Reservoir fluids are usually characterized by phase equilibria measurements and volumetric properties. In this paper, we show that integrating ultrasonic velocity in the database increases the predictability of the model. Acoustic sensitivity to the fluid characteristics also permits direct detection of phase transitions.
Ultrasonic velocity is related in a simple way to isenthalpic and isentropic compressibility, hence the importance of its measurement and modelling in temperature and pressure ranges encountered in oil and gas processing where choke (mainly isenthalpic) and compressors (mainly isentropic) are widely used.
Ultrasonic velocity measurements have been performed on pure hydrocarbon compounds, binary mixtures and reservoir fluids at different temperatures and pressures up to 130°C and 70 Mpa. The measurements are much more accurate than for any other thermodynamical property, the accuracy of our determination is about +/− 0.5 m/s and the typical range is from 1000 to 1500 m/s.
Furthermore, phase transitions are determined easily by this method even in the case of liquid/liquid equilibria.
The experimental results are compared to the values calculated using different thermodynamic models and different mixing rules. A calculation procedure, applicable in the case of any model, is described.
It is shown that cubic Equations of State, poorly adapted to the description of pure compounds and binary mixtures, are very reliable to model the behaviour of complex mixtures such as reservoir fluids.