Typical Natural Gas Components

Chapter 7
Separating and Treating Well Fluids
- - Well fluids produced - - Mixture of oil, gas and water
 the force of gravity
Separation of oil, gas and water
Principles and Operation of Production Separators
─ Gas, oil and water separation was achieved by
the difference in gravity, or weight, of each fluid.
─ Production separators do the same job, except they
are built to handle a continuous-flow stream and
have features to improve separation efficiency
under flow conditions.
Three general types of separators
─ Horizontal separator
─ for high-pressure and medium-pressure service
─ Vertical separator
─ for low-pressure service (generally)
─ Spherical separator
─ more compact and cheaper
─ limited separation space and liquid surge capacity
─ for low-volume remote platforms
Internal Structure of Separators
Separation methods
Stage separation method
Low-temperature separation method
Stage separation method (two stage separation)
Any number of separators may be used in stage separation as
long as stage operates at successively lower pressures.
Low-temperatures separation method
It is used to handle the production from high-pressure gas
Well fluids – a mixture of gas & some light liquids
Dehydration – the removal of water vapor from gas
Low – temperature separation method
uses the cooling effect of expanding high – pressure gas
Low-temperature separation method
For high-pressure gas wells (well fluids: mixture of gas and
some light liquids)
─ Dehydration to remove water vapor from gas
Water-removal — before the oil can be delivered to the pipeline
Free water – some of the water produced with the oil will not be
mixed with it ; this is known as free water .
Water removal
─Treatment of free-water
Free water knock out (FWKO)
Treatment of oil-water or water-oil emulsion
Water and oil are immiscible.
 These two liquids will form an emulsion only
(1) if there is sufficient agitation to disperse one liquid as
droplets in the other , and
(2) if there is an emulsifying agent , or emulsifier , present.
Emulsifying agents – asphalt 瀝青 柏油
resinous substances
oil-soluble organic acids.
Emulsion – a mixture in which one liquid, termed the
dispersed phase, is uniformly distributed (usually as
minute globules, in another liquid, called the
continuous phase or the dispersion medium (phase))
Oil – water emulsion –
the oil is the dispersed phase, and
the water (is ) the dispersion medium (or continuous phase )
Water-oil emulsion –
the water is the dispersed phase, and
the oil the dispersion medium
Emulsions -- continue
Stable emulsion – will not break down into its components
without some form of treating.
Tight (difficult to break) or loose (easy to break) emulsion
Depending on –
(1) the properties of the oil & water
(2) the percentage of each found in the emulsion
(3) type and amount of emulsifier present .
Treatment of oil-water or water-oil emulsion
Treatment of oil-water or water-oil emulsion
(4)Combination of these (heater- treater)
flow treater or emulsion treated
(5)Gun barrel or wash tank if emulsion is not stable
Application of heat
One theory assumes that very small droplets, like those found
in emulsions, are in constant motion even when the emulsion
itself is not rest .
Application of heat
(1) Heat increases the movement are makes the droplets strike
each other with greater force and frequency.
(2) Heat also reduces the viscosity – the resistance to flow – of
the oil.
Heater (加熱器)
(1) direct heater
(2) Indirect heater—water bath (fig5.22 P.161)
Application of chemicals
One theory suggests that chemical should be used strictly to
neutralize the emulsifying agent.
Thus, to break a water-in-oil emulsion,
another emulsifying agent to produce oil-in-water emulsion
should be added.
Another theory suggests the chemicals should make the film of
emulsifying agent around the water droplet in a water-in-oil
emulsion very rigid.
Thus, to break rigid film
apply heat or
add chemical.
Heater-treater also called a flow treater or an emulsion treater.
Heater-treater (or call flow treater, or emulsion treater)
apply the effects of
Chemicals, heat , settling, and often electricity
Any or all of the following elements may be included in a treater:
oil-gas separator,
free-water knockout,
water, wash, filter section,
stabilizing section,
heat exchanger, and
electrostatic field.
Treater can be operated at atmospheric pressure,
often be operated under low working pressure.
 A low-pressure, second-stage separator as well as
treating unit.
When flow-line pressure are low, it can be used as a
primary separator thus eliminating the need for a regular
separator. (fig 5.25, P.164)
Treating natural gas
Field processing of natural gas consist of four basic processes:
the gas must be separated from free liquids such as crude oil,
hydrocarbon condensate, water , and entrained solids,
(2) the gas must be processed to remove condensable and
recoverable H.C. vapors
(3) the gas must be treated to remove condensable water vapor,
which might cause hydrate formation
(4) the gas must be treated to remove other undesirable components,
such as hydrogen sulfide or carbon dioxide.
Treating Natural Gas
The primary treatments for natural gas involve
(1)prevention of hydrate formation
by applying heat and/or
adding hydrate inhibitor:
glycol (乙二醇)
methanol (甲醇)
absorption (吸收) – Liquid 乾燥劑
adsorption (吸附) – Solid 乾燥劑
(3)The removal of undesirable components (H2S、CO2)
Alkanolamine process
Iron-sponge process
Glycol/amine process
Sulfinol process
Molecular-sieve removal
Prevention of hydrate formation
Water is always necessary for hydrate formation.
 water always accelerates corrosion.
Phase diagram for a pure single component
Dehydration means removal of water
Water vapors (as well as certain other vapor) are removes from
gas by either
(1) the absorption process, or
(2) the adsorption process
Absorption process -Water vapor may be removed from natural gas by bubbling the
gas counter currently through certain liquids that have a
special attraction or affinity for water.
Dehydration means removal of water
Adsorption -Some solids also have an affinity for water, and when gas
flows through a bed of such granular solids, the water is
retained on the surface of the particles.
Desiccant -The liquid or the solids that has the affinity for water in either
process is called a desiccant.
Dehydration equipment
-- The liquid-desiccant dehydrator
-- The solid-desiccant dehydrator
Removal of undesirable components
Hydrogen sulfide (H 2S)
  acid gases
 Carbon dioxide (CO 2 ) 
Alkanolamine process
-- It is a continuous-operation liquid process that uses absorption
for the acid-gas removal, with subsequent heat addition to string
the acid-gas component from absorbent solution.
Other processes that are used to removal H2S & CO2 are
The Iron-sponge process,
The glycol/amine process,
The sulfinol process, and
The molecular-sieve removal.
Typical Natural Gas Components
Casinghead gas
-- gas produced with oil from an oil well.
Residue gas
-- any gas suitable for as commercial natural gas that comes
from a processing plant.
Sweet gas
--the content of hydrogen sulfide, other sulfur compounds,
and carbon dioxide is low enough that gas may be sold
commercially without further effort to remove these
Sour gas
--the opposite of sweet gas.
Types of Natural Gas Liquids
Commercial propane (丙烷)
-- propane and/or propylene (at least 95%)
-- vapor pressure < 215 psig at 100 0F
Commercial Butane (丁烷)
-- butanes and/or Butane (at least 95%)
-- vapor pressure < 70 psig at 100 0F
-- At least 95% must evaporate at 34 0F or lower in a
standard test.
Liquefied Petroleum Gas (LPG)
-- A mixture of commercial propane and commercial butane.
-- maximum vapor pressure < 215 psig at 100 0F
At least 95% must evaporate at 34 0F or lower in a
standard test.
Types of Natural Gas Liquids
Natural Gasoline
--This petroleum product is extracted from natural gas
• Vapor pressure:10-34psi
• Percentage evaporated at 140 F: 24-85%
• Percentage evaporated at 275 F: not less then 90%
Vapor pressure, usually referred to as Reid vapor pressure
(RVP), used to designate grades.
 Motor fuels: 5-8psi RVP
 Very light oils (60-70AIP): 12psi RVP
 Natural gasoline product: 14-26 psi RVP
The Storage System
The Storage System
Stock tanks (or storage tanks)
─Tank battery
─separation equipment
treating equipment
storage facilities
tank battery
─stock tanks
Bolted steel tank – 500 bbls or larger; assembled on location
Welded steel tank – 90 bbl to several thousand bbls;
welded in a shop and then transported
as a complete unit to the site
─Vapor Recovery System
─LACT (Lease automatic custody transfer) unit
Oil sampling
thief-sampling method
bottle-sampling method
Gas sampling
Treatment of emulsion
To break down a petroleum emulsion
 the properties of emulsifying agent must be
neutralized or destroyed .
By application of
(1) heat
(2) chemicals
(3) electricity, or
(4) combination of these (heater – treater )
The well stream
The well stream -- high-velocity, turbulent,
constantly expanding mixture of gases and hydrocarbon liquids,
intimately mixed with water vapor, free water solids and
other containment.
The bottle test
-- It is used to help determine which chemical can most
effectively break the emulsion from a given well, lease, or
Result from a bottle test also indicate the required ratios of
treating compound to emulsion. That is,
the smallest around of the proper chemical need to
satisfactorily break the volume of emulsion being
Application of electric current
Usually in conjunction with heat & chemicals.
Gun barrel or Wash tank
Sometimes an oil-water emulsion is not stable,
Given enough time, the water will settle to the bottom of a
tank and oil will rise to the top.
The settling vessel used for this kind of separation method is
called a gun barrel or wash tank.
Stock tanks (storage tanks )
Once the oil is clean enough to meet pipeline specifications, it is
flowed into storage tanks, sometimes called stock tanks.
Tank battery-
Stock tanks (storage tanks )
Two basic types of stock tanks:
bolted steel
welded steel
Bolted steel tanks
- 500 bbl or larger; assembled on location.
Welded steel tanks
- 90 bbl to several thousand bbls.
Welded in a shop and then transported as a complete unit
to the site.
Most tanks are equipped with a bottom drain out let for
draining off basic sediment and water (BS&W).
Vapor Recovery System
when oil is treated under pressure and then goes to a stock tank at
pear atmospheric pressure, some of the liquid hydrocarbons flash, or
convent, to gas.
-- In past years, flash gas or vapor were vented to the atmosphere.
-- Governmental agencies now insist on vapor recovery in order to
reduce air pollution.
A vapor recovery unit consists of
* a control pilot mounted on a tank for compressor control,
* a scrubber to keep the liquid hydrocarbons out of the
* a compressor, and
*a control panel.
Treating oil-field emulsions
Emulsion 之定義
Oil - Water Emulsion (oil in water) 
 oil - dispersed phase
Water - oil Emulsion (water in oil)
Oil and water to form an emulsion only
(1) sufficient agitation
asphalt 瀝青
(2) emulsifyin g agent or emulsifier exist  resinous substances
 oil - soluble organic acid
Tight 
 difficult to break
Loose 
 easy to break