Oil and gas production handbook An introduction to oil and gas production

sweetening/acid removal is the process of removing them. 
 
Natural gas sweetening methods include absorption processes, cryogenic 
processes, adsorption processes (PSA, TSA and iron sponge) and 
membranes. Often hybrid combinations are used, such as cryogenic and 
membranes. 
 

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60 
 
Gas treatment may also include calibration. If the delivery specification is for 
a specific calorific value (BTU per scf or MJ per scm), gas with higher values 
can be adjusted by adding an inert gas, such as nitrogen. This is often done 
at a common point such as a pipeline gathering system or a pipeline onshore 
terminal. 
 
Raw natural gas from the well consists of methane as well, as many other 
smaller fractions of heavier hydrocarbons and various other components. 
 
Component Chemical 
Formula 
Boiling Point  
at 101 kPa 
Vapor pressure 
at 20 °C approx. 
Methane CH
4
 -161,6 
°C
T
cri t
−82.6 °C
 @ 4,6 MPa
Ethane C
2
H
6
 -88.6 
°C
4200 
kPa
Propane C
3
H
8
 
-42.1 °C
890 kPa
Butane n-C
4
H
10
 
−0.5 °C
210 kPa
Higher order HC 
 Alkenes 
 Aromatics 
 
C
n
H
2n 
e.g. C
6
H
6
Acid gases 
 Carbon dioxide 
 Hydrogen sulfide 
 Mercaptans ex. 
   Methanethiol 
   Ethanethiol  
 
CO
2
 
H
2
S 
 
CH
3
SH 
C
2
H
5
SH 
−78 °C
-60.2 °C
5.95 °C
35 °C
5500 kPa
Other Gases 
 Nitrogen 
 Helium 
 
N
2
 
He 
-195.79 °C
-268.93 °C
Water H2O  0 
°C
Trace pollutants 
  Mercury 
  Chlorides 
 
Data source: Wikipedia, Air Liquide Gas Encyclopedia 
 
Natural gas is characterized in several ways dependent on the content of 
these components: 
 
o
  Wet gas is raw gas with a methane content of less than 85%. 
o
  Dry gas is raw or treated natural gas that contains less than 15 liters 
of condensate per 1,000 SM
3
. (0.1 gallon per 1000 scf). 
o
  Sour gas is raw gas with a content of more than 5.7 mg hydrogen 
sulfide (H
2
S) per scm (0.25 grains per 100 scf); this is about 4 ppm. 

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61 
 
o
  Acid gas has a high content of acidic gases such as carbon dioxide 
(CO
2
) or H
2
St. Pipeline natural gas specification is typically less than 
2% CO
2
. Acid gas fields with up to 90% CO
2
 exist, but the normal 
range for sour raw gas is 20-40%. 
o
  Condensates  are a mixture of hydrocarbons and other components 
in the above table. These are normally gaseous from the well but 
condense out as liquid during the production process (see previous 
chapter). This is a refinery and petrochemical feedstock. 
   Raw gas is processed into various products or fractions: 
o
  Natural gas in its marketable form has been processed for a 
specific composition of hydrocarbons, sour and acid components, 
etc., and energy content. Content is typically 90% methane, with 
10% other light alkenes. 
o
  Natural gas liquids (NGL) is a processed purified product 
consisting of ethane, propane, butane or some higher alkenes 
separately, or in a blend. It is primarily a raw material for 
petrochemical industry and is often processed from the condensate. 
o
  Liquefied petroleum gas (LPG) refers to propane or butane or a 
mixture of these that has been compressed to liquid at room 
temperature (200 to 900 kPa depending on composition). LPG is 
filled in bottles for consumer domestic use as fuel, and is also used 
as aerosol propellant (in spray cans) and refrigerant (e.g., in air 
conditioners). Energy to volume ratio is 74% of gasoline. 
o
  Liquefied natural gas (LNG) is natural gas that is refrigerated and 
liquefied at below -162 °C, for storage and transport. It is stored at 
close to atmospheric pressure, typically less than 125 kPa. As a 
liquid, LNG takes up 1/600 of the volume of the gas at room 
temperature. Energy to volume ratio is 66% of gasoline. After 
transport and storage it is reheated/vaporized and compressed for 
pipeline transport. 
o
  Compressed natural gas (CNG) is natural gas that is compressed 
at 2-2,2 MPa to less than 1% of volume at atmospheric pressure. 
Unlike higher alkenes, methane cannot be kept liquid by high 
pressure at normal ambient temperatures because of a low critical 
temperature. CNG is used as a less costly alternative to LNG for 
lower capacity and medium distance transport. Methane for vehicle 
fuel is also stored as CNG. Energy to volume ratio is typically 25% of 
gasoline. 

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62 
 
5.3 Gas processing 
Raw natural gas must be processed to meet the trading specifications of 
pipeline and gas distribution companies. As part of the purification other 
components such as NGL is produced, and pollutants extracted.  
 
The diagram shows an overview of a typical gas plant. Marketable products 
are listed in blue and the production process is shown in grey as it is not 
considered part of the gas plant.