Summary
A model to calculate the theoretical critical flow rate of nitrogen
(N2) or natural gas through a Venturi gas lift valve is described
herein. This new model considers real-gas effects not only in density
calculations but also in other thermodynamic properties that are relevant
during gas isentropic evolution.
For the properties of N2, the Bennedict, Webb, and Rubin (BWR)
equation of state and an accurate correlation for the ideal-gas isobaric heat
capacity were used. For natural gas, the Dranchuk and Abou-Kassem equation,
which reproduces the well-known Standing and Katz chart, was used, and, for the
ideal-gas isobaric heat capacity, it was assumed that the natural gas was a
mixture of methane and ethane only, their individual ideal-gas heat capacity
being calculated by updated correlations.
To validate the use of the proposed equations of state, a comparison of
calculated with experimental or reference data on properties of N2
and natural gas (including pure methane and some relevant mixtures) was
performed with very good results for N2 and for natural-gas
compositions usual in gas lift operations (dry gas with very small amounts of
contaminants). For natural gas with moderate amounts of N2 and
carbon dioxide (CO2), accurate results were obtained after
correction of critical conditions and of ideal heat capacity. The model was
also compared with other theoretical models found in the literature, which use
compositional approaches for natural gas, with excellent results. Some
experimental results obtained with commercial Venturi valves manufactured in
Brazil are also presented.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
29 April 2009
- Revised manuscript received:
3 March 2010
- Manuscript approved:
2 April 2010
- Published online:
19 August 2010
- Version of record:
15 March 2011
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