Experimental and predictive PC-SAFT modeling results for density and isothermal compressibility for two crude oil samples at elevated temperatures and pressures

Abstract

Rapid growth in world oil demand is driving the oil and gas industry to search for petroleum in deep wells where elevated temperatures and pressures prevail. Accurate knowledge of thermodynamic properties such as density and isothermal compressibility at in-situ conditions is necessary to estimate the recoverable oil from these deep reservoirs. Yet, crude oil density data in the open literature for pressures exceeding 155 MPa are scarce or nonexistent.In this study, we present density data for two Gulf of Mexico crude oils at pressures from 3 to 276 MPa and temperatures up to 523 K. Measurements on the liquid fraction of the crude oil were made along three isotherms at temperatures T = (323, 423, and 523) K and at pressures from near-ambient platform to ultra-deep reservoir conditions. The liquid fraction of the crude oils or “dead” oils contained numerous components including alkanes, branched-alkanes, cyclo-alkanes, aromatics, resins, and asphaltenes. For simplicity, the composition of each dead oil was represented as three pseudocomponents - saturates, aromatics and resins, and asphaltenes. Methane gas at two concentrations (5 and 10 wt%) was added to the dead oils to obtain synthetic live crude oils. Experimental density data for the two live oil samples are also reported at four isotherms and similar pressure conditions. The uncertainty in the reported densty data is less than ±1%. A Tait relationship was used to correlate the measured density data; from this relationship, a reference value for isothermal compressibility was obtained. The present density and isothermal compressibility data have been compared with predictions from the Perturbed Chain version of the Statistical Associating Fluid Theory (PC-SAFT) without the use of any fitting parameters. A special set of new PC-SAFT parameters accurate for one-phase calculations at high-pressure, high-temperature conditions was used; these parameters were predicted with a knowledge of the measured aromaticity of the oil under investigation. Oil density values were predicted to within 1–4% of experimental values on average, and isothermal compressibility values were within 1–10% on average.