Study of Liquid–Liquid and Liquid–Liquid–Vapor Equilibria for Crude Oil Mixtures with Carbon Dioxide and Methane Using Short-Wave Infrared Imaging: Experimental and Thermodynamic Modeling
Résumé
In this work, phase behavior was analyzed for recombined mixtures of a Brazilian pre-salt crude oil with carbon dioxide and methane. This oil was sampled at stock tank conditions during a well-test, and mixtures with gas were prepared with a gas molar composition ranging from 20.0 to 86.0 mol %. Their phase behavior was investigated from 293.15 to 378.15 K and up to 100.0 MPa. Direct phase transition detections were achieved by using a full visibility PVT cell, coupled with a short-wave infrared camera. At this wavelength, crude oil has a lower absorbance, and it has been noted that macroscopic phase transitions can be easily detected by visual inspection. For the pseudo-binary system crude oil + carbon dioxide systems, liquid–liquid phase transitions were observed when the gas content is higher than 70.0 mol %. In addition, at the higher carbon dioxide composition, an asphaltic phase was formed at high pressures together with liquid–liquid phases. Similarly, crude oil and methane systems presented a liquid–liquid immiscibility region at methane composition above 60.0 mol % in all temperature domain studied. The Peng–Robinson equation of state was used for modeling purposes, and liquid–liquid and liquid–liquid–vapor transitions were qualitatively described. Additionally, it was observed that liquid–liquid equilibria behavior was highly dependent on the crude oil heavy fraction immiscibility because of the increasing system asymmetry by increasing mixture gas content.