Fundamental properties of crude oils such as density are important in the estimation of the amount of recoverable petroleum in an oil reserve. In addition, the isothermal compressibility, which is a density-derived property, is needed for estimating the fraction of the total hydrocarbons present that can be produced in the primary recovery period.
We have investigated the performance of several equation of state (EoS) models for predicting the high-temperature, high-pressure (HTHP) volumetric properties of hydrocarbons and their mixtures. This HTHP designation, which encompasses, temperatures to 250 ˚C and pressures to 275 MPa, covers the conditions encountered on the platform as well as ultra-deep formation conditions such as those that exist beneath the deep sea floor in the Gulf of Mexico. As such, the density data for two dead crude oil samples from the Gulf of Mexico have been measured from ambient to HTHP conditions at temperatures to 250 ˚C and pressures to 275 MPa. The density measurement for simulated live crude oil samples is in progress, where the simulated live oil is obtained by adding gases to the two aforementioned dead oil samples.
A variable-volume view cell was used in the measurement of the crude oil density data. The view cell has previously been successfully used to measure the density data for various pure component and binary hydrocarbon mixtures at temperatures to 250 °C and pressures to 275 MPa. From the experimental density data for the crude oil samples, the isothermal compressibility values for the oils are then calculated as a function of pressure. The perturbed-chain statistical associating fluid theory (PC-SAFT) EoS has been used to successfully predict the density and isothermal compressibility of the crude oils. The performance of the PC-SAFT EoS is studied using the sets of original PC-SAFT parameters and the sets of parameters obtained from the fit of the PC-SAFT EoS to HTHP pure component density data. To obtain PC-SAFT parameters for the crude oil pseudocomponents, correlations were used wherein the pure component parameter values are defined as a function of molecular weight and aromaticity. Preliminary results show that the predictions using the PC-SAFT EoS with the HTHP parameters are in good agreement with the isothermal compressibility data to within 7%. This number represents a significant improvement over the 25% agreement obtained when using the original set of PC-SAFT parameters. In summary, the PC-SAFT EoS predictions, particularly those for the isothermal compressibility, show great promise over those predicted by traditional cubic EoSs.