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

摘要

Highly accurate data for density measurements are required for engineeringcalculations as well for developing equations of state (EOS) for use in the custodytransfer of natural gas through pipelines. The widely used present industry standard, theAGA8-DC92 EOS, was developed against a database of reference quality natural gasmixtures 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 producegas from deep and ultra-deepwater of the Gulf of Mexico where the pressures andtemperatures encountered are much higher. Produced gas mixtures have compositionscontaining higher percentages of the C6+ fraction. As AGA8-DC92 is a statistical fitequation developed for one set of conditions, time has come to evaluate its performanceto assess whether it is still viable for gas custody transfer with a new set of conditionsencountered.A highly accurate, high pressure and temperature, compact single sinkermagnetic suspension densimeter has been used first to determine densities of purecomponent??????s densities for which very reliable data are available. After validating itsperformance, the densities of four light natural gas mixtures, containing no C6+ fractionand 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 of250 to 450 K and in the pressure range of 10 to 150 Mpa (1450 to 21,750 psi); the heavymixtures 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 gasmixtures have been presented in the dissertation. Data on two heavy mixtures have notbeen published due to reasons of confidentiality.Measured densities of light mixtures, not containing the C6+ fraction show lessthan expected relative deviations from the AGA8-DC92 EOS predictions except at lowtemperature. The deviation with the recently developed GERG02 EOS was morepronounced. A force transmission error analysis and uncertainty analysis was carriedout. The total uncertainty was calculated to be 0.105 %.The data measured as a part of this research should be used as reference qualitydata either to modify the parameters of AGA8-DC92 EOS or develop a more reliableequation of state with wider ranges of pressure and temperature.