High-temperature, high-pressure viscosity of n-octane and isooctane

Experimental viscosity data are reported for n-octane and isooctane (2,2,4-trimethylpentane) at pressures between 4 and 242 MPa and temperatures between 303 and 523 K. These extreme conditions are representative of those encountered in ultra-deep petroleum formations beneath the deepwaters of the Gulf of Mexico. The measurements are performed using a windowed, variable-volume, rolling-ball viscometer. A comparison of reported viscosity values with available literature data for n-octane and isooctane covering limited pressure and temperature ranges shows that the average absolute percent deviation (AAPD) varies between 1.0% and 2.5%. The data are correlated by a non-linear surface fit as a simultaneous function of temperature and pressure that yields AAPD values of 0.6% and 0.8% for n-octane and isooctane, respectively. The viscosity data are also correlated with a model suitable for use in reservoir simulators; the free volume theory model with density values obtained from the Peng–Robinson equation of state (PR EoS), the volume-translated PR EoS (VT-PR EoS), the perturbed chain statistical associating fluid theory (PC-SAFT EoS), and the PC-SAFT EoS in which the parameters are fit to high temperature, high pressure density data (HTHP PC-SAFT EoS). If the parameters of the free volume theory model are fit with the EoS incorporated into the model, comparable results are obtained with each of the density models. However, if the parameters of the free volume theory model are first fit using experimental density data and then an EoS is incorporated into the free volume theory model, the best viscosity results are obtained with the most accurate density models, which are the HTHP PC-SAFT and VT-PR EoS.