Commissioning of a magnetic suspension densitometer for high-accuracy density measurements of natural gas mixtures

Abstract

High-accuracy density measurement data are required to validate equations of state (EOS) for use in custody transfer of natural gas through pipelines. The AGA8-DC92 EOS, which is the current industry standard has already been validated against a databank of natural gas mixtures with compositions containing up to 0.2 mole percent of the heavier C6+ fraction and is expected to predict densities of natural gas mixtures containing higher mole percentages of the C6+ fraction with the same accuracy. With the advances in exploration, drilling and production, natural gas streams containing higher percentages of the C6+ fraction have become available from the deepwater and ultra-deepwater Gulf of Mexico in recent years. High-accuracy, density data for such natural gas mixtures are required to check if the AGA8-DC92 EOS covers the entire range of pressure, temperature and compositions encountered in custody transfer. A state-of-the-art, high pressure, high temperature, compact single-sinker magnetic suspension densitometer has been used to measure densities of two simulated natural gas mixtures named M91C1 and M94C1 after validating its operation by measuring densities of pure argon, nitrogen and methane in the range (270 to 340) K [(26.33 to152.33) oF, (-3.15 to 66.85) oC] and (3.447 to 34.474) MPa [(500 to 5,000) psia]. Measured densities of M91C1, not containing the C6+ fraction show larger than expected relative deviations from the AGA8-DC92 EOS predictions in regions 1 and 2 but agree well with predictions from the recently developed REFPROP EOS, implyingthat the AGA8-DC92 EOS may be unreliable in its present state even for natural gas mixtures not containing the C6+ fraction. Measured densities of M94C1 containing more than 0.2 mole percent of the C6+ fraction deviate from the AGA8-DC92 EOS predictions by more than the expected values in region 1 which is not surprising but the agreement with AGA8-DC92 EOS predictions in region 2 is misleading which becomes evident when the measured densities are compared to the REFPROP EOS predictions. The measured data can be used to recalibrate the parameters of the AGA8-DC92 EOS or to validate an entirely new EOS.