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

 
Volume is measured in standard cubic meters (scm) defined as 1 m
3
 at 0 ºC 
and 101.325 kPa or standard cubic feet (scf) as 1 ft
3
 at 60 °F (16 °C) and 
14.73 PSIA. 
 
Calorific value specifies the total amount of energy per unit generated 
during combustion of the gas. The value is used to calculate the amount of 
energy delivered. Several values are listed: 
 
•  Gross calorific value or gross heat of combustion is the heat 
released when a specific quantity of fuel in mixture with air is ignited 
and the end products have returned to the initial temperature, 
normally 25 ºC. EU specifications are typically 38.8 MJ (10.8 kWh) 
±5% per scm. In the US 1030 BTU ±5% per scf. 
•  Net calorific value or net heat of combustion is the net heat 
generated when the water vapor in the gas does not condense 
(water forms during combustion) and can be 10% lower. 

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Wobbe index measures the heating effect that a burner is exposed to during 
combustion. A higher value means a greater thermal load on the burner. 
Different gases with the same Wobbe index will impose the same load on 
the burner. An excessively high value is a safety hazard, as it can lead to 
burner overheating and to excess production of carbon monoxide during 
combustion.  
 
Calorific value and Wobbe index can be adjusted by blending gas from 
different sources as well as by addition or removal of nitrogen (N
2
). 
 
Methane number is a value similar to octane value for gasoline, and is 
important when the gas is used for internal combustion engines (as CNG).