The global demand for oil is still increasing and expected to stay at a high level for the near future. This has driven many production activities to deep reservoirs at high pressures and high temperatures (HPHT). The description of the density and phase equilibrium of highly asymmetric mixtures related to reservoir fluids is an important issue especially for HPHT reservoirs.
This PhD thesis is dedicated to the study of the high-pressure phase behaviour of asymmetric mixtures related to reservoir fluids, whose data are generally scarce in the literature. Instead of studying well-defined mixtures mimicking reservoir fluids, we prepared live fluid systems by combining a light gas component (carbon dioxide, nitrogen, or methane) and stock tank oil (STO). Their density and phase equilibrium data were systematically measured at temperatures from 298.15 to 463.15 K and pressures up to 1400 bar. The densities of the three gas + STO systems as well as the STO itself were measured through a vibrating tube densitometer while the phase equilibrium of the three systems was studied through a full visibility PVT cell. Since the systems mimic reservoir fluids better, the obtained data provide a more relevant evaluation of various thermodynamic models in describing highly asymmetric reservoir fluids.
We used the measured data to evaluate several equations of state (EoS), including the classical SRK and PR, and the more theoretical PC-SAFT. It is impossible to find one model that performs consistently better than the other models. The performance of these models in saturation pressure is case dependent. For volumetric properties, it is found that the deviation in the calculated STO density somewhat correlates with the deviation in the live oil density. This indicates that the live oil density modelling can be improved by a better modelling of the STO density.
Overall, the study has produced valuable HPHT density and phase equilibrium data for evaluating and further improving thermodynamic models for HPHT reservoir fluids, thus supporting the relevant industrial applications on exploring and developing the high-pressure reservoirs. Part of the PhD thesis is dedicated to the measurement and modelling of the high-pressure interfacial tension and density of the mono ethylene glycol+water+methane/natural gas systems, which are of direct relevance to offshore natural gas processing.