Practically all metering devices used in the oilfield today require some correction factor for gas gravity and gas compressibility factor (Z factor) in their calculations. An error in either gas gravity or Z factor could potentially make a significant difference in these calculations. The accepted method of calculating the Z factor uses critical property and gas gravity correlations. The proposed method allows for the calculation of the Z factor by direct field measurements.

A critical flow nozzle can be used in conjunction with a thermometer, turbine meter, and differential and static pressure gauges to allow direct measurements at the wellhead. A simple computer program then uses these measurements to calculate both Z factor and gas gravity. The paper describes the test apparatus and the calculation procedure.

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