화학공학소재연구정보센터
Journal of Chemical and Engineering Data, Vol.66, No.3, 1356-1366, 2021
Measurement of Fluid Phase Equilibria for High Gas Ratio Mixtures of Carbon Dioxide, Methane, and Brazilian Presalt Crude Oil
The phase behavior of petroleum fluids under reservoir conditions provides fundamental information for an adequate crude oil production scheme. Recently, complex phase transitions have been studied for presalt crude oils, involving atypical liquid-liquid-vapor and liquid-liquid-asphalt phase transitions. In this paper, the phase behavior of synthetic mixtures containing carbon dioxide, methane, and a Brazilian presalt crude oil was investigated using PVT, coupled with near-infrared (NIR) transmittance and high-pressure microscopy (HPM) measurements. Crude oil (API 28.0, 0.68 wt % of asphaltenes) was mixed with 25.0 wt % gas (for a CH4/CO2 ratio from 0 to 62.5 wt %), and the phase behavior of the mixture was evaluated at a reservoir temperature of 343.15 K and pressures up to 100 MPa. Black oil phase behavior was detected for systems with a lower CH4/CO2 ratio and lower gas volume fraction, with no asphaltenes precipitation observed. As the CH4/CO2 ratio increased, pressure-volume curves showed a slight phase transition, with a not evident sharp break in the slope because of the minor difference of fluid compressibility. Moreover, a phase insolubility was confirmed by NIR and HPM tests for a high CH4/CO2 ratio that can be associated with the formation of an asphaltic phase. When total molar gas content increases by increasing the CH4/CO2 ratio, the asphaltic phase is formed at higher pressures. However, asphaltenes were detected as an uncommon fine dispersion with no larger fractal aggregates formation, even at pressures far below the asphaltenes onset pressure (AOP). Additionally, a nontypical behavior was observed in HPM tests with a total asphaltene dissolution when pressure reached the bubble pressure point. This atypical redissolution is in accordance with an instantaneous phase dissolving at pressures higher than the AOP, observed for the same oil at high methane ratios.