Natural Gas Measurements: Volume, Heating Value and Gas Richness

Natural Gas is typically measured in volumetric units, at surface conditions of temperature and pressure, for flow in reservoirs and pipelines and in thermal energy units for commercial purposes.

Volumetric Measurement of Natural Gas

Because gas is very compressible its volume expands significantly as it goes from high pressure in a reservoir to surface conditions and then shrinks again as it is compressed into a pipeline for sale (see Figure 1). For this reason the industry has established standard conditions for referring all gas volumes. In the US and many other countries it is the Standard Cubic Foot (SCF) measured at 60°F and one atmosphere or 14.73 psi (pounds per square inch) of pressure. In Europe and many other countries the standard unit is the Normal Cubic Meter (Nm3) measured at 0°C and one atmosphere of pressure. (Note: some Europeans may use the Standard Cubic Meter, which is measured at 15°C and one atmosphere). Note that the cubic meter is much larger than the cubic foot: 1 m3 = 35.31 ft3.

Unit of Gas Volume Measurement Standard Conditions Area of Common Usage
Standard Cubic Foot (SCF) 14.73 psi (one atmosphere), 60°F US, Latin America, Africa, Middle East.
Normal Cubic Meter (Nm3) One atmosphere, 0°C Europe, Canada, Russia.
Conversions: 1 m3 = 35.31 ft3; 1 ft3 = 0.0283 m3

Table 1: Standard Gas Conditions in Various Regions of the World.

Figure 1

Figure 1: Actual Volume of 240 SCF of Natural Gas in the Reservoir and Pipeline.

Figure 1 demonstrates the compressibility of natural gas. The gas has a volume of 1 unit (ft3 or m3) in the reservoir at 4,000 psia pressure and 140 °F (271.6 atm and 60 °C). When it is produced to surface temperature and pressure 14.73 psia and 60 °F (1 atm and 15 °C), referred to as Standard Conditions, it expands to 240 units, and when it is compressed into a high-pressure pipeline at 1,000 psia pressure and 60 °F (67.9 atm and 15 °C) it becomes 3.14 units. In all three conditions the engineer will refer to the gas volume as 240 Standard Units ft3 or m3.

Because the cubic foot and cubic meter are quite small the industry uses larger standard quantities with appropriate symbols as shown in Table 2. Thus a well may produce at the rate of 10 MMSCF, many wells in the field may deliver 10 BCF per month from a field with reserves of 2 TCF.

Units Quantity Symbol Application
Thousand cubic feet 1000 SCF MCF Basic unit of sale
Million cubic feet 1,000,000 SCF MMCF Daily well production
Billion cubic feet 1,000,000,000 SCF BCF Annual field's production
Trillion cubic feet 1,000,000,000,000 SCF TCF Field reserves

Table 2: Volumetric Quantities and Symbols.

If gas volume is measured in m3, simply replace SCF with m3 within the above symbols. Some companies use Giga ("G") and Tera ("T") in place of billion and trillion.

Heating Value of Produced Natural Gas

The value of natural gas to a customer who uses it as a fuel depends on its heating value, usually measured in BTUs per unit volume. (British Thermal Units: the thermal energy required to raise the temperature of one pound of water by 1 °F). The heating value of natural gas will depend upon its composition, which is obtained by laboratory measurement of a collected sample. The measurement yields the mole percent, which is equal to volume percent at standard conditions, of each component in the sample.

Each component has its own heating value, expressed in BTU/SCF, which depends on the number of carbon atoms in the molecule. Table 3 shows the composition of a rich gas, in this case the Hassi R'Mel Field in Algeria, which contains a number of hydrocarbon components, and three components that will be essentially removed during treatment (N2, CO2 and H2S). The composition of the gas after treatment is calculated on a weighted average basis to be 1,130.9 BTU/SCF.

Hassi-R'Mel (Algeria) Gas Composition
  Gas Composition Mole % After * Treatment Mole % Component Gross Heating Value BTU/SCF** Gas Heating Value BTU/SCF
C1 83.50 89.11 1,010 900.1
C2 7.00 7.47 1,770 132.2
C3 2.00 2.13 2,516 53.7
i-C4 0.40 0.43 3,252 13.9
n-C4 0.40 0.43 3,262 13.9
C5+ 0.40 0.43 4,009 17.1
N2 6.10 0.00 0 0.0
CO2 0.20 0.00 0 0.0
H2S 0.00 0.00 0 0.0
Total 100.00 100.00   1,130.9
* Nitrogen, Carbon Dioxide and Hydrogen Sulfide will be removed during treatment.
** Source: Engineering Data Book GPSA.

Table 3: Calculation of Heating Value of Natural Gas.

Because the heating value of this gas is too high for most pipelines to accept, it must go through gas processing to recover the propane+ and perhaps even the ethane+ components as natural gas liquids ("NGLs"). Stripping out the liquids will cause some shrinkage in the gas stream and a change in its composition to mostly methane and a much lower heating value. Typically we may recover 90% of the ethane and 100% of the heavier components during gas processing and so the heating value of the gas going into the pipeline will be 1,016.3 BTU/SCF as shown in Table 4.

  After Treatment Mole % Recovery in the Gas % After Processing Mole % Gas Composition Mole % Component Heating Value BTU/SCF Gas Heating Value BTU/SCF
C1 89.11 100.00 89.11 99.17 1,010.00 1,001.62
C2 7.47 10.00 0.75 0.83 1,770.00 14.72
C3 2.13 0.00 0.00 0.00    
i-C4 0.43 0.00 0.00 0.00    
n-C4 0.43 0.00 0.00 0.00    
C5+ 0.43 0.00 0.00 0.00    
Total 100.00   89.86 100.00   1,016.3

Table 4: Calculation of Gas Heating Value after Gas Processing.

Because mole % is equal to volume % we see, by comparing column 2 to column 4 that the liquid recovery reduced the gas volume by 11.14% (100-89.86). We also note that the 1,000 SCF of natural gas, 1 MCF, contains 1.0163 million BTUs. If the gas price is $4.00/million BTUs then you would receive $4.065/MCF for your gas. The recovered liquids would receive an oil equivalent price.

Gas "Richness"

Another important value that indicates the NGL content in a natural gas stream is the GPM or gallons per thousand cubic feet. Traditionally GPM was meant to apply to C3+, but nowadays is commonly apply to C2+. The calculation of the GPM value in the example shown previously, is indicated in Table 5. This value is determined before processing and is very important in the economics of NGL recovery and process selection.

In order to calculate the GPM content in natural gas, these unit conversions are used (standard conditions): 379.3 SCF/lb-mol, 2,636 lb-mol/MMSCF.

In the case example of Hassi R'Mel Field in Algeria, its GPM equals to 2.82, i.e. there are 2.82 gallons of C2+ per one thousand CF of gas, as shown in Table 5.

Gas richness can also be indicated as barrels per million cubic feet (1 barrel is equivalent to 42 US gallons). In this example, 2.82 GPM is equivalent to 67.1 bbls/MMCF.

Please refer to the Midstream Gas Petroleum Online module on Gas Processing and Fractionation for further details.

  Composition % Gal/Mol GPM
C1 83.50    
C2 7.00 10.126 01.87
C3 2.00 10.433 0.55
i-C4 0.40 12.386 0.13
n-C4 0.40 11.937 0.13
C5+ 0.40 13.713 0.14
N2 6.10    
CO2 2.00    
H2S 0.00    
Total 100.00   2.82

Table 5: Calculation of liquid content in a natural gas stream (GPM or gallons per thousand cubic feet).