The accuracy of existing thermodynamic models under HPHT (High Pressure, High Temperature) conditions is evaluated, and improvements are suggested.
With dwindling easily accessible oil and gas resources, the industry is driven towards deeper geological formations. In deep reservoirs, temperature and pressure can become extremely high, e.g. up to 250 °C and 2,400 bars. It is well known that established thermodynamic models have shortcomings when applied to such extreme conditions. The thesis evaluates the accuracy of existing models for HPHT (High Pressure, High Temperature) exploration, while also suggesting improvements.
HPHT reservoirs are technologically and economically risky to develop, but highly rewarding if successfully produced. A critical parameter is the length of time which equipment must withstand the HPHT conditions. Therefore, accurate knowledge of the reservoir fluid behavior is required, including density and viscosity of oil and gas at reservoir conditions. Also compressibility is critical, as compression is necessary for HPHT production.
Currently, there are neither accurate databases for these fluid properties, nor adequate equations of state (EoSs) for density and compressibility able to predict these properties at extreme conditions. Presently, oil companies use correlations based on lower temperature and pressure databases despite their unsatisfactory predictive capability at extreme conditions – with margins of error as large as +/- 50%.
The project evaluated both cubic and non-cubic EoSs. It was found that the non-cubic models are much better than the cubics for density, compressibility, heat capacity, and Joule-Thomson coefficient calculation of both light and heavy components in reservoir fluids over a wide temperature and pressure range. The GERG-2008 model gave the lowest deviations of all. Still, this model gave large deviations for bubble point pressure calculation on certain heavy and asymmetric binary systems, suggesting that the model may still be improved for some binary pairs.
Soave-BWR gave the closest prediction of the thermal properties to that of GERG-2008 among the other EoSs tested.
For non-cubic models like PC-SAFT, the characterization method is less mature than cubic models. Therefore, a new reservoir fluid characterization method for PC-SAFT is proposed. In addition, this method was improved by adjusting the correlations with a large PVT database.
Importantly, a general approach to characterizing reservoir fluids for any EoS has been established. The approach consists in developing correlations of model parameters first with a database for well-defined components and the adjusting the correlations with a large PVT database. It is shown that this approach can be applied to PC-SAFT, and to classical cubic models like SRK and PR.
With the developed methods, a comparison in PVT calculation involving 17 EoScharacterization combinations and 260 reservoir fluids was made.
Finally, several new sets of mixing rules were developed for Soave-BWR for mixture calculation to improve the value of this model. It was shown, that some problems with the original Soave-BWR can be fixed by the new mixing rules. However, the overall performance was not significantly improved. Development of mixing rules for non-cubic EoS models is still a semi-empirical process, requiring extensive testing.
Illustartion:
In addition to non-cubic EoS models, advanced characterization of heavy fractions is crucial to HPHT PVT modelling