(74c) Evaluation of the Methods for Predicting Liquid Densities of High-Temperatures and High-Pressures Deep Petroleum Formations

Six currently used methods for predicting liquid densities of high-temperature and high-pressure (HTHP) deep-water petroleum formations are evaluated for ease of implementation and applications for flow assurance modeling, pipeline flow simulation and reservoir simulation of carbon dioxide sequestration in the depleted oil-reservoirs. The evaluated techniques include the various modified versions of the Peng-Robinson EoS, the high-temperature, high-pressure, volume-translated Peng-Robinson EoS, the volume-translated Soave-Redlich-Kwong EoS, the generic Van der Waals cubic EoS, the modified Tait equation and the perturbed-chain statistical associating fluid theory EoS. The HTHP liquid density data used in the evaluation of the methods for HTHP liquid densities are reported in the literature by Enick Group at U of Pitt, Caudwell-Trusler-Wakeham Group at Imperial College London, Ducoulombier et al. in France, the Doolittle specific volumes of n-Alkanes (nC₇ to C₄₀H₈₂) and the Sage-Lacey volumetric data of CO₂, N₂, H₂S, C₁ to nC₁₀, including the isomers of iC₄ and iC5. While all the evaluated methods gave various degrees of uncertainties at HTHP conditions (due to the judicious choice of the parameters required to implement PC-SAFT and the volume-translated techniques), the VDW generic equation of state (with covolume temperature function) performs better and in overall agreement with the measured liquid densities at the HTHP conditions of deep-water petroleum formations.

The analysis of model results highlight the capability of the various techniques to account for the liquid densities measured for some isomers investigated at HTHP conditions as well as for high-molecular-weight fluids. The HTHP VT-PREoS provides minor improvements over the original 1976 design of PREoS, but neither EoS is capable of accounting for the effect of isomer structural differences on the measured liquid densities at HTHP conditions and significant deviations are observed at HTHP liquid densities of heavier molecular-weight hydrocarbons. However, the PC-SAFT EoS, with pure component parameters derived from the literature provides modest improved liquid density predictions over those obtained with the PREoS or HTHP VT-PREoS or volume-translated SRK EoS.

Overall, the VDW generic cubic EoS (with embedded covolume temperature function) provides more accurate predictions of liquid densities at HTHP conditions associated with the deep-water petroleum formations than all the methods of predicting liquid densities that are analyzed in this investigation. Even though the PC-SAFT EoS shows improved predictions of liquid densities over the Tait correlations and over those EoS (without the embedded covolume temperature function), it still over predicts the experimental liquid densities at HTHP conditions. The VDW generic cubic EoS is recommended for all analyses of flow assurance modeling. But, we are still performing similar analysis for derivative properties (isothermal compressibility, volume expansivity, bulk modulus, isobaric and isochoric heat capacities, Joule-Thomson coefficient and speed of sound) at HTHP conditions associated with the deep-water petroleum formations.

Keywords: cubic equation, Tait equation, liquid density, covolume temperature, heavy hydrocarbon