For many years the AGA 8 method has been the basis for all fluid property calculations related to natural gas supply, and forms an integral part of the measurement infrastructure throughout the world. However, application outside its valid range has the potential to cause significant errors. Case studies are presented that show the potential errors in density and compressibility, and resultant financial exposure, when using the AGA 8 method outside its range of viability.
If all the components are within the specified composition ranges for pipeline quality gas, densities calculated using the AGA 8 method are claimed to have an uncertainty of 0.1 % (at k=2) for temperatures between -10 and 65°C at pressures up to 120 bar(a). However, for mixtures with components in the extended composition range and / or at temperatures and pressures outside the standard limits, the uncertainty increases to between 0.2 and 0.5 % (at k=2) and is effectively indeterminate for compositions outside the extended range of applicability. Comparison with values calculated using the GERG-2008 method generally confirms these claims. However, even for compositions that are within the extended AGA 8 range, the differences can exceed 1 % and the effect of ignoring pseudo-components can cause differences of over 5 %. Given the known limitations of the AGA 8 method and the lack of clarity of the likely uncertainties in calculated densities and compressibilities when used in its extended range, the use of an alternative method would be preferable. Although the GERG-2008 method overcomes some of the limitations of the AGA 8 method, there still remains the problem of ensuring that all the components in the gas stream are measured.
By raising awareness of the limitations of calculating gas properties from composition, this work will help metering system designers, operators and regulators understand the issues that must be addressed when dealing with gas fields whose compositions are outside the pipeline quality gas specification required by the AGA 8 method, as the potential financial exposure can run to hundreds of millions of dollars per annum for a region such as the North Sea.