Phase behavior of systems with high CO2 content: Experiments and thermodynamic modeling

Abstract

Pre-salt reservoirs are located in ultra-deep Brazilian waters. The petroleum production from these reservoirs is characterized by a high gas-oil ratio (GOR) and high content of CO2 in the associated gas. The challenges for production in such the pre-salt reservoir conditions include understanding the unusual phase behavior observed in the laboratory from field samples. In this paper, we discuss the phase equilibrium data and thermodynamic modeling of the systems containing CO2/n-C16H34, CH4/CO2/n-C16H34 and CH4/CO2/n-C16H34/C14H10. Transitions observed during this study were liquid-liquid (LL), vapor-liquid (VL), and vapor-liquid-liquid (VLL). The phase behavior investigated includes equilibrium conditions observed at temperatures from 293.1 K to 353.1 K and at pressures up to 58 MPa. The Peng-Robinson equation of state with classical van der Waals mixing rule was used to correlate the obtained experimental data. Results show a complex phase behavior for the binary system CO2/n-C16H34 at 298 K, featuring two liquid-liquid regions and barotropic inversion near the VLL transition. The addition of methane to the previous system (1/1 mol ratio relative to CO2) induces a phase behavior alteration in which the liquid-liquid transition disappears. The addition of a polyaromatic component (phenanthrene), whose influence over the mean molecular weight of the mixture under test is little, increased the saturation pressure. This study contributes to show that the use of a model representing the live-oil is a good strategy to understand how chemical and physical-chemical characteristics can induce complex phase behaviors, such as those observed in reservoirs with high carbon dioxide content.