Ward A. Burgess

Effect of isomeric structures of branched cyclic hydrocarbons on densities and equation of state predictions at elevated temperatures and pressures.

The cis and trans conformation of a branched cyclic hydrocarbon affects the packing and, hence, the density, exhibited by that compound. Reported here are density data for branched cyclohexane (C6) compounds including methylcyclohexane, ethylcyclohexane (ethylcC6), cis-1,2-dimethylcyclohexane (cis-1,2), cis-1,4-dimethylcyclohexane (cis-1,4), and trans-1,4-dimethylcyclohexane (trans-1,4) determined at temperatures up to 525 K and pressures up to 275 MPa. Of the four branched C6 isomers, cis-1,2 exhibits the largest densities and the smallest densities are exhibited by trans-1,4. The densities are modeled with the Peng-Robinson (PR) equation of state (EoS), the high-temperature, high-pressure, volume-translated (HTHP VT) PREoS, and the perturbed chain, statistical associating fluid theory (PC-SAFT) EoS. Model calculations highlight the capability of these equations to account for the different densities observed for the four isomers investigated in this study. The HTHP VT-PREoS provides modest improvements over the PREoS, but neither cubic EoS is capable of accounting for the effect of isomer structural differences on the observed densities. The PC-SAFT EoS, with pure component parameters from the literature or from a group contribution method, provides improved density predictions relative to those obtained with the PREoS or HTHP VT-PREoS. However, the PC-SAFT EoS, with either set of parameters, also cannot fully account for the effect of the C6 isomer structure on the resultant density.

Experimental Investigation and Molecular-Based Modeling of Crude Oil Density at Pressures to 270 MPa and Temperatures to 524 K

A predictive crude oil density model reliable over a wide range of temperature and pressure conditions is increasingly important for the safe production of oil and accurate estimation of oil reserves. While hydrocarbon density data at low-to-moderate temperatures and pressures are plentiful, data and validated models that have reasonable predictive capability for crude oil at extreme temperatures and pressures are limited. In this investigation, we present new experimental density data for crude oil sample obtained from the Gulf of Mexico region. Density data are measured at pressures to 270 MPa and temperatures to 524 K. These conditions simulate those encountered from ultra-deep formations to platforms. These density data points are then used to validate both empirical-based and molecular-based equations of state models. Results show that the molecular-based perturbed-chain statistical associating fluid theory (PC-SAFT) models, without the use of any fitting parameters, predict the crude oil density within 1% of the experimental data. These results are superior to the density predictions obtained with the high-temperature, high-pressure, volume-translated cubic equations of state.

Equation of state density models for hydrocarbons in ultradeep reservoirs at extreme temperature and pressure conditions

The necessity of exploring ultradeep reservoirs requires the accurate prediction of hydrocarbon density data at extreme temperatures and pressures. In this study, three equations of state (EoS) models, Peng-Robinson (PR), high-temperature high-pressure volume-translated PR (HTHP VT-PR), and perturbed-chain statistical associating fluid theory (PC-SAFT) EoS are used to predict the density data for hydrocarbons in ultradeep reservoirs at temperatures to 523 K and pressures to 275 MPa. The calculated values are compared with experimental data. The results show that the HTHP VT-PR EoS and PC-SAFT EoS always perform better than the regular PR EoS for all the investigated hydrocarbons.