia.
Gas flows through the space between the vanes, while the
pack mist extractors remove liquid from
s in the
s s
and falls.
packs are effective, but they tend to foul because the rings are close together.
tors in a separator:
liquid coalesces on the vane surfaces.
Coalescing
gas streams. The ringcoalescing pack change the direction of flow. Gases are whirled as they curve aroundimpinges on the rings, collect
Coalescing
each ring. The heavy liquid traveling with the ga
Mist extrac
Break up the incoming fluid stream.
Maintain line and separator pressure.
Remove liquid drops from the gas.
Answer
2004, Resource Development Company, LLC.
-Phase Separators OIL AND GAS SEPARATORS
©
Horizontal Two
This is a horizontal two-phase separator. When well fluids first enter the separator they strike the angle baffle. Forward motion is temporarily stopped and heavy liquids fall immediately to the bottom of the separator.
Gases and oil spray continue through the defoaming element, or primary mist extractor. There, the baffles change the direction of flow. The change of direction will cause the oil to:
Stick on the baffles and the gas to flow around.
Rise over the baffles and the gas to sink.
Answer
The oil sticks to the baffles and flows down the baffles to the liquid section. Gas continues through the secondary mist extractor where almost all of the liquid is removed from the gas.
Oil and water leave the separator through:
The liquid outlet.
Separate outlets.
Answer
Solids, such as sand, mud, and salt, separated out of the fluids settle on the bottom of the separator. Periodically, this residue is scraped and flushed through drain connections in the bottom of the separator. Manways, cleanout openings, and washout connections provide accessibility to the separator interior. Openings to the separator must not be blocked by equipment and must be kept accessible at all times.
Because horizontal separators have a larger bottom area than vertical separators, they are harder to clean. A horizontal separator is also not as good at handling liquid surges as a vertical separator. However, if you compare the capacity of a
vertical separator and a horizontal separator of the same size at equal pressures, you can see that the horizontal separator has the greater capacity.
Size Pressure Gas Capacity Oil
Capacity
36" x 10', vertical 125 PSI 8.3 MMSCFPD 3920 BPD
36" x 10', horizontal 125 PSI 13.6 MMSCFPD 8800 BPD
© 2004, Resource Development Company, LLC.
