High-accuracy P-p-T measurements of pure gas and natural gas like mixtures using a compact magnetic suspension densimeter



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Texas A&M University


Highly accurate data for density measurements are required for engineering calculations as well for developing equations of state (EOS) for use in the custody transfer of natural gas through pipelines. The widely used present industry standard, the AGA8-DC92 EOS, was developed against a database of reference quality natural gas mixtures with compositions containing less than 0.2 mole percent of the heavier C6+ fraction. With the advances in technology in the late nineties, it is possible to produce gas from deep and ultra-deepwater of the Gulf of Mexico where the pressures and temperatures encountered are much higher. Produced gas mixtures have compositions containing higher percentages of the C6+ fraction. As AGA8-DC92 is a statistical fit equation developed for one set of conditions, time has come to evaluate its performance to assess whether it is still viable for gas custody transfer with a new set of conditions encountered. A highly accurate, high pressure and temperature, compact single sinker magnetic suspension densimeter has been used first to determine densities of pure component??????s densities for which very reliable data are available. After validating its performance, the densities of four light natural gas mixtures, containing no C6+ fraction and two heavy gas mixtures containing more than 0.2 mole percent of the heavier C6+ fraction, were measured. The light mixtures were measured in the temperature range of 250 to 450 K and in the pressure range of 10 to 150 Mpa (1450 to 21,750 psi); the heavy mixtures were measured in the range of 270 to 340 K and in the pressure range of 3.45 to 34.45 MPa (500 to 5,000 psi). Out of those, the data of only two light natural gas mixtures have been presented in the dissertation. Data on two heavy mixtures have not been published due to reasons of confidentiality. Measured densities of light mixtures, not containing the C6+ fraction show less than expected relative deviations from the AGA8-DC92 EOS predictions except at low temperature. The deviation with the recently developed GERG02 EOS was more pronounced. A force transmission error analysis and uncertainty analysis was carried out. The total uncertainty was calculated to be 0.105 %. The data measured as a part of this research should be used as reference quality data either to modify the parameters of AGA8-DC92 EOS or develop a more reliable equation of state with wider ranges of pressure and temperature.